AMA | December 2021

 

AMA questions December 2021

Joye Colbeck
Why are Black Holes bald?

Preston Justice
A couple of months ago, my Professor called your “Poetic Naturalism” pop philosophy. He is a philosopher and did not mean this condescendingly but what is your response to that? I understand that your books are written for a very wide audience to target those of us who are non-physicists but you also give us the math and make us work and you put in the work for your philosophical inquiries as well. So how would you distinguish “real” Philosophy from “pop” philosophy? I also follow the work of David Wallace and loved your podcast with him and know you have been associated with him for some time. I assume you have him and many other Philosophers to consult with whenever needed, right?

Regular Apistevist
Can you give a very simple explanation about the difference between primordial gravitational wave vs. gravitational wave from inflation?

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Perry Romanowski
How do black holes interact with dark matter? Does it fall in? Are there dark matter black holes?

Adam Berger
Since dark matter has mass and therefore generates gravitational force, can it form "heavenly bodies"?
Can dark matter form black holes, and do we have a Schwarzschild radius for that scenario?

Chris Chautard
In the LCDM model, could it be that dark matter black holes can form?
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Marian Marcali
Is the DM transparency a matter of scale, energy or magnitude mismatch in our tools: similar to the circle and point metaphor. If we only use a point as a probe, we can never find the circle.

LINEU D MIZIARA
How can an electron have zero size if It has mass?

Pete Harlan
What are your options for typesetting equations in the upcoming "Biggest Ideas in the Universe" book? Despite Knuth, equations in Kindle books seem to fall into two camps: static graphics that don't scale and look terrible in dark mode, or inline text formatting as in Susskind's "The Theoretical Minimum" books that is unreadable. Is there an option for an author that properly accommodates electronic books?

Björn Engström
If Jeff Bezos were to give you all his money, say 250 billion dollars. What would you spend it on. Also you can’t keep any of the money for yourself. What project or causes would you fund?

Geek God
The scientific method consists of going back to the drawing board with your theory if the data doesn’t support it – not to just postulate the existence of supporting data. How does it then make sense to say that dark matter and dark energy exist – just because we don’t see the amount of matter that we expect to see as per the theory? Shouldn’t we say instead that the theory is wrong?

Michael
PRIORITY QUESTION -- How fast are we moving hurdling through space if we are still, say, sitting in a chair? I.e., if we are ‘still’, aren’t we moving at the speed of the Earth's rotation -- while, at the same time, the Earth is also traveling at a certain speed hurdling through space, which is, at the same time, traveling through space which is expanding at a certain speed, and so forth?
Of note, if one were to say something like, “how fast are we moving relative to what?”, I would then wonder: what is the speed of the expansion of the universe relative to?

Jeremy Dittman
I am wondering about your views on the future of scientific progress (say 100-1000 years from now) and whether or not major advances on big problems (eg quantum gravity, rules of complexity and interesting forms of emergence, nature of space and time, etc…) would ultimately be taught to the equivalent of high school students in the future.

Chris Mortlock
Is the process of acquiring knowledge limitless or limited? Do you think it's possible a civilization could ever acquire 'total' knowledge with no further branches of scientific discovery possible?

Kunal Menda
Do you tell your guests what you intend to ask them, before a podcast? How much have they typically prepared their answers?

Frank Lehman
Do you think Twitter is a net good or negative for physics and science more broadly. It seems that, for every positive interaction one has on the social network, there are 10 others that confirm its reputation of being a "hell-site.

Paul Torek
In Everettian QM, individual branches get “thinner” over time (their amplitudes get smaller), so they make smaller and smaller contributions to the total energy of the universe. Suppose we fashion half the mass of a solar system into spaceships, and toss a quantum coin. Heads we send out the fleet, tails we stay a few more decades to improve the ships. We observe tails. Do we expect the local curvature of spacetime to be reduced as our other-branch siblings fly away, resulting in a lower all-branches local mass/energy density?

Richard Graff
I’m reading and enjoying Cixin Liu’s Three Body Problem series. In it a character describes a proton in a high dimensional space being “unfolded” into a lower dimensional space. One of the results is that the object’s size grows immensely large in the lower dimension, and increasingly so as it unfolds into even lower dimensions. Does this unfolding concept and the described size increase have any mathematical or physical validity or is just a clever literary device? If the former can you explain the process in any more detail?

Paul Hess
Can you explain the relationship, if any, between Entropy and Weak Emergence.
I began to think about this when Anil Seth described weak emergence as higher level behavior that can not be predicted from the lower level components without "exhaustive" simulation.
An exhaustive simulation sounds to me like it represents information that is not compressible, and in computer science they use the idea of entropy to talk about how compressible something is. Does it take any sense for me to think of Entropy and Emergence as related concepts?

Brendan
How would you apply Bayesian reasoning between a deity and an advanced alien being? If an “entity” suddenly appeared and performed actions that defied our current understanding of physics, how would you update your Bayesian probability?
I think many people would be quick and weight heavily for a deity, but it wouldn’t be completely unreasonable to think an alien race millions (or billions) of years more advanced to perform things that we think are not possible.

Kathi Seeger
I've read an article on wikipedia about "dark fluid" theory in which they aim at unifying dark matter and dark energy.
The theory proposes that "dark matter and dark energy are strongly linked together and can be considered as two facets of a single fluid. At galactic scales, the dark fluid behaves like dark matter, and at larger scales its behavior becomes similar to dark energy". What do you think of it?

Sam Hartzog
Do mathematical "functions" like integrals, differential equations, matrix and vector and tensor calculus etc have some kind of metric tied to them to represent something like "how many independent bits of information" are required to carry out meaningful "operations"? It occurred to me that a notation like tensor calculus allows you to write things pretty concisely and wondered if there was any kind of metric for "hidden computational complexity".

Thijs Janssen
You have me convinced of Everettian QM. So much, I can't see any problems with it. I listened to the podcast with David Albert twice, and read something deeply hidden twice, but I still can't see the problem with probabilities. [cut]

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Graham Clark
Do you think that a philosophical "zombie" is actually conceivable? Your arguments against panpsychism seem to indicate that, but then I wonder why the zombie concept hasn't been rejected.

Keith
I enjoyed your blog post on p-zombies for physicalism and visit on Mind Chat. I followed the reasoning for the most part, but I really get hung up a bit up stream.
As a fellow physicalist I don't really get how a p-zombie is even conceivable, or maybe my notion of conceivability is too strict.
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Alejandro Ordetx Gonzalez
Let us suppose we discover that consciousness is a fundamental property of nature (which I know you are inclined to think is emergent). What could be the implications of that for the current understanding of physics and the model of the universe we have now? Just entertain us with some plausible possibilities, if there are any at all.

krathorlucca
regarding your theory on the arrow of time, in the last months AMA you mentioned something allong the lines of: ‘I dont necesarily think my theory with Jeniffer Chen is true, but I think there’s a non-zero chance that it’s true. However, I know of no better theory regarding the arrow of time’
If you believe you have the best theory on the arrow of time, isn’t the rational thing to do then to regard it as true? We can never truly justify our beliefs, so what other reason can we have for believing something is true besides it being the current best theory of some part of reality?

Tarim Shahab
What is energy? In high school they would always say 'energy is the ability to do work', and then define work in terms of energy. I've also heard it described as 'book-keeping'. Is that all it is, or is it more meaningful?

Horst Wurst
What is your Bayesian prior of finding compelling evidence of biosignatures in the atmosphere of exoplanets in the next couple of decades?

Robert Ruxandrescu
A few days ago Elon Musk responded to a Bernie Sanders tweet about the rich paying their fair share to society by commenting "I forgot you're still alive". So I asked Elon Musk if he thinks living in a world where one man has one trillion dollars and other people die of hunger is a good world.
What ensued afterwards was a ton of attacks from the "sympathizers" of Elon Musk calling me all sorts of things, from "communist" to "idiot".
My question is this: where do you think all of this aggressiveness and "rich man idolatry" is coming from? Why are people so aggressive, why are people so obsessed with money and why are people so rude and violent?

Dvij Mankad
I was thinking that borrowing some physics jargon can bypass some of the free-will discussion pitfalls -- at least among physicists. In particular, do you think that the apparent conflict of the phrase "could have done otherwise" with determinism can be avoided by separating the off-shell and the on-shell descriptions?
I was tempted to think this way when I read the classic "Freedom of the Will and the Concept of a Person" paper by Frankfurt -- which, I think, is essentially formulating the concept of free will as the existence of degrees of freedom for the human will -- albeit in a different language.

Chris
What do you think are the most important things to know? Not in science, but in life in general.

Napolean's Corporal
DNA codes pass on physical traits, and the specific genes that carry these traits can be identified. But what about behavioral traits? Can we identify genes that result in things like the 'herding' instinct that Sheltie dogs have been bred to have? The broader question here is how can any physical thing influence something like behavior?

alexandra bates
Do you believe there are certain traits/paradigms that separate scientific theories from pseudoscientific ones, or do you believe all theories have the same capacity to be scientific, and what makes them so is their agreement with the data?

Liam McCarty
Why do you think some ways of solving a given problem are easier than other ways? I’m sure part of it has to do with human psychology, but I’m wondering if there’s anything beyond that. For example, arguably Feynman diagrams are more “efficient” than Schwinger integrals, and Newton’s laws are more efficient than Einstein’s field equations in a particular regime, beyond just matters of preference or style. Perhaps we could define such a notion of efficiency for problem solving approach or physical theory? Perhaps there are “complexity classes” of a sort here, not for different problems but for the same way of solving a single problem?

P Walder
Is the interference pattern observed in Youngs slit experiment to be considered as evidence for the Many Worlds Interpretation of reality?

Lukas Derhaschnig
I have been recently looking into the cosmic speedlimit postilated by Minkowsky. It was quite interesting to find out we all move through spacetime at the speed of light. My question is if we would completely stop moving through space how fast would time go by?

Marco Taucer
My question is: how is an electron's wavefunction different from the electron field? Do you picture them differently in your mind?

Dan O'Neill
What are we? When I think of the four-dimensional spacetime of relativity theory, and how the passing of time is just an illusion, I think we must be the braided worldlines of all the particles that have made up our bodies over the course of our lifetimes. Then, when I remember the questionable status of a "particle" in quantum mechanics, I think we must be an intricate inter-rippling of quantum wave functions. But then, when I remember the problem of wave function collapse, and the solution posed by Many Worlds, I try to picture myself as a massively complex branching bush of wave functions in an unpicturably vast multiverse. Ignoring emergent properties, is this what a human being is? If so, do you see it as a remarkable, perhaps beautiful, phenomenon even at this fundamental level?

Chris Murray
Near the end of "The Biggest Ideas in the Universe | Q&A 9 - Fields" you introduce the Reeh–Schlieder theorem. Can you make an educated guess as to what e.g. a person on a space station would see if they looked at the Taj Mahal being created on the moon from acting on the fields somewhere on Earth?

Nate Waddoups
Standard sirens came up in an episode that I listened to last month (sorry, I don't recall who it was with). What are the forces that come together to tune them to "standard" behavior? I mean, why do they all act the same rather than being random sirens?

Teresa Robeson
In your opinion, what are a couple of the most indispensable science magazines for the layperson (with a science background but not grad degree level) to read to keep abreast of recent scientific research and advances?

Michael Shillingford
Pre-Socratic philosophers used to debate whether reality was made up of discrete things e.g. Atomism or Stuff like water. I’ve recently seen similar argument centered around particles and fields. Which ontology is correct between things and stuff or is it defective debate?

Gregory Kusnick
Should parents lie to their kids about Santa Claus?

Varun Narasimhachar
Do you see a problem with basing day-to-day epistemology, ethics, and other essential aspects of human life on very rigorous calculi (e.g. Bayesianism, utilitarianism)? While it is possible to do due diligence to these calculi in academic research, implementing them in their "pure" form in our daily lives would require impractical amounts of thought, computation, and possibly knowledge. Are there philosophers who seriously work on developing "heuristic-only" versions of such calculi that are (1) feasible for everyday use and (2) robust against variations in the users' backgrounds, biases, self-serving motives, etc.? By seeking such heuristics, do you think we would merely rediscover intuitions that we already have, or are there potential surprises?

Emmet Francis
Any thoughts on some strategies for maintaining good mental health as a graduate student (or as an academic more generally)? From my experience as a current PhD student and from talking to other grad students, I know a lot of us struggle with imposter syndrome, anxiety from the pressure to "publish or perish", etc.

Jeff B
I was reading about Lord Kelvin's idea that different atoms were just different types of knots in the ether. Although this is obviously false, can we think of string theory as a quantized and relativized upgrade to this idea, or is there more to it?

Casey Mahone
I sense a stigma in our culture against being single. I am starting to find that I may be the kind of person that would prefer to just be alone, but people seem to view this as childish. They wonder how you can really be an adult if you don't have a partner to build something with. Do you sense this stigma, and do you think that it is justified?

Robert
PRIORITY QUESTION
If our current formulation of QM hasn’t appeared to answer what happed at or before the Big Bang, the Blackhole Fire wall and singularity questions. Is it better to think the Wave Function is a complete description of QM. Or that it needs to be modified to account for these mysteries.
Could you help shine a light on why you are taking the position you are.

Anonymous
Regarding the “many worlds” interpretation of quantum mechanics is information conserved in the same way that energy is conserved?

Seth
Did your discussion with Sylvia Earle on the ramifications of consuming animals change your view of the morality of animal consumption ?

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Sid Huff
You have mentioned a few times on previous AMAs that you had a distinctly (your word) non-academic upbringing.
Do you think it was extra-difficult for you to succeed so well academically because of that?
Have you ever felt the “imposter syndrome” – i.e., the sense that don’t really know enough, aren’t really capable enough, to be where you are, that somehow you’ve managed to fool everyone so far… ?

Jim Murphy
Do you have any advice about developing discipline? I am always impressed by how much you are able to do without seeming exhausted at all.
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Einar Wennmyr
PRIORITY QUESTION
Why is the uncertainty principle not enough to explain the collapse of the wave function at measurement in quantum physics?
A measurement of a quantum property means to amplify a phenomenon from the microscopic, atomic domain to the classical domain. To amplify means to add a substantial amount of energy to the microscopic domain to make it noticeable in the classical domain. When adding energy to one of a superposition of quantum states, the object being measured will have an uncertainty in its energy. This is determined by the added energy. The uncertainty in energy will imply an uncertainty in the life time of the superposed state through the uncertainty principle, causing the superposed state to decay quickly into one state.

Blake Brasher
On a somewhat recent podcast you mentioned that we shouldn't think of the nucleus of an atom as being composed of discrete protons and neutrons, but rather as a swirling mass of quarks without any distinct boundaries between the quarks that make up on proton or neutron and those that make up another proton or neutron. Does this mean that neutron stars are actually giant blobs of quark soup, or is it also wrong to think of a neutron star as being the same thing as an atomic nucleus?

Simon Kitt
PRIORITY QUESTION Is the discussion and research of consciousness an example of strong emergence? As much as I struggle with the strong emergence concept, I find it hard to have a coherent picture of endeavours to understand consciousness that doesn't involve some top down causal power. I cant imagine a race of philosophical zombies having this field of inquiry. What could possibly be causing discussion and research into the nature of consciousness if not consciousness it self?

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James Nancarrow
Why don't we know the actual size of the universe?
I understand that it may be that our observable universe is only what we can see and there is likely matter further away beyond our horizon But doesn't the CMB or early universe nucleosynthesis calculations allow some scaling of the universe's total size, that is does the proportion of primordial H and He determine the total amount of matter, the bigger the universe the more time for cooking up more Helium at the start?

Aleksadar Marash
Hi Sean, I think you mentioned in one of the previous AMAs (while discussing Inflation theory) that the Universe, while obviously being super-small at the beginning and during the inflationary epoch, could have been infinite in size right after the Inflation ended. I heard professor Alan Guth saying many times that our Universe could have been the size of a big marble or a baseball at the end of the inflationary period. Taking this piece of information on board, did I, therefore, get your point wrong? More broadly, when we say that our Universe could be infinite in size, either right after Inflation or today, I get confused: do we mean our Universe, which, as per Guth's opinion was once the size of a marble, or some other (which?) broader reality?
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Thomas Prunty
In your podcast with David Wallace you guys had a bit of disagreement about how to think about the low entropy of the big bang. He said something like "The early universe had as high an entropy as it could have given that it was uniform", and you said that uniformity was a huge constraint.
I don't see why uniformity should be surprising or need explanation, it seems like a very simple and sensible boundary condition. I understand it's low entropy when gravity is considered, but I don't see how that implies that it's unlikely in this case. What am I missing?

Bijan Warner
Here's a question on the relationship between quantum mechanics and chaos theory:
To what extent does the principle of sensitive dependence on initial conditions (i.e., the butterfly effect) apply to quantum systems? Does this framework matter at all, or is there something about quantum systems that make this not an interesting thing to ask?

andrew vickerstaff
Many recent Mindscape episodes have looked at the fallibility of the Human thought process and how susceptible we can all be to bias, manipulation and irrational ideas. Given that we understand this problem today much more than we did, say, 20 years ago, and given the exponential growth in misinformation the average person is now exposed to, do you think that the Jury system, in its current form, is still a fair why to decide criminal trails? In particular do you have any thoughts on the jury selection process in America and whether it helps or hinders the justice system?

Brandon Lewis
Do you imagine you will use quantum computing in your research, once they become available to you, and mature enough to be useful? If so, what kinds of problems would they help you to solve?

Peter B
Priority Question: You have discussed before how the many worlds of quantum mechanics may be just the other side of the same coin as the cosmological multiverse. I am still struggling to understand this idea. I can understand how I am in a superposition with other versions of me in the quantum multiverse but how can I be in superposition with other versions of me trillions of light years away? Or are you saying these two types of multiverses redundantly contain the same set of events but they they are still distinct from each other?

Matt Hickman
PRIORITY QUESTION: Can I have another priority question please?

Rebecca Lashua
You've mentioned in the past that if you weren't a theoretical physicist, you would consider being theoretical computer scientist. Are there any particular results in the theory of computation that you find interesting or aesthetically pleasing?

Peter C. Harris
Do you have any thoughts about the 1619 Project and the New York Times' decision to endorse it? My understanding is that the person leading the project is a brilliant journalist but not an historian and that many historians, including African American ones, say that the project contains significant historical errors. Isn't this roughly analogous to a science journalist creating a project that endorses the Copenhagen interpretation of QM and having that project endorsed by the Times, even though credible rival interpretations, such as Many Worlds, exist? Should a newspaper of record even be making such endorsements?

Paul Cousin
The last episode with Tai-Danae Bradley was fascinating. In it, you discussed the philosophical question of whether it could be conceivable to find all of the semantics in the statistics of words alone. Would it make the idea more realistic if we include in these statistics all the sensory inputs that humans receive from the world while experiencing words? In which case a machine would be strongly limited in its abilities to understand language unless we provide contextual sensory information as well in the dataset. What do you think?
naveed alam
which genre of music do you consider to be your favorite? do you have any interest in opera, say operas by wagner, richard strauss, or mozart?

Alex Borland
Do you have any favorite restaurants in Boston, especially in Chinatown?

Justin Bailey
What do you think is the most credible explanation for why we experience a “single” reality in a branching universe?

Josh Charles
When thinking about the far flung future, do we have an idea of how much matter would escape going through the process of entering a blackhole and being radiated back out?

Zach McKinney
How do you see differences in seemingly esoteric philosophical and metaphysical questions such as the nature of consciousness or free will translating into concrete and consequential differences in how people approach real-world problems and conflicts?

Sam Buck
And now for something entirely different… What’s your favorite kind of sandwich?

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0:00:00.0 Sean Carroll: Hello everyone, welcome to the December 2021 Ask Me Anything episode of the Mindscape Podcast. I'm your host, Sean Carroll. The podcast episode is being released a little bit before December comes along, because it's the Thanksgiving holiday here in the United States, so I'll be travelling for that and then travelling for a workshop directly afterward. I wanted to get to your AMA questions before I started travelling rather than waiting until afterward. So, you know that... Or long-time listeners know that, I do take vacation for real over the Christmas holidays/New Year's holidays. So, there's two weeks when I will not have a real podcast episode, and I will try to do a holiday message, short, brief, thanking everyone for their support kind of thing, just before the holidays. And there will be no Ask Me Anything episode for January, so no Ask Me Anything episode with questions asked at the end of December, answers given at the beginning of January. So this is it, this is the last Ask Me Anything episode for this year. The next call will be near the end of January.

0:01:04.7 SC: Again, those of you who are long-time listeners know if you want to ask a question for the Ask Me Anything episode, the important thing is to support Mindscape on Patreon. You can go to patreon.com/seanmcarroll, and you sign up there, dollar, or whatever you want, per episode of Mindscape, will get you both ad-free versions of the podcast as well as the ability to ask questions for the monthly Ask Me Anything episodes. It used to be, back in the old days, when it was just a few of us around the fire, that everyone who asked a question got it answered. We've grown since then, it can't be quite the same anymore, but I do try to pick the best questions in terms of things I can give interesting answers to. Do not feel bad if you ask a question that I don't get around to it. It's me, it's not you, don't worry about it.

0:01:53.1 SC: The other thing, of course, is I'd like to remind people we have a web page for the podcast, preposterousuniverse.com/podcast, and all the Mindscape episodes are there. You can search them, there's a wonderful little search tool, you can search not only the titles and my show notes, but also complete transcripts of all the podcasts, transcripts that are paid for by the Patreon supporters we have here. And of course, you can also get links to the people who are talking, their web pages, their publications, whatever it is. And the final thing, which I never remember to mention but I always remember it when I'm not recording, is that we do appreciate reviews of the podcast on iTunes and elsewhere. Getting good iTunes reviews helps you get more visible to other people who might like to listen to podcasts. So, as good as the podcast is, don't hog it for yourself. Share it around, share the word that Mindscape is a good thing. Let me see, anything else? Nothing else right now, I have a lot of packing to do. There is a trip coming up and a lot of questions to get to, so let's go.

[music]

0:03:16.6 SC: Joy Colbeck says, "Why are black holes bald?"

0:03:20.9 SC: This is of course in reference to the famous statement by John Wheeler that a black hole has no hair, an idea that was semi-rigorized by Roger Penrose into the what we call "the no-hair theorem", but a mathematician would not really call it a theorem, it's not been proven yet, because of course, you don't know the ultimate laws of physics yet. What does it mean to prove something, unless you have some good axioms? Now, what you can do is assume some laws of physics, like it's just general relativity plus electromagnetism, and then you can prove a no-hair kind of theorem that says that once you make a black hole, unlike a planet or a star which has all sorts of interesting features on it, all black holes settle down to a uniform configuration, it's always the same kind of configuration, given the mass, the charge, and the spin of the black hole. There's no other hair on a black hole other than that, it's not like you have continents and oceans or something like that, like a planet does. So why, is Joy's question. "Why" questions in physics are always tricky, it depends on what assumptions you're gonna start with here, but the way I like to think about the black hole situation is, it's entropy. It's usually entropy that is at the heart of these kinds of questions.

