AMA | June 2023

0:00:00.1 Sean Carroll: Hello everyone, and welcome to the June 2023 Ask Me Anything edition of the Mindscape Podcast, I'm your host, Sean Carroll. The only thing really on my mind these days, the semester is over here at Johns Hopkins, I'm not traveling for the next several weeks at least, so I'm finishing the book, volume two of The Biggest Ideas in the Universe, which is going to be called Quanta and Fields. This one has been a lot of work. Of course, it's taken longer than it should have because I got a new job and flew across the country and helped to fix up a house before moving into it, and taught two courses and a whole bunch of other things. There's a lot of going on.

0:00:37.5 SC: So we originally had hoped that volume two would be coming out a year after volume one which would put it this fall, I'm not done, and it takes almost a year to get a book out once you've handed it in. So no, it's not gonna be coming out this fall, it will come out sometime... First half of next year, I presume 2024. I'm hoping that since I am not gonna move across the country again, and my job is already settled here, that the book after that book three in the series, which will be Complexity and Emergence, that will come out about a year after volume two.

0:01:14.4 SC: So volume two that I'm finishing up right now, I go back and forth because it's super ambitious. Volume one was pretty ambitious. Volume one taught you relativity both special and general, it starts very slowly going with Newtonian mechanics, what is space, what is time, there are lot of words. But eventually the equations start coming and I teach you how to do calculus and things like that, and then Riemannian geometry, I should say, non-Euclidean geometry, culminating in general relativity. And that's a lot for a trade book type of audience.

0:01:48.9 SC: This book, I'm doing quantum mechanics of course. But actually in book two, the quantum mechanics is gonna go by pretty quickly. I'm not dwelling on the intricacies of quantum mechanics, certainly not the interpretation thereof or foundational questions like we did in Something Deeply Hidden. Because most of the book is going to be about quantum field theory and particle physics. So I'll be talking about Feynman diagrams and Lagrangians and effective field theory and renormalization and gauge symmetry and the spin-statistics theorem. And unlike book one, I can't do all of the equations, book one we zip through some equations, but there was nothing missing, it was all there, and the amount of material being covered in book two is just far too humongous to do that. It would be three times the length and much more intricacy if I did it all in book two. So I need to judiciously choose which equations to go through and where to wave hands.

0:02:48.2 SC: And so far have been erring on the side of showing some equations. Like I want... To you can see using the equations why the photon is massless, why charge is conserved, things like that. I'm gonna define what a a non-abelian symmetry group is, you're gonna know what SO3 and SU3 stand for why they appear in different equations. And you're not gonna go through everything, so I'm not talking about spinners and the direct way of constructing the electron field and direct matrices and stuff like that. So I can skip over some things, but there's a lot there and it adds up to quite a bit of demand on the reader, but I think it's an absolutely unique pay-off. I don't think that any other book does this, the effective field theory stuff, the gauge theory stuff and so forth. So I really want to have a book where even if it is a challenge for most people to read, there's a reward.

0:03:47.8 SC: It's useful to go through this. Those of you who have gone through the videos for The Biggest Ideas in the Universe series, know what I'm talking about. I did very similar things there and we got pretty deep into the weeds, but I think that it's all worth it. Anyway, that's what's going through my brain right now these days. I'm in the home stretch in finishing that book, and I think... You never know I gotta take a break from it. After I'm done the first draft or the... It's not the first draft, by any means, but the draft that I will hand in, take a break from it, and then a little while later I will come back, read through the whole thing and start editing it and we'll see what I think of how it's going. The only other thing I wanted to say, this being the Ask Me Anything edition, is I really should remind people some of the rules of the AMA. For one thing, of course, we know that the questions are asked by Patreon supporters.

0:04:38.2 SC: If you're out there and you're listening and you enjoy this, you could be a Patreon supporter. Even if you don't want to ask questions. That's fine. The number of people who ask questions is much smaller than the total number of Patreon supporters. There's just a feeling of good fellowship and happiness and warmth by being a Patreon supporter of Mindscape. So you can be such a supporter by going to www.patreon.com/seanmcarroll throw in a dollar or two or whatever for every episode, it's not really that painful. And for those of you who are asking the questions, you know that I give instructions at the beginning of every post where I ask for the questions. And I've noticed that the instructions are kind of being ignored a little bit by some people, and perhaps the two most important instructions are keep the questions short and you only get one question. And having three or four or five question marks in a single paragraph, doesn't really count as one question, that's actually multiple questions.

0:05:39.7 SC: So I usually just try to edit them down, but if the questions are too long that I'm not even gonna contemplate including them 'cause we can't include every question, and if one question is very, very long, that's an easy way for me to decide to ignore it. So that's advice, if you're out there contributing questions to the AMA and wondering why yours are not picked. But having said that, most people are very good about obeying the rules and I do appreciate it. We have a lot of good questions this month, so let's go.

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0:06:24.1 SC: Our first question comes from Sid Huff, who says, "Most universities in the US, Canada, and some other countries have instituted a tenure system. While originally designed to provide academic freedom to protect academics from social or political backlash against their research or ideas, nowadays the original rationale for tenure seems questionable in light of other civil and criminal protections the law provides. Critics argue that tenure may induce post-tenure laziness. Top universities in other countries survive nicely without any tenure system at all. What is your view of the worth of the tenure system? Would you support getting rid of it?"

0:06:57.7 SC: No. I would absolutely not support getting rid of it for many reasons. And I say this, by the way, as someone who does not have tenure and never will have tenure, most likely. The job I have right now is permanent and they're not gonna fire me, so maybe effectively, it's like having tenure. But by the letter of the law, because I have this Homewood Professor job at Johns Hopkins, that floats above any individual departments without being a tenured member of any individual department. And therefore being beholden to serve on all the committees that the departments have, I have a better than tenure job in many ways. But anyway, I don't technically have it, and yet I'm super duper in favor of it.

0:07:36.0 SC: For one thing, it does still provide protections against attacks on freedom of speech and things like that, because it's not the same thing, academic freedom, and the government protected speech that you have, those are two different things, and sometimes one can protect certain kinds of things and not the others. People can say really terrible things in their social media and not be fired from their job as a professor, and I think that's the right thing to do if that speech that is very terrible, even if they're a terrible, terrible, terrible person, if it is not affecting their jobs.

0:08:14.3 SC: I don't think that tenure is or should be an absolute band against losing your job. If you do things like sexually harass your undergraduates or commit criminal acts or start teaching astrology in your astronomy classes, things like that, there's all sorts of reasons why you still can be fired from your job even though you have tenure. But you should not be able to be fired from your job, just because of opinions that you hold and how you express those opinions outside of the university context. That is a stronger and different kind of protection than you get just from the First Amendment, let's say here in the United States.

0:08:51.2 SC: And of course, it protects your right to do research in various ways as well. You're not pushed around by the university, ultimately, you get to decide what kind of research it is you're doing. But the other thing that I think is very, very under-appreciated by non-academics is that tenure makes professors much cheaper. There's a lot of people who want to become professors, the number of people who get PhDs and hope to become professors is much larger than the number of actual professorship jobs. What is it that makes being a professor such a good job?

0:09:26.6 SC: Well, there's various things, academia is fun, the intellectual challenge of it all and so forth. But given all the training that you have as a typical academic, given the skills that you have, is typically very easy to earn, not necessarily 100% possible, but typically easy to earn way more money in the private sector then by being a professor. And one of the reasons why people wanna be professors anyway is because it's a different kind of life than you have in the private sector. Good professors work extremely hard, they work weekends and summers and nights and the whole bit, but they do it not because they are being forced to by their bosses, they are sort of self-motivated people. They want to get this research done, teach classes well, etcetera.

0:10:16.0 SC: Not everybody does it. Not everyone is that motivated, there's plenty of lazy professors out there who don't work that hard, but many, many, many people do. And there's a feeling that there's a bargain that is implicitly struck where I will take less money for this job in return for the job security provided by tenure. Almost all academics in countries that have tenure are in favor of expanding it, not getting rid of it. The academics in other countries that you mentioned, where tenure is not there are often very, very upset that there is not tenure. That's one reason why the United States has historically been able to attract people from other countries, and vice versa doesn't happen very often when it comes to professorships.

0:11:04.1 SC: When I was leaving CalTech and I was talking to various places, other universities that might have in principal been interested in hiring me, that was an absolute consideration. The lack of tenure, even though I didn't think I was gonna get fired or anything like that, I think that it shows a misunderstanding of the academic system. Even though the academic system is very far from perfect, we should aspire to be a system where we let people think and do research without worrying about their job security. It saves university's money, it's good for the well-being and mental health of the professors, and I think that's the best system that we have right now.

0:11:45.6 SC: Now again, there are people who just get tenure and then stop doing their work, become lazy, etcetera, there should be other incentives for people like that, and there are. Guess what, your salary doesn't go up as fast, if you're not doing your job, you don't get raises and things like that. There are people who are rather up there in age who never made full professor, they're still associate professors and things like that, for one reason or another. You don't get perks, whatever it is, funding for grad students or for travel or whatever. There's various ways that the university has of indicating its displeasure with your work as a professor, and that's why the research productivity of the faculty in the United States Universities is unprecedentedly high. It's actually a very good system, not perfect but I still think it's the best one I know of.

0:12:38.4 SC: Someone whose handle is R then VLN, that stands for something, probably says, "If a pair of spin-entangled particles traverse separated paths through curved space, what happens to the directions of their spins?" So this is referring, of course, to a version of the famous EPR thought experiment, where you have two spins that are entangled with each other, and let's say they're entangled in the sense that they're opposite spinning. So you don't know which measurement outcome you're gonna get for either spin, but you know that when you do both of them, they will both be opposite, and that's supposed to be true even if they're very, very far away.

0:13:17.7 SC: So the question quite correctly points to this idea of the spins being opposite. What does that mean that the spins are opposites if the spins are located at different locations in the space, when the space in between them is curved? It is a feature of relativity of curved spacetime, that there is no unique way of comparing vectors like spins at different locations in curved space uniquely. You can... What you can do... Sorry let me just say that again, so it sinks in before I give the answer. If you... We're very used to taking vectors like velocities and momenta or things like that, and adding them and subtracting them, that's what vectors are supposed to be able to be done.

0:14:00.3 SC: But in general relativity, in a curved spacetime background. You really can't do that. There's no unique way to do that. It's true that you have vectors, but vectors are always defined as living at a point. There's even a whole bunch of technical jargon that goes with this. You have the tangent space of the manifold at each point, and the vector lives in a certain tangent space, it lives in the tangent space located at that point. And there's no unique way of comparing vectors at one point in spacetime to vectors at another point in spacetime.

0:14:31.0 SC: There are sometimes situations where you can sort of approximately talk about such comparisons, like when we talk about the apparent velocity of a distant galaxy, right? That is in a very apparent velocity, where we know we're really cheating when we talk about that, but it kind of makes sense unless you really care about the subtleties. You don't need to be more specific than that. But in something like the EPR experiment where you say, "Okay, I'm gonna compare the spin outcomes of entangled particles, how are we to do that?"

0:15:03.5 SC: And the answer is that even though there is no unique way to compare vectors at different points in spacetime, if you give me a certain path between two points in spacetime, then I can do my best to keep the spins constant. That is called parallel transport to keep the vector constant as I move along the path. It's called parallel transport, you can read about it in volume one of The Biggest Ideas in the Universe. And so that is what will happen. Some version of that, depending on what kind of spin you're moving around will happen as you move a spin without literally pushing its direction of motion.

0:15:42.6 SC: If you just sort of do your best to keep it constant as you go from one point to another in spacetime, when the two entangled particles are created, they're the same point. That is a feature of the laws of physics, they are local, when you have entangled these particles in the right way, something started at one point in spacetime and then spread to different points. And you just have to tell me exactly what path that traveling happened on, and then I can figure out what the relative spins would be and what the prediction for measuring them should be.

0:16:12.6 SC: Stevie CPW says, when he was in his early teens, "My brother told my devoutly Christian mother that he was an atheist and it made her very sad. It took me longer to transition to atheism and then to declare it. But it was still hard for my mother to take when I did. When did you realize you were an atheist? How long did it take for you to declare it to your parents, and what was their reaction? Also, what advice do you have for people who want to come out of the atheist closet, but are concerned about disappointing or hurting their religious loved ones."

0:16:40.2 SC: Well, this is... It's a complicated question because it's gonna depend very much on your individual situation. I don't think there's any one-size-fits-all answer to questions like this. It's part of an issue involving the relationship that you already have along many dimensions between yourself and your parents. My parents were religious, but not that religious, that was not central to their life in any way. So there wasn't any grand reveal of my atheism or anything like that, it just sort of crept up and became clear and it never became a big deal. But that doesn't mean that it's not a big deal to other people, as with many other ways in which you could come out, right? To come out about many things, most famously, I suppose, about being gay or being trans or being something like that. It's actually quite analogous.

0:17:28.8 SC: Hopefully, in all of these cases, your family will be mature enough and loving enough to support whatever choices you have. And it is your... On your side, the responsibility is to be as understanding and loving about their feelings as you can. You should stick by your guns when it comes to believing whatever it is you believe about the universe, but the fact that they can be upset by it is real, and you can hope that you don't upset them. Or you could at least say, I know I'm going to be upsetting, but I wanna do it in the gentlest way, possible.

0:18:05.4 SC: And hopefully, the fact that you are members of the same family and care about each other is more important than your attitudes towards atheism and theism and things like that. Look, sometimes it's not. Sometimes people have to cut off contact with their family because they disagree about something pretty profound. I think that's also okay. I'm not someone who thinks that just because you have family members means you have to put up with anything that they do, no matter how terrible it is. And they should accept you for who you are, you have to... Maybe let's put it this way. You have to respect them enough to imagine that they will eventually accept you for who you are, and to try to give them whatever space and time they need to do that.

0:18:52.8 SC: Okay, I'm gonna group a bunch of questions together, there were a bunch of questions that were engendered by the recent podcast we had with Thomas Hertog, and I'm gonna try to read several of them, but there were even others that I'm not gonna read 'cause it's all... Along the same basic ideas. Jeffrey Segal says, "The conversation with Thomas Hertog inspired me to read his book, which was very interesting. However, although he emphasized that this top-down principle, which was used to deduce the previous history of the universe differed from the anthropic principle, I did not see the distinction. Can you perhaps clarify what the difference is?"

0:19:25.4 SC: Bruno Tesnière says, "I'm not sure I was able to understand Thomas Hertog's quantum cosmology. What struck me... With me was something like the distant past and future, space are uncertain, but what causes the decoherence of the near time and space is it us?" Nick Gal says, "What's the difference between your multiverse cosmology and Hawking and Hertog's top-down worm's eye holographic cosmology. I had difficulty understanding the details on how the two differ from your podcast with Hertog." And then Red Antonov says, "Can you clarify what Thomas Hertog was driving at in the quantum cosmology episode?" So clearly there was something that was unclear about that episode. Sorry about that. But we'll try our best.

0:20:06.5 SC: Anyway, Red continues. "Was he saying that at the largest scales, the correct theory is one that calculates probability amplitudes just like quantum mechanics does at the microscopic scale, thus what we observe with our telescope is one of many possible outcomes albeit each having a different likelihood. Can a theory like this be tested. Does it have anything to say about dark energy or predict an effect akin to dark matter?" So I will confess that I've glanced through the Hertog and Hawking technical papers here, but I've not read them in great detail. I do think I have the basic idea of what's going on, or at least I understand what I take to be the most important underlying issues so let me try to explain that. Maybe we'll shed some light on what is going on.

0:20:51.7 SC: So think back, we were just talking about Alice and Bob and their... I don't think I said Alice and Bob, we're talking about the EPR experiment with two different spins, very far away. There's an implicit thing going on, and how we usually talk about the EPR experiment, which is we have two particles, they're entangled. We move them far away, we measure one and then in the many world's way of talking, we say decoherence happens in the branch the way function branches. And there is a branching into one branch where Alice observed spin up, Bob observes spin down. And another where Alice observed spin down, Bob observes spin up. But as we said with general relativity, there is a little bit of an ambiguity as to how you compare what you mean by the direction of spins at different points in space.

0:21:39.5 SC: The amount of curved spacetime between us and Alpha Centauri is actually not that large, it's still pretty close to flat spacetime, so in practice, this is not really a worry. But in cosmology, it becomes more of a worry. So, one looming issue in many-worlds quantum mechanics that people disagree about is how the branching of the wave function extends to... Throughout space and time. Let's put it that way. David Wallace in his... Former Mindscape guest, in his book, The Emergent Multiverse, talks about a picture where... What happens is you make a quantum measurement at some point in spacetime, and then in your future light cone, there are two branches. But outside your future light cone, there are not two branches, there's still just one branch of the wave function of the Universe, just as there was before you did the measurement at all.

0:22:36.0 SC: So somehow there's this interplay between spacetime location and branching, that is actually not the way that I like to think about it. I like to just let the universe branch throughout spacetime all at once. And this looks like it's violating special relativity, but it's not really because, as I often say, spacetime lives within the branches, it's not that the branches live within spacetime. So what does this all have to do with Hertog and Hawking? They are dealing with a situation where they have a wave function of the Universe, they are truly quantum from the start. Okay. It's not supposed to be a semi-classical spacetime with quantum fields in it, like many physicists often think about, it's truly a quantum mechanical story of spacetime itself.

0:23:23.1 SC: So here I am in my location in spacetime, I make some measurements, I look around, I see that things are located in certain places. So what I've done is I have decohered and branched the wave function, and now I am on a branch with a certain semi-classical arrangement of stuff around me that makes sense, okay? The question is, how do I extend that branch out to the edge of the universe, out to the edge of the observable universe and beyond it. Okay? There's no edge to the universe, but there is an edge to the observable universe. And how do I extend my semi-classical branch? In fact. It's perfectly okay to extend that branch out to the edge of the observable universe, because after all, it is observable, you are looking at the positions of galaxies and the temperature of the cosmic microwave background all of this branches the wave function causes decoherence, etcetera.

0:24:16.6 SC: But what about extending it past the horizon of the observable universe. That's a much dicier thing, it's much less clear how we should extend our branch into regions of the universe that we're not observing and therefore not decohering and collapsing and branching, etcetera. So what I think is going on, and I've translated to my language, so maybe Thomas wouldn't actually even agree with me. But the way that I think about it is they're emphasizing the fact that outside your observable part of the universe, the universe is still ineluctably quantum in nature, there is no such thing as the classical state of the universe outside the part of it that you can observe, you should just talk about the whole wave function all at once.

0:25:05.4 SC: And of course, even at one location or what we think of as one location, there are many branches going on with many different kinds of observers. So what they're really doing is trying to zoom in on what is the regime of the wave function where it consistently makes sense to talk about a semi-classical universe that has observers in it, and things like that. And there may be many, many such regions, many, many as physics talk for a huge number, maybe an infinite number, of regions with different kinds of observers seeing different things. So it's a different picture than the traditional cosmological multiverse.

0:25:45.5 SC: The traditional cosmological multiverse, as you know, is different from the Everettian multiverse, the Everettian multiverse says, when I observe a spin right here in the room, there's now... Well, I shouldn't say right here in the room. But right here, I have caused to come into existence two different branches of the wave function. Whereas the traditional cosmological multiverse has a single semi-classical spacetime spread throughout a distance much, much larger than what we can observe. And I think they're calling that latter picture into question that you shouldn't talk about the actual semi-classical spacetime that we live in outside our observable universe. You should talk about the quantum-ness of it all.