0:04:30.9 SC: Black holes have the highest entropy that you can fit into a region of fixed size. They are, in some sense, local maxima of entropy, and what that means is operationally that they're all gonna look like. That's a feature of high-entropy states, is that they're not that interesting. Think about black body radiation, think about taking an object, a relatively heat-proof, melting-proof object, okay, so an object that can sustain some high temperatures, but it doesn't matter what it's made out of, it doesn't matter what colour it is, it doesn't matter what shape it is, or anything like that. Put it in a tight oven, a tightly-sealed oven, at a very fixed temperature, and let it come to equilibrium. So let the entire object inside come to the same temperature that the oven is, and then open the oven.

0:05:20.6 SC: What you will see is that the object is glowing, giving off radiation, and the radiation it gives off and the way it gives it off is completely independent of the material it's made out of, of the colour it is, of the shape it is, or anything like that. That is because it has come into equilibrium with the thermal bath that it is in, and it has a fixed temperature now, given its shape, etcetera, it's in a maximum entropy state, and that means it's featureless, in terms of the radiation it gives off. And black holes are the same way. Black holes, as Hawking taught us back in the '70s, give off thermal black-body radiation, another sign that they are maximum-entropy states. So maximum-entropy things, given their macroscopic constraints, like the size, the mass, the charge, the spin, etcetera, whatever quantum states are inside the black hole are as uniform and high-entropy as they can possibly be. That is why black holes are bald.

0:06:21.7 SC: Now, that's a very different explanation than you would have gotten back in the late '60s, early '70s, when they first started talking about this stuff, because they were not thinking about quantum states or entropy; they were just thinking about classical general relativity. It's a feature of classical general relativity that if you make a black hole, it can have bumps and wiggles, but those bumps and wiggles quickly radiate away. That's what happens, for example, when two black holes in spiral, like we see at LIGO or something like that, all the individual idiosyncrasies of the specific states that went into making the black hole are radiated away in the form of gravitational waves, and the black hole settles down very, very quickly. So that's just a feature of classical general relativity, but in some sense, it makes a little more sense to us once we understand the quantum underpinnings of that and the fact that it's evolving towards a maximum-entropy state. In fact, this was the inspiration, right? There was a noticeable set of similarities between black hole mechanics and thermodynamics, that's what eventually pushed Hawking and Bekenstein to show that there is an entropy and a radiation coming out of black holes.

0:07:25.3 SC: Preston Justice says, "A couple of months ago, my professor called your poetic naturalism pop philosophy. He is a philosopher and did not mean this condescendingly, but what is your response to that? I understand that your books are written for a very wide audience, to target those of us who are non-physicists, but you also give us the math and make us work and put in the work for your philosophical inquiries as well. So how would you distinguish real philosophy from pop philosophy. I also follow the work of David Wallace and love your podcast with him, and know you have been associated with him for some time. I assume you have him and many other philosophers to consult with whenever needed, right?"

0:08:00.3 SC: So I almost didn't answer this question because I don't wanna feel like I'm just defending myself or something like that, but I think this touches on a much bigger and really important issue, so I will try to address it directly, specifically, and then go on to the general thought here. Look, I don't care if someone calls poetic naturalism pop philosophy. The book, The Big Picture, which I wrote about poetic naturalism, is a popular-level book, no question about that. It is not a technical philosophical tome. It is not written in the style that you would find professional philosophical writings appearing in. Professional philosophy would be much more careful about not just name-checking all of the great minds and other people who've thought about these issues, which I did a little bit but nowhere near comprehensively. It would also be much more careful, typically, professional philosophy, especially in these days, tries its best to go through every single possible permutation of the argument in front of you. In fact, this can be frustrating sometimes as a scientist who likes philosophy. Sometimes philosophers seem to be content with just listing every possible answer to a question rather than telling you what is the right answer. [chuckle]

0:09:11.5 SC: As a scientifically-oriented guy, I just wanna get the right answer. And so, that's the spirit, both as someone who just wants the right answer and someone who is writing a book for a popular audience, that's the style in which all of that was talked about in The Big Picture. Now, what I do care about is whether it's right or wrong. [chuckle] If the characterization of it as pop philosophy was in some sense meant to imply that it was not serious, that it was wrong somehow, then I would like to know why it's wrong. Then, just labelling it "pop philosophy" is kind of meaningless. That is just then... Carl Bergstrom in the podcast episode we did here, labelled "bullshit gatekeeping", [chuckle] making sure that the interlopers from outside your discipline don't come in. But I did talk to plenty of philosophers while writing the book, I've gotten very good responses from philosophers since then, some philosophers have used the book as texts in their class. Others have disagreed with the book, which is fine, that's what I care about, I care about either people agreeing with it or disagreeing with it less than whether it's called "pop" or "serious".

0:10:20.7 SC: But the bigger-picture question which this touches on is there's sort of two dimensions along which we're plotting our discourse here: One is the disciplinary dimension, there's science, there's philosophy, there's history, there's etcetera, etcetera, and the other dimension is the sort of academic versus popular dimension. And so, when I'm writing about philosophy, I'm both being interdisciplinary, crossing over a disciplinary boundary within academia, but also writing for a wide audience, not writing a sort of technical thing. And so the big question is, "Where do we place our trust?" Forget about me, forget about my book, but when we're reading some book or listening to some source, maybe it's a YouTube video or a podcast or an article in the New York Times, but it's by some person, and we are not individually experts in the area that they are writing about, how do we judge how seriously to take them?

0:11:15.4 SC: That's a really hard question. We would like to find some simple algorithm for thinking about it, but there is no such simple algorithm; we have to really kind of think carefully in each individual case. Now, certainly, you would like to say that if someone is a credentialed expert in a certain field and that's the field they're writing about, then you take them a little bit more seriously. You don't take them just at face value, you don't say, "Well, they must be right because they're an expert," because experts disagree with each other sometimes, but you would put more weight on someone who is writing about exactly their field. But you also know that even people who are experts sometimes say silly things, even within their own field. [chuckle] I know physicists who say silly things about quantum mechanics all the time. Some physicists would say I say silly things about quantum mechanics all the time. So you can't use that as a completely reliable guide.

0:12:08.9 SC: I am a huge believer that people in academia should write for both wider audiences and for other academic audiences in different fields, while keeping in mind that the process of doing that is necessarily going to open you up to making mistakes and being criticized, right? So, I think that there's some kind of attitude that says that if you're an expert in field X, you just should not talk about any other field other than X. And I think that's a terrible attitude. How in the world are we gonna make progress if different fields can't talk about and to each other? But I would take a philosopher talking about physics less seriously than a philosopher talking about philosophy, and vice versa for physicists talking about philosophy. So I think you need to really take attempts for people to talk outside their credentialed area of expertise with a grain of salt, but there are techniques we can use to decide who is trustworthy and who is not. And these techniques have more to do with the person talking than the thing they're talking about. Once you know that they're not exactly in their expertise, you shouldn't completely dismiss them, but you should ask questions about how credible they seem. So what does that mean?

0:13:25.6 SC: Well, you can ask do they seem to be familiar with the field they're talking about, even if it's not their technical area of expertise? When they're talking, do they seem to be overly sure of themselves? Are they very clear about what is known and what is not known? Are they clear about what they're trying to say with confidence versus speculating about? Are they willing to be wrong, or are they fallibilistic, or are they just sort of bombastic? Are they clearly trying to be sensationalist or contrarian just for the sake of being contrarian, just to sell books or get clicks or something like that, versus are they trying to work through difficult issues in good faith to the best that they can, right? So all of these are things that you can think about and try to take seriously. At the end of the day, you have to judge it, whether if someone is... If you're an expert in philosophy or physics or anything else, when you read my book or anyone else's book, you've got a judge how seriously you wanna take that. At the end of the day, that's what matters, whether or not the ideas are worth something, not whether or not they're popular versus academic, philosophy versus physics, or anything else.

0:14:34.8 SC: Regular Apistevist says, "Can you give a very simple explanation about the difference between primordial gravitational waves versus gravitational waves from inflation?"

0:14:44.8 SC: The zeroth-order explanation is, no, there is no difference, so there's not that much to say, but there is a tiny difference depending on how exactly you wanna get into it, namely, gravitational waves from inflation are a subset of all possible primordial gravitational waves. So inflation is this idea that in the super-duper early universe, the primordial universe, the universe underwent a phase of exponential super-fast expansion. And due to quantum fluctuations, that would naturally generate some gravitational waves that could leave an imprint on the universe. Cosmologists have not found evidence for such an imprint yet. There was a moment a couple years ago when we thought maybe we had, but that went away. It turned out not to be right. So, inflation predicts that there should be gravitational waves, but it's silent on exactly how strong they should be. So, it's very possible that inflation is correct, and yet we don't ever find the gravitational waves predicted by inflation. It is also possible that we do find them, and that would be evidence for inflation. It is also possible that we find gravitational waves and yet they come from something other than inflation, and that's tricky, because if you don't know what the specific model is, you don't know whether it predicts gravitational waves or not.

0:16:00.0 SC: So, in other words, I can imagine primordial gravitational waves that are not from inflation, I just don't have any specifically good scenario to make them. I know there are all sorts of bouncing and cyclic scenarios and things like that, I find these generally utterly unconvincing for completely other reasons, so I don't even know personally which of them would predict primordial gravitational waves and which would not.

0:16:27.0 SC: Okay, I'm gonna group some questions together here, three of them. Perry Romanowski says, "How do black holes interact with dark matter? Does it fall in? Are there dark-matter black holes?" Adam Berger says, "Since dark matter has mass and therefore generates gravitational force, can it form heavenly bodies? Can it form black holes? And do we have a Schwarzchild radius for that scenario?" And Chris Chotard says, "In the Lambda-CDM model, could it be that dark-matter black holes can form?"

0:16:54.0 SC: So I don't know why [chuckle] there were simultaneously a bunch of questions about dark matter, black holes, maybe there was a news article or something or a tweet that I missed out there. But the short answer is, yes, sure, dark matter can make black holes, but in the real world, it's much, much, much less efficient than ordinary matter. The point is that ordinary matter is not dark, it couples to electromagnetic radiation; and dark matter is dark, it does not couple to electromagnetic radiation. And what that means is that when dark matter particles come close by, they just go right through each other, they do not dissipate. When atoms bump into each other, they can lose energy by bumping into each other, temporarily getting excited, and then kicking off a photon, by radiating. And radiating that energy away, they lose energy. So when a bunch of atoms or individual ions or electrons or whatever come near to each other, they can stick together, they can clump, they can coalesce.

0:17:57.1 SC: And black holes are very, very, very dense. To make a black hole, you have to put a lot of mass, a lot of energy, into a very, very small region of space. Even though there's more dark matter in the universe than ordinary matter, it's not condensed; it is thinly spread throughout the universe, and it has no forces acting on it that want to condense it. I mean, there's gravity, but the problem is you have two dark-matter particles, they pull each other through their mutual gravitational pull, and they come right by and they go right past each other, 'cause they don't stick, 'cause it's only gravity, and gravitational waves are far, far too weak to make that happen. So in principle, you can make black holes out of dark matter. In practice, it doesn't really happen.

0:18:39.1 SC: Now, there is a loophole there. It is possible that in the early universe, ordinary matter and dark matter coalesce in the same place. So, the dark matter might be a little over-dense in a region where the ordinary matter also is, and they both condense together, and even though the dark matter just goes right through and the ordinary matter can stick, if the ordinary matter is going along with the dark matter and it gets so dense that it makes a black hole, that will trap some of the dark matter in the black hole. So dark matter can contribute to the formation of black holes in the early universe, and there's some theories that that's actually important astrophysically. But by itself, it would be very hard. If ordinary matter were not helping it along, it'd be very, very hard to make a dark-matter black hole. Not impossible, but not easy, either.

0:19:29.3 SC: Relatedly, here's another question, from Marian Marcally, "Is the dark matter transparency a matter of scale, energy, or magnitude mismatch in our tools? Similar to the circle and point metaphor, if we only use a point as a probe, we can never find the circle."

0:19:45.0 SC: This is an interesting question. So, I think I get it, I think I get what you're asking, let me try to rephrase it. We say that dark matter is dark, and the easy way for that to happen is that dark matter doesn't interact with photons at all, with electromagnetism at all. It's neutral, it's therefore dark. But I think the question being asked here is, "Can we imagine that, for some reason, dark matter is transparent to certain wavelengths of light but not to other wavelengths of light?"

0:20:19.1 SC: I think that the short answer is that in easy models of dark matter that you would write down, that's hard to make work, [chuckle] because, basically think about it this way: In particle physics, a particle is either electrically charged or it's not. Now, in the real world, we can make condensed collections of particles that are either transparent or opaque. We can make a piece of wood or a piece of glass, and one of them will stop light from going through it and the other one will not. But the individual particles, if you just dispersed them, all of these particles are charged and they will interact with photons very, very readily. And if they're not interacting with photons, if they're not charged particles, it's hard to make them stick together like we just said, and form substances, okay?

0:21:09.9 SC: So, the reason why ordinary matter can either be transparent or not to different wavelengths of light is because of the sort of chemistry and condensed-matter physics of it when they come together to make different arrangements of atoms and molecules, not because of the properties of the individual particles themselves. And we don't think that dark matter does come together in that way, so we think that the dark-matter particles are either charged or they're not. If they're charged, they would not be dark; they'd be visible, we could see them. And if they're not charged, then they're gonna be dark, no matter what you want. So I don't see any simple way to make dark matter particles interact with certain wavelengths of light but not with other ones. There might be something more clever that I'm not thinking of right now, like maybe the dark-matter particles are not charged but they have like an electric dipole moment or something like that, I don't know, that would be a more complicated model that I would have to think about.

0:22:05.2 SC: Parenthetically, I cannot help but mention that there is the possibility of dark radiation. So not only dark matter but dark photons, dark light, right? This is an idea that I wrote about with some students. Well, one student, Lottie Ackerman, and one post-doc, Matt Buckley, who's now a professor at Rutgers, and my colleague at the time at Caltech, Marc Kamionkowski, who's now at Johns Hopkins, we wrote a paper: What if there was an entirely different copy of electromagnetism that only coupled to dark matter, not to ordinary matter? And interestingly, it can work, it's possible. But still, that would not affect how dark matter interacted with ordinary matter... Or with ordinary visible radiation. Sorry about that.

0:22:51.8 SC: Lenieux Miciara says, "How can an electron have zero size if it has mass?"

0:22:58.9 SC: So, the first answer I wanna give is, why not? Who says that you need to have size to have mass? You're clearly kind of visualizing some very classical kind of lump of clay model for the electrons. But elementary particles are not like that. The better answer is electrons don't have zero size. I know people say that they do, but electrons are wave functions, and now, I'm just gonna assume that my version of quantum mechanics is the correct one, my Many-Worlds version of quantum mechanics. If you think that the wave function is really what is going on in the world, whether it's in many-worlds or any other version of quantum mechanics, then electrons don't have zero size, you cannot localize the electron wave function to a point. You can get an approximate theory of how electrons behave by starting with the classical theory of a point particle and quantizing it. That's the reason why physicists will sometimes claim that electrons have zero size.

0:24:01.0 SC: But that's just because they're not taking quantum mechanics seriously. I mean, the quantum theory doesn't have any memory that you made it from the classical theory of a point particle. It is in some sense as if when you measure the electron it has no spatial extent. You can measure it with an arbitrary precision, its position or something like that, but that's a feature of the measurement process of quantum mechanics, not an intrinsic feature of the electron itself. So, real world electrons do not have zero size, therefore it shouldn't bother you that they have mass.

0:24:37.0 SC: Pete Harlan says, "What are your options for typesetting equations in the upcoming Biggest Ideas in the Universe book? Despite Donald Knuth, equations in Kindle books seem to fall into two camps: Static graphics that don't scale and look terrible in dark mode, or inline text formatting as in Susskind's The Theoretical Minimum books that is unreadable. Is there an option for an author that properly accommodates electronic books?"

0:25:00.2 SC: So the only reason... I have no idea what the answer to these questions is, they're good questions. The reason why I'm reading it out loud is because I had never even thought about this. But your right, of course. So, implicitly in the question, you're asking about the electronic editions of books. Believe me, it's hard enough to get the paper editions published. A publisher, like my publisher, Dutton, who has done all of my books so far, all of my trade books so far... And every one of my trade books has one or two equations in it, right? So they know how to typeset an equation. But this book has a lot of equations in it, The Biggest Ideas in the Universe books coming up. And it's gonna be a little bit tricky to make sure that everything is italicized correctly, formatted correctly, numbered correctly, all of that stuff. And I was already fretting just about getting the paper edition done correctly. And you're pointing out correctly that we're gonna have to think again about the electronic edition.

0:25:56.7 SC: So I don't know what the answer is. I do know that my publisher tries really hard. They don't brush off these concerns, so they will try. And now that you've put the idea into my head, I'll at least make sure they're warned ahead of time about that question.

0:26:18.1 SC: Alright. Bjorn Angstrom says, "If Jeff Bezos were to give you all his money, say $250 billion, what would you spend it on? And you can't keep any of the money for yourself. What project or causes would you fund?"

0:26:29.4 SC: I'm very sad I don't get to keep at least one billion dollars out of 250 billion. I mean, it seems like you're giving me a lot of work to do and then I get no recompense for it at all. And I don't have a simple answer to this kind of question. I think when you're talking about that scale of question... So here's how I interpret the question, How to be a good global citizen in the case where you have a huge amount of money, okay? So not just like "What is the one single thing you would do?" I think there's more than one thing worth doing with all that money, and therefore you need to sort of set up a system for divvying out different amounts of money to different kinds of causes.

0:27:04.5 SC: And I also think that it's still worth giving money to some causes even if they're not the single highest priority cause that you can think of, like things like solving global poverty or educating people worldwide, or just building roads and infrastructure around the world, these are the most obvious things to do. Healthcare for people around the world, these are things that I probably couldn't get done for $250 billion, but you could make a dent in it, and I think that's well worth doing.

0:27:34.6 SC: But then what about supporting graduate students in theoretical physics [chuckle] or supporting local libraries or blood drives or things like that? Those are also worth doing, even if they might not have a sort of "saving lives per dollar" kind of aspect. What about supporting arts organizations or other good charitable things? I think that all of these things are worth doing, to some extent. So I would want to have a foundation or something like that, try to have a lean and mean foundation that did not soak up a lot of the money itself, less than 1% of the total, but with only 100th of a percent of 250 billion, you can still fund a pretty good foundation. Or maybe, I don't know, put it in the bank and live off the interest. Not live off the interest, but give money off the interest. And like I said, I would wanna dole it out to many different things at once, starting with the most high impact things in terms of saving lives and making the lives of poor people around the world better, but also setting aside some for other aspects of life: Cultural, scientific, educational aspects and so on.

0:28:46.0 SC: Geek god says, "The scientific method consists of going back to the drawing board with your theory if the data doesn't support it, not to just postulate the existence of supporting data. How does it then make sense to say the dark matter and dark energy exist just because we don't see the amount of matter we expect to see as per the theory? Shouldn't we say instead that the theory is wrong?"

0:29:06.3 SC: Well, what theory are we talking about here? [chuckle] I mean, is the theory general relativity? General relativity doesn't make a prediction all by itself for what the rotation of a galaxy should look like, it depends on how much mass is in the galaxy. So the theory is the combination of the theory of gravity plus the theory of what is causing the gravity, the mass, the energy, and so forth. And when we say there's dark matter and dark energy it is precisely that the theory is wrong, that is what we're saying. We said, "We had a theory, namely that the matter that you see is everything there is, plus general relativity is the right theory of gravity, we made a prediction, it came out wrong, and therefore that theory is wrong." But adding more matter, dark matter or more dark energy or whatever, is just as much changing the theory as changing the theory of gravity. And of course, cosmologists have tried to change the theory of gravity, I've tried to do it. It just doesn't work. It doesn't fit the data. So we're very interested in fitting the data. And dark matter and dark energy fit the data really, really well. That's why they're the popular way to go in these particular situations.

0:30:17.5 SC: So Michael asks a priority question... Remember the priority questions, you have the rule that I made up, which says that every person gets to ask a priority question once in their lives, and I will always try to answer the priority question, so I can't guarantee... I answer roughly these days half of the total questions that are asked, I answer half the total questions that are asked, but if you ask a priority question, if you give that one try, I will do my best to answer it. Sadly, there's no follow-up priority questions, so if I don't answer it in a way that makes you happy, sorry about that, I can't really tell you what to do. Anyway, so Michael's question is, "How fast are we moving, hurdling through space, if we are still, say sitting in a chair? If we are still, aren't we moving at the speed of the Earth rotation, while at the same time the Earth is also travelling at a certain speed, hurdling through space, which is travelling through space, which is expanding at a certain speed, and so forth? If one were... " So, now Michael is anticipating the answer here, "If one were to say something like, 'How fast are we moving relative to what?' I would then wonder what is the speed of the expansion of the universe relative to?"

0:31:24.7 SC: So the correct answer to the first part of the question is, "Relative to what?" I mean that's what relativity says. There's no such thing as "the speed" with which you're travelling, there just is no such thing. You really have to accept that, it's not just like a fun little motto. There is only the relative speed between different objects. And so there's no such thing as the speed that you're moving at when you're sitting on a chair, you always have to say relative to what. And then you say, "What is the speed of the expansion of the universe relative to?" but the answer there is that the expansion of the universe does not have a speed, it just doesn't. It has a rate, you can ask, "In a certain period of time, by what percentage will the scale factor of the universe expand?" but that is not a quantity that is measured in metres per second, it does not even have the units of speed, it has units of one over time. And that time is, roughly speaking, the time scale for the universe to double in size. So that's not a velocity that is relative to something else, that is a rate of increase in some quantity which can be perfectly well defined. So it's a different kind of thing than the velocity with which you're travelling through space.

0:32:33.3 SC: Jeremy Dittman says, "I'm wondering about your views on the future of scientific progress, say 100 to a 1000 years from now, and whether or not major advances on big problems, quantum gravity, rules of complexity, emergence, etcetera, would ultimately be taught to the equivalent of high school students in the future."

0:32:51.0 SC: That's a good question. You can certainly imagine what people would have said if you asked this question 500 years ago, and they're like, "Yeah, you might come up with crazy new theories, but they won't ever trickle down to high school students," but these days we do indeed teach high school students natural selection or Newton's laws of motion. You know, we don't teach them Maxwell's equation, we don't even teach them Boltzmann's equation for entropy or anything like that. So there's plenty of things over 100 years ago that we do not teach them. And so the short answer is I don't know. I can see it either way. I would imagine that some things that we learn over the next 100 or 1000 years will turn out to be both so fundamental and so readily understandable that it is completely possible to teach them to high school students, and we will end up doing that. I can also imagine that the things we learn just remain a little bit too esoteric, in the sense that to really understand them you would need to have a lot of background, you would need to do a lot of pre-requisite work to learn how to manipulate equations or some abstract mathematical concepts or something like that. So I think it'll be a mixture of those things.

0:34:09.3 SC: It's hard to be more definite than that, 'cause I don't know what the scientific advances are going to be, right? I don't really think that high school students are going to be doing a lot of tensor analysis or something like that, like you need for general relativity. They could, in principle, I could imagine, but it takes a lot of work to learn the background concepts to get up to that point. So I don't think it's simply a matter of as time passes we compress these ideas into earlier and earlier educational stages, but there might be some brand new ideas about emergence or about space-time or whatever, that are actually simple enough to teach the high school students. I just don't know.

0:34:52.5 SC: Chris Mortlock says, "Is the process of acquiring knowledge limitless or limited? Do you think it's possible a civilization could ever require total knowledge with no further branches of scientific discovery possible?"

0:35:06.0 SC: No, I don't think that's possible. I think that it is possible to know the laws of physics. I've said many times that we know the laws of physics that are relevant to our everyday lives, but that we don't know the laws of physics outside our everyday lives, we don't understand quantum gravity or dark matter or what happened with the Big Bang, etcetera. I think it's possible that we could know them, however. So I certainly imagine that someday we might either know all the fundamental laws of physics, or at least have a theory that purports to be the fundamental laws of physics, that is both completely consistent and fits all the data. You always wanna keep open the possibility you're gonna have to revise your ideas later on down the line, right? So even if it is the correct theory, you're not sure.

0:35:52.7 SC: But here's why I don't think you'll ever know all the laws of science, broadly construed, is because you can always just get more detailed, right? A human being cannot carry in their brains enough information to completely describe another human being, because by a complete description you would mean the position and the momentum of every single atom in that other human being. And there's just not enough room in your brain to hold all of that information, right? So that's why we coarse-grain and do higher-order approximations and things like that, so we might have theories of psychology and sociology, etcetera, that are really, really, really good, but still are based on incomplete information. And so I think there will always be more we can learn because there is just... Until we are ourselves as big as the universe, there is an obstacle in principle for us to understand everything that is happening in the universe, right? And that's why none of us is or ever will be Laplace's demon.

0:36:53.0 SC: Kunal Menda says, "Do you tell your guests what you intend to ask them before a podcast? How much have they typically prepared their answers?"

0:37:04.6 SC: Very little. Look, most people who I talk to, they have a spiel, they have something they like to talk about that is kind of pretty obvious, so we don't need to do a lot of negotiations. Sometimes there are people who... I've certainly done a lot, like when you talked to Dan Dennett, he's done a lot of different things. You talk to Roger Penrose, he's done a lot of different things, and so you might want to say, "Well, okay, let's pick this to focus in on." And I do generally say that ahead of time in the invite letter, but I also offer them the opportunity to say, "Actually, what we should talk about is this other thing." But that's it, it's really just a couple of sentences back and forth. It's certainly not the case that I tell them the questions I'm going to ask, since I don't know those, oftentimes, before we even start the interview. I have a kind of bullet-point outline at best and then try to work from that.

0:37:52.9 SC: But you want the interview to be not overly planned. I think that my very early days of podcasting I planned too much, I really had my questions plotted out ahead of time, which is no good way to do it. You want some points you wanna hit, but the specific questions and the specific give and take has to come organically out of the interview. And again, most of the people who I have, it's not their first rodeo, they've done this before, so they're pretty good at offering answers that sort of make sense. I do try to have, as I've said before, like diversity along many axes, and one axis is youth to age and seniority. So I have some famous senior people and I have some younger people relatively just starting out. And unsurprisingly, young people just starting out are less practised at hitting the sweet spot of long enough answers to give some meaty details without just giving a little mini lecture every time. So that's a skill you develop over time. That's okay, I'm happy to play a tiny role in exposing certain young promising people to a wider audience and giving them a chance to sort of hone their technique at addressing these questions in real time like that.

0:39:10.5 SC: Frank Layman says, "Do you think Twitter is a net good or negative for physics, and science more broadly? It seems that for every positive interaction one has on the social network there are 10 others that confirm its reputation as being a hell site."

0:39:25.8 SC: You know, look, I don't know whether it's a net good for physics or science. That's very hard to tell. Most physicists and scientists don't use Twitter. I think that you can get a very distorted view of the world by taking Twitter too seriously as reflecting what is going on in the wider world. Okay? And that's just as true for... It's even more true for physics than it is for something like politics or sports or whatever. And physicists are the worst, like physics don't do use social media at all compared to other things. And there's a broader point here that I've said before but is worth emphasizing, that both the people who come to mind when you say "a physicist in the modern world", and the issues that come to mind when you say "What physicists care about in the modern world" are completely different in the public sphere and in the academics sphere. There's some overlap, but it's kind of small, this overlap.

0:40:26.4 SC: So the things that get talked about in physics departments among physicists, there's a certain set of issues, a certain set of people who are looked up to as, you know, wise voices that we should pay attention to, etcetera, certain set of questions that we care about, and they're not utterly unrelated to what we talk about on Twitter and elsewhere in public, but they're very different. And so I kind of sometimes have to roll my eyes when people give their lists of the top five physicists alive today. Look, if I appear on that list then you know the person is not credible. I'm a good physicist, but not anyone's idea of one of the top five physicists alive today. But people very naturally attribute slightly more importance to their favourite public figures than they would do to professors who they've never heard of.

0:41:17.9 SC: Anyway, the point about is Twitter good for physics or for science, one thing about Twitter is it's optional, they have not yet made it compulsory, they have not yet made it mandatory. If you yourself find Twitter to be dragging you down then just don't do it. That would be my advice. I use Twitter and I like it, and I'm not trying to make light of the fact that Twitter can be a downer, because there's a lot of people on there who, with the best of intentions... I mean, some obviously have bad intentions, but forget about those people, but even with the best of intentions, there are people who use social media to highlight problems in the world, to point out injustices or mistakes or falsehoods or misinformation. And if you're just being reminded of that aspect of the world all the time, even if all the people you're listening to are the ones you agree with, it drags you down. It just weighs on you. You need to have some joy in there. In my own tweeting I like to try to mix in me railing against the injustices of the universe with more fun, uplifting, enlightening things as well.

0:42:34.0 SC: And you get to pick who you follow on Twitter, right? So if you're constantly being bombarded with things that bring you down then you have to ask why you're following the people who are doing that to you. And if you just can't avoid it, then you have to ask why you're on there at all. I think it's obvious that Twitter is a wonderful source for broadcasting information. Like, some people like to have conversations on Twitter, they like to go back and forth and discuss things, and I just have no interest in doing that, and I don't mean to insult anybody but when I tweet something and someone responds to it, the chances that I will respond to their response are almost zero, because I just don't think it's useful for that. It's a broadcasting medium. If I wanna learn about when the LIGO experiment found evidence for gravitational waves, that news spread on Twitter faster than anywhere else, right? That is the kind of thing Twitter is super duper useful for, linking to things that are longer, more careful things, but not for having in-depth conversations about things. So I find Twitter very, very useful and helpful for that, but different people's mileage will vary.

0:43:40.0 SC: Paul Torac says, "In Everettian quantum mechanics, individual branches get thinner over time. Their amplitudes get smaller. So they make smaller and smaller contributions to the total energy of the universe. Suppose we fashion half the mass of the solar system into space ships and toss a quantum coin. Heads, we send out the fleet, tails we set day, a few more decades to improve the ships. We observe tails. Do we expect the local curvature of space-time to be reduced as our other branch siblings fly away resulting in a lower, all branches local, mass energy density?"

0:44:13.6 SC: So in other words, in case that thought... I edited it a little bit so it's not entirely Paul's fault here. But in case that wasn't clear, imagine we do a quantum experiment where there's two different possible outcomes and in one outcome, we send off a lot of mass, and in the other one we don't. Would we feel a different gravitational field? No, we do not. Otherwise... I mean, to say that we would, is to say that the existence of this other branch of the wave function of the universe would be observable. That would be great. [chuckle] That would be wonderful. But within each branch, the laws of physics of ordinary classical physics are more or less obeyed. So the matter that is in the branch, when you say the Earth...

0:44:55.5 SC: Just do a simpler version of your thought experiment. Say that there is a spin you're measuring, 50-50 chance to be spin up or spin down, and you have a bomb that destroys the Earth with 50-50 probability. And you do the experiment and you find yourself on the branch where the Earth is not destroyed. Does the Earth suddenly get half of its mass disappearing? No, that is not what happens, because the mass exists within each branch and the branching does not change the amount of mass in a single branch at all. Modulo, a little tiny effect, which is what I wrote about in my recent paper with Jackie Lodman that of course, if the two branches... The two branches can be slightly different in energy, as long as their average is the same as the energy that you had before. But there's a difference, and this is just why it's...

0:45:44.8 SC: The math is again, completely, crystal clear about this. Completely unambiguous. There's a difference between the energy you perceive from within a branch of the universe and the contribution of that branch to the energy of the whole wave function of the universe. The energy you perceive from within a branch doesn't change no matter what's happening on the other branches.

0:46:06.1 SC: Richard Graph says, "I'm reading and enjoying Cixin Liu... " Sorry, I can never pronounce this person's name, I'm sorry, Mr. Liu. "Cixin Liu's three-body problem series. In it, a character describes a proton in a high dimensional space being unfolded into a lower dimensional space. One of the results is that the object's size grows immensely large in lower dimension and increasingly so as it unfolds into even lower dimensions. Does this unfolding concept and the described size increase have any mathematical or physical validity, or is it just a clever literary device?"

0:46:43.4 SC: I don't know, is the short answer. Whenever you have something like a physics process translated into a science fiction novel, two things are happening. One, is the novelist will ignore the laws of physics in favour of telling the story they wanna tell, but also number two, even if they're obeying the laws of physics, they're not writing down the equations. They're using metaphorical descriptions to try to give you a feeling for what is happening. So nothing in those words corresponds cleanly and crisply to anything that I know about that could actually happen in the real world.

0:47:20.5 SC: The closest is the idea of living on a brane, B-R-A-N-E, which is sort of a sub-space of a larger, higher dimensional space. So we see a three-dimensional world around us, it is possible that that three-dimensional world is a three-dimensional brane, back construction from the word membrane, so a three-brane embedded in a four-dimensional space or five-dimensional space or something like that. And there are particles, the reason why we don't notice the extra dimensions is because the particles of which we are made are stuck to the brane. Now, as far as I know, there's no sensible theory in which something like an electron or a proton can be stuck to a brane and then escape and then fly off into the extra dimensions. Particles and fields either are or are not stuck to the branes. What you can imagine happening is two particles annihilating. Two particles that are stuck to the brane can annihilate and change into particles that are not stuck to the brane. And that just appears to us on the brane as missing energy. Energy would disappear from our observable universe. Only the observable universe, not the real universe, it has nothing to do with quantum mechanics here. But that is a possible thing. So I think that's the closest thing that I can think of that as an analogy to that particular example in the novel.

0:48:45.9 SC: Paul Hess says, "Can you explain the relationship, if any, between entropy and weak emergence? I began to think about this when Anil Seth described weak emergence as higher level behaviour that cannot be predicted from the lower level components without exhaustive simulation. And exhaustive simulation sounds to me like it represents information that is not compressible. And in computer science, they use the idea of entropy, to talk about how compressible something is. Does it make any sense for me to think of entropy and emergence as related concepts?"

0:49:15.1 SC: Yes, I think that they are related. In fact, I would go so far as to say that we're not sure... Or at least I'm not sure, and I've done a little bit of work on this, I don't think we can completely articulate right now what the relationship is. 'Cause frankly, I think that despite the fact that we babble about it all the time, I think that our understanding of how emergence works is pretty primitive and not very general. We've looked at it in certain cases, but the general theory of when and how one description emerges from another one is very primitive, I think. Very underdeveloped. So here is the sense in which entropy does...

0:49:52.3 SC: Well, there is a sense, potentially, in which entropy is related to emergence. And there's a trivial sense in which that's true, namely the following. What if we were in equilibrium? [chuckle] What if we were in the highest entropy state it is possible to be in? Then there would be no emergence. Entropy would not be increasing, but also we'd be in equilibrium, everything would be whatever configuration it would be in, presumably smooth and featureless. And so there'd be no stuff to be higher level objects in the emergent theory. In the real world, emergent things like human beings or whatever are constantly increasing the entropy of the universe. Now that's not perfect or exact, what I just said, because another example of emergence is the Earth moving around the Sun.

0:50:42.4 SC: The Earth can be described by its centre of mass, coordinates. And it doesn't need to be increasing in entropy for that to happen. It's true that the configuration of the Earth plus the Sun is very low entropy compared to thermal equilibrium, but it's not increasing in entropy in any noticeable way. That increase in entropy is not playing an important role in the emergence of the Earth as just a point mass going around the Sun. So that's why it's complicated. That's why it's hard. And so I do think that there is a relationship. One more fact about that, and so if you don't have the general theory, I can just throw out facts and you can make of them what you will, there is a relationship to this word that you use, compression. The whole idea of emergence is that there are many, many states that you could imagine specifying in the underlying microscopic theory that look the same. They correspond to the same state in the macroscopic theory. So for the Earth moving around the Sun, there's 10 to the 50th particles in the Earth, there are many, many ways to arrange them so that the centre of mass, position and velocity of the Earth is the same. And so that's a compression.

0:51:55.5 SC: The whole idea of emergence absolutely takes advantage of the compressibility of the microscopic theory in some way. And whenever you compress, there's an associated entropy, you can ask about, how many ways are there to arrange the microscopic constituents so as to make the macroscopic thing in that, and so the logarithm of that is going to be an entropy. It's not like necessarily the thermodynamic entropy, etcetera, but it's related. So I think this is all a very rich vein to imagine mining as we work towards a better understanding of emergence, going forward.

0:52:30.5 SC: Brendan asks, "How would you apply Bayesian reasoning between a deity and an advanced alien being? If an entity suddenly appeared and performed actions that defied our current understanding of physics, how would you update your Bayesian probability? I think many people will be quick and wait heavily for a deity, but it wouldn't be completely unreasonable to think an alien race millions or billions of years more advanced to perform things that we think are not possible."

0:52:56.1 SC: Yeah, that's a very good question. I think it would depend on the specifics. Well, okay. Let me be even more fair than that. I'm not at all sure that the deity concept is even coherent. So I would tend to put low prior probability on the deity idea. Number one, because it requires violating our understanding of how the universe works, whereas aliens do not. Number two, I think that a lot of our ways of talking about god-like creatures come from starting with our ways of talking about ordinary human beings and exaggerating them, and exaggerating them so much to talk about them being infinitely bigger or better or more powerful. But I don't really think that move is very rigorous or very easy to make sense of. There's a standard line about God being omnipotent and omniscient and omni-benevolent. And there's standard objections that say, "Well, you can't be all of those things because if you were perfectly good, you wouldn't let all this bad stuff happen in the world." And there's clever theological moves and maneuvers you can use to get around to that conclusion, but the simple conclusion is the whole idea was just half-baked from the first. From the start.

0:54:19.9 SC: So my immediate priors would be mostly on highly advanced technological civilization rather than deity. But I'm willing to update them. Let's talk to these people. I don't see why an infinitely powerful being would talk to me, [chuckle] or hasn't talked to me yet but would start... There's a million ways in which the concept doesn't quite make sense to me. But if they claimed that there was some reason that they could demonstrate that they were more supernatural than natural, then I would give them the chance to do that and see what happened.

0:54:56.8 SC: Kathy Seager says, "I read an article on Wikipedia about dark fluid theory in which they aim at unifying dark matter and dark energy. The theory proposes that dark matter and dark energy are strongly linked together and can be considered as two facets of a single fluid. At galactic scales, the dark fluid behaves like dark matter, and at larger scales, its behaviour becomes similar to dark energy. What do you think of it?"

0:55:19.6 SC: So I don't know exactly what is being referred to here, there's sort of more than one idea that has been labelled dark fluid in some sense. It's a natural kind of thing. Look, I already talked about dark radiation and dark photons, it's like once you invent dark matter and dark energy, you're gonna start inventing a lot of different dark stuff just to see how much you can get away with. I certainly think that any of these sort of more elaborate, baroque theories should be given much lower credence than the simple theory, because we have a very simple theory for dark matter and dark energy. Dark energy is a cosmological constant, and dark matter is some cold, weakly interacting or non-interacting massive particle. Those are two very, very simple ideas that fit all of the data, up to your usual collection of tiny anomalies, like we talked about with Priya Natarajan before, there's a tiny anomaly there in the gravitational lensing, there's other anomalies here and there. But with astrophysical anomalies, there's a lot of phenomena that you're observing. Occasionally, you will find an anomaly and occasionally it will go away.

0:56:21.9 SC: So there's no super strong killer evidence against that very, very simple theory. So it's worth trying to invent better theories 'cause there's absolutely open questions about the simple theory, but until there's really a very well-motivated and empirically successful version of them, I would not put too much credence in them. I know that Justin Cory at the University of Pennsylvania has a very interesting theory about some superfluid dark matter, I don't think he's trying to explain the way dark energy in the acceleration of the universe, but he's trying to explain the behaviour of dark matter in galaxies and clusters, and he says he can do that in a very specific way. I think that kind of thing is promising, the particle physics behind it is a little clunky and unnatural, but if you can really explain something, astrophysically, maybe it's worth it. And it's certainly worth at least thinking about it, I'm in favour of work going forward in these directions.

0:57:21.5 SC: Sam Hartsog says, "Do mathematical functions like integrals, differential equations, etcetera, have some kind of metric tied to them to represent something like, how many independent bits of information are required to carry out meaningful operations? It occurred to me that a notation like Tensor Calculus allows you to write things pretty concisely, and I wondered if there was any kind of metric for hidden computational complexity."

0:57:46.8 SC: So I debated whether to answer this question or not 'cause it kind of violates my rules. It's a good question, but I don't have anything interesting to say about it. The short answer is no, as far as I know. There's not a kind of metric... A metric on the space of operations you need to do to perform a certain operation, to transform a certain function or something like that. There are certain very restricted domains in which you can do something like that, and that's what made me think of it. So in particular, let's say you're doing quantum mechanics, and let's say that you're doing quantum mechanics in a finite dimensional setting. So not finite dimensions of space, but finite dimensions of Hilbert space, of the quantum mechanical space of possibilities. So in particular, let's be even more specific, let's imagine you have qubits, let's imagine you're doing something like a quantum computer. And so you have a bunch of qubits and you're gonna act on them.

0:58:45.6 SC: And it's very much like a regular computer, you act on bits in a regular computer using gates. AND gates, NOT gates, XOR, whatever. If you've studied computer science, there's a set of things you can do with two bits to turn them into one bit or another two bits or etcetera. Similarly with quantum information and qubits. And so if you have a set of qubits and you have a set of allowed gates, so you have gates that can act on these two qubits one at a time, and so you can't just act willy-nilly on any two qubits, but there's a set of qubits, like maybe the qubits are arranged in a line or they're arranged in a lattice on a two-dimensional plane or something like that, and you allow gates to interact on nearest neighbours. Then, you could talk about the complexity of the resulting quantum state or the amount of entanglement in the resulting quantum state. So you start with a quantum state that is completely un-entangled and you act on it, pairs of qubits at a time with your allowed gates.

0:59:51.5 SC: And then there's a simple metric on how complicated your quantum state is, namely, what is the smallest number of allowed gates, that will get you to the state you want to get to from some given starting point? As you might gather from that explanation, it's a very, very specific situation you're looking at there. It's very far away from a general theory of transforming one function into another or one vector into another or anything like that. So I think that's the state of the art as far as I know. Because if you don't make those restrictions, let's say you just start with one quantum state and go to another quantum state, and you say, without restricting on the set of gates I can use and whatever, in principle, how many steps does it take me to get from one quantum state to the other? And the answer is always one. One step. If you really cleverly pick your operation, you can always go from any one orientation of a vector in some vector space to any other orientation whatsoever. So I think the general theory there is gonna fall short unless you make some other choices about what operations are allowed and then maybe you can make some progress.

1:01:01.4 SC: Theiss Yanson says, "You've convinced me of Everettian quantum mechanics so much I can't see any problems with it. I listened to the podcast with David Albert twice and read something deeply hidden twice, but I still can't see the problem with probabilities." And then he goes on to explain what his issue is with the problem of probability. So rather than reading your issue with the issue, let me just do my best to say how I interpret David's objection to many-worlds.

1:01:30.9 SC: The way that I think about the origin of probability in many-worlds is via self-locating uncertainty chips, Evans and I wrote a paper about this to try to formalize it, etcetera, based on ideas that other people use, Lev Vaidman, [1:01:43.2] ____ and others. And the basic idea is when you measure a quantum system or when a quantum system is measured, it doesn't need to be a person or even an apparatus, but when a tiny quantum system decoheres, when it becomes entangled with its environment, and therefore the wave function of the universe branches into multiple copies, I know I can predict ahead of time with certainty that there will be a version of me on every one of the copies. Okay?

1:02:10.4 SC: And it happens very rapidly. This splitting of the wave function happens far more rapidly than any one of those copies are aware of. So after the splitting, there are many copies of me or just two or whatever, some number, and none of them know where they are. That's the self-locating uncertainty. So is there a way for those people, those future versions of me to assign a credence to being on one branch of the wave function in the universe versus another? And Chip and I argue that if you try to answer that question in some sensible way, there is only one possible reasonable answer, which is the Born rule of quantum mechanics, you assign a probability or a credence to be on any one branch that is given by the amplitude of that branch squared. And David Albert would like to say, "No, I have another perfectly reasonable possibility," as he says in the podcast, "namely, I have no clue." [laughter]

1:03:04.1 SC: And that's his attitude. He says, "You can't force me to say, Well, what is the most reasonable probability measure to put on this? 'Cause there's n copies of me, they are all in identical situations, how can they say anything at all about the probability of being on one branch or another? Especially, 'cause none of them can observe that. None of them can observe the amplitude of the branch that they are on, you can ahead of time predict, you can set up the initial quantum states so that you know what the branch amplitudes will be ahead of time, but once you're on a branch. It's just a branch. You don't know what the amplitude is sitting outside. It has no effect. The amplitude doesn't change the dynamics of your branch once you are on it." So David just says the alternative to choosing the sensible Born rule set of credences is not to choose credences at all. Just to say, I don't know what branch I'm on and therefore, I cannot predict anything. So I think the mistake there is... There's two... Well, there's two... It's not even a mistake. It's an attitude. You're allowed to take it, so it's not a mistake, you're allowed to take that attitude.