0:26:27.6 SC: But nevertheless, I think it will be fair to argue that still, for all intents and purposes, you have something that is very, very much like a multiverse, maybe you're not locating different universes and different legitimate physical locations because you're not in the classical spacetime when you go outside what we can observe, but there are still different observers, maybe some of them see different laws of physics. Certainly some of them see different physical conditions about the energy density, the amount of dark matter and things like that. Okay?

0:27:03.4 SC: So that's my general picture of what's going on, and I'm not at all sure that it's right or that it makes sense or anything like that, it might be. I'm very much in favor of taking seriously the quantum nature of the whole universe. Similar words were said before by Raphael Bousso and Lenny Susskind and also Yasunori Nomura in a bunch of papers from a few years ago, asking whether or not in a world with holography and complementarity, many-worlds and the cosmological multiverse aren't secretly the same thing. So I think that these are all really good issues that should be thought of very, very carefully, and I think that Hawking and Hertog have an angle on them. I don't know whether it's the right angle just 'cause I haven't thought of it. Flipped through it too quickly. Final note on that, to go to Red Antonov's question, "Can a theory like this have anything to say about dark energy or predict an effect akin to dark matter?"

0:28:00.4 SC: For dark matter? The answer is clearly no. And I would dissuade you from trying to think of weird physical effects that have an effect akin to dark matter. It is overwhelmingly likely. The dark matter is just matter of some sort, it looks like a particle, it acts like a particle, it walks like a particle, it quacks like a particle. By which we mean, it has an energy density that evolves with time, just like particles do. It changes its spatial location and density just like cold particles would, and so on and so on. We don't know what particle it is, but I wouldn't look to weird physical effects to be dark matter. I would look to new particles to be dark matter as opposed to... As concerned in saying something about dark energy, famously, one of the best successes of something like the anthropic principle was accounting for the fact that the dark energy density, the cosmological constant that we observe is much smaller than what you might naively predict. I presume that Hawking and Hertog would claim that kind of success for their own model, but I don't... I haven't actually seen them explicitly do it.

0:29:11.3 SC: Okay. Roloba Jess say, "I think that you describe yourself as a Humean about laws of nature." Contemporary Humeanism was set out by David Lewis as regarding the world as a vast mosaic of local matters of particular fact, just one little thing than another. So by the way, this is Sean talking now and not Rolo. Yeah, humanism in this sense about laws of physics is, laws of physics are just convenient descriptions, summaries of the set of the infinite number of events that actually happen in the world. As opposed to the idea that laws of physics have a separate existence and have some oomph they help bring about the actual physical configuration of reality.

0:29:56.8 SC: Okay, the question continues, "Surely, modern physics views the world as various types of fields described by Lagrange in which very continuously the value of the field at a given point is completely not independent of the value at neighboring points. Isn't this a modal structure constraining matters of particular fact? I.e., exactly what the Humean denies. Unless you say the fields might have taken any value, but just happen to take values described by continuously varying functions, which seems insanely improbable."

0:30:24.4 SC: So there's two things going on here. One is a physics issue and one is a philosophy issue. The physics issue is, it is not true to say that modern physics views the world as various types of fields which vary continuously. That would be true if the world were described by a classical field theory but it is not. To just described as far as we know by quantum field theories, maybe not even that when it comes to gravity, but certainly the fields that we're looking at are quantum. And these ideas of continuity only makes sense if you think that the field has definite values at different points in space, but quantum fields don't. They have wave functions, okay?

0:31:00.7 SC: So you can't just think of it as smooth fields rather than ones that are that have discontinuities all over the place. In fact there is a... This is not exactly the same point, but there's an important point to keep in mind. If you know about the Feynman's sum over histories, the path integral take on quantum mechanics. Where you say that in quantum mechanics or in quantum field theory, you derive the amplitude to go from some initial condition to some final condition by summing over e to the iS, where S is the action summing over every single possible path that takes you from the initial state to the final state. And you can prove mathematically that the set of smooth paths that take you from the initial state to the final state are a set of measure zero. They contribute nothing to the path integral. All of the work in the path integral is being done by paths that are discontinuous everywhere.

0:31:58.6 SC: So this is not a statement about field theory, but it is just a reminder, the things that you would have taken for granted being smooth in classical mechanics are not always smooth in quantum mechanics. So the more philosophical point is I think that you're making sort of a... I think it's a mistake. But anyway, let me say it as a presumption that many non-Humeans often do, which is they attribute some element of surprising-ness or improbability to the fact that the world obeys the laws of physics. So I could say everything that you said in principle in the context of classical mechanics of point particles moving on trajectories, right?

0:32:39.8 SC: If I thought that Newton had been correct and he thought that the Earth and the Sun had equations of motion that were perfectly classical, and they had trajectories. In the space of all trajectories that the Earth could take around the Sun, the set of trajectories that are smooth and continuous is a set of measure zero. But that's what the actual one that it seems to take according to Newtonian laws of physics. So one is tempted to say, That would seem insanely improbable. But that's only because you are somehow attributing some probability distribution to the measure on the space of possible paths that you made some choice about what to be surprised about. But there's no principle that anyone has ever advocated in favor of that says that every imaginable path is a priori equally likely, and we should be surprised when the universe obeys certain patterns and not others.

0:33:39.6 SC: You can worry about it, you can ask the question, but you have to realize that it is a question. It's not some pre-existing result that we should rely on that says that if it weren't for laws of physics, all sorts of crazy behaviors would in some sense, be equally probable. What does that mean? Probable. Where did this probability function come from? It is a fact that the universe obeys patterns, which implies that its evolution in the real world describes a very, very, very tiny and special subset of all conceivable evolutions that it could.

0:34:14.0 SC: Now, it's true that the anti-Humean has an explanation for that, namely that there are laws of physics that force it to do that. But the Humean says, you haven't really explained anything by saying that you just restated it. How do these laws of physics govern, how do they actually exert an influence, is it like the particles in the fields are trying to do something and the laws of physics step in and prevent them from doing it? This just seems like a bad anthropomorphic analogy, not an actual amount of insight. So you can choose to believe whichever one you want, but I think that both of those positions make sense on their own terms.

0:34:52.6 SC: Rue Phillips says, "What do you see is the connection between large language models and emergence? What interests you about it, if anything? For example, the opening statement of a recent research paper states, 'We present evidence that language models can learn meaning despite being trained only to perform next-token prediction on text, specifically a corpus of programs.'"

0:35:13.7 SC: Well, I think that your question ends up in a place different than it began. There is this question, what is the connection between large language models and emergence? But then there's this specific example from the paper you quote, and I would put those two things on very different footings. I have seen talks and I've looked at papers claiming that large language models exhibit some understanding that they have not just an ability to predict the next word or a sentence or whatever, but they actually attach some meaningfulness to it.

0:35:47.6 SC: But when you look closely at these talks and these papers, they fall apart, they're just not that convincing. They're defining what they mean in such a narrow way as that it's really not what you and I mean, when we think about meaning an extrapolation, etcetera. So I think that I'm not moved in what I've seen so far by the idea that these large language models are actually not just learning what to say next, but learning what things mean. Or more specifically, that they are implicitly forming a picture of the world in their minds. I don't wanna say brains, but in their minds. The way that we think of human reasoning is that we have a picture of the world, we have a model of the world. And so when you say construct... A typical thing, that you ask large language models to do is, "Here are five objects, a table, a box, a coffee cup, a marshmallow and a pencil. Stack them on top of each other in a way that is the most stable."

0:36:53.0 SC: And they're pretty good at that actually, right? And you say, "Oh my goodness, somehow they must have a model of what is stable and what is not stable." No, they don't. [laughter] And what they know is that lots of people have talked about putting boxes on tables, and very few people have talked about putting tables on boxes, that's what they know. So I think that there is the evidence that any of this is actually happening in large language models, any of this construction of a model of the world is very, very flimsy. And in almost all these cases, you can then try a little bit harder to make the LLM say something nonsensical, and it's not that hard to make them say something nonsensical. Again, that doesn't mean that I'm not enormously impressed with what they do, but let's be accurate about what they do.

0:37:38.7 SC: So having settled that, your very first question, "What is the connection between large language models and emergence?" I'm not quite sure what you mean. I'm not sure if you're defining emergence says something like what we just talked about, do you mean the emergence of meaning somehow. That's not what I usually mean when I talk about emergence. So I'm not sure what the connection might be, I don't generally try to attribute such a connection, but maybe there is one that I'm just not aware of.

0:38:05.6 SC: Nikola Ivanov says, "I have been struggling with the following question, unitarity is a foundational principle of quantum theories, it implies the preservation of information and determinism. The last two principles imply time symmetry, and yet time symmetry is broken in the weak sector of the standard model and has to be saved by CPT Symmetry. I know that CPT Symmetry arises naturally from the Dirac equation. I'm not implying it is ad hoc, I just can't reconcile the principles above with results from experiments. Is my reasoning flawed? Am I missing something? Or does this indicate the Quantum Theory is just not complete?" It does not indicate the Quantum Theory is not complete. These are perfectly good questions to ask, but we know the answers to these questions 100% perfectly well.

0:38:50.6 SC: The basic answer is that once you have reversibility, which you get from unitarity, that is to say if you know the state of the system at one point in time, you can uniquely predict what will be another point in time. So that applies to quantum mechanics as long as you don't measure, you don't collapse the wave function, etcetera. Just the Schrödinger equation has that feature. Then you can show that there must be some definition of time reversal invariance, okay? In the following sense that you have your state, you can evolve it forward in time, do some transformation to it to the future, call that the time reversal operator, then evolve it backward in time to where you started. Apply that time reversal operator again and you will come back to where you started.