1:04:12.0 SC: I think it's not the right or best attitude to take for two reasons. Number one is, there is an obvious probability measure to use, it's not like we're completely clueless. Everyone agrees on what the obviously correct one is if you think that there is one. And that's the Born rule. As I often say, there's never any possibility that would be the wave function cubed or the logarithm of the wave function, it's always gonna be the wave function squared. That's the set of numbers that add up to one and are conserved over time. But the other is, more importantly, that there is sort of a normative reason for choosing a set of credences, normative in the sense of it's a judgmental thing, it's not forced on you by the world, but it's something that you want to do if you know what is going on. And it's the same normative reason that we want to correctly describe physics at all, that we correctly want to have a theory of how the world works. When you are going to jump off of a building, you have a theory of what's gonna happen when you jump off the building. You can't say, "Well, maybe I'll fall down and hurt myself, or maybe I'll float off into the sky. Both are possible. Right? So who knows?" [laughter]

1:05:26.6 SC: I mean, they are both possible, if you really strictly go by the meaning of what possible means. It is possible that this time, you will fly up into the sky rather than go down to the ground, but you don't give it equal credence to the idea that you will go down to the ground, because you're trying to understand the world the best you can, you have some experience with the past, you have some rules for assigning credences to different possible worlds that you could live in. That's a kind of self-locating uncertainty. You don't know what the laws of physics are in the world you're in. There's a world, a possible world where the laws of physics say that gravity is attractive and you will fall down off the building. There is a possible world where the laws of physics say that gravity is attractive right up until the moment you jump and then gravity becomes repulsive. [chuckle] It's possible, right? You assign very, very low credence to that world, and there's reasonable reasons to do that. They're not metaphysically certain, you could be wrong, but it's the best you can do under the circumstances. That's how I think about the Everettian probabilities. You're there on a branch.

1:06:32.0 SC: You can just decide not to assign credences at all, I can't stop you, it's a free country. But it's not the best way to do the best job you can do of understanding how the world works and where you are in the world. And once you accept that, then you're gonna put the Born rule credences on those branches. Okay, I'm gonna group two questions together about zombies. Oh, if any of you know, I should have mentioned this. But in case you didn't know, I did a podcast, a long podcast with Philip Goff and Keith Frankish. Philip, of course, has been a Mindscape guest before. He is a promoter of panpsychism in the theory of consciousness. And Keith is on the opposite side of the consciousness debates, he's also a philosopher, he's a proponent of illusionism, he doesn't think that qualia in the strict philosophical sense are even real, they're just illusions. So we talked about why I am a physicalist about these things, and it was a useful conversation, we talked about zombies a lot and then I wrote a follow-up blog post on the zombie argument for physicalism, if you're interested in that. So this is partly what is instigating these questions.

1:07:38.4 SC: So one is from Keith. I don't think it's Keith Frankish. [chuckle] It's a first time Keith who says, "I enjoyed your blog post on P-zombies for physicalism and visit on Mind Chat." Mind Chat is the name of the podcast, and p-zombies are philosophical zombies. So for those of you don't know, a p-zombie is the idea that we could imagine exactly two different, again... Two different possible worlds. One of which I have real human beings and they have all their conscious experiences and all of that stuff, the real world as we think and we know it, but I also, according to this thought experiment, could imagine a different possible world made of the same kinds of matter, same kinds of quantum mechanics and electrons and protons and neutrons, etcetera, obeying exactly the same laws of physics, doing exactly the same things, but no consciousness. And the people who live in that world, I would call them zombies.

1:08:33.9 SC: And so... Sorry, I'm departing from Keith's question here, but I'm giving you background, if you don't already know. So the zombie argument is the idea that because in principle, I can conceive of the same exact physical collection of matter with and without consciousness, consciousness must be something other than some way of talking about the physical behaviour of that collection of matter. Of course, I would say that that's begging the question, because you can only conceive of those zombies if you believe that consciousness is not physical. If you believe that consciousness is just an emergent thing to talk about, of describing the underlying physical behaviour of matter, then you cannot conceive of zombies, 'cause the same exact physical collection of matter behaving in exactly the same way would be conscious under that definition. Okay. That is the argument that goes back and forth.

1:09:26.5 SC: So Keith's question is, "I followed up the reasoning for the most part, but I really get hung up a bit upstream. As a fellow physicalist, I don't really get how a p-zombie is even conceivable or maybe my notion of conceivability is too strict." And then another question from Graham Clarke that says, "Do you think that a philosophical zombie is actually conceivable? Your arguments against to panpsychism seem to indicate that, but I wonder why the zombie concept hasn't been rejected."

1:09:51.0 SC: Yes. So I think you're both correctly putting your fingers on something that I didn't make very clear in my blog post. The answer is, zombies are not conceivable. That's what you have to say if you are a physicalist who thinks that consciousness is just weakly emergent. That's just what I said. If you think that consciousness is just a way of talking about the collective behaviour and higher level ways of talking about a collection of particles doing certain things, then there's no way to remove consciousness from that, and therefore zombies are not conceivable.

1:10:24.0 SC: So I wrote that in the paper that I wrote. I wrote a paper for the journal of consciousness studies. And I'm thinking of writing a philosophy paper. [chuckle] That was responding to Philip Goff's ideas, but I didn't make that very clear in the blog post, 'cause I was in a rush by the time I finished it. It was already too long. So yes, the punchline is that taking the zombie argument at face value as it is usually given only works if you assume ahead of time that consciousness is not physical. Otherwise, zombies become not conceivable. So that is the correct way to think about it, yes.

1:11:00.9 SC: Alejandro Gonzalez says, "Let us suppose we discover that consciousness is a fundamental property of nature, which I know you're inclined to think is emergent, what could the implications be of that for the current standing of physics and the model of the universe we have now? Just entertain us with some plausible possibilities, if there are any at all."

1:11:18.3 SC: Well, this is hard to do because I don't think it makes sense. I don't think it's true anyway. And part of the reason why I don't think it's true that consciousness is a fundamental property of nature is that I haven't seen even the slightest gesture toward a serious theory of that. I mean, you can attach words to the existing theories of quantum field theory or whatever, and say, "Oh, it's all made of consciousness," but I don't see any operational meaning to how that is supposed to happen. As I emphasized in the podcast with Philip and Keith, and in my blog post and elsewhere, either you change the dynamics of the particles and fields we know about, or you don't. That is true about anyone's theory of consciousness. You either do that or you don't. And if you don't, then I just don't see what it matters. There's just no implications of that idea. And if you do, show me the equations, show me exactly how the equations that we think work are wrong. Show me what the additional parts of them are. I mean, that's what you do. If you are a physicist, inventing a new theory of gravity or a new model of dark matter or explanation for the hierarchy problem, you show us the equations and we can work out from them what the implications of those are.

1:12:31.9 SC: And I don't know what those would be for a theory of consciousness as a fundamental property of nature. So I truly don't know. I'm not trying to be coy about it or anything like that, I'm just not even sure how that would work. I think that the sort of a feeling that people have that consciousness can't be just physical stuff because of their introspection, but I don't know whether that feeling actually leads anywhere sensible in terms of how the universe works.

1:13:00.6 SC: Crather Lucas says, "Regarding your theory on the arrow of time, in last month's AMA you mentioned something along the lines of, 'I don't necessarily think my theory with Jennifer Chen is true, but I think that there's a non-zero chance that it's true.' However, I know of no better theory regarding the arrow of time. If you believe you have the best theory, isn't the rational thing to do then to regard it as true? We can never truly justify our beliefs, so what other reason can we have for believing something is true besides it being the current best theory of some part of reality?"

1:13:28.4 SC: So no, I don't think that's exactly right. I think that for me to say it's the best current theory, requires that I should give it... In this case, it's not really a theory of the arrow of time, so much as... The theory of the arrow of time is entropy increases because of Boltzmann's explanation for what entropy was, plus the fact that the entropy near the Big Bang was low. That's the arrow of time. But then the question is, why was the entropy low near the Big Bang? That's what I and Jennifer Chen had a theory of. And I still think of it, as far as I know, as the best theory that I've heard of. So I think that what that requires in order to be self-consistent is I give more credence to that theory than to any other theory. However, the total number of other theories might be very large and there might be a large amount of credence I give to some theory and the set of theories no one has articulated yet.

1:14:22.9 SC: It's a little bit different than to say something like, "My theory in many-worlds is... My credence in many-worlds is 95% or something like that." That's a situation where we have several competing theories, we know what they are, we can articulate them pretty clearly, and they're all in some ways successful and I can judge them against each other. Whereas for something like this, we have no really good theories including my own, because there's a lot of un-understood physics that is involved in our own theories about how baby universes could be created or something like that, how you calculate a measure to talk about probabilities and make predictions and all of these things. So there are no great theories. Our theory, I think, is the most promising one among the not very good theories. And in that situation, you should still give a hefty amount of credence to I just don't know yet, we haven't thought of the right answer. So I would not say that I believe it's true even if I think it's the best theory currently out there.

1:15:23.0 SC: Tarim Shahab says, "What is energy? In high school, they would define... They would always say, energy is the ability to do work. And then define work in terms of energy. And I've also heard it described as bookkeeping, is that all it is, or is it more meaningful?"

1:15:36.0 SC: Well, it's certainly not the ability to do work, that's not a good definition, because there are forms of energy that literally are defined as the kinds of energy that can't do meaningful work, you know, the heat. The thermal energy in an equilibrium system in its highest entropy state, you can't do useful work with it. That's what free energy is. Free energy is the energy that is available to do work. Non-free energy is the energy locked up in the equilibrium high entropy configuration. Okay. So it's not that, but what is it?

1:16:12.4 SC: There's different definitions, and I think that the favourite one and probably the right one, even though it doesn't make a lot of instant intuitive sense, is the quantity that is conserved because the laws of physics are independent of time. If you think about... And this is easiest to sort of gain intuition for by thinking of the converse. What if the laws of physics were not invariant over time, right? So that would be like masses of particles or strengths of forces or something like that, just changing as time goes on, and as you might imagine, if I have a particle moving just in a straight line and according to Newton's laws, but its mass is changing then if momentum is conserved, the velocity of the particle would be changing even though it's mass... Because its mass is changing. Whereas if momentum is not conserved, but velocity is conserved, then its momentum is not conserved over time and in neither case is the energy conserved over time, because basically, intuitively, and this might not be exactly correct, but intuitively, it's kind of like something is pumping energy into the system by changing the mass of the particle.

1:17:26.1 SC: So because we live in a world where the particles have more or less constant masses, the forces are more or less locked in with their strengths, and so the overall laws of physics are basically constant over time, there is no extra thing, ommph, impetus put into the system and we call the quantity that is conserved because of that symmetry energy. Now, this is a handwavy way of saying in a much more rigorous result, Noether's theorem. I mean, Noether proved that whenever you have certain kinds of classical mechanical systems that have a symmetry, that symmetry is associated with the conservation law. Momentum is associated with the fact that things are translationally invariant in space. You can move in either X, Y or Z directions, the laws of physics don't change, so you have three conserved quantities, the three components of the momentum vector. And then you also have translations in time and the quantity that is conserved with respect to that is energy. It's not a very intuitive definition, I know that. But that is what the mathematical definition is. And it matches up with our intuitive definition of some combination of potential energy and kinetic energy and heat and stuff like that.

1:18:43.9 SC: Horace Deforest says, "What is your Bayesian prior of finding compelling evidence of bio-signatures in the atmospheres of exoplanets in the next couple of decades?"

1:18:53.2 SC: This is a great question because it's specific enough to make sense, [chuckle] like one should have a Bayesian prior about this, but it's really hard to know what it is. I'm going to put it low, I'm gonna put it at, let's say, 10%. And the reason why this is difficult is because, of course, we have two pieces of information, I don't even wanna say data points, but two pieces of information, one is life exists here on Earth, we're flourishing here on Earth, the other is we haven't seen already sort of in an your face kind of way, life elsewhere, either elsewhere on other planets in the solar system or aliens flying by and saying hi to us from elsewhere in the Milky Way. So those are two pieces of information you have to reconcile with each other.

1:19:43.5 SC: The easiest reconciliation is that in some sense, life elsewhere is rare, and of course as a selection of fact, we're only gonna exist where life exists, so the fact that we exist here on Earth is not really telling us that much. But there are lots of steps along the way from no life at all to an intelligent civilization on another planet, and any one of those could be a bottleneck that explains why we haven't seen other intelligent civilizations yet. So it might be that there's no life, but it also might be that life is everywhere, it's ubiquitous, it's very common, but it's always only single-celled or something like that, not very technologically advanced. That's why it's very hard to make a good Bayesian prior, so I'm gonna put it at 10%, don't ask me to justify that, I might change my mind if we learn more about chemistry or geology or something like that.

1:20:39.0 SC: Robert Ruxin Drescu says, "A few days ago, Elon Musk responded to a Bernie Sanders tweet about the rich paying their fair share to society by commenting, 'I forgot you're still alive.' So I asked Elon Musk if he thinks living in a world where one man has $1 trillion and the other people die of hunger is a good world. What ensued, was a ton of attacks from the sympathizers of Elon Musk calling me all sorts of things from communist to idiot. My question is this, Where do you think all this aggressiveness and rich man idolatry is coming from? Why are people so aggressive? Why are people so obsessed with money? And why are people so rude and violent?"

1:21:13.0 SC: Well, I'm not gonna give you a fully blown theory of why people are rude and violent. I think... This is a difficult question, and one point to keep in mind is that you have an extremely strong selection effect here. It's not that people are rude and violent, it's just that you poked a certain hornets' nest that has some very rude and violent people in it, and you got a reaction that was kind of predictable, to be honest. Now why those people exist at all, it is a more complicated psychological question. The reason why I wanted to address this question is I think that we have a weird relationship in modern society with the concept of super rich people, with billionaires or whatever, in part, because we have a lot of inequality in society and it's getting to be too much inequality.

1:22:01.5 SC: We have both very ostentatiously wealthy people, and a lot of people who are struggling. We have a societal system that makes it really, really hard to climb out of very, very low income situations. It's just too easy to get into debt, to spiral downward, like life is much more expensive for poor people than it is for rich people, in a very real sense, because as a fraction of whatever income they have, there are fixed costs that rich people can just laugh off and poor people can't. You know, parking tickets are a big deal when you're poor, and what you do is you end up not paying them and then they grow, or overdraft fees on the bank, we literally charge you money for not having enough money. Whereas, a rich person doesn't care about parking tickets, they can laugh it off and still go make more money off of their investments or whatever it is. And that kind of system that has this built-in stratification dynamics to it is troubling if you don't have a lot of economic and social mobility built-in, so I think that people are upset for a lot of good reasons.

1:23:12.7 SC: And then, other people are aspirational, other people want to be those rich people who are very, very successful. So I think that the conversation about wealthy people has become too polarized, honestly. They're just people. Like, I know Elon. We're not best buddies, but I've met him, I argued with him on Twitter about extraterrestrial life, talking about the previous AMA question, and he has said, nice things about my book, so that's good. I think that he's a person. I know lots of people who are quite wealthy, and there's good ones and bad ones, smart ones and dumb ones, good ones and evil ones.

1:23:50.4 SC: I think that it's possible with respect to Elon Musk, to both say... To criticize him, to say we don't like how Tesla workers are treated, or the ability to unionize or something like that, or the fact that he doesn't wanna pay his fair share of taxes, while also recognizing that he has almost single-handedly done more to change the world than almost anyone else I can think of, by really converting a whole industry from gas engines to electric cars. That was unthinkable a short time ago, and without Elon, that probably would not have happened.

1:24:27.1 SC: So I think that people are complicated, and I think that we should develop the mental capacities to have nuanced views of human beings. And when it comes to rich people, that's becoming harder. People either want to worship them, like you say, to hero worship them or to vilify them. And I'm not in favour of either one of these. I don't wanna hero worship anybody. I have a general theory of not hero-worshipping people. I don't worship Richard Feynman or Albert Einstein or Martin Luther King, or Ruth Bader Ginsburg, or however you wanna pick. I think that it's just a bad idea. People, you can admire people for what they've done. But hero worship involves extending that admiration to a general feeling that they're good people just because they did this one good thing, and that's just setting yourself up for disappointment. A person can both change the world and be bad in other ways, and you have to be able to accept those nuances there.

1:25:21.0 SC: In fact, Liam Koffi Bright, who was on the podcast, when I first invited him on the podcast, I wanted to talk about this 'cause he has a whole spiel about how we should have no heroes, which is very interesting and compelling, we got distracted talking about truth, but you can look that up on his website.

1:25:40.0 SC: But I don't wanna villainize... Villainize? Yeah. Villainize rich people either. I'm reminded of Alexandra Ocasio-Cortez, AOC. Remember when she went to the MET Gala? This happened a few weeks ago or months ago, so she went to this gala full of rich people and she wore a dress, and on the back of the dress was written 'Tax The Rich'. And now, do me a favour, just for a moment, put aside whatever pre-existing feelings you might have about AOC or the tax system, or the MET Gala or whatever. I wanna make one very small, specific point. People objected on the basis that it was hypocritical or somehow there was some tension between the fact that she was at a party full of rich people and had a dress that said, "Tax The Rich."

1:26:30.3 SC: And I objected to this objection. This makes no sense. This is not in any sense hypocritical or even slightly tense. It is no contradiction whatsoever to hang out with rich people, have a party with them and have a good time, and also think they should pay more taxes. Those two things are completely compatible with each other. The only reason why you might think that they're incompatible with each other is if you think that the fact that rich people should pay more taxes is a reflection of the fact that we don't like them, that they are bad or they are evil, and that's why we wanna tax them more. And I don't think that at all. Like I said, rich people are just people and they have the same span of abilities and virtues that other people have.

1:27:17.0 SC: I want to be rich myself, I want everyone else to be rich. That's what I want. I don't wanna get rid of rich people, I want everyone to be rich. That's the thing I think that we should be aiming for. But I also want the current rich people to pay more taxes because rich people are part of society and benefit from society in very, very obvious ways. Like, of course, you can inherit money, and then we can talk about the justice of that, that's a perfectly good set of questions, but if you made money to become a billionaire, it's not because you worked really hard at your job in Walmart. No Walmart worker ever saved up enough money to become a billionaire. That's literally never happened. What happened is, you either invented a system or you participated in some existing system to arrange for a billion people to give you a dollar each, roughly speaking at the end of the day.

1:28:07.6 SC: Or maybe a 100 million people gave you $10 each. There's different ways to work it. But the point is that the only way to accumulate a billion dollars is if a lot of people send you their money. It only happens if you're embedded in a system that allows that to happen, a system with people, but also roads and education and commerce systems and the whole supply chain that we talked about with Christopher Mims a couple of episodes ago, and it is completely fair for society then to say," In return, give us some of the money. We set up a system that allowed you to make this money, now we wanna keep the system working, give us some of the money." Asking rich people to pay taxes is not a bad thing. It's certainly not unnecessary.

1:28:52.2 SC: Even if you think it's a bad thing, you shouldn't mix it up with some distaste for the idea of rich people at all, I think that's just entirely misguided and it's a reflection of this idea that every intellectual disagreement has to become a moral emotional stance. And I think that's a bad attitude, so anyway, I got very far away from your question, Robert, sorry about that, and I probably haven't even answered your question, but your question provoked me to say some things, and that's how I treat these AMAs these days.

1:29:19.5 SC: Davij Mankhurd says, "I was thinking that borrowing some physics jargon can bypass some of the free will discussion pitfalls at least among physicists. In particular, do you think that the apparent conflict of the phrase "could have done otherwise" with determinism can be avoided by separating the off-shell and the on-shell descriptions. I was tempted to think of this way when I read the classic Freedom of the Will and Concept of a Person paper by Frankfurt, which I think is essentially formulating the concept of free will as the existence of degrees of freedom for the human will, albeit in a different language."

1:29:55.6 SC: My guess is no, my first guess, like, maybe there's a theory to be developed here that you wanna go and develop, but that is not the way that I think about free will. Even if I were to restrict myself to physics jargon, I don't think that's the main point. So for those of you who don't know the physics jargon, think of a particle moving through the air. You all know the classic example, when we learn in classical physics, Newtonian mechanics, you throw up a ball with a certain velocity and given the acceleration due to gravity, it travels on some kind of parabolic trajectory, and you can calculate what that trajectory is. Okay?

1:30:32.3 SC: So the fact that the ball moves along a certain trajectory, both its position as a function of time and its momentum as a function of time obey equations of motion, that's how we say it, and there are reasons which I'm not gonna go into, from particle physics that when you obey the equations of motion in that way, we say that you are on-shell, you were on the mass shell, you were obeying certain equations. It's more complicated than that, but I'm just trying to get you the essence of it if you don't already know, whereas off-shell means you don't necessarily obey the equations of motion, we can imagine all sorts of trajectories for a particle through space that don't obey the equations of motion that the real physical particles would obey.

1:31:15.3 SC: So what Davij is getting at is, even given some fixed initial conditions, we can distinguish between the actual behaviour of the system that obeys the initial conditions from... Sorry. That starts with those initial conditions and obeys the equations of motion, from some more arbitrary random kind of motion. But I think that that's not the right distinction to make, because I think that the point is that we don't know the initial conditions in the real world, like if the world were just a simple harmonic oscillator, if that was the whole world, or if the whole world were just a few planets moving around the Sun, there'd be zero reason to talk about free will in any sense, 'cause you can just see what happens and it's pretty clear and you can predict what's going to happen. The reason why free will, if you believe it is, if you're a compatibilist, if you believe it's a useful concept is because human beings are so complicated that we don't know their initial conditions.

1:32:10.5 SC: So we don't know what they're going to do, so we invent concepts to describe both their current state and their possible future actions, and those concepts involve things like reasons and choices and deliberations, and this is part of how we talk about human beings. And as I've said many times before, even the most devoted free-will skeptic uses those words, they can't help but talk about human beings. Free will skeptics are constantly trying to persuade you to behave in certain ways, which is slightly incompatible with their underlying metaphysical view of the world, but that's why it's easier to be a compatibilist. And the formulation of free will as I could have done otherwise, I think is completely consistent with a deterministic universe and everything obeying the equations of motion. In other words, you don't need to go off-shell to explain it, because the point is, you don't know what the initial conditions are. You know some basic macroscopic coarse-grained features of the system, but not all the microscopic details.

1:33:16.9 SC: So to me, the question of, "Could you behave differently?" The reason why free will skeptics get it wrong, in my view, is they formulate the question, could the system have behaved differently as if we had exactly the microscopic information about the state. And in that case, the answer would be no. The system could not have behaved differently, but we don't have that information. So the right question to ask is, Given the macroscopic information we have about the system, in the space of all possible microscopic configurations that fit into that macroscopic description, are there some that would act this way and are there some that would act that way? And if the answer is yes, then that corresponds in the higher level vocabulary to, you could have acted differently. That's where I think is the right connection between free will and the underlying laws of physics.

1:34:07.8 SC: Chris says, "What do you think are the most important things to know? Not in science, but in life in general."

1:34:14.8 SC: So this is not gonna be very clever or original, but I'm gonna give a sort of teach-you-how-to-fish kind of answer. I don't think there's a set of facts that are the important things to know, I think the most important things to know are how to learn things, techniques for gaining new knowledge, 'cause then rather than just giving you some knowledge, I think you should have... If I teach you to learn things, then you can get knowledge for yourself. So I don't know what those things are.

1:34:40.8 SC: Basically, the answer is along the lines of techniques for gathering information about the world, converting them into models and hypotheses, testing them and being a good scientist about it. Okay? In the broadest possible sense, so I'm gonna include, you know, human relations and things like that in that set of things, the theories that I'm talking about developing are not just theories for how people behave, but your theories for how you should behave, how to get ahead, how to live a meaningful life, to develop purposes and how to matter to other people and be nice. Those are all things you need to learn, and so that's what I would hope that you would start out by knowing.

1:35:23.7 SC: Napoleon's Corporal says, "DNA passes on physical traits and the specific genes that carry these traits can be identified, but what about behavioural traits, can we identify genes that result in things like the herding instinct that Sheltie dogs have been bred to have? The broader question here is, how can any physical thing influence something like behaviour?"

1:35:42.2 SC: Well, I don't know any specific examples of DNA base pairs that are correlated or even genes that are correlated with specific behaviours, but 100%, they exist. Yeah, that is absolutely something that neuroscientists and psychologists, etcetera, geneticists do figure out, and honestly, I'm not quite sure what you mean by, "How can a physical thing influence something like behaviour?" What else influences behaviour? The behaviour of the ball is influenced by the fact that I throw it, right? Physical things influence behaviour, behaviour is a way of talking about what physical things do, I think. So I'm not even quite sure what you're worried about there.

1:36:29.0 SC: Alexandra Bates says, "Do you believe there are certain traits or paradigms that separate scientific theories from pseudo-scientific ones, or do you believe all theories have the same capacity to be scientific and what makes them so is their agreement with the data?"

1:36:42.0 SC: No, actually, I don't think there's a hard fast line. People would love it if they could find a single way to demarcate scientific theories from non-scientific theories, and there's a name for this, the demarcation problem. I don't think that's gonna be quite that easy, I think it's more of a sliding scale. And I've famously pointed out the Karl Popper's attempt to answer this question by talking about falsifiability is not up to the task. I don't think anything is up to the task, but I do think that Popper was on to something, as I often say, and people ignore that part. What he was on to was there are two properties that good scientific theories tend to have. One is that they're definite, that they're incompatible with something.

1:37:30.0 SC: So a good scientific theory is not one-size-fits-all, a good scientific theory says that some things happen and some things don't, and the reason why this is philosophically not crystal clear is because when you talk about falsifiability, then you're taking this ontological feature of the theory, some things happen and some things don't, and turning it into an epistemological theory, I can see some things happen or not see them happen and learn about it, and that's where things get fuzzier, because if the things that happen are other universes, then they might very definitely happen or not happen, but you can't observe them, so that's where it gets a little bit messy, but nevertheless, the basic idea that a scientific theory is definite, that it says certain things happen and certain things don't, I think is absolutely part of scientific theory. And the other thing that I think Popper was right about in his direction that he was trying to move in is we ultimately judge scientific theories empirically. We do not intuit our way to the right answer, or at least we don't intuit our way to knowing that the answer is right, we have to test them against the data.

1:38:38.9 SC: So scientific theories are not ideas that had to be right, they're not deductive arguments from a priori principles to correct answers, they are hypotheses that are tested. Hypotheses that we put forward and we assign the credence to, and then the credence goes up or down on the basis of observations and collecting new data about the universe. So I don't think that there is a simple, clear dividing line, but I think that these two properties of being definite and being amenable to empirical testing are two parts of being a good scientific theory.

1:39:15.7 SC: Liam McCarty says, "Why do you think some ways of solving a given problem are easier than other ways? I'm sure part of it has to do with human psychology, but I'm wondering if there's more to it than that. For example, arguably Feynman diagrams are more efficient than Schwinger integrals, and Newton's laws are more efficient than Einstein's field equations in a particular regime. Perhaps we could define such a notion of efficiency for problem-solving approach or physical theory, maybe there are complexity classes of the sort here, not for different problems, but for the same way of solving a single problem."

1:39:45.8 SC: So this is similar to the previous question about a metric on the space of operations on functions or something like that. I'm sure that human psychology plays some role here. Human beings are specific arrangements of stuff and specific ways of thinking, so it would be weird if the specific kind of thought process that went on in human brains had nothing to do with why some problems are harder and some are easier, but having said that, I think we tend to overrate that aspect of the issue, I think that there are some aspects of problems that are just hard.

1:40:24.9 SC: This is... If you just say, the initial question you asked is, "Why are some ways of solving given problem easier than other ways?" This is very closely related to complexity classes. Complexity classes are about how many steps it takes to solve a problem rather than different ways of solving the problem, but complexity classes are usually about the minimum number of steps it can take. And this is a pretty objective fact, I mean, if it takes a certain number of steps to solve the problem, it doesn't matter whether it's a human being doing it, or a dolphin or a computer.

1:40:56.4 SC: So I think that we tend to not credit enough the fundamental aspects of the problem and we tend to leap too quickly to some particular fact of human psychology when we think about these things, but the basic question that you're asking, why are some ways easier than others? Yeah, I'm not gonna give you the right answer. I don't even know how to formulate what an answer would look like, but I will, again, make the point that it probably has to do with the features of the problem more than the features of the human brain. Most of the time.

1:41:32.0 SC: P. Walter says, "Is the interference pattern observed in Young slit... Yeah, Young's slit experiment to be considered as evidence for the many world's interpretation of reality?"

1:41:42.7 SC: Nope, because the double-slit experiment is predicted by any decent interpretation of quantum mechanics, so therefore it literally cannot possibly count as evidence for many-worlds. I mean, you might think that the explanation for it for many-worlds is nicer or more attractive or whatever, but as a Bayesian, what is the likelihood that you will get that data given the theory is equal in many-worlds or Bohmian mechanics or any other respectable theory of quantum mechanics, because if you didn't predict the double-slit experiment correctly, you would not have gotten off the ground, no one would have paid attention to you.

1:42:17.5 SC: Lucas Der Hashing says, "I've been recently looking into some of the cosmic speed limit postulated by Minkowski. It's quite interesting to find out that we all move through space-time at the speed of light. My question is, if we would completely stop moving through space, how fast would time go by?"

1:42:34.9 SC: So I think that I'm gonna disagree with your middle sentence there, we do not move through space-time at the speed of light. I don't know who said that. [chuckle] I'm not even sure what it means, and how fast does time go by, it always goes by one second per second. That's the only speed at which time can possibly go by, and that's true whether you're speeding or staying still or in a gravitational field or not. There's no such thing as the speed with which we move through space-time, the idea of a speed is the amount of distance we travel as a function of time, that's what speed is. So it just doesn't apply to going through space-time, it's a different kind of thing. And I'm not gonna go into many details here, but this is just something you have to wrap your mind around when you really get into what is meant by special relativity and the interval in space-time and why the twin paradox works. I will recommend reading the book that I just wrote and will eventually come out like a year from now, that explains these things in great detail, or failing that, you could just check out the videos on the Biggest Ideas in the Universe.

1:43:41.2 SC: Marco Touser says, my question is, "How is an electron wave function different from the electron field? Do you picture them differently in your mind?"

1:43:50.7 SC: Well, they are different. They're very different in fact, but there's sort of a limit in which they become kind of similar to each other, and that limit is where the field is only vibrating enough to give you a single electron. And honestly, there's technicalities that arise with electrons that make me reluctant to talk about that, it has to do with the difference between fermions and bosons, and it's harder, although not impossible, to talk about the electron field in classical visualizable terms because of the exclusion principle, because you only have one fermion like an electron doing the same thing at the same time. It's easier to talk about a bosonic field, like the photon field or the Higgs field or something like that, but the point is that when you have the quantum field, which is the more fundamental thing, then you have a wave function of that quantum field, and the single field can describe any number of particles, it's not just one electron or one, whatever, Higgs, boson or photon, it's any collection of these things can be described by the wave function of that single field. But then when you boil it down, when you say, "Okay, I'm only going to be interested in configurations of the field or wave functions of the field that represent a single particle," then there's really no...

1:45:09.9 SC: The difference between the wave function and the field kind of dissolves, not exactly, certainly the case of the electromagnetic field, it does not dissolve very well because the electromagnetic field is a vector and it's real valued, it is not a complex valued scaler number like the wave function is. But my point is, the only useful thing I'm trying to say here is that the wave function of the field is a much bigger, grander thing. It can describe not just one particle at a time, but any collection of particles, including the creation and annihilation of particles. That's one of the crucial features of quantum field theory. So the single electron is sort of a limit that you get, and then in that case, the distinction between the electron wave functional and the electron field becomes a little bit harder to distinguish.

1:46:00.3 SC: Dan O'Neil says, "What are we?" I like these big questions. "When I think of the four-dimensional space-time of relativity theory and how the passing of time is just an illusion, I think we must be the braided world lines of all the particles that have made up our bodies over the course of our lifetimes. Then when I remember the questionable status of a particle in quantum mechanics, I think we must be an intricate inter-rippling of quantum wave functions, but then when I remember the problem with the wave function collapse and the solution opposed by many-worlds, I try to picture myself as a massively complex branching bush of wave functions in an unpicturably vast multiverse. Ignoring emergent properties, is this what a human being is? If so, do you see it as a remarkable, perhaps beautiful phenomenon even at this fundamental level?"

1:46:44.4 SC: So there's one problem with your question, Dan, which is, you say, ignoring emergent properties, is this what a human being is? But a human being is emergent property. There's no human beings in the fundamental ontology of the world. If you're a modern particle physicist, you would say the fundamental ontology of the world is quantum field theory, if you're even more extreme like myself, you would say it's just a vector in Hilbert space, it's a single quantum wave function, but either way, there's no human beings there, right?

1:47:15.8 SC: Human beings are concepts that only makes sense at the higher emergent level, so you can't say, tell me what a human being is, but don't talk about the higher emergent level. That's just not fair. [chuckle] And it's also crucial to actually answering the question more seriously because as I said in different contexts, you can always take the purely microscopic point of view, and just never talk about higher level emergent concepts, and this is what you would do if you were the Laplace's demon, if you literally knew the exact micro-state of the universe and how it evolved over time, you wouldn't need to talk about these higher levels, you wouldn't need to describe the Earth going around the Sun as just a centre of mass and its centre of velocity.

1:48:00.5 SC: You would talk about every single particle in the Earth, why not? You have access to it all, if you have that fundamental microscopic description, you can purely talk in that language. So the question of what is a human being is by its nature a question about the relationship of the higher levels, including the ones that have human beings as part of them, to the lower levels. And there might not be a unique answer 'cause it might depend on exactly which levels you're comparing to each other.

1:48:31.0 SC: So it's perfectly okay to talk about human beings in a single world at a time, you can know, you can be convinced that the fundamental theory of reality predicts that there are copies of you being made all the time, but you can still talk about the one that is in the world that you're in right now. That's okay. And you don't need to worry about all these other ones, there's certainly... Once they've branched off, they're not you anymore, they are descendants of your past you, but they are not related to your present you in any useful way. So I think that to correctly tackle this... I'm not giving you the right answer to this question because I think it's a tricky one and probably not a unique one, but to think about this question in a useful way requires taking seriously and biting the bullet about these higher emergent levels, you can't really get around them.

1:49:17.7 SC: Chris Murray says, "Near the end of the biggest ideas in the universe, Q&A, number nine fields, you introduce the Reeh-Schlieder theorem. Can you make an educated guess as to what a person on a space station would see if they looked up at the Taj Mahal being created on the Moon from acting on the field somewhere on Earth?"

1:49:35.4 SC: So the Reeh-Schlieder theorem is a reflection of the fact that in quantum field theory, in the vacuum state, the fields in any one region of space are entangled with fields everywhere else in any other region of space, all throughout the universe. At least, if you're truly in the vacuum, which maybe you're not, and if it's just Minkowski space, which we know we're not, but anyway, something like that is likely to be true. And because of that, because of that entanglement, you know how if you have two particles that are owned by Alice and Bob and they're entangled with each other, making a measurement on one particle tells you something about what the other particle is going to be doing, even if you don't know it immediately, it might take time for any information about what is going on to go back and forth, but something happens.

1:50:26.1 SC: The difference is that in quantum field theory, even in one small region of space, you don't just have a spin that is up or down, in quantum field theory, you literally have an infinite number of things that could be going on in one region of space. You have fields that can take on literally any value. There might be a probability that they take on certain values or certain other ones if you were to observe them, and there is, but every value is represented as an infinite number of possibilities, and this is one of the reasons why I don't think quantum field theory is right, that's kind of a lot to put into one tiny region of space, in an infinite number of possibilities. So this infinite number of possibilities is entangled with infinite numbers of possibilities in every other region of every other point in the universe. So the Reeh-Schlieder theorem says that when you make a measurement on a region on the quantum fields in some region of space, one possible outcome is to bring into existence somewhere else in the universe literally anything.

1:51:28.3 SC: In the Alice and Bob with spins case by measuring your spin up or down, you bring into existence, the spin is either up or down, whereas it was for Bob's particle, even though it wasn't a superposition. In the quantum field theory case, empty space can be thought of as a superposition of all sorts of crazy things, and you don't notice it 'cause it's in the superposition that gives you a minimum energy. But in principle, there is a measurement outcome that corresponds to bringing into existence a copy of the Taj Mahal on the Moon. That's why the Reeh-Schlieder theorem is sometimes called the Taj Mahal theorem. And so the question is, what would that look like?

1:52:04.7 SC: You know, the honest question is, it could look like almost anything because that's part of the theorem. On the one hand, you bring into existence the Taj Mahal on the Moon, on the other hand, there are some photons that travel from the Taj Mahal to you. And guess what? They could be anything, 'cause you could bring anything into existence. The whole point of the theorem is you're supposed to suspend your interest in the probability that any of these things actually happen. You're just proving rigorous mathematical theorem about what could happen, so what could you see? Anything, literally anything. But the probability that you see anything at all is gonna be very, very small. The probability that the Taj Mahal exists... Comes into existence is extraordinarily small, this is absolutely a case where just 'cause something in principle could happen, I wouldn't worry about it in any realistic way.

1:53:00.3 SC: Nate Waddoups says, "Standard sirens came up in an episode that I listened to last month. What are the forces that come together to tune them to standard behaviour? I mean, why do they all act the same rather than being random sirens?"

1:53:13.0 SC: So the point of standard sirens is you're imagining two black holes that are coalescing, just like we observed in LIGO or whatever. The point is in a typical LIGO event, what you don't know is the red shift, because you don't have any visible light coming from them, to get a red shift, you need visible photons that have spectral lines and you could ask how red shifted they are. So what you're really looking for is the special sub-class of events that have both gravitational waves and electromagnetic radiations. You can measure a red shift to them.

1:53:44.8 SC: And then the point is that, you can figure out the distance if all goes well and you get enough data in principle, in practice, this hasn't been done yet, but they're working on it. The point is that you can follow the hole inspiral. So it's not just you see a blip, you see a wave that grows over the course of a few seconds and it then chirps. Okay. And so you see both the total amount of time it takes for that to happen and you see the frequency, you see the space between the different peaks in the amplitude of the wave and how it develops over time. So that's actually a pretty good amount of information, and from that, if all goes well, you can discern the actual masses of the inspiraling black holes, and so you can predict the amplitude of those gravitational waves, and therefore you compare that to the amplitude you actually observe and you get the distance. So the reason why they're standard is just because there's... All that's happening is gravity with two black holes. There's not a lot of degrees of freedom, not a lot of weirdness, it's not like a star that can have different chemical composition inside or different... Be in a different environment with different accretion disks or anything like that.

1:55:00.8 SC: It's two black holes that have two masses. That's literally almost all the degrees of freedom, it's not quite 'cause they could be tilted and they can be spinning, so it's a little bit more complicated than that, but in principle, you can work all that out. And there's certain special cases where you have reason to believe that it's even simpler than that, so you can have more confidence in your reconstruction of what the masses were and therefore the amplitudes and therefore the distance to the thing. So really the ultimate answer is you're looking at a very simple system, two black holes, that are being very simple, direct set of equations, the equations of general relativity, most things in the world are not that simple. It goes back to the very first question in the AMA, black holes have no hair. There's not a lot of details when it comes to describing black holes.

1:55:45.7 SC: Teresa Robeson says, "In your opinion, what are the couple... Of a couple of the most indispensable science magazines for the lay person to read to keep abreast of recent scientific research and advances?"

1:55:57.9 SC: Yeah, I don't know, honestly. My science, my popular science input comes actually mostly from Twitter, because I follow science journalists and scientists on Twitter, and when something interesting happens, they link to it, and so then I will click on it and it might be wherever it is. So I don't regularly go to the home pages of magazines or subscribe to magazines. The magazines I subscribe to are like the New York Review books or something like that, right? Not specifically science or physics magazines. I will put in a plug for two of my favourites, one of them is Ars Technica, because my wife Jennifer writes for them. So you can trust them. Absolutely. And the other, of course, is Quanta, Quanta Magazine does a very, very good job at modern physics and math and so forth. If anything, it likes math a little bit too much. So if you don't think there's enough math in your popular science diet, I would definitely check out Quanta. But the great thing about Quanta is they really care about the ideas, it's not a excuse to write profiles of scientific heroes or things like that, it's really about sometimes quite abstract ideas in math and physics, they tackle things that other science outlets will not tackle.

1:57:11.1 SC: The firewalls paradox, have you ever heard of the firewalls paradox in black hole physics? Jennifer actually wrote an article for Quanta about that, it was the first article about the firewall problem to appear in the popular media because everyone else was afraid of doing it, but Jennifer would tackle it and Quanta would publish it. So it appeared there, and the New York Times came along a few months after that, and then it caught on even more broadly than that.

1:57:35.4 SC: Michael Schillingford says, "Pre-Socratic philosophers used to debate whether reality is made up of discrete things, e.g. Adamism or stuff like water. I've recently seen a similar argument centred on particles and fields. Which ontology is correct between things and stuff, or is it a defective debate?"

1:57:54.4 SC: Yeah, I think that it depends. What do you mean here? So there's the question of the ultimate ontology of the world, which I've recently written a paper saying it's a vector in Hilbert space, that's what I claim that is our best current understanding of it. But to get there, we work through various levels of description and there's absolutely a much more solid ground, a level of description, which is the quantum field theory level, so I will take your question to be at that level of quantum field theory, at the best tested version of fundamental physics we currently have.

1:58:31.1 SC: What does the fundamental ontology look like? And unsurprisingly, given that it's called quantum field theory, it's a theory of fields, but it's a theory of the quantum theory of fields, not the classical theory. Okay, let's take that, but it is definitely not a theory of particles, and this is a case where physicists and philosophers think about things differently, because philosophers think there is a debate about whether or not the fundamental ontology is more particle-like or field like, and they debate it. Physicists, for the most part, know it's all fields, and they just think it's fields and it's not really much of a debate, and part of it is because the difference is you start with the quantum theory of particles, and you can kind of bootstrap your way up into a quantum theory of fields by insisting that the particles can be able to interact with each other and annihilate and create and so forth, so, many body physicists do that, not many body physicists, but many body physicists.

1:59:34.8 SC: Or you can go the other way. You can start with the theory of fields, quantize it, show that you get particles out in the low energy perturbative limit, so it seems like they're on an equal footing in some sense. But if you take the field perspective seriously, there are non-perturbative effects of particle is a tiny fluctuation in a quantum field, and so it's an appropriate concept to think about when the field is nearly quiet, it's a tiny perturbation around that quiet vacuum state. But sometimes fields do big things, collective things that are not tiny perturbations in any sense. They do non-perturbative things, like when the Higgs-boson field rolls from a value of zero to its non-zero expectation value in the vacuum, that's a big non-perturbative thing that's easy to understand from the field theory perspective, it's almost impossible to make sense of that if you thought the world were fundamentally made of particles and not of fields. So for those reasons, particle physicists tend to think that really the world is made of fields, but there could be something else underlying the whole thing, as I said. Okay.

2:00:45.7 SC: Gregory Kushnick says, "Should parents lie to their kids about Santa Claus?"

2:00:50.1 SC: If they want to. I don't care. There's bigger problems in the world to worry about, for one thing, but I think that there is a danger that people can fetishize always telling the truth a little bit too much, okay? I try to be a reasonable person about this, I care about the truth, but that doesn't mean that there's some sort of Kantian principle that says that you should only ever tell the truth and never lie, no matter what the consequences might be. If someone just got their hair cut and says, "Don't you think it looks nice?" Just say it looks nice, who cares what you actually think? If they say, "Should I get my hair cut in this way or that way?" and it hasn't yet happened, then by all means, be honest, but if it's already there and they're just looking for some reassurance not really a judgment, or an opinion, or a helpful fact that they can use to make a decision, be nice, say the nice thing to them.

2:01:48.0 SC: Likewise, if your kid is five years old and is excited about Santa Claus, tell him that Santa Claus is gonna come and give him presents. That's fine. Most people who I know who grew up in the sort of same environment that I grew up in, we all thought that Santa Claus is real when we were very, very young kids, none of us were seriously brain-damaged or psychologically impacted by the realization eventually that it wasn't true. And if you don't wanna tell them that Santa Claus is real and you wanna tell them the truth, go ahead and tell them that too. That's also fine.

2:02:22.8 SC: Naroun Narasimacha says, "Do you see a problem with basing day-to-day epistemology ethics and other essential aspects of human life on very rigorous calculi, e.g. Bayesianism or utilitarianism? While it is possible to do due diligence to these calculi in academic research, implementing them in their pure form in our daily lives require impractical amounts of thought, computation and possibly knowledge. Are there philosophers who seriously work on developing heuristic only versions of such calculi," this is calculi in the sense of plural of calculus, by the way, in case you're wondering, "that are both feasible for everyday use and robust against variations and the users backgrounds, etcetera?"

2:03:00.6 SC: Basically, yes, I do agree with the thrust here in the following sense, and I think I've said this in other context, but I think that in many ways, the fact that human beings are bounded, finite beings, computationally and informationally circumscribed, is really, really important. We don't either have the time or the capacity to do perfect reasoning about anything, we will always be using heuristics, etcetera. And this fact has been put to use in psychology and things like that, for example, remember the podcast with Karl Friston, who thinks about the Bayesian brain and the free energy principle. And the free energy principle is a way to sort of make sense of what the brain does, thinking that it's trying to be a good Bayesian reasoner, but it doesn't have perfect resources to do that. So the free energy principle is sort of a shortcut to do Bayesian updating to get a picture of the world.

2:04:05.8 SC: And I absolutely think that in the real world, there are going to be constraints on how precise we can be and how precise we can reason. Another relevant podcast episode is the one with Elizabeth Anderson, where she talks about ideal theory. This is an ongoing debate right now in moral and political philosophy, how useful is it to figure out how to be better morally or politically by first imagining how to be perfect, it will be the ideal society, etcetera, and then moving in that direction, right? That's a feasible thing to say, if we knew what the perfect society was and we know what our society is now, we could say, "Good, let's move to make things closer to the perfect society," and I think that would work.