0:39:42.6 SC: So there's four things you did. You evolve forward, time reversal evolved back, time reversal again, come back to where you started. Such a time reversal operator will always exist if the dynamics are reversible. If they're not reversible, so think about billiard balls on a table. But I put glue on all the bumpers, so all the billiard balls bang around, but when they hit a bumper, they just stick there. That is not reversible dynamics and from knowing what the state is in the future, I cannot predict what the state was in the past likewise if we functions truly collapse in quantum mechanics.

0:40:16.1 SC: So... But what this theorem set does not say is that you know what time reversal is. Time reversal is often involves some massaging of the states, and that's true just in classical mechanics. Think about positions and momenta in the Hamiltonian version of classical mechanics, your phase space is positions and momenta, and there is a priori relationship between velocity and momentum, that turns out to be an equation of motion, but the fundamental variables are independent of each other in their position and momentum. And guess what? To get time reversal, you keep the positions fixed, but you reverse all the momenta, right?

0:41:00.9 SC: So that was a choice that you made. Reversing all the momentum was a non-trivial thing that you did. Likewise in quantum mechanics, you can do the same thing, but you need to take the complex conjugate of the wave function as part of your time reversal operation. So when you say in the standard model, time symmetry has to be saved by CPT, I don't think that's the right way to think about it. I would say time symmetry is CPT. CPT which is charge parity and naive time reversal invariance is just what you should mean by the time reversal invariance of the standard model. It involves C and P as well as the naive complex conjugation operation. And nothing wrong or surprising or weird about that. It's 100% compatible with unitarity, the Schrödinger equation, etcetera.

0:41:53.0 SC: David Maxwell says, "For the first 40 years of my life, I adored science and dismissed philosophy as the musings of ancients devoid irrelevance. You bridged that gap for me by revealing over years of Mindscape that a lot of what I love is actually a philosophy. I just didn't know how to call it that. I've come to think of them as two sides of the same coin, each the application of logic to gain insight, but one applies to the testable the other to the un-testable. Science has become cool and interest... In it widespread. Basic scientific literacy is why they thought of as useful to life, yet philosophy is thought of as a niche area of little relevance outside its own academic bubble. As someone who's indirectly bridged that gap for me, how would you encourage the reframing and marketing of philosophy to new generations so that it can one day be as cool and ubiquitous as science has become?"

0:42:42.5 SC: Well, that's a very good question. You know, I also... I'm not sure if they're are two sides of the same coin, but I see them as continuous with each other. Science and philosophy, they are both ways of trying to understand our world that use slightly different tool kits, but have the same ultimate goal. In each subset there are different problems that are considered to be central and most important, but that's okay, there's a lot of overlap where they have common problems like what happens when the wave function collapses in quantum mechanics.

0:43:11.8 SC: I don't know if I have an accurate data-based statement to make about popular philosophy. I'm not as familiar with that as popular science. Clearly, we teach science in high school and junior high school in a way that we don't teach philosophy here in the United States. I never had a philosophy class until I was in university. I know that in other countries, they do teach philosophy much earlier, and that's an interesting fact why here in the United States we're relatively slow to start people learning about philosophy. But then outside of the context of... And by the way, I also don't know what fraction of college undergraduates take a philosophy course. I would hope it's large, but I somehow suspect that maybe it's not. Anyway, in addition to within the school systems, there's also the question of just in general life, how ubiquitous is philosophy, how easy is it to find a good popular philosophy book on the book shelves.

0:44:08.4 SC: There's certainly lots of books that kind of seen quasi-philosophical meaning of life, self-help books, those kinds of things, and there are books that are actually quite respectable by people like former Mindscape guests like David Chalmers or Daniel Dennett and so forth, that are very readable to the outside. But it's nothing like the popular science industry, which is much more successful and much more ubiquitous. So I think that, I mean, let's turn it around. This is a market opportunity. I think that there's a lot that can be done to share the delights of good philosophy with a much wider audience. And part of that is that just like scientists, philosophers will have to learn the difference between what they do in their everyday research endeavors versus what is interesting to the outside world, right?

0:45:02.2 SC: Scientists have to recognize that that's true, you can't use the same jargon or even necessarily care about the same puzzles when you talk to a general audience, but some of the stuff that you do is interesting. And likewise for philosophy. Some of the stuff that philosophers do is absolutely interesting to a wider audience. And there have been examples of very successful popular philosophy, the novel Sophie's World for example, was very popular not too long ago. And Harry Frankfurt wrote this book Bullshit that's sold a tremendous number of copies.

0:45:36.5 SC: So it absolutely can be done. And I'm kind of just hopeful that philosophers try harder to do it. They're a little less motivated because they don't get that much funding from the government, right? There's no large grant proposals, there's no large experiments that need to be funded that philosophers can motivate their colleagues to reach out to the wider public. Because otherwise, their funding will have evaporated, because their funding is already evaporated, there's very little funding in the field. But nevertheless, my take has always been that if you spend your life thinking about deep mysteries of the universe, and then you answer some deep mystery of the universe. Or at least make some progress, gain some traction on learning about the deep mysteries of the universe, and then you don't tell anybody, that's just bizarre, or you only tell your 12 friends who read your paper, right?

0:46:32.3 SC: I mean, I think that not as an individual, because individuals have different skill sets, different interest on whatever. But as a field, it's perfectly clear to me that philosophy should care much more than it does about spreading the message of what is going on and the cool stuff that they think about to a much broader audience. I have no institutional influence over these things, but I can try to set an example. I can try to write books and do podcasts and give talks in which philosophy and science are intertwined with each other until eventually people catch on. Like perhaps David you have that this is all smart people trying to understand the universe a little bit better, and it's all worth following and thinking about.

0:47:18.2 SC: Nickel Pickle says, "After listening to the latest episode regarding the offshore financial system as a complex system. I'm curious about the stability of complexity in general. Some complex systems such as life or the brain or the entire world evolved for banking, seem to have error correction and stability enhancing measures incorporated. Whereas some systems such as the venture finance banking system that is teetering in Silicon Valley at the moment, seem to lack some important in-build error correction mechanisms. In your opinion, does complexity fundamentally include instability and the possibility of system collapse? Or is a system's overall stability even without additional error correction mechanisms more case and fact specific?"

0:48:01.7 SC: I love this question in part 'cause I don't know the full answer, I don't have a full theory of this, but my vague impression is that you're gonna have both. In other words, a complex system that has any non-trivial lifetime at all that exists over... Well, thinking as a physicist, you would say there's some dynamical timescale, right? For any given system, there's some timescale it takes for the system to change in an order one way, and if a complex system lasts for many dynamical time scales, it's gonna have to have some stability mechanisms, some negative feedback mechanisms where if it begins to get out of alignment, something nudges it back into alignment. But at the same time, most complex systems, I think, are going to have failure modes, are either going to have fluctuations that can grow or they will have lifespans built into them. After all, living organisms are not designed to last forever, for the most part. Some of them are, but the ones that are tend to be the simple ones, like a bacteria will not...

0:49:13.2 SC: A bacterium will not die of old age, okay? But it's a single tall organism, it's not that complex. The greater complex systems maybe require some input from the outside world in order to do that error correction. They require some food, right? Some free energy that they can use to repair themselves and keep going, and maybe this is a process that isn't going to last forever. And of course, there are very, very ephemeral things that are temporarily complex and then go away and maybe something like a startup company or something like that, or the junk bond system or something like that qualify under those criteria.

0:49:57.7 SC: So I think there's probably a rich landscape out there of different things going on. It's probably not a simple answer, you're probably gonna need some error correction, stability, negative feedback loops there somewhere, but they're never gonna be guaranteed to last forever. It'd be a fun thing to study more in detail. Ahmad Shaker says, "How exactly do you get from entropy increases in a closed system and entropy was lower in the past to this explains the arrow of time? Surely these events are correlated, but how do we know they're actually related?"

0:50:34.1 SC: Yeah. I mean, we don't know a priori they are related, we do work to show that they are related. So there's a way of talking about the arrow of time that is sometimes popular in which people talk about many different arrows of time, right? There's the thermodynamic arrow of time, there's the aging arrow of time, there's the arrow of memory, there's the electromagnetic arrow of time, whatever you want. Different kinds of arrows, and then you would like to ask why they are related to each other and if they are related to each other. My own personal way of talking is there's one fundamental underlying arrow of time, which is the thermodynamic arrow. The fact that entropy is increasing and then other arrows that we know about are parasitic on that. They arise because of that one.

0:51:16.5 SC: So the fact that we remember the past, not the future, the fact that we age, the fact that causes come before effects, these are all because entropy is increasing. I'm not actually gonna answer the question because connecting those things, explaining why, for example, increasing entropy is behind the fact that we remember the past but not the future, is work. That's stuff that I talk about at great length in my book From Eternity to Here, and it's ongoing work. We don't know all the answers to all those questions. So it is absolutely something you need to do the effort, put in the effort to establish that it's true it's not automatic, but I think that the effort has been put in at least enough to say that it's going to be true even if not all of the I's are yet dotted and the T's are yet crossed.

0:52:05.2 SC: Eric Davigi says, "When matter collapses into a black hole, does it have to overcome The Pauli's Exclusion Principle? To expand on the question, What happens to the various parts of an atom when they get smashed down into the singularity, at what point is a quark de-quarkified?" Well, I think that there's a lot going on here, and part of the answer is going to be, there aren't any singularities in nature. You know, singularity is a prediction of classical general relativity, but classical general relativity is not right 'cause it's not quantum. And in particular, you're asking a question here about the quantum behavior of matter. So even at the level of the behavior of quantum fields in a classical spacetime, you can't quite invoke the Pauli's Exclusion Principle because you really should be talking about fields not particles. Right? Now there are fermionic fields, there are the fields that make up electrons and neutrinos and things like that, that obey a field version of The Pauli's Exclusion Principle, but what you would expect is that even though you squeeze particles into the same position in space, that they're not in the same quantum state because they're vibrating or they have a different momentum in different ways.