2:04:53.4 SC: What Anderson says and what other people would argue is, there's no such thing as the ideal theory, because as the real society changes or as conditions change, you might learn more, change your mind about what you want society to be. It's much more feasible to talk about how we can start with where we are and make things a little bit better, than to start with where we are and say, "Well, over there is the perfect thing, let's move in that direction." And I think a similar thing is probably true for more everyday life kinds of things, you know, how should I choose what to do from day-to day or even make big picture decisions about getting married or choosing a new job or having kids or something like that. There will always be heuristics involved, and the short answer is, I don't know of detailed philosophical investigation about exactly, focused on exactly that feature of day-to-day decision making, but that's not to say it's not out there, it might just be out there and I'm not aware of it.

2:05:51.2 SC: Emmet Francis says, "Any thoughts on some strategies for maintaining good mental health as a graduate student or as an academic more generally. From my experience as a current PhD student and from talking to other grad students, I know a lot of us struggle with impostor syndrome, anxiety from the pressure to publish or perish, etcetera."

2:06:11.2 SC: I don't know if I have great thoughts on strategies for that. I'm very sympathetic to the issue. I like to say I think that graduate school should actually be fun. I mean, there's no question, it should be a challenge, it should be pushing you to your limits, but in a healthy and rewarding way. Very few people go to graduate school and something like theoretical physics for the fame and fortune, right? They go into it because they love the material, the topic, the subject matter, and if that love is squeezed out of you by being in some sort of psychological pressure cooker, then it's not doing what it's supposed to be doing.

2:06:53.0 SC: There are absolutely abusive advisors and mentors and lab PIs, etcetera, that if possible, you should stay away from. So this is one of the things you should take very seriously when you're looking at possible graduate schools, not only which is the most intellectual powerhouse, but of the potential advisors of the places you're looking at, do they treat their students well, what are the reports back from the students who are there. Having said that, there are certain things that are always going to be true about the graduate school experience. Not only is it hard to learn the things you need to learn to be a successful PhD, but usually you enter graduate school hoping to become a professor, and there are not that many professor jobs, it's a very tight market and there's just nothing to be done about that, there will always be more people who want those jobs than there are jobs. It's like being a concert pianist or a professional basketball player, there's a lot of people who want that, and the competition is very stiff to do it.

2:08:00.0 SC: In some sense, as difficult as graduate school is, it's a lot easier than trying to make a living as a Hollywood actor, because at least graduate school, postdoc, etcetera, is a clear-cut set of steps that you can go through and it's either up or out, either you succeed or you... It's clear, you should get a job doing something else, whereas if you're an actor or musician, etcetera, you can struggle for years and never know, and it might be that tomorrow you get a big break, it's a lot more random and a lot less structured. So, hold on to that as a little shiny beacon of hope, but otherwise... What can I say? Other than not choosing an abusive advisor, even if you choose a really nice advisor, people can still put pressure on themselves, like you say there is impostor syndrome, there's the competition to eventually get jobs and things like that, I don't have good advice there, I really don't and I'm sorry, because I think that I care a lot about the mental health of students. I try to make my own students... If anything, I don't push my students hard enough, 'cause I know that it's hard and I let them push themselves and that's a tricky thing as an advisor.

2:09:13.0 SC: For one thing, by the way, you get no advice on how to be an advisor, no one teaches you how to do it, so it's a hard thing to do because you have a tremendous impact on the future lives of your students when you're an advisor and you have no training and how to do a good job at it. And one of the things that's crucially important about being an advisor is that different students need different kinds of advising, some students are better if their advisor's hands off and they let them do their own thing, others really need to be guided. Others need to be constantly encouraged, others need to be told, "No, you're slacking off, try to do better," and it's hard to know ahead of time what the right strategy is. So even if your advisor seems to be taking the wrong strategy, don't necessarily blame them because they're not experts in this game either.

2:10:02.2 SC: I think that if I'm gonna give one piece of advice, well, it's not very helpful, I know, but I'm gonna give it anyway, which is to just do your best not to lose sight of whatever that little bit of delight and passion and wonder it was that got you into this field in the first place. If you went to grad school because you didn't know what else to do, then I don't know what to tell you, but I think most people go to grad school 'cause they're really passionate about the material, they wanna learn about this stuff and hopefully increase the total amount of human knowledge about this stuff. And that's what to keep in mind. Not comparing yourself to other people or worrying about the job market or whatever. Getting a PhD, whether or not you get a job as a professor ultimately down the road is an intrinsically worthwhile endeavor. It's like climbing a mountain, right? It's an obstacle there that is maybe a little artificial, but it's a sense of accomplishment upon doing it, and it's better than climbing a mountain because you learn something that is really useful for other purposes.

2:11:07.4 SC: I don't wanna disparage mountain climbers, sorry about that, but you get something out of it that is both a sense of accomplishment and something a little bit more tangible. So it's always the comparisons with other people that get you in trouble psychologically here. "I'm not good enough. I'm not gonna get a job. My advisor is mad at me," it's always those things that get you in trouble. The material, the learning... And keep reminding yourself how much more you know now than you did two years ago. It's remarkable because at any one moment from week to week, it's easy to lose track with the fact that you're learning a lot, but then you look back a couple of years, and you're like, "Boy, I didn't know anything two years ago," and that is a wonderful time of your life. You're really learning to go from being a student to being a researcher when you're in graduate school, and that's just a singularly wonderful thing to get to do. So impostor or not, don't forget to be glad that you're in this wonderful position, to really take advantage of the intellectual life that humanity has been able to cultivate for a few of its people and not too many of them.

2:12:19.9 SC: Jeff B. Says, "I was reading about Lord Kelvin's idea that different atoms are just different types of knots in the ether. Although this is obviously false, can we think of string theory as a quantized and relativized upgrade to this idea, or is there more to it?"

2:12:33.4 SC: Well, there's definitely more to it, but maybe there's a family resemblance there also. There's no ether, so that part is wrong. And they're not knots. Strings are generally not knotted, they're generally either single line segments or circles. Okay, they're not tied topologically interesting ways around each other. If they did, they would just untie right away. So they're neither knots nor the ether. But I know what you mean, I mean, they're vibrating, one-dimensional things and the kinds of vibrations are what specify what kind of particle it is, what kind of field it is, etcetera. So that's the family resemblance, but there's no direct intellectual lineage from Kelvin's idea to string theory.

2:13:16.2 SC: As you may know or may not, string theory arose in a really backwards way. They were trying to understand the strong interactions... Well, there's a couple of different threads that came together, people were trying to understand the strong interactions. There were certain patterns in the observed particles out there that particle accelerators were giving us, and people were trying to fit them to different functions and make predictions and so forth. And Veneziano came up with a formula that said he could predict some of these patterns of masses and spins in the observed spectrum of particles. And then other people like Leonard Susskind is one of them, and from a very different angle, Yoshiro Nambu was one, and Goto, his collaborator, pointed out that by starting with one-dimensional strings and quantizing them, you could derive this Veneziano amplitude.

2:14:10.9 SC: And they were still trying to understand the strong interactions and it failed, it didn't work for the strong interactions. There's a little bit of lingering remnant of it in the fact that you could take two quarks and stretch them apart, like a one-dimensional string of gluon field stretches between them. So there's something slightly stringy about the strong interactions, but what we now call string theory is a completely different version of that that gives us gravity and things like that, and hopefully everything else. But the strong interactions come along for the ride rather than being the central point. None of this has anything to do with what Kelvin was talking about. So it's at most, a family resemblance rather than a direct relationship.

2:14:49.8 SC: Casey Mahone says, "I sense a stigma in our culture, against being single. I'm starting to find that I may be the kind of person that would just prefer to be alone, but people seem to view this as childish. They wonder how you can really be an adult if you don't have a partner to build something with. Do you sense this stigma and do you think that it is justified?"

2:15:09.1 SC: Well, I certainly don't think it's justified. I get what you mean. There's both sort of a deep sense and a shallow sense. The shallow sense is a lot of our society's social activities are constructed around the idea of being part of a couple, or a family, go to dinner with your loved one or whatever, or show up at a table for two or two people hotel room or whatever it is. We group society's functions around the idea that either now or some day you're going to be a member of a couple. That is true. And then I think, like you're hinting at, there is sort of a deeper thing going on where people judge you. It's not just we build structures to make it easy to be part of a couple, but it is assumed that you want to be part of a couple, and if you're not, it's because you failed in this universal goal.

2:16:03.7 SC: It's not exactly the same, but there's a close analogy with the idea of being sexual versus being asexual. Remember, we did a podcast with Angela Chen, where she talked about how difficult it is to realize that you're asexual, that you just don't have that kind of relationship between love and lust that most people have, and society is not built to deal with you and judges you to be bad if you don't fit in. This is what society does: Society takes the fact that human beings are very diverse, looks at the most common ways that human beings are and judges those to be correct, and judges all the other ways to be wrong. And this is a flaw in how society works, but it's hard to...

2:16:53.1 SC: Hopefully, we're getting better at it, etcetera, there's a lot of resistance to change along these directions. So it's certainly not justified, like, I mean, there's softer but related prejudice against not having kids, for example. It's assumed you should want to have kids. That's just what you wanna do. Getting married, etcetera. What if you're in a couple and you don't get married? What if you're in a couple for... The same couple for 50 years, but don't get married? People will look at you funny.

2:17:20.6 SC: What can you do? Be proud of who you are in your individual peculiarities, rather than worrying about them. It's not the biggest kind of prejudice out there in the world, so I get that you might feel like you're being judged, but lots of people are being judged for different reasons, and you just gotta tell society to suck it up and deal with it, because you are who you are and you like it that way.

2:17:44.6 SC: Robert asks a priority question. "If our current formulation of quantum mechanics hasn't appeared to answer what happened at or before the Big Bang, the black hole firewall and singularity questions, is it better to think of the wave function as a complete description to quantum mechanics or that it needs to be modified to account for these mysteries?"

2:18:07.7 SC: Good. This gets to the issue of quantum mechanics as a framework versus a theory. Quantum mechanics is not a theory of physics, it's a framework in which you develop theories of physics, just like classical physics is not a theory all by itself. You can have the classical theory of gravity, of electromagnetism, of the simple harmonic oscillator, whatever. Likewise, you have the quantum theory of simple harmonic oscillator, electromagnetism, gravity, whatever, you have different versions of all of these different specific theories, classical versions and quantum versions. So when you struggle to understand some particular quantum mechanical phenomenon, it is possible to ask, "Is this a case where it's not just we haven't found the right model within quantum mechanics, but the quantum mechanics itself is not up to the task?" Maybe. In some sense, that's what Stephen Wolfram would say, if you remember our podcast with him.

2:19:10.2 SC: I don't see that at all. I think that quantum mechanics, number one, there's no experimental problem with it at all. In other examples of times where we've had to throw out a theory or improve upon it, there's always been some experimental evidence that it wasn't quite fitting.

2:19:26.5 SC: But number two, I don't think that we've tried hard enough. It seems to me, to be extremely premature to say, "Well, we haven't quantized gravity yet, therefore, quantum mechanics is wrong." We just haven't tried hard enough to quantize gravity. Honestly. I know there's a lot of people out there working on quantum gravity, but most of them are still starting from some classical theory and trying to quantize it. I don't think that taking the quantum-ness of it seriously has been tried nearly hard enough. So that's why I am not in favour of doing that. Quantum mechanics as a framework is extremely robust and successful, and there's also the small data point that people have tried to change it and failed. It's really, really hard to modify quantum mechanics in a way that doesn't immediately run afoul of something you know about the world, like the speed of light limit or something like that. So you're welcome to try, but there's no experimental evidence that says you have to, and there's a lot of theoretical hurdles to overcome if you're gonna give it a shot.

2:20:26.0 SC: Anonymous says, "Regarding the many-worlds interpretation of quantum mechanics, is information conserved in the same way that energy is conserved?"

2:20:34.1 SC: Yes. In fact, precisely the same way, namely, in many-worlds, both information and energy are exactly conserved in the wave function of the universe. They are not exactly conserved on individual branches of the wave function, where observers find themselves. And this fact is very, very well-known when it comes to information. It is less well appreciated when it comes to energy, but I think as I mentioned earlier in the AMA, it is true. Energy can vary a little bit from branch to branch of the wave function.

2:21:08.5 SC: Seth says, "Did your discussion with Sylvia Earl on the ramifications of consuming animals change your view of the morality of animal consumption?"

2:21:17.3 SC: I think I might have edited this badly because we were talking specifically about sea animals, not land animals. So the short answer is no. Sylvia Earl had very meaningful and important and interesting things to say about the health aspects of eating seafood, both your individual health as a person and the health of the planet. I mean, there's the issue of over-fishing and ruining the resources that we have in the oceans, that's very, very important, and I think we should take that seriously. Then there's the extra issue of the point she made, which is kind of very obvious in retrospect, but I had never really thought about it before, that unlike... If you're a meat-eater, as a human being, and you eat land animals, most of the land animals or even most of the birds that we eat, so chicken, pork, beef, whatever, these are themselves vegetarians. Herbivores. The cows eat grass and wheat and things like that. They don't eat other animals.

2:22:25.4 SC: We don't eat jaguars and crocodiles that much. I guess we do eat crocodile sometimes. But anyway, most of the animals that we eat for our daily diets are themselves herbivores. So when chemicals that we human beings nastily or thoughtlessly put into the environment go into the environment, there might be some bad, poisonous chemicals that fall on to plants and then the cows eat them and then we eat them, but it's relatively sparse, all things considered. Whereas the fish that we eat are themselves carnivores, the fish we eat generally eat other fish to live, and those fish eat other fish, etcetera. And what happens is, a focusing and a concentrating effect that all the bad chemicals in the oceans are consumed by the little fishes or the plankton or whatever, and they keep getting consumed by bigger and bigger fish, and they don't go away. So they get concentrated in the fish that we eventually eat, so seafood can have a much higher level of these toxic chemicals than our land food can, and I think that was Sylvia Earl's biggest point about the health aspects of eating fish.

2:23:35.8 SC: And so I think it did certainly make me think twice about the health benefits of eating seafood, 'cause if you think just... If you don't think about the toxic chemicals, then seafood is thought of as a healthy thing to eat, compared to bacon or hamburgers or pizza, But when you do think about those chemicals, then it changes your mind a little bit and so... Yeah, I think maybe... It made me a little bit less eager to seafood and count it as a health move than I otherwise would have.

2:24:06.4 SC: Okay, I'm grouping two questions together. Let's see if I can remember why. Sid Huff says, "You've mentioned a few times on previous AMAs that you had a distinctly non-academic upbringing. Do you think it was extra difficult for you to succeed so well academically because of that? Have you ever felt impostor syndrome, the sense that you don't really know enough, aren't really capable enough to be where you are, that somehow you've managed to fool everyone so far?" And Jim Murphy says, "Do you have any advice about developing discipline? I am always impressed by how much you are able to do without seeming exhausted at all."

2:24:41.7 SC: So yeah, I was reluctant to answer both these questions 'cause I don't like to talk about myself personally that much, I like to talk about my ideas and so forth. But these are decent questions, and maybe we can help other people by talking about them together. And I especially liked Jim's question about how impressed he is about I'm able to do so much without seeming exhausted at all because, man, I am exhausted all the time, and I'm very happy to hear that it doesn't seem that way. Especially... Yeah, I'm really good at just saying yes to requests to do things that have deadlines in the future, and then being in trouble when the deadlines get here, and it can be very exhausting. And there's a lot of things I wanna do. There's so many books I wanna write and papers I wanna write, and things I wanna learn that I don't yet know, that I could easily imagine living for another 200 years and not doing nearly everything that I wanna do.

2:25:35.1 SC: So there's a kind of rush to get things done, in some sense. But at the same time, at some point, you wanna enjoy life, and I'm actually not that bad at enjoying life. I'm not a workaholic who sort of deprives oneself of leisure time or anything like that, but then that just contributes to the fact that you're working really hard when you're not having the leisure time, when you're not watching movies or basketball games or whatever it is, and I'm not especially disciplined either. I feel like I waste some time and I'm very productive at other times. I'm fortunate enough, when it comes to writing or doing science that I can go in bursts, and different people work in different ways. But I'm the kind of person who can be really unproductive for long periods of time, and then a burst of activity and get a lot done. And so far so good. Kind of good.

2:26:25.9 SC: Like I said, I'm not one of the world's five best physicists, so maybe it could be better if I were productive all the time, but I've done okay, and I'm appreciative of the fact that other people seem to appreciate some of the things that I do. That makes me feel good. But as far as developing discipline and having some technique or something like that, no, I really don't. I muddle through as best I can. People who write books are always asked about their writing process. Like, "Do you wake up at 6:00 AM and meditate and then write for an hour and then answer emails," and that's just not my life at all. Every day, it might be completely different, process-wise. I need to write when I'm inspired to write, when I'm ready to write, I can't force it on some schedule or in some routine. Maybe it'll be better if I had a routine and did that, but I just don't.

2:27:10.7 SC: As far as Sid's question goes, imposter syndrome was never the right way to say it. I think, I'm, like I said, pretty good at physics, but not as good as the absolute best people in the world, and I think that is correct syndrome. That's more or less accurate. I don't feel like I'm an imposter. I think there are people who I'm better than at and people who I'm worse than at. I didn't construct that sentence very well, because like I said, I don't like talking about myself. But I am an odd duck in some ways. Obviously, I have a podcast. So if nothing else, that makes me very unusual in the set of all theoretical physicists. And I write physics papers, which makes me very unusual in the set of all podcasters, even about physics. So it's always been an awkward fit, and I've never been a big... Non-academic family, big public high school, middle of the road Catholic university, etcetera. It's always been me trying to find my own ways in ways that my environment didn't always train me for in the best way. It was certainly never something where this was all expected to happen.

2:28:29.0 SC: And for better, for worse, I just wanted to do things in my own quirky ways, like when I was at Chicago, I eventually got denied tenure at the University of Chicago, and I'll be very honest that I worked hard and wrote papers, and some of those papers became super highly cited, but I didn't like devote myself to getting tenure. I didn't just say like... I didn't live in fear of not getting tenure, I just thought, "Obviously, I will get tenure. What's the problem? I'm doing good work, I'm writing good papers. What is the thing to worry about?" And in retrospect, that was pretty dumb on my part. But it is how I live my life. I would like to do things like I would like to do them, and I've been extraordinarily fortunate enough to mostly get away with it. That was one example where I didn't get away with it in a very big and loud and noisy and impactful way, but otherwise, I've done okay. So I don't think it's impostor syndrome, I've never...

2:29:26.2 SC: I know that there are people who are better at physics than I am, but I don't think that's a psychological mistake, I think that's just true, and I admire those people, and I try to learn from them, rather than feeling like I don't belong with them because I do think I'm better than them at other things. And there's probably people who are better than I am at everything, but I don't... Those people, I don't run into that often. I'm lucky enough that for most people, even if they're better at me, at 99% of the things I care about, there's one thing that I'm better at than them even if it's just like playing poker or... I don't know. Making stir fries or something. There's gotta be something. So that's how I avoid getting too much imposter syndrome in my life.

2:30:05.3 SC: Heinar Venmir says, "Priority question. Why is the uncertainty principle not enough to explain the collapse of the wave function at measurement in quantum physics? A measurement of a quantum property means to amplify a phenomenon from the microscopic atomic domain to the classical domain. To amplify means to add a substantial amount of energy to the microscopic domain, to make it noticeable in the classical domain. When adding energy to one of a superposition of quantum states, the object being measured will have an uncertainty in its energy. This is determined by the added energy. The uncertainty in energy will apply an uncertainty in the lifetime of the super-posed state through the uncertainty principle, causing the super-posed state to decay quickly into one state."

2:30:47.2 SC: No, so I don't think it works at all. Sorry about that, Heinar. For many reasons. For one thing, to amplify a microscopic quantum superposition to a macroscopic superposition has nothing to do with energy whatsoever. You don't need to put energy into the system at all. The classic example is Schrodinger's Cat, where you amplify the superposition of a decaying nucleus to a superposition of a cat, but the point is not that you put energy into the cat. The awake cat and the asleep cat have the same amount of energy, roughly speaking, it's just that they're in a superposition of two microscopically different things, and that's absolutely typical of quantum mechanical superpositions. There need not be some large energy difference between the two parts of the superposition, so you need not add any energy at all to put things into a superposition. That's one thing.

2:31:40.2 SC: Second thing is, you say at the end, there is an uncertainty in energy that implies an uncertainty of lifetime and the super-imposed state will decay quickly. But that's already assuming an answer to the measurement problem. Super-imposed states don't decay quickly, they involve according to the Schrodinger equation, unless you have some theory of the measurement that then collapses the wave function on to one thing or the other, that's what we call a decay. So when you observe a Gieger counter click because some nucleus decayed, that is a measurement of the system that has collapsed the wave functions. You need already, a theory of the collapse of the wave function before you can talk about that.

2:32:17.9 SC: Finally, in some very real sense, the uncertainty principle is not about measurements at all. So you really, in principle, can't bootstrap your way up from the uncertainty principle to a theory of the measurement problem, 'cause the measurement problem has to do with the fact that there is a set of processes under which wave functions appear to not obey the Schrodinger equation. They appear to collapse. And so either they don't obey the Schrodinger equation or they just appear not to, because as Everett would say, "You're not observing the whole wave function."

2:32:51.6 SC: But the uncertainty principle isn't about that, it's not about the measurement process, it's about the existence of quantum states without even measuring them. Certain quantum states just don't exist, there exist no quantum states where there is a perfectly... The technical way of saying it is, you cannot be in an Eigenstate, a state of perfect determination of both position and momentum at the same time, 'cause there are no states like that. And I did not use the word measurement anywhere in that statement that I just made. There are consequences for measurement of the uncertainty principle, obviously, that's not surprising, but it's fundamentally a statement about the fact that there's a relationship between momentum and position, that with one is perfectly fixed, the other one is perfectly uncertain. That's it. Nothing to do with measurements.

2:33:44.9 SC: Blake Brasher says, "On a somewhat recent podcast, you mentioned that we shouldn't think of the nucleus of an atom as being composed of discrete protons and neutrons, but rather as a swirling massive quarks without any distinct boundaries between the quarks. Does this mean that the neutron stars are actually giant blobs of quark soup, or is it also wrong to think of a neutron star as being the same thing as an atomic nucleus?"

2:34:08.9 SC: You know, this is way too down to earth physics for me to really be an expert, so I believe that the answer is yes. It does mean the neutron stars are actually giant blobs of quark soup, and this is a part of the weirdness of quantum mechanics. There's going to be a lowest energy wave function for a certain collection of up and down quarks, that is also feeling gravity if it has enough mass to have an appreciable gravitational field, such that that lowest energy state looks like a neutron star. So it's not that there's individual neutrons bumping up against each other, 'cause there's a lower energy configuration than that, and the system will settle into that. At least that's what I think, on the basis of my basic physics knowledge, but I don't know nearly enough about the details of neutron stars. You should ask a real neutron star person or maybe even Google it. I honestly don't know.