0:53:19.7 SC: So when the Pauli's Exclusion Principle really says... What it says is not that you can't put two particles at the same point in space, but that you can't put them in the same quantum state. So in principle, in a quantum field, you could always just excite it and give it different momenta and fit in a lot of particles into a tiny region of space. But again, the real answer is questions like this involve quantum gravity, if you want to discuss what happens in the real world, and we don't understand quantum gravity yet enough to give you the correct description of what happens at that point near the... Near the point in a black hole where you would have had a singularity if classical general relativity had been correct.

0:53:58.4 SC: Brendan says, "Are you a fan of the John Wick movies or are those type of movies not interesting to you?" I am a fan and I have no objection to those type of movies. I'm very happy to have fun action movies that are not trying to be too, I don't know, intellectual or whatever. But you know, the idea of an intellectual movie is a slippery one. I think the original John Wick movie was brilliant. I thought that it was a fresh take on a very old genre, which is very, very hard to do. It was a good story telling, it started small and then the world grew a little bit. The action sequences were novel and different and interesting. A lot of these days in superhero movies or whatever, or spy movies or thrillers or whatever, I actually tune out during the action sequences 'cause I know the hero is not gonna die, and I know the basic moves that are gonna happen.

0:54:53.4 SC: So I wait for the plot to restart again. But in the original John Wick movie, it was really very, very well done and interesting, stunt choreography in those action scenes. I think the subsequent movies in the series have not been as good, they sort of lost the focus of the storytelling aspect, it's become a little bit bloated in the... The universe expands, but it's not really coherent. In the first John Wick movie there were hints at this wider mythology, and they were all intriguing, you wanted to know more, why were these hotels there, what kind of assassins are there? But you didn't need to know all of them for the purpose of the story. So the fact that you didn't know didn't bother you. In the later... In the sequels, I suppose, to me, as they showed you more and more of that background, it still doesn't make sense, but it should make more sense because you're being shown more of it and it doesn't.

0:55:51.3 SC: And the most recent one, which people are really raved about, I just didn't like it all. I thought it was just boring, I was ready to leave, honestly. I thought I really had lost the heart that was at the center of the original movie. Like why are these things happening? It's clearly, you know, just an excuse to get from one choreographed action sequence to another. So I think the first one was a little bit of a lightning in a bottle of magical event, but it was hard to reproduce in later installments.

0:56:20.1 SC: Aksel, spelled as A-K-S-E-L says, "Can we assume that any particle is always being observed by its interactive environment? So why would the detector be treated differently or could it be that a detector is breaking the environment symmetry? So no, we cannot always assume that a particle is being observed by its interactive environment, so there's two things going on here that are very important. One is for observation in the sense of a quantum mechanical observation. You know, a quantum mechanical observation that splits the wave function into different possible observable... Eigenstates would be the technical term. In other words, an observation that entangles the system that you're looking at with some measuring device, that's a specific kind of physical interaction, but it's not the only kind.

0:57:12.3 SC: The example I like to use is if I have a spinning particle, right? If it's in a superposition of spin up and spin down, and I drop it in a gravitational field, there's no question that the gravitational field is interacting with the particle, it's pulling the particle in the direction of increasing gravity. But it pulls the spin up and the spin down parts of the particle in exactly the same way. So it's not entangling, it's not measuring the spin of the particle. So particles are constantly interacting with the world around them. But that doesn't mean they're being observed in the technical sense that we talk about measurement in quantum mechanics.

0:57:49.8 SC: The other thing... That's the first thing. The second thing is particles are tiny. So an individual particle is relatively easy to shield from being observed by the rest of the universe. Like we said, it can be interacting with the rest of the universe, it can be trapped in a certain location in a crystal and some sort of potential inside of a solid or something like that. But it's not interacting in a way that measures any feature of it. Whereas when you have a macroscopic system, like a cat or a pointer on a device or something like that, if you try to put that into a superposition of two different things with different locations in space, it's very, very difficult, essentially impossible, to shield it from being measured by its environment. That's why according to the standard explanation of decoherence, we see in the real world things with relatively localized positions in space that need not be the case for a single elementary particle because that's not bumping into things and having its position measured all the time.

0:58:57.0 SC: Walter E. Miller says, "What is your expectation for the next generation of particle accelerators, a more powerful version of the LHC for proton smashing or a completely different version, perhaps a muon or electron accelerator. What will physicist be probing for?" I'm not gonna say a lot about this because I don't really have a lot to say. I am a huge supporter of experimental elementary particle physics. I think we should build new particle accelerators. I don't know what they're going to be. I remember participating in a Snowmass Workshop. Snowmass is kind of process, it's literally a place, right? In Colorado where you meet and you... I think it's Colorado.

0:59:39.0 SC: Anyway, in the Rocky Mountains where it's a ski resort, but in summer times on occasional years, particle physicists will get together and map out the future of the field. And so this was when we already agreed that the large Hadron Collider is going to be built but it was 20 years ago. So it's like 2004, the LHC was going to come online, but we were thinking about what's gonna come next. And everyone rallied around the idea of building a Linear Collider, okay? A collider that would take electrons and positrons or something like that, rather than protons and neutrons... Rather than protons or antiprotons smashing them together at very high energies, But the thing is that with a Linear Collider, you can't reach quite as high energies as you can with a Circular Collider, but you can get more precision because you can control what individual electrons and positrons are doing in a way that big protons smashing into each other like at the LHC are a lot messier.

1:00:33.6 SC: So the thought was, the HLC would turn on, we would discover a bunch of new particles and the LHC would be a discovery machine, and then the Linear Collider would be a studying machine learning about all of those new particles that we had discovered at the LHC. As maybe you've heard, we haven't found that many new particles at the LHC. We found the Higgs Boson and we re-found the Standard Model. So far, that's it. So the motivation for a Linear Collider has diminished since then, and we still don't have a consensus 20 years later on what collider we will build next or even if we will build one.

1:01:10.3 SC: Probably the two most active areas of interest here are in China or at CERN once again in Europe for building a next generation collider. But I'm not up on what people's favorite versions are. I know that Muon Colliders, which are sort of like electrons, but heavier, have received a lot of recent attention. They can, in some sense, be a best of both worlds kind of thing where you can get the precision of an electron, but also the higher energy of a proton. The problem is muons decay. You can't keep them alive for very long, so there's special technical challenges there. As far as what they'll be looking for, they'll be happy to find anything. There's no more targets, there were targets for the LHC and they didn't find them, so now it's a fishing expedition, but it's a good kind of fishing expedition.

1:02:00.5 SC: There's something out there. It's very, very, very unlikely in my mind to think that we've discovered all of the particles there are to discover. What I can promise you is that they're discoverable at any particular new particle accelerator, so it's a cross your fingers and hope for the best kind of situation. Prometheus says, "I understand that mass and electric charge are two different fundamental properties of particles. But as far as I know, there are no massless particles that have an electric charge. This makes me wonder if the electric charge is somehow an intrinsic property of the mass of a particle, what is your take on this?"

1:02:35.3 SC: Well, you could imagine... This is a good question, it's fundamentally a quantum field theory question because the field theory-ness matters in this case. You really can't really think just about individual particles. The reason why it is the following, if you have a massive particle, so forget about electric charge for a second. If you have massive particle, there's energy that you associate with the particle, and part of that energy is its rest energy, E=mc squared. And part of it is it's kinetic energy if it's moving. And in relativity the relationship is the E squared equals P squared plus M squared. Where P is the momentum, and we've set speed of light equal to one. So it's a Pythagoras's theorem kind of thing. The total energy is the hypotenuse of the mass and the momentum. So in that case, there is a minimum energy that is not zero, the minimum energy you can have is the mass. As the mass goes to zero, the minimum energy gets smaller and smaller, and for massless particles, you can have arbitrarily small amounts of energy, okay?

1:03:39.1 SC: Now, if you consider charged particles rather than neutral particles, there's always some energy there because there's energy in the electric field around the charged particle. So I think that even though I haven't thought about this, you should do it much more carefully. I think that if you have charged particles, they will always have at least a little bit of mass. But that might depend on details of the quantum field theory, it might be possible to build some quantum field theory where you effectively have massless charged particles. I kind of doubt it. You could write it down and try, but I think that something would get in the way of having the physical mass actually be zero.

1:04:16.7 SC: Tracy P says, "How is it that black holes can grow if observer is sufficiently far away, never seeing falling objects across the event horizon?" I've heard answers to this question that sidestep the issue by using different coordinates, etcetera, but the black hole... Does the black hole only grow in certain reference frames but not others. No, as black holes really do grow because who cares what observers far away see, right? What does the black hole see is the much more relevant question. Also that's the glib but correct answer, but there is a subtlety here that it's worth remarking on. When we talk about what observers see when particles fall into black hole... Black holes. We very often make an approximation, which is, we imagine that the particles falling into the black holes themselves have no gravitational field. I don't mean by that a massless particle, 'cause even massless particles have a gravitational field, they have some energy, so they have some gravity, okay?

1:05:14.5 SC: Which actually makes me rethink the answer to the previous question. Because the gravitational field is kind of like an electric field. So maybe just like there are massless particles with a gravitational field, they are also massless particles that can be electrically charged. Maybe I was wrong about that one. Shows you what I know anyway. Anyway, I'm not familiar with any viable field theories that have electrically charged massless particles, but maybe that's just 'cause they don't appear in nature, so we don't try too hard to construct them, maybe they are possible after all. I'm talking myself into it. Anyway, the point is that real particles do have their own gravitational field, and we ignore that when we talk about what you see when you throw a particle into a black hole.

1:05:55.9 SC: So as a result, when a real particle falls into the black hole, it pulls on the black hole, just like the black hole pulls on it. So what physically happens, and people have actually thought about this is that the event horizon of the black hole when the in-falling particle comes close enough to it, that event horizon expands a little bit because it's being pulled by the particle and it sort of engulfs the particle and eats it up. And the result is a slightly bigger black hole that includes the mass and energy of the particle itself. So I think that from the point of view of the outside observer, you would see the particle move more and more slowly, but eventually in the real world, you would see it swallowed by the black hole. It's certainly true one way or the other, that the black hole does grow. Whether or not an observer sees it from far away is not really the relevant question.

1:06:49.2 SC: Okay, my mind is still thinking about this massless charged particle question. I should actually think a little bit before answering these questions in real time. I still don't... I'm not sure what the answer is, but let me say two other true things that might be relevant. One is, certainly it's possible to write down a classical field theory that would correspond to massless charged particles. My suspicion is that quantum effects renormalization of virtual particles would effectively give such particles mass. But I don't know about that, I'm not 100% sure about that.

1:07:19.9 SC: The other thing worth saying is that there clearly are not massless charged particles in the real world, that much is perfectly clear because they'd be very easy to create in virtual particles... Sorry, interactions, etcetera. We could just bump any charge particles into each other and they would spew out a very large number of massless charged particles. That doesn't happen in the real world, so it's a completely hypothetical question, which is fine. But just so I was not... If I was not clear about that, there's no lingering question that they exist in the real world, it's a purely question about what we could hypothetically imagine.

1:07:57.5 SC: Okay, even if I have more thoughts on it, I'm not gonna say anything more, that's enough to chew over. Let's move on. Brandon Lewis says, "I sometimes wonder whether industrial civilization might have arisen on Earth more than once. In other words, are we sure ours is the first one given that the evidence of previous civilizations might have been wiped out in some cataclysm? What do you think?" And I think it is very, very, very, very unlikely that anything like that happened. It's fun to imagine that there were previous industrial civilizations, but I think that it's... You would underestimate the extent to which we would imagine such a civilization would leave its mark. Like even if you got rid of all of the literal physical artifacts of our civilization, we have also changed the atmosphere and the geology of the planet, and future scientists would be able to recognize that, right?

1:08:53.1 SC: But also historians and archaeologists and paleontologists are really remarkably good at figuring out things about the evolution of life and the evolution of civilization and things like that. It doesn't seem to make sense to me that you could literally have an advanced industrial civilization that completely escaped our notice. The only way you could possibly do it would be if it were completely localized, if they were like an advanced industrial civilization that lived on an island like Atlantis and then it collapsed under the sea. So that it was really left a... Had a small impact on the earth as a whole, and then all of the physical evidence was just hidden forever.

1:09:36.0 SC: I don't think that a single island could support the coming into existence of an advanced civilization, so I'm not very worried about that. And by the way, Atlantis is not real, so that's just a colorful analogy to what it would have to be. But I think that the real answer is, it's very, very, very unlikely. Aaron C says, "I read recently that aleph-null is the set of all real numbers, and it's called the smallest possible infinity. This had me thinking about the sizes of infinities relative to each other, and there are a few things that I'm having trouble wrapping my head around. How could there be a bound to how small or how... Yeah, how could there be a bound on how small an infinity could be? Couldn't you just find an arbitrarily smaller subset of all the real numbers? For example, wouldn't the set of all prime numbers be smaller than set of all real numbers? Which leads me to the question, is there any real world physical implication or utility to comparing the different sizes of infinity?"

1:10:33.0 SC: So there's a lot going on here. As usual, it's a longer than question that it needs to be, but that's okay. I'll just ramble on about a little bit. So the first thing I wanna say is, aleph-null is not the number of real numbers, okay? It's the number of integers or the number of natural numbers, but the number of real numbers is a strictly larger infinity. This was one of the discoveries by Georg Cantor years ago, that there are different sizes of infinity and the set of real numbers, which is the continuum, right? The real numbers are all the numbers, including Pi and e and all those things, square root of 2. The integers are 0, 1, 2, 3, -1, -2, -3, etcetera. And Cantor proved that the number of real numbers is strictly larger than the number of integers. But infinity is not a number in the usual sense, and so there are things you can say about infinity, but they don't always map on to things that we're used to saying about non-infinite numbers. Such as if you take the... Just take the integers, okay? Which do have what we call a cardinality, which is the size of the set. How many numbers are there? Cardinality of the integers is aleph-null.

1:11:48.8 SC: But you can take half of that set like just take the even numbers, right? What is the cardinality of that? It is also aleph-null. The size of the set, all the even numbers is equal, it's not approximately equal to, is exactly equal to the size of the set all of the integers, okay? And why? Well, because we can put them in one-to-one correspondence, namely for every integer, multiply it by two, and then you get an even number. So the rules are a little bit trickier. A subset of the integers may or may not be infinite in size, but if it is infinite in size, it is equal in cardinality to the integers as a whole.

1:12:29.3 SC: Whereas the real numbers are a strictly larger set. There's a lot that is either unknown or choice dependent about the transfinite numbers as they are known. So there's the continuum hypothesis, for example, that hypothesizes that there are no infinite numbers with sizes between the real numbers and the integers. So there's the set of the integers, aleph-naught and then under the continuum hypothesis aleph-one would be the number of real numbers that's strictly larger, but there's no infinite numbers in between. But that's not something you can prove in the ordinary language of set theory. You can assume it, you can assume the continuum hypothesis as an axiom, and that's completely consistent, but it's also consistent not to assume it.

1:13:18.0 SC: So that's where I stop paying attention, really like, not because it's not interesting, but because it's super weird and difficult and things that we're not used to thinking about in ordinary arithmetic. So you kind of have to specialize in it and bite the bullet to really keep everything straight in your mind and I have not put that work into it. As far as physics is concerned, I think that the natural answer is there is no... Well, what, what should I say? The tempting answer, let's put it that way. The tempting answer is, it doesn't matter if there are different size, infinite numbers, because we human beings can only sort of experience finite things, right? Even if there's a continuum of points in space, for example, we can't measure exactly the location of points in space. We measure things to within a certain precision and therefore there's in effect only a finite number of outcomes we can get by measuring things.

1:14:13.9 SC: Having said that, there might be examples of, for example, cosmological scenarios, the universe being either infinite or finite. Maybe there, it does matter whether you have different sizes of infinity or not, but I think all of that is very much in the realm of speculation. It's certainly not something where your typical working physicist needs to worry about the sizes of different infinities.

1:14:40.6 SC: James Tronson says, "Given that the universe is expanding, what is it expanding into? Rather, if nothing exists outside the universe, how does it expand?" So this is a classic question. This is an old saw, but that's okay. We can dip into those occasionally. The answer is it's not expanding into anything. And the answer is it doesn't need to have anything outside the universe in order to expand. And the reasons why are, because when we're talking about the universe, some of your intuitions about objects inside the universe are not going to apply. Okay? So we have experience with balloons or raisin bread or things like that, objects within our universe that expand and by expanding, they grow into the space that is around them.

1:15:26.7 SC: So when cosmologists start talking about the expanding universe, they will very naturally use vocabulary words that have grown up over the years to refer to things that we know about in our everyday experience, like expand, etcetera. And your intuition then wants to attach the features of things you know about to the universe, but they might not apply. Okay? So when we say the universe is expanding, what do we mean? We mean that the physical distance between distant galaxies is larger now than it used to be and smaller than it will be in the future. There is literally more physical distance in-between galaxies. That doesn't mean that there has to be anything outside the universe. The universe doesn't have to expand into anything. This is a feature of non-Euclidean geometry and general relativity that if you have... Even if the universe is finite in size, okay, the universe is like a three dimensional sphere in space, then that sphere can have different sizes and that sphere can grow or shrink over time.

1:16:30.5 SC: Even if the universe is infinite, we were just talking about there's an infinite number of numbers, and if you double them, it's still an infinite number of numbers. Likewise, if the universe is infinite in extent, you can move everything apart from everything else. It's still infinite in extent, but in a very real sense it has expanded. Okay? So the mental switch you have to do when you start thinking about general relativity is that the geometry you're talking about is intrinsic to the space or spacetime that you're talking about. This was actually an idea that came about from Gauss and Riemann and those folks back in the 1800s. You don't have to imagine embedding a geometrical figure into a larger space. You can just talk about its properties intrinsic to the space itself and that's what we do when we talk about the universe.

1:17:25.3 SC: Craig Vandervest says, "Well, ChatGPT's deep language learning has been the rage in 2023. A few years ago, it was deep reinforcement learning. DRL provided exciting Atari Go and Chess demonstrations of superhuman game skill. I've heard that deep learning techniques have been used on particle physics data sets, but are there other areas of physics or philosophy that are or may be productively explored? For instance, could or are ontologies being explored using DRL brute force techniques to help find or outline paradoxes and conceptual limitations that a human may yet be blind to?"

1:18:01.5 SC: I think that this is a very interesting field that is being thought about right now, the use of AI models in physics. I don't know about philosophy. That one I have not actually come across, but maybe it could be. Look, there's certainly very down to earth uses of deep learning, machine learning, reinforcement learning in physics and astronomy because we have giant data sets to analyze, right? So it is a very active field to use these techniques deep neural networks, things like that to sift through large piles of data looking for new particles or trying to understand the distribution of galaxies, things like that. Okay, that's not going away. That is a very exciting area, but you're not looking for new kinds of things. You're telling the computer what to look for and it's... It will find it or not. It's much harder to imagine using the current kinds of AI programs to do truly creative work in physics or philosophy.

1:19:09.0 SC: And that's... I want to very quickly say that's not because it's impossible, it's just because it's... That's not what the current set of models are trained to do. The current set of models are not even very good at logical or mathematically rigorous thinking, right? Because they're really very good at a specific thing and that specific thing is completing sentences or drawing images or playing a game. And that's not enough to come up with a truly new picture of spacetime or something like that. You could imagine... Imagine going back to 1910 and you could ask a... But go back to 1910, but bring along with you a modern computer with AI programs installed on it. You could certainly feed into it all the physics that was known at the time and say, solve the following equations or look for the following pattern or so forth.