2:34:58.9 SC: Simon Kit says, "Priority question. Is the discussion in research of consciousness an example of strong emergence? As much as I struggle with the strong emergence concept, I find it hard to have a coherent picture of endeavors to understand consciousness that don't involve some top-down causal power. I can't imagine a race of philosophical zombies having this field of inquiry. What could possibly be causing discussion and research into the nature of consciousness, if not consciousness itself?"

2:35:28.5 SC: No. I don't think it is an example of strong emergence. So you've already undermined your own question when you say, "I can't imagine a race of philosophical zombies having this field of inquiry." Remember, by definition, zombies behave in precisely the same way as non-zombies do. So every book that a non-zombie would write could also be written by a zombie. Every discussion, every field of inquiry, every passionate defense of the redness of red and the experience thereof, would be done by a zombie just as much as it would be done by a conscious person. And this is why I don't think that people who advocate the zombie thought experiment really think it through, for exactly this reason.

2:36:12.9 SC: But I don't think that... Strong emergence is a slightly ill-defined concept because it's kind of defined negatively. You say that, I have a theory and of small constituents making up a bigger system, and I have a theory of how the small constituents behave. And if I put that theory of how the small constituents behave on a computer and simulated it, what it would predict for the collection of all those constituents is different from what happens when I actually physically get those constituents together. In other words, it's just a statement that your theory of the small constituents is insufficient, it's incomplete, is not enough to say what happens when all these constituents come together. That's strong emergence.

2:37:04.4 SC: As I talked about in the podcast, with Philip Goff and Keith Frankish, there's no room for that, if your microscopic theory is the standard model of particle physics. Particles don't have... Particles behave locally in space and they respond locally to what fields are doing at the same point in space where the particles are. So there's nothing... No way within that theory, unless you wanna change the laws of physics, which is good for you, try to do that, but if you don't change the known laws of physics, strong emergence is not gonna happen from the level of particle physics to the level of human beings.

2:37:37.0 SC: The question you're asking is... I think you put it best in the final line, "What could be causing discussion and research into the nature of consciousness, if not consciousness itself?" Well, sure, the research is being done by conscious creatures. There is no school of consciousness research that is populated by researchers who are not themselves conscious. That's true. But this is not something that is in any way in conflict with the idea that conscious creatures are just collections of physical particles obeying the laws of physics. Your brain is a thinking machine and is thinking about itself, among other things, and there's a very good article recently in... Where was it? I don't know, but I tweeted it out. It was by Jenann Ismael, previous podcast guest, and the reason why I tweeted it out, not only 'cause it was a good article, but because the headline on the article was abysmally bad. The headline was something like, "Physics is wrong. Human beings play a crucial role in the cosmos," and of course, Jenann would never say something like that, and those words are nowhere to be found in the actual article.

2:38:45.0 SC: What she was saying was, there is a problem... Well, not a problem even, but a feature of how we describe the world that needs to be taken into account the fact that we are in the world. So it's a self-reference problem, it's not a problem of incompleteness of the laws of physics, but it's a problem of one of these famous philosophy problems of a... If you're able to completely predict the future, if you're a prophet, or if you're Laplace's demon, but then if you know what's gonna happen in the future, 'cause you can predict it, what happens if you do something else? What happens if you violate the prediction? This is always the problem with time travel stories or prophet stories or whatever. And the answer is that real physical systems are not completely accurate predictors about the future, they model the universe and they do their best. And Jenann's point was that in that model of the universe needs to be a model of you, because you can affect the universe. But your model of you is always gonna be crude, you don't have enough capacity to model yourself perfectly. This is my way... And now I'm translating... Now, I'm riffing on what Jenann said, but that's the basic point.

2:40:02.0 SC: But that's not an obstacle to describing yourself at all. There's an obstacle to describing the world perfectly, given that you're in it, but we never try to describe the world perfectly. As we talked about earlier in the AMA, we use heuristics, we use coarse-grained descriptions of the world, and they're always going to leave wiggle room. That's why there is something that is useful to call our ability to make choices and have volition and have free will in the world, but there's nothing about any of that that makes you think that it would be impossible to describe all that in terms of purely physical motion of matter, as far as I can see.

2:40:35.2 SC: Okay, I'm going to group two questions together. James Nancaro says, "Why don't we know the actual size of the universe? I understand that it may be our observable universe is only what we can see, and there is likely matter farther away beyond our horizon, but doesn't the microwave background or early universe nucleo-synthesis allows some scaling of the universe's total size, that is, does the proportion of primordial hydrogen and helium determine the total amount of matter, the bigger the universe, the more time for cooking up more helium at the start."

2:41:08.9 SC: And then Alexander Marash says... Alexader, sorry, Alexader Marash. "I think you mentioned in one of the previous AMAs, that the universe, while obviously being super small at the beginning and during the inflationary epoch could have been infinite in size right after inflation ended. I heard Professor Allen Guth saying many times that our universe could have been the size of a big marble or a baseball at the end of the inflationary period. Taking this piece of information on board, did I get your point wrong? More broadly, when we say that our universe could be infinite in size, either right after inflation or today, I get confused. Do we mean our universe, which as [2:41:45.3] ____, was once the size of a marble, or some other broader reality?"

2:41:49.7 SC: Okay, so there's a couple of different things going on here. One is, cosmologists are often sloppy when they talk about the universe. You would think that they would be precise when talking about the universe. It's their job, it's what they do for a living, but in fact, they're sloppy. Sometimes when you say the universe, you mean the collection of everything, absolutely everything, the whole shebang; what we see, what we don't see, the whole bit. Other times, we just mean our observable universe. In fact, it's probably more frequent when a cosmologists talks about the universe and says, "Oh, there's 10 to the 88th particles in the universe, they mean our observable part of the universe, they mean our observable radius of the horizon that we're able to see out to.

2:42:32.6 SC: So that is the part that when we say the universe at the end of inflation was the size of a marble or a baseball, that's what we're talking about. We're only talking about the observable part of the universe. We're not necessarily talking about the whole thing. The whole thing could be either pretty much that size, it may be a little bit bigger, but not too much, or it could be infinitely big. We just don't know. There are theoretical models either way, and there's zero experimental data because we can't see outside our observable universe by the nature of what it means to be the observable universe.

2:43:07.5 SC: So all of the things that Alan Guth was saying about the size of the universe refers to the observable universe. My statement... So you said the universe was super small at the beginning and during the inflationary universe, but could have been infinite in size right after and right after inflation ended. So I didn't say that... Or if I did say that, I just really didn't mean that. What I might have said, what I have said sometimes is that, at the moment of the Big Bang, which is a hypothetical singularity, which is really just a way of saying, "There is no singularity, we just don't know what's happening, but if you just naively trace the equations backward, there's a singularity. And that's the Big Bang," it's impossible to define the size of the universe at the Big Bang, because it's a singularity. It just means your equations are breaking down and you don't know what's going on. But at any time after the Big Bang, the universe could have been infinitely big. That's completely compatible with everything we know about general relativity.

2:44:07.2 SC: So the Big Bang could be the beginning of an infinitely big universe. It doesn't need to start at some finite size and expand. So it's nothing to do with inflation whatsoever. That's just a feature of general relativity, that the whole universe could come into existence infinitely big, when it's there, and that's not our observable universe, of course. Our observable universe was marble-sized or whatever it was.

2:44:30.6 SC: And then for James's question, I got mixed up there in the middle of your question, because CMB and nucleosynthesis and things like that, all of these data points we have from the early universe, they are completely independent of the size of the universe. They had nothing to do with the size of the universe. What they have to do is with the local density and expansion rate and temperature of the universe. So the CMB and the nucleosynthesis result from micro-physical processes of protons and neutrons fusing, or electrons being captured by nuclei, things like that, things that happen in points in space. And they depend on the local conditions, the temperature, the density, etcetera. But they don't know how big the universe is. The CMB and nucleosynthesis would happen exactly the same way in a finite sized universe and an infinite sized universe, with the same temperature and density everywhere.

2:45:23.0 SC: Okay, Thomas Prunty says, "In your podcast with David Wallace, you guys had a bit of a disagreement about how to think about the low entropy of the Big Bang. He said something like the early universe had as high an entropy as it could have, given that it was uniform, and you said that uniformity is a huge constraint. I don't see why uniformity could be surprising or need explanation, it seems like a very simple and sensible boundary condition. I understand that it's low entropy when gravity is considered, but I don't see how that implies that it's unlikely in this case. What am I missing?"

2:45:57.0 SC: Well, in some sense, this is all just fuzzy talk, like, "What should the universe be? What should be surprising? What demands explanation?" Who are we to say what the universe should look like or be surprised by? But I don't think that the fact that something is simple... Sorry. Let me be a little bit more definite than that. Simplicity is a very useful thing to keep in mind when thinking about laws of physics, dynamical rules that tell you how systems evolve over time or something like that. We found empirically, through the history of science that looking for such simple rules is a very fruitful endeavor. But it's completely different, in my mind, to think about conditions or configurations that the universe can be in. That's not something where we have any good reason to think, a priori, that the configuration of stuff in the universe should take a simple form. I don't see why that should be true.

2:46:58.4 SC: I do see why it should be a high entropy form, and the answer is, because there are more ways to be high entropy than to be low entropy. If you just sort of randomly consider all the different ways the universe could be, many, many, many more of them are gonna be high entropy than to be low entropy. So that's not to say the universe can't start out with a low entropy, but it says if it does start out with a low entropy, there should be some reason why, there should be some explanation, that's a clue that there's something about the dynamics of the universe that you haven't figured out yet. And so, when David says the early universe had as high entropy as it can given that it was uniform, that's true, in some sense. It's not exactly true, but it's pretty darn close to being true.

2:47:41.8 SC: But that doesn't say that that statement is completely independent from whether or not the fact that the early universe is uniform suggests that we need to think more deeply about what was going on or should just be the stopping point. I don't see why the fact of the early universe is uniform should in any sense be something where we go, "Oh yeah. Well, what else could it have been?" That's a natural starting point. The obvious thing to think about is, as Hugh Price likes to point out, if you imagine a collapsing universe, if you run the clock backwards, a collapsing universe does not become more and more uniform as it collapses, it becomes lumpier as it collapses. So that's the arrow of time, it's a very, very unusual, weird, finely tuned configuration for the early universe to be so uniform.

2:48:29.4 SC: Bejenn Warner says, "Here's a question on the relationship between quantum mechanics and chaos theory. To what extent does the principle of sensitive dependence on initial conditions apply to quantum systems? Does this framework matter at all, or is there something about quantum systems that make this not an interesting thing to ask?"

2:48:47.7 SC: This is a complicated question, actually, and I'm not even an expert on it, so let me try to restrict what I say to things I'm pretty sure are true. Okay? There's a whole subject called quantum chaos, and here's the reason why it's a non-trivial question, because in classical chaos, classical systems are often chaotic. Not always, there are non-chaotic classical systems, but you don't need to work very hard to get a classical system that is chaotic, like you put a couple of pendulums and hang them from each other, and you get a chaotic motion for the triple pendulum, as it is called.

2:49:22.5 SC: But quantum systems are a little bit different because the reason why classical systems can be chaotic has to do with the non-linearity of the interactions between them. And what I mean by that is, linear is when everything just depends on the variable you're looking at to the power one, X to the first power. As soon as something depends on higher powers, X squared, etcetera, that's non-linear. And the difference is that when you have a linear system, if you perturb it a little bit, it moves a little bit, and that's it. A small perturbation leads to a small difference in what is happening. But when you have a non-linear system, a small perturbation can grow very rapidly, and that's where you get chaos, that's where you get sensitive dependence on initial conditions, because the tiny perturbation you give it can feed on itself and grow very, very quickly.

2:50:17.1 SC: In classical mechanics, the fundamental equations of motion are often non-linear. In quantum mechanics, the Schrodinger equation itself is linear. The Schrodinger equation has a wave function as its fundamental variable, and the wave function appears to the power one on both sides of the Schrodinger equation. One side is the Hamiltonian, which is asking how much energy is there, and it says H, operating on one power of PSI, the wave function. The other side is the time derivative, how fast is the wave function changing, acting on one power of the wave function. So the kind of evolution that is chaotic and classical systems naively doesn't appear at all in quantum mechanical systems. The wave function itself does not evolve chaotically, full stop.

2:51:10.4 SC: But of course, in quantum mechanics, we have a classical limit. And this is where it becomes a subject that requires study and papers written about it, and why it's a whole thing called quantum chaos, because even though the wave function evolves linearly and non-chaotically, the classical observables, the classical limit of that wave function gives rise to classical mechanics. The triple pendulum that is a chaotic system is a classical limit of some quantum system. So there has to be some classical limit of this quantum system that does behaves non-linearly and chaotically. So there is an interesting thing to talk about, which is, this relationship between the underlying behaviour of the wave function and the emergent classical world and the emergence of non-linearities in that classical world. So I'm not gonna say anything specific about that, but that is an interesting intersection, overlap to talk about.

2:52:08.4 SC: Okay. Andrew Wickerstaff says, "Many recent Mindscape episodes have looked at the fallibility of human thought processes and how susceptible we all can be to biased manipulation and irrational ideas. Given that we understand this problem today much more than we did, say 20 years ago, and given the exponential growth in misinformation the average person is now exposed to, do you think that the jury system in its current form is still a fair way to decide criminal trials? In particular, do you have any thoughts on the jury selection process in America and whether or not it helps or hinders the justice system?"

2:52:41.6 SC: That's an interesting thing to put together. The jury system that came out of hundreds of years ago, it was thought to be important that people were tried by a jury of their peers. Now, there's well-known issues there, because only property-owning white men were considered people at the time, in terms of who got to vote and who got to serve on the juries and all that stuff. But nevertheless, the idea was that it was a participatory part of the democratic process, that deciding guilt or innocence should not be led to a professional elite, but should involve the participation of the common people. In part, a reflection of going way back to ancient Greece, in Socrates' trial, everyone in Athens who wanted to be part of the jury could be part. They all came and they all voted if they wanted to. We don't do it quite that badly.

2:53:37.6 SC: But by the way, for those of you who are not in the US; in the US, there is a right to trial by jury. We say there's a right to trial by jury. In fact, it's only for sufficiently serious crimes. If you get a parking ticket, you do not have a right to trial by jury for that. But just because there's a right, doesn't mean people take it. In fact, jury trials are a tiny percentage of all of the legal actions going on in the legal system at any one point. Sort of the big ticket corporate legal procedures don't even go to trial. You can sue somebody, but they're usually just settled out of court after it becomes clear who will win at court. And even in criminal defense situations, you can easily just have a judge trial. You don't need to have a jury trial. It's something that the defendant can request under appropriate circumstances. So juries are not a big part of the criminal justice system, overall.

2:54:36.9 SC: I certainly appreciate the problems with the fact that people who appear on a jury are not going to be experts. They're chosen specifically not to be experts, but I think that... And so, I'm not an expert on this, but here is my feeling, and then real experts can let us know. There's both good and bad aspects of that, okay? I mean, one aspect that is good, is just the fact that it is participation. Just the fact that it's a reminder to the people on the jury that they play a role in the government, that the act of governing their country, and including judging people's guilt or innocence in a court of law is not purely the domain of some distant elites, it is something that everyday citizens have some role in doing. So there's some civic virtue kind of aspect to it that way. And then the other...

2:55:37.5 SC: From the other side, from the point of view of the defendants, etcetera, I think it's the case that usually juries take their jobs pretty seriously, right? It's an interesting phenomenon, I talked about this with Astra Taylor when she was on the podcast, the political system called sortition, where it's government or if not the whole government, at least certain decisions being made, not even by voting, but by just a random selection of people are chosen to sit and think about an issue. So not the guilt or innocence of some criminal defendant, but you know, a law, right? Should this be a law or not?

2:56:16.0 SC: So sortition says, "We're gonna pick some random representatives from the population, and let them sit and think about it." And what you find is that when people are put into that situation, they usually take it really seriously. That doesn't mean they're good at it, necessarily, they still have their own biases, etcetera. We're not always very good at making sure that all of the representative voices are heard on juries and things like that, but there are some surprisingly uplifting features of that kind of situation. So, I don't... So clearly, juries do the wrong thing sometimes, that's absolutely true, 100%, but I'm not at all convinced... I don't know enough to have any strong feelings about, but certainly I'm not at all convinced that we would have a better system overall if we didn't have juries. Like, judges and lawyers can be bad too, right? Even though they are professionals. So, I would need to see actual data-based, evidence-based studies that are trying to be very fair about whether or not juries overall make things worse or better. I really don't know.

2:57:20.2 SC: Brandon Lewis says, "Do you imagine that you will use quantum computing in your research once they become available to you and mature enough to be useful? And if so, what kinds of problems would they help you to solve?"

2:57:32.3 SC: I mean, probably not, is the short answer. The research that I do is very pencil and paper, I do some minor computer simulations, like in the paper I wrote with Ashmeet Singh on dividing up quantum systems into sub-systems. We did a little example, a little toy example with some simple harmonic oscillators, and we put it on a computer and simulated it. And in principle, on a quantum computer that will be way more efficient, that's what quantum computers are really good at, is simulating quantum systems. And so in that sense, some of my research projects would be good fits for quantum computation, but I'm a little skeptical that you're gonna get very powerful quantum computers like that, at least in an affordable way, any time soon, by which I mean, like, my research career, okay?

2:58:21.3 SC: I'm a little skeptical that we're gonna build up a lot of qubits and have just so much capacity that people are gonna have quantum laptops or something like that, you know? I'm not at the level of using supercomputers even right now, so I don't think that I will ever get to the level of needing a quantum computer for my research more than people who are really doing quantum field theory, simulating on a quantum computer or chemistry on a quantum computer, those are the kinds of things that would be higher priority than little old me.

2:58:50.9 SC: Peter B. Says, a priority question, "You've discussed before how the many-worlds of quantum mechanics may be just the other side of the same coin as the cosmological multiverse. I'm still struggling to understand this idea. I can understand how I am in a superposition with other versions of me in the quantum multiverse, but how can I be in a superposition with other versions of me trillions of light years away? Or are you saying that these types of multiverses redundantly contain the same set of events, but they are still distinct from each other?"

2:59:18.4 SC: Well, this is a subtle thing, so I guess I've discussed it before, but probably not very often, because it is a very subtle thing. I have a blog post about it that goes into more detail. This was an idea that however many years ago, before these two papers came out, two papers came out, one by Yasunori Nomura and the other by Rafael Buso and Lenny Susskind they both were on along very similar lines, drawing a connection between the cosmological multiverse and the many-worlds of quantum mechanics. And before that, if you would ask me, I would've said, "No, they're obviously very, very different, they have nothing to do with each other." But the missing ingredient, which is highly non-trivial is the idea of Horizon complementarity, okay? Which is a little complicated, but this is a priority question, so you're... It is good that you put priority question in here because otherwise I might have skipped over it, 'cause this is complicated to explain.

3:00:10.8 SC: But the idea is the following. Due to Susskind and some of his collaborators, first in the context of black holes, they were trying to understand the black hole information loss puzzle, and there's certain features of classical black hole physics that are a little bit puzzling, not super puzzling, but a little bit counterintuitive at face value. If you throw something into a black hole, right? The famous thing to say for the information loss puzzle is, throw a book into a black hole, the information falls in, it disappears. Classically, that'll be fine, but quantum mechanically it evaporates away, how does the information get out? But we all know because there is something called gravitational time dilation, that from the point of view of the outside observer, at least in some approximation, where the book does not have its own mass or something like that, but the book... You don't see the book disappear past the event horizon, right?

3:01:04.7 SC: What you see is the book move more and more slowly as it comes close to the event horizon. The book doesn't feel that way, if you were riding the book, if you fell in with the book, you would just pass right through the event horizon, but from the point of view of an outside observer it just seems to slow down and approach... Creep up closer and closer to it. So Susskind and friends said, "Well, maybe there's a souped-up quantum version of this," which says that there's one thing that is going on in the world, but there are two different ways of talking about it, that depend on which observer you are. There's the observer that falls in and everything is just perfectly normal, and to the observer far away, all of what falls into the black hole, all the information that falls into the black hole is somehow stuck on the horizon. And this is an incompatible picture, but that's why it's called complementarity, because you get to choose, you can't describe it both ways, at the same time, you can either describe it from the point of view of the in-falling observer or the distant observer, but not both.

3:02:04.7 SC: There's a version of this idea that works for cosmology, if you're in an accelerating universe like we're in, then you have a horizon around you, that horizon approaches a fixed size, and so the horizon complementarity view of cosmology would be that there is no outside universe. There's different versions of this idea, I'm telling you the most radical hardcore version. The most radical version is, there's the universe we see from inside, and what you and I think of as the external universe, all of its information is just on our horizon, there's nothing more to it than that. But that includes everything that you might have been tempted to say is outside our observable universe, it's all just encoded on our horizon, and that's a finite amount of information that you need that would ever be relevant to describing the outside world. What does that have to do with many-worlds and the multiverse?

3:03:01.0 SC: Well, it would seem that taking that idea at face value rules out the multiverse, you're saying that there is no universe extending infinitely far away. There's just our horizon. But now remember, there is the many-worlds interpretation of quantum mechanics, and in these cosmological theories of the multiverse, what you're imagining is there are quantum mechanical transitions between different vacuum states that would look like different local laws of physics. So, much like a decaying nucleus or let's say, just pick an individual neutron out there in space, a neutron can decay, we say, into a proton, an electron and antineutrino, in a correct many-worlds version of that story, we say that the neutron wave function evolves into a superposition of a neutron plus proton, electron and antineutrino.

3:03:54.9 SC: In a very similar way, the universe evolves into a superposition of our vacuum state plus other vacuum states. And in all of them, according to this version of the story, there's a horizon and this horizon has a certain size, and that's the whole universe. So what this theory says is that the cosmological multiverse still exists in the presence of horizon complementarity, but it exists because of the many different branches of the wave function, all the different parts of the universe where there's different local laws of physics are sitting on top of each other at the same point in space and some ill-defined sense, but all the branches of the wave function describe individually finite-sized cosmological horizon-sized patches. That's the story.

3:04:48.0 SC: So you can sort of instantiate the cosmological multiverse in one patch of space-time by evoking the many-worlds of quantum mechanics. That's the idea. Is the idea right? Yeah, we don't know, we have no idea. This is doing our best to draw conclusions from sketchy assumptions about how both gravity and quantum mechanics work, so it's possible, but I couldn't tell you whether it's right or not.

3:05:16.7 SC: Matt Hickman asks a priority question to it, "Can I have another priority question, please?" Nope, you can't. That's the whole idea of the priority question, you only get one, so you should use it wisely or you should have used it wisely, however you want to parse that.