1:20:02.7 SC: It's very hard to imagine the current kind of AI programs that we have, saying, "Oh, you should think of spacetime as curved." Or, "Oh, you should replace positions and velocities of particles with a complex valued quantum wave function." That's just not the kind of thing that it is trained to do. You shouldn't hold that against the AI programs, just like you shouldn't hold it against the Go playing programs. They're not good at horseback riding. They're really very good at playing Go. Right? How should you design an AI program to come up with truly novel ideas in physics or philosophy or anywhere? Well, I don't know. That's a good one. That's a good question. I'm not saying it can't happen. I'm just saying that that's not what we've tried to do yet.

1:20:54.6 SC: Johan Faulk says, "Do you think large language models and AI chatbots will affect the way we look at knowledge?" Good. I think that this is a very good question because the answer is maybe yes. Does an AI... Does a large language model know things in the sort of most strict philosophical sense? I'm not sure what you should say about that. Clearly, you can ask it questions and it can tell you answers. Sometimes those answers will be wrong, absolutely, but very often if you ask it a well posed question, the answers will be right. If you ask questions of human beings, they will sometimes be wrong too, right? We don't hold that against them, the fact that they're sometimes wrong if they have some very large swath of knowledge that a human being can reliably tell you the correct answer to, we would say it knows something, it has some knowledge. Why shouldn't we attribute the same statements to AI models? But I should say not and, but the kind of knowledge is different, right?

1:22:01.7 SC: As we discussed earlier, and this goes back to podcast conversations we've had with people like Melanie Mitchell, Gary Marcus, etcetera. The way that large language models have that knowledge is a different way than human beings have that knowledge. Human beings take the world and compress it into models, models that we might call common sense or folk physics or whatever, but models of pieces of things that have relationships between them and interrelate to each other in various ways. Whereas large language models are really looking at LLM that is right there on the tin. They're looking at language, they're looking at what sentences people have said in the past and trying to figure out how words will fit together in a certain way. It's a very different way of coming up with the true statements that they come up with. It's not they don't come up with true statements, but they generate them in a very different way than human beings do. So that's why I think it's a very interesting question to ask, Does that count as knowledge?

1:23:05.6 SC: And I suspect that what's going to happen is that we will begin to distinguish between different kinds of knowledge. We will attribute a kind of knowledge to a large language model, which is a different kind of knowledge that a smart human being has. Which isn't to say, once again, that you couldn't invent a different kind of AI program that also had the same kind of knowledge that a human being has. It's just not what we have in an LLM. Adam Rotmil has a priority question. Remember that priority questions are things that you get once in your life for every Patreon supporter. You get to ask a question, I will do my best to answer it. Adam has split his questions to two parts, which are really two different questions, and you are not allowed to do that. So I'm going to answer the first one.

1:23:48.6 SC: It's still only one question per question. That's still a rule, even for priority questions. The question is, "When I think about the first, second after the Big Bang, my understanding is that as we look backward toward T=0, the temperature becomes hotter and hotter until we approach infinite temperature. But when I think about what temperature is, I know it has to do with motion. I know there were many interactions in that first second. Carlo Rovelli tells us that time is the order of events. From that perspective, I wonder if we can say that the temperature becomes time-like as it reaches infinity, such that the first, second might as well have been an eternity." Well, no in all sorts of ways, but I'm not exactly sure what you mean at some points in the question. So when you say, I wonder if we can say the temperature becomes time-like, I'm not sure whether you mean temperature is like time in the sort of casual sense or time-like as in the technical relativity sense, the difference between a space like trajectory, one that goes faster than the speed of light and a time-like trajectory, one that goes slower. I presume you mean the former rather than the latter.

1:24:54.0 SC: It's kind of like time, but maybe I'm getting this wrong. I'm gonna do my best. I'm not quite sure what it means to say the temperature becomes like time. Of course, in an expanding universe, the temperature evolves essentially this... There's footnotes here and, and caveats as always, but essentially the temperature evolves monotonically with time, right? The temperature of the universe only ever goes down as the universe expands. Sometimes it goes down faster, sometimes it goes down slower, etcetera. Or thought the other way around, given the temperature of let's say the microwave background now about 2.7 kelvin, if we contract the universe for example, by going to the past, it becomes hotter monotonically. It never starts cooling down.

1:25:44.6 SC: Now, it's true that sometimes you, would you hear people say that as you approach the Big Bang, you approach infinite temperature, that's a little bit sloppy for a couple reasons. Number one, there is no Big Bang, remember that. The Big Bang singularity is just a prediction of general relativity that we all expect to be wrong. So don't spend too much time thinking about what life is like literally at the Big Bang until you have some quantum theory of gravity. The Big Bang model where the universe expands and cools and galaxies form, that's in a hundred percent good shape, but that's entirely different than the Big Bang event at T=0. So you will hear people sometimes nevertheless say you approach infinite temperature, but that's just an extrapolation. It's an extrapolation beyond what we're allowed to talk about, right? If you have a box of gas and you squeeze it to smaller and smaller sizes, the temperature goes up, and as you hit zero size, the temperature would go to infinity. But you don't really ever hit zero size. That's not a thing that happens.

1:26:44.9 SC: So the third thing to say about this is of course the world is not a box of gas. It's not made of particles in that way. It's made maybe of quantum fields, but maybe in quantum gravity, it's made of something even more subtle, something more purely quantum mechanical. In quantum field theory, there are definitions of the word temperature other than what it has to do with motion, right? When Adam, when you say, I think the temperature has to do with motion, that's not necessarily true in a quantum field theory. There are other definitions of what you're talking about because you don't have particles moving, you have fields interacting and vibrating and so forth. So I think that in the spirit of the question, there's nothing really bizarre going on as you approach T=0. At T=0, there's gonna be something bizarre going on that we don't know what it is, but the approach is pretty ordinary and normal.

1:27:44.6 SC: There could be things like phase transitions, you could have inflation and stuff like that, but the laws of physics don't need to be changed in order to make that happen. Having said all of that, there are way more speculative ideas, not about conventional standard Big Bang cosmology, but about quantum gravity and the emergence of time. In fact, Carlo Rovelli is the co-author on one such idea called the Thermal Time Hypothesis, where indeed it's not that temperature is playing the role of time, but the temperature characteristics of a certain quantum system allow us to show how time emerges in the Thermal Time Hypothesis. That's a complex and subtle story. I'm not sure that I believe it entirely, but it is out there. Maybe that's what you're referring to, I'm not sure. If it is, then I don't have that much to say about it. I'm interested in it, but I don't know enough details to say much more.

1:28:42.9 SC: Brad Malt says, "In your recent podcast with Thomas Hertog, you explore applying the rules of quantum mechanics to the creation of the universe. That got me wondering. Presumably, right after the Big Bang, everything in the quantum universe was in a state of superposition, but what would cause the first branching of the wave function of the Universe? There was no observer to make the first measurement or no macroscopic object for something to interact with and cause decoherence." Well, this is actually a subtle question. Parts of it are easy to answer and parts of it are more complicated. Remember that observing has nothing to do with anything. If you believe many-worlds at all, which I presume is going on beneath this question, one of the achievements of the many-worlds formulation of quantum mechanics is to erase words like observer or measurement or anything like that from the laws of physics.

1:29:32.5 SC: Those words are in there in the Copenhagen interpretation, but in many-worlds, it's just physical systems interacting with each other. Okay? So what do you mean by what happens in the real world when you do a quantum measurement, it is something like decoherence and branching of the wave function. In order for that to happen, you have an environment that is to say the degrees of freedom that you're not keeping track of in your system, etcetera. And then you have the system that you're looking at and they become entangled, and that's decoherence. You can do all of that right there near the Big Bang. You can just call the environment, the specific locations of all the photons or the specific quantum state of all the fields that you're not paying attention to, right? You're paying attention to some things like maybe the large scale distribution of density of matter or something like that. But there's other things that you're not paying attention to, and those are the environment.

1:30:25.7 SC: If this sounds a little fuzzy, then that's fine. It is fuzzy. It's not just you who thinks that it is a little bit fuzzy. I wrote a paper a few years ago with Jason Pollack and Kim Boddy about decoherence during inflation and its effect on eternal inflation, because sometimes when people talk about eternal inflation, they act as if the collapse of the wave function is a truly objective event that happens stochastically, which would not be true in many-worlds. It's an apparent event caused by decoherence. And so we ask the question, this sometimes happens when you're a theoretical physicist. We ask the question, if you do it right, how does that change the answer from what answers people have been quoting? And you find that it doesn't change it very much at all. It changes it a little bit, but not in a way that really matters very much.

1:31:13.5 SC: So this can be done, it's just decoherence. There's just an environment and you can do it, but it's not quite as easy as it is in the laboratory. In the laboratory, it's a little bit more straightforward, which parts of the world to call the environment and which parts to call the system. In the early universe, that's not so straightforward so that's why it's work. That's why I have to write papers about it rather than just quickly calculating it and getting on with your life.

1:31:40.9 SC: Bill McIntosh has a question. It's a long question so kind of editing it down to the essence here, which is, "Isn't it possible or even likely that the universe is so vastly large that curvature is difficult to detect, similar to how an ant would assume the earth is flat?" In other words, we talk about the geometry of space and the universe on very large scales, and we say the universe looks flat to us, and Bill is asking isn't it possible it's just a little bit curved and we don't notice. Yes, that's a hundred percent possible. And so cosmologists are not dummies. They know that. They try to measure it. And what you do is you don't say the universe is flat, we're done. You say the universe is... The data that we have on the universe is compatible with it being flat and we can put error bars on that. We can say that it is true to plus or minus a certain percent. I don't know what the percentage error bars are now. It's something like 1%, 5%, something like that.