3:05:32.4 SC: Rebecca Lashwa says, "You've mentioned in the past that if you weren't a theoretical physicist, you would consider being a theoretical computer scientist. Are there any particular results in the theory of computation that you find interesting or aesthetically pleasing?"

3:05:46.6 SC: It's very vague 'cause I'm not a theoretical computer scientist, but I find fascinating the restrictions on computability and computation, and as we discussed before in the AMA, all of the structure that sort of comes to life because of our boundedness and finitude. And this has to do with emergence and things like that that I am working on. So in John Conway's Game of Life, the famous cellular automaton with the black and white squares on the checker board, there's a sense of emergence in that game because there are structures like gliders and glider guns that you can talk about as individual phenomena without specifying the specific details of every little black and white squares.

3:06:32.0 SC: In some sense, in some sense, theoretical computer science has something to do with artificial worlds, and you can study general principles of dynamics or emergence or things like that in the context of these artificial worlds, and then worry later about applying it to our world. As a physicist, I kinda gotta worry about our world, that's my job, but as a theoretical computer scientist you are less constrained. So I think that's the kind of thing I'd be interested in, but who knows, maybe I would be doing artificial intelligence or something like that, it's a whole new brand new world that we're inventing in that whole field, so many, many interesting questions out there.

3:07:11.4 SC: Peter C. Harris says, "Do you have any thoughts about the 1619 project and The New York Times decision to endorse it?" My understanding is that the person leading the project is a brilliant journalist, but not a historian, an historian," Peter correctly wrote. Sorry, I mispronounced it. "And that many historians, including African-American ones, say the project contains significant historical errors. Isn't this roughly analogous to a science journalist creating a project that endorses the Copenhagen interpretation of quantum mechanics and having that project endorsed by the Times, even though incredible rival interpretation, such as many-worlds exist. Should a newspaper record even be making such endorsements?"

3:07:49.3 SC: I think the short answer is... I try to give the short answer to long questions first, so the short answer is, no, I don't quite agree with where you're going here, and I don't even think that it's correct to formulate it to saying The New York Times has endorsed this. The New York Times published, the 1619 project, just like it publishes op-ed pieces and book reviews and news articles and a whole bunch of other things that it published. And certainly it rose to the level of being publishable in the New York Times, but I think that the framing is a little bit off, I would say, at least my opinion versus your opinion, I would frame it a little bit differently. The 1619 project made a lot of claims and some of them were mistakes. That's not surprising to me at all. Like any big project... I've written books, as someone who writes books, a couple of mistakes will creep into those, that is by itself not really determinant of anything at all.

3:08:44.9 SC: I do think there's a slightly more pointed critique you could make that maybe some of the mistakes they made were... The direction of the mistake was influenced by an underlying political perspective that they had. I think that's a better criticism to make that they erred... There were systematic errors and not just random ones, let's put it that way. And that's okay, it's a perfectly good criticism, but to me, that criticism completely pales in comparison to what they're trying to do to the bigger picture point or view of what they're trying to do. Look, any society, any country, any nation tells stories about itself. History is a set of stories that we choose to tell, the number of facts about the past that we could potentially write down is infinitely big. The whole thing about history is we pick some of those facts to focus on and to tell our stories and to relate to ourselves, and unsurprisingly, countries, nations tend to tell more or less flattering stories about themselves. They tend to gloss over the parts that are a little bit less flattering, like Japan doesn't really like to talk about what happened in Manchuria at the beginning of World War II.

3:10:00.3 SC: They don't really like to talk about that. France, I have friends who are French who really are genuinely puzzled about why other people from European nations don't like Napoleon. I mean, after all, Napoleon just brought civilization to them and the rule of law. Shouldn't they be thankful to Napoleon? British people are genuinely puzzled that the people around the world mind that they were colonized. Again, weren't they just bringing peace and love to them? They don't get it. Because we tell these stories about ourselves... That in China, any idea about Tiananmen square has been scrubbed clean. And the United States is no different than that. The United States tends to, in its sort of history lessons, etcetera, teach a sanitized version of the real history. There's plenty of examples, the Trail of Tears is a terrible, awful incident in the United States history, which is not put front and centre, certainly when I was in high school, and maybe it's different now. Maybe a more well-known example is the controversies over Columbus Day.

3:11:11.3 SC: By any, even halfway, objective judgment, Christopher Columbus was a terrible, terrible person. I mean, he was a genocidal rapist, racist maniac, and he also had some ships that travelled across the ocean. Even though his Spanish sponsors got completely tired of him by the end of it, but no objective historical account of what Columbus did would paint him as a hero, but there's a story in the United States that he discovered America and we brought civilization to it, and this is not just from hundreds of years ago, I remember when I was growing up, we had Schoolhouse Rock, which was great. Mostly Schoolhouse Rock were these little cartoons you would watch on Saturday mornings that would teach you about different educational things, sometimes about grammar or science, but sometimes about history. And there was one called Elbow Room, about how when the colonists came from Europe to the United States, they needed Elbow Room, they needed space, so they basically spread across the country and everyone was happy. [chuckle]

3:12:22.2 SC: That's a story that we're telling that leaves out certain aspects of the story, namely like all the indigenous people who were killed, and there's their cultures wiped out in the process of that expansion across the continent. And the idea of slavery and its role in the whole history of the United States is one that absolutely we don't always face up to in the United States. And so the goal of the 1619 project was to put slavery front and centre as an aspect of what was being thought about at the founding of the United States, and did they go too far sometimes, did they over-emphasize the role of slavery and why the American colonies chose to declare their independence from Britain and so forth? Maybe, very plausibly. I'm not a political expert there. But look, the point is, it's them versus the world. It's not like they're the only history of the United States that's ever been written.

3:13:28.5 SC: They're going up against every other single history of the United States where the role of slavery has been under-emphasized, you need a correction in these situations. And so good for them for moving in that direction. I think that this is part of being honest, being factual and evidence-based is that you have to look at your potential biases, and this goes for countries just as well as it goes for individuals.

3:13:56.6 SC: You want to tell a story of your past that makes you look good. And this, again, for societies as well as for individuals, and the United States has been just really, really... It's consistently failed to examine the role of slavery in the history of the United States. We talk about the Civil War. Plenty of people in the South still think that the South was... That the Confederacy was virtuous in one way or another, and they try to tell themselves stories about how it really wasn't about slavery and things like that. And so I am perfectly willing to criticize factual mistakes that are made by the 1619 Project or anywhere else, but the project is well worth taking seriously. We need to examine our flaws, and again, this is part of being honest and truthful and factually-based. We need to be able to face up to the parts of our history personally and nationally and humanly that make us the least comfortable. That's not always an easy thing to do, but I applaud them for pushing us in that direction.

3:15:08.4 SC: Paul Cousin says, "The last episode with Tai-Danae Bradley was fascinating. In it, you discussed the philosophical question of whether it could be conceivable to find all of the semantics and the statistics of the words alone. Would it make the idea more realistic if we include in these statistics all the sensory inputs that humans receive from the world while experiencing words, in which case, a machine will be strongly limited in its abilities to understand language unless we provide contextual sensory information as well in the dataset. What do you think?"

3:15:40.2 SC: Again, this is something where I don't have thoughts that are necessarily very expert level. This is the kind of question that people take very, very seriously in AI robotics, language studies, linguistics, etcetera, semantic, semiotics, but let me just say that maybe in principle, you would get a different view of the role of language and how it was used if you coupled it to observations of the outside world. So you look at not only what words are being said, but in what context to those words are said. What is going on in the world around you? What data you have in front of you when you say those words?

3:16:21.9 SC: Sure. That's more data. So it gives you a better view of the world, but it also sort of pulls the rug out from underneath the dramatic claim that is being made, which in some sense... There's a claim being made, which is that you don't really need that extra information, and that claim could be true or false. But the idea is that there is enough information just in the relationships between words and how they appear, that it greatly constrains the possible meanings of those words, even if you don't know what the person is looking at or talking about or whatever. So, yeah, you might be able to get even better understanding by adding more stuff in, but the question is, can you get a really, really good understanding without it? I think that is the issue that is trying to be addressed by this research, and again, the answer might be yes or no, or yes sometimes, and no other times. But that's what they're trying to do.

3:17:16.5 SC: Naiv Alam says, "Which genre of music do you consider to be your favourite, do you have any interest in opera, say by Wagner, Strauss or Mozart?"

3:17:25.6 SC: So I'm pretty ecumenical in my musical taste. I like music, overall. I like all sorts of different kinds of music, but classical music is something where I'm not an expert, I know that there are layers there that I am not sufficiently discerning or familiar enough to really talk about or judge in any reliable way, so I'm very happy to go to the opera and enjoy it, but I'm not going to claim to be appreciating the nuances that a real expert would be able to experience. Somewhere between jazz and classic rock are my favourite genres of music. I grew up in the '70s, I guess, my formative musical years, '70s, early '80s, and I like the music of the '60s and the early '70s and in popular music-wise, so that's my bread and butter. And then later on, I discovered jazz and would go to jazz clubs, and I really do love jazz probably... Maybe a little bit more even than rock and roll, but everyone from Mingus to Thelonious Monk, to a whole bunch of people. There was a reason why I had Wynton Marsalis as one of my first podcast guests.

3:18:42.0 SC: But I'm not very judgy about music. It'd be weird since I'm not even a moral realist for me to be an aesthetic realist, I don't think there is a fact of the matter about what music is good and what music is bad. Music is what moves you. Well, what matters about music to me is what moves you, personally. You can like and enjoy the simplest, dumbest bubblegum pop music, as far as I'm concerned. I do not think any less of you, for that, I don't think it's wrong to like that. If it brings you pleasure, then that's great, and there's no special accomplishment in liking the weirdest, most challenging music. Like, it's good, if you like it, that's cool. But it doesn't make you better than anybody else. Liking John Cage is not intrinsically superior to liking Britney Spears or whatever it is, whatever comparison you wanna do.

3:19:34.0 SC: Alex Borland says, "Do you have any favourite restaurants in Boston, especially in Chinatown?"

3:19:39.6 SC: So I debated whether to answer this question because the answer is no. I don't have any really super favourite ones, but I wanted to just remark on the fact that, look, I love Boston, and it's why I'm spending some time here, and I spent eight years of my life here living here. The restaurant scene, man, it's just not up to snuff. Sorry. Before this, the two cities I lived in, or in between when I was here for grad school and postdoc and now, the two cities I lived in the longest were Chicago and Los Angeles, both of which in their ways are amazing food towns. Weirdly, Chicago is great for fine dining, for like 3-star Michelins or restaurants, kind of things. Alinea, which is arguably one of the world's greatest restaurants is there, and there's plenty of other restaurants in that stratosphere, for getting dressed up a little bit, spending a lot of money, Chicago was just amazingly good.

3:20:33.0 SC: And it's not even a lot of money, to be honest. There are cities that are a lot more expensive than that, whereas Los Angeles, the sort of street food, ethnic food, a variety of different cuisines and different mixtures is unprecedented in my experience. And of course, there's other cities, New York and San Francisco and whatever that have amazing food scenes. And in their own ways, New Orleans and Austin or other places around the US. And Boston is fine. It's okay. Look, I live in Chinatown right now, from where I live for a couple of months, and I'm surrounded by dumplings, so many dumplings, so many Hot Pot restaurants, a few noodle restaurants, and they're fine.

3:21:17.6 SC: There's a little bit of variety, there's a Malaysian place that is... Of the ones I've sampled, this Malaysian place is definitely the best. But it's not like transcendently good. It's not like you're gonna get in the San Gabriel Valley and if you're in Los Angeles. Probably, I've been to a couple of fine dining restaurants here in Boston and they were fine, but not as good as you get in Las Vegas. In Vegas, you get really good fine dining, much less Chicago or New York. And probably the single best restaurant experience I had was at a place called Ruka near Chinatown, near Downtown Crossing, which is sort of like... It belongs in Vegas, honestly, and maybe it has an outpost in Vegas, it's like a hip Japanese Peruvian fusion place. And that's the kind of recipe for a disaster, if you do it badly, but they really do it well, like the cold sesame noodles, and the rolls that they had were really, really top notch. I gotta give Ruka some kudos there, R-U-K-A, if you're near Chinatown in Boston.

3:22:21.9 SC: Justin Bailey says, "What do you think is the most credible explanation for why we experience a single reality in a branching universe?"

3:22:31.1 SC: So I think this is one of those like sneakily difficult but really fascinating questions, actually. So I'm more hardcore about quantum mechanics and many-worlds than even most other many-worlds people are, than more than David Wallace is, in the sense that I think that the ultimate way to think about reality is a pure quantum vector, not something that sort of is based on some pre-existing classical precursor theory. So to me, there's work to be done in explaining why the world looks classical at all, more work than for most people who sort of presume some classical superstructure hidden in how they define the correct theory of the world, like they put in space and fields or particles or whatever. I think that all those have to emerge somehow from the wave function, so it's a tricky question why we experience a single reality, so first you have to say why... There's different levels of this question. One is, how is it that a classical-looking world can emerge from a quantum mechanical underlying structure? And I think that there seems to be a lot of prerequisites to that. The underlying quantum structure has a lot of properties that are necessary to allow for a classical limit to exist.

3:23:51.2 SC: So when you say single reality... Sorry, let me back up, there's just too many things going on here with this question. So let me finish that thought I was making. Only certain quantum theories will have classical limits at all. That is one fact, and we're still trying to investigate that and understand it better. The other, which I should have said first, is that the word 'we', is sneakily important in this question you ask, "Why we experience a single reality?"

3:24:24.5 SC: So, again, if you were the quantum version of Laplace's demon, you can just talk about the wave function in the universe, you wouldn't even talk about branches or anything like that, but Everett's insight is that when the branching happens, what occurs on one branch has no effect whatsoever on what occurs on other branches. So that is a subset of a sort of deeper principle, and I'm not sure what that deeper principle is. And this is why I think this is good research level question to ask. How, given some big theory of everything, do you divide it up into subsets which interact with each other in some interesting ways so that they would count amongst themselves as a separate world? We have examples in the case of many-worlds, when there's decoherence, it's pretty clear you can sort of point to the reasons why things that happen on one branch of the wave function will not affect things that happen on another branch.

3:25:26.4 SC: In the cosmological multiverse, it's also clear because, and forget about all the craziness we just talked about about horizons and complementarity and things like that, in the naive picture of the cosmological multiverse where something is just trillions of light years away, you can't get to it. So that's a pretty straightforward reason why you can't interact with it and therefore you're a part of a separate universe, self-contained kind of universe. But the general theory of that, I don't know.

3:25:53.5 SC: I don't know what the general statements are about when to divide things up into different universes, which you would call... I cannot treat myself and the table in front of me as part of two different universes 'cause I can easily interact with it and the fact that it can affect me, but I don't know the general theory of that, so that's point zero. Point one is, the way that individual branches of the quantum wave function become separate worlds in this sense, is by being classical, is by being well approximated by the rules of classical mechanics. Like, we got along for hundreds of years in human history, describing the world, the world with the rules of classical mechanics pretty well, we didn't need to know quantum mechanics. Quantum mechanics imposes itself in unavoidable ways, only when we get down to the level of individual particles or atoms. Why is that? Why are their classical limits to the world and why do we need there to be classical limits to call it a single world.

3:27:00.0 SC: And then finally, so there's the question of, Given a quantum theory, how do you get a classical limit out of it that you would call a single world?" Then there's the very deep question of, Why does the quantum theory of our universe have the properties that are required to give you a classical limit at all, why are we allowed to have a classical limit? I don't know that one either. That's another set of rules that we don't have written down yet, so all this is very difficult to answer stuff. So the vague answer to your question is, there is some feature of the real world that is fundamentally quantum mechanical, but it's amenable to an emergent higher level description as a series of parallel classical approximations. Why that's true, or under what conditions it could be true, are things that we don't really know very well.

3:27:52.8 SC: Josh Charles says, "When thinking about the far-flung future, do we have an idea of how much matter would escape going through the process of entering a black hole and being radiated back out?"

3:28:03.5 SC: So I think what you mean is, if you just let the universe evolve for a long time, some matter is gonna fall into black holes and then be radiated away eventually, and some maybe won't. I'm sure this is a very answerable question if you really sat down and thought about it very carefully, there have been papers written by Fred Adams and Greg Laughlin about exactly these kinds of questions, about the ultra-long term fate of the universe. And I think the question is that most matter falls into black holes if you wait long enough. Some matter was just like flung out into the space in between the black holes, but our galaxy, for example, most particles in our galaxy are gravitationally bound to the galaxy, they're not just passing through on hyperbolic orbits.

3:28:48.5 SC: And what that means is if you wait long enough, they will spiral into the black hole at the centre. Now, you have to wait a very, very long time. And the spiralling in is caused both by mixing between different parts of the stars and gas and dust in the galaxy, by the emission of gravitational waves, by just cooling off and emitting photons by a whole bunch of different things. Plenty of things won't fall into black holes, plenty of photons will never fall into black holes, but I think most of the actual massive particles in the universe will eventually get there. That's my belief. Don't ask me to do a calculation to test it.

3:29:22.7 SC: Zack McKinney says, "How do you see differences in seemingly esoteric, philosophical and metaphysical questions such as the nature of consciousness or free will translating into concrete and consequential differences in how people approach real world problems and conflicts?"

3:29:39.4 SC: Well, they do, and they don't. I think some do and some don't. I think that there's an obvious way in which these deep philosophical esoteric questions do have an effect on real world outcomes, namely meta-ethics. So we have ethics, which is your view, what is right and what is wrong, we have meta-ethics, which is how you justify your view of what is right and what is wrong. So if you say, "Do unto others as you would have them do unto you."That is an ethical precept. If you say, "There are universal rules that everyone should follow," that is a meta-ethical precept. And then the do unto others is the actual rule that you say everyone should follow.

3:30:27.0 SC: Or if you say, "You should be a consequentialist," that's a meta-ethical precept, and then if you say, "Here are all the consequences that matter, and here's how you add them up," that is a specific ethical way of doing things. So here's a meta-ethical perspective, "Do what God tells you to do." [chuckle] And I think if you have that meta-ethical perspective, which many people in the world do by the way, you might end up with different ethics than people do who are more naturalist about it. Theism versus naturalism is a really important philosophical question with very obvious, practical, real world consequences. I've used this argument with friends before that because a lot of physicists and philosophers, they tend to be atheists, they tend to not be theists, they tend to be naturalists.

3:31:23.0 SC: Not all of them. There's obviously many counter examples, but many, most are atheists, but they don't talk about it a lot, they don't draw a line from their scientific or philosophical view points to a public declaration of atheism or naturalism. And I try to say that, look, what we do for a living as theoretical physicists or philosophers or whatever, this is sort of the implicit issue in your question from Zach, it doesn't have a lot of impact on people's everyday lives. Whether or not it's Bohmian mechanics or Everettian quantum mechanics, you can go through your life pretty much okay.

3:32:05.9 SC: But the one example where it does have an impact is in whether God exists, that has a big difference. And you can sort of fill in the gaps, fill in the details about how that might affect people's behaviour in the real world. And then also, even if you specialize to people who are naturalists, I don't think that naturalists have done a good job of telling people how to live, or if you don't wanna be quite so prescriptive, of suggesting ways that we can find meaning and purpose in our lives even without God. There's some cliches that we bandy about, but really, when you get right down to it, it's not quite as convincing and full of a story, and it could be and should be, but we're still...

3:32:56.2 SC: We've abandoned these underlying philosophical justifications for a lot of the choices and modes of life that we've had throughout human history without actually changing the modes because we haven't really thought through what the implication should be of this new view of the fundamental nature of reality. Look, again, I'm quoting a lot of old podcasts here, but go back to the very first week of podcasts I did, and I talked to Anthony Penn, who is an atheist theologian at Rice University, and also he's Black, and he cares a lot about how the Black community is very religious, and he thinks that they should be more atheist, but he gets why they're not. And he says, "Look, you have...

3:33:40.5 SC: That Black people get something out of going to church, out of being part of a religious community, and you can't just tell them that the force of reason is against their belief system, you have to give them a soft landing, you have to tell them how they can live just as fulfilled and rewarding a life without that religious super structure, and you haven't," he says, "you atheists who," and he includes himself and myself among them, "You haven't done that." So I think that even within naturalism or atheism, there's still a lot of work to be done and in doing exactly what you are asking about, "What are the everyday consequences of this view of the world? I mean, how in the world can you imagine changing your completely deepest fundamental nature of reality without any consequences for how you live your everyday life? Surely, there must be there, we haven't done a great job of explaining what those differences are. Alright.

3:34:36.6 SC: And the very last question from Sam Buck is, now for something completely different, "What's your favourite kind of sandwich? And so I'm gonna... I helped myself to a few flattering questions earlier, actually, I started with an insulting question, about the professor telling me that it was pop philosophy, but then there were some flattering questions about how I was disciplined and things like that, so I wanted to end with a self-deprecating question, because I do not have a sophisticated theories of sandwiches.

3:35:07.0 SC: I think that's too bad. Maybe I should have a sophisticated theory of sandwiches, but my favourite sandwiches are kind of the down-to-earth everyday sandwiches, like a good tuna melt, I like. There's, of course, the famous question about whether hot dogs are sandwiches? But if they were, and I don't have a strong feeling about whether how dogs are sandwiches. But if they were sandwiches they would absolutely be some of my favourites, like a really good hot dog. I know a lot of people have never had good hot dogs before. That is... Part of the Chicago culinary scene is their hot dogs. Like again, the East Coast as much as I love it, and I grew up here in Boston or in New York, for that matter, the street level food is whoof, like the hot dogs just sitting there in the lukewarm water or the pizzas that they make in the morning, and then the pizza sit there cold for hours and they will re-heat a slice for you. I'm like, "How do you live like this?" This is just terrible.

3:36:03.1 SC: In Chicago, you grill that hot dog when you order it, and then it's a good hot dog, it tastes completely different. Even in O'Hare, even in the airport in O'Hare I can get a really good hot dog better than I can get on the streets of New York, by far. Anyway, cheesesteaks, I grew up in Philadelphia, I love a good cheesesteak. I like sandwiches. I like the idea of sandwiches, I like them in pita. One of the best hot dogs I ever had was in Paris of all places, like in a baguette with really good French mustard, but I don't have a good sophisticated theory of sandwiches, that I couldn't tell you why certain sandwiches are good and certain sandwiches are bad. They're easy, they're good comfort food, you can get a lot of variety in every bite. It's a good system. I'm pro sandwich. I can go that far, even without telling you what the best one is.

3:36:49.9 SC: And with that, we'll call an end to the AMA, the final AMA of the year 2021, what a year it was, in many ways. Thank you once again, as always, for supporting the Mindscape Podcast. I appreciate it very, very much. I hope you're getting something out of it. I am constantly kind of amazed that people will go with me on this journey through so many different questions in so many different fields, talking to so many different people. It's great fun and great privilege to me, and I'm glad you're coming along, so take care. And for those of you who have holidays in front of you, have good holidays, give thanks. Be well.

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