1:32:36.6 SC: But by the way, beyond the part of the universe, we can observe beyond our observable horizon, not only is it completely possible that the universe is not flat, it's also completely possible it's not smooth, right? It's not homogeneous and isotropic. There is this game that gets played in the popularization of cosmology where people talk about three different geometries the universe could have. It could be flat, or it could be positively curved, or it could be negatively curved. Well, those are only the three different geometries you can have if you assume that the curvature of the universe is the same everywhere, both at every point and in every direction. And that seems to be an assumption that is borne out on large scales in the universe that we observe. But there's zero reason to think that that assumption needs to continue to be true beyond the universe that we're actually able to see. So there's an infinite number of possible geometries the universe could have. It's not just those three, it could be wildly different from place to place.

1:33:37.7 SC: James Allen says, "How are we supposed to make sense of fractional charges in quarks? It makes sense to say there's a symmetry that charges can exist in units of plus one or minus one. But what is guaranteeing the fractional charges add up to integer multiples of that of the electron? Could you have a charge, particle with charge 0.7 or e or pi?" Well, we don't know is the short answer here. The fact that the quarks have one third charges or two thirds charges is pretty much irrelevant to this question. What matters is that they're rational numbers so that there is always a certain number of charges that add up to integers, right? If we discovered quarks before we discovered electrons, we would've assigned them charge one and we would've assigned electrons charge three or minus three, nothing wrong with that at all. It's completely conventional. The overall scale doesn't matter.

1:34:29.5 SC: So the right thing to say is that in nature, in the charges that we know about, they're all multiples of each other. They're integer multiples or divisors of each other. And that's a question you can ask, is that necessary or is it optional, etcetera. And the answer is actually a little bit interesting because if... You might know that electromagnetism can be thought of as being based on a U [1] gauge theory. If you don't know what any one of those words mean, just wait until volume two of The Biggest Ideas in the Universe comes out. They will all be explained in great detail. But if that's all you knew, and it's basically Maxwell could have said that almost not in those vocabulary words, but he knew the basic physics there, then in principle, you could be allowed to have charges of any sort at all.

1:35:18.4 SC: A gauge theory like that, like electromagnetism is, does not by itself put any restrictions on what the charges can be. However, it turns out that if you extend electromagnetism to broader contexts, then you will often find that there is now a restriction on the charges of particles. For example, Paul Dirac showed very long time ago that if you have magnetic monopoles in the universe, so he was asking we have electric charges that have plus or minus charges, and they're monopoles as far as electrical charge goes, which is to say they're just a plus sign or a minus sign, and all the lines of force go directly toward the particle and end there. Whereas for magnets, we have a north pole and a south pole. We always have both poles and the lines of magnetic force kind of wrap around inside. So Dirac said, could we have magnetic monopoles like a north pole without a south pole.

1:36:16.9 SC: And he came up with a way to do it, and he showed that as an implication of that way, if there's even just one monopole in the universe, magnetic monopole, then the restrictions of quantum mechanics imply that all of the electrical charges have to be quantized. In other words, have to be multiple... Integer multiples of some basic unit. We have never seen a magnetic monopole, so that may or may not be relevant to nature, but there are certain theories that predict them. Likewise, if the U [1], which is the symmetry that leads to electromagnetism is embedded in another kind of gauge group, like it is in grand unified theories that try to unify the strong and electroweak interactions, then you can get charge quantization also. So they're speculative ideas for why charge needs to be multiples of some basic unit, but we don't know whether that any of those speculative ideas are true. And they might, they may or may not be. And it may just be a fact that all the charges are multiples of a basic unit that maybe that's just one of the things we need to live with.

1:37:22.3 SC: Jesse Rimler says, "Estimates are that at least 500,000 Iraqis died as a result of the US' invasion of Iraq. And the resulting war between 2003 and 2011. The pretext for this invasion is now seen even in the US as unjustified. During this conflict, do you think it would've been morally correct for a third nation to supply weapons intel and even troops to the Iraqis in order to defend against the US' forces?" So there's kind of two levels at which I want to answer this, one is the surface level, then there's the deeper level. At the surface level, I don't think that the phrase supply weapons and intel to the Iraqis is well defined. Like who exactly are you supplying these weapons and intel to? We have the obvious analogy right now of the US supplying weapons and Intel to Ukraine in its war against Russia. But there, I think that there's very good moral reasons here since you're asking about moral justifications here. I think the moral reasons are very good to help Ukraine in staving off the Russian invasion. They were invaded and they didn't want to be invaded.

1:38:34.4 SC: In Iraq, it was a much more complicated thing. I think you can say in some vague sense that Iraq did not want to be invaded, but the government of Iraq at the time was a bad place, right? It's not at all obvious that you would have been morally correct to give weapons and intel to Saddam Hussein to help fight off the United States. Having said that, I was not in favor of the invasion myself. I thought it was, like you say, cooked up for the wrong reasons and that's bad. But in the real world of geopolitics, you're not just abstractly giving weapons and intel to countries, you're giving them to people or governments or whatever. And in the initial stages of the Iraq War, that would've been at least a little slightly morally questionable thing to do.

1:39:20.2 SC: Now that you would have a better case if you said, well, okay, after Hussein had fallen and we were later into the war in Iraq, there was still an insurgency going on where Iraqis were fighting against the United States occupiers. And there I think you would've had a better case for giving them weapons or intel or whatever. But it's still a tricky case because when you look closely at who was leading those insurgencies, they were also not the nicest people in the world. So again, maybe there was a case, I don't remember the details well enough to go into names and places and things like that, but I think you have to think it through.

1:40:01.1 SC: So that's the surface level of the question. There's a sort of subtext level of the question inspired by the comparison between this situation and the Ukraine situation, which is a lot of people criticize the US and NATO giving support to Ukraine on the basis that you wouldn't want it done to you, you wouldn't want other countries to give support to the places that you are invading. Would you? And I think that's a very dumb bad argument. Or even sometimes the argument is put in as the United States has no right to do this kind of thing, or you have no right to pass judgment on Russia or Ukraine or whatever because the United States has done so many bad things in its past.

1:40:50.2 SC: I think that's just a silly thing to say. The United States has absolutely done all sorts of bad things in its past, and the right thing to do with respect to that is to criticize it for doing that. It's not as if Americans think that it's bad to criticize America. Americans criticize America all the time and some more than others of course, and you should be thoughtful about how you do that and think about it. But there's no moral guideline that says that you just can't criticize other countries until your country is completely correct. I think you should criticize all the countries that are doing bad things, your own country as well as other countries.

1:41:31.1 SC: Sugar Pine Press says, "Do you suppose that in the same way that the contradictions that exist among a multiplicity of religious beliefs lends credence to the theory that none of them are correct, or even that God doesn't exist? The contradictions between numerous meta-ethical theories increases the likelihood that none of them are correct." Not really, no, I don't think so. Forget about difficult things like religion and ethics and so forth, there's plenty of cases where we have multiple theories because we don't know what the right theory is, and yet one of them is going to be correct, right?

1:42:05.2 SC: The fact that there are multiple theories disagree with each other, I don't think it's very strong evidence that any one of them is incorrect. In the case of religions, it's a lot deeper than that. The story is a lot more complex than that. It's not just that there are a lot of religious beliefs, but that the specific kinds of religious beliefs that there are and the ways that they develop in different countries and things like that, are to me exactly what you would expect if none of them were true, right? If it were really just human beings telling each other's stories rather than saying something true and important about the nature of the universe.

1:42:43.2 SC: So that may or may not be true in these other cases where you want to compare them. In the cases of meta-ethical theories, for example, I could very easily imagine that one of them is true and the others just are not true. I don't see what the slightest impact that there is on the existence of other theories should be for the theory that I think is true. Maybe there is a more detailed analogy to be spelled out there that I don't know about. But as it is, I don't think that that has a lot of weight.

1:43:18.7 SC: Simon Carter says, "I know that Bell's theorem does not apply to many-worlds because it just proves that a hidden variables interpretation can only be non-local. Does many-worlds assume non-locality at the fundamental level?" Well, it goes back a little bit to what we've already talked about, that the way that you choose to describe the branching of the universe is up to you in some sense. There are wrong ways to do it for sure, but there's not necessarily a single right way to do it. And I think that someone like David Wallace would say he has a way of doing it that recovers locality at the fundamental level. I don't... My way of doing it does not have no... Locality at the fundamental level, but I also don't think you should have locality at the fundamental level, because I don't think that physics has locality at the fundamental level. There is such a thing as gravity. And in a world with quantum gravity, you don't expect physics to be purely local. So as I've said before, from my perspective, the thing to puzzle over is not, Oh my goodness, how are we going to include non-locality in physics? Rather, it is, oh my goodness, why does the world look pretty local to us in our classical world? I think you can try to answer that question, but I think that's the one you should be focusing on. The surprise is not non-locality, the surprise is locality.

1:44:39.6 SC: Peter Musgrave says, "As a lapsed relativist who went into software long ago, I've continued to monitor the new physics textbooks on GR, collecting some along the way. This week, I get yet another email from a publisher with yet another new GR book on offer. It seems like there are several new textbooks on GR per year. This seems like a lot. Why do you think people are writing GR books at this rate? What made you write your GR textbook?" I'm not sure if there are several new textbooks on GR per year that would sound like a lot. By the way, GR, General Relativity for those of you who are not in that subfield. But I'm very happy that there are multiple books out there because the books are different. They're not just Xerox copies of each other. Different people have different ways of coming to terms with these different ideas. And so just among famous books, if you compare Bob Wald's book to Misner, Thorne and Wheeler to my book, to Jim Hartle's book, to Steven Weinberg's book, these are all radically different books. Like none of these two books look alike, okay? They're all teaching you general relativity, but all in very different ways.

1:45:43.6 SC: As far as what made me write my GR textbook, when I was writing it, I did have at least one famous general relativist to say, "Why are you writing another book? We already have Wald's book. We don't need any more books." But if you've ever looked at Wald's book, it is a brilliant, wonderful, gorgeous book. A true tour de force, and a work of art in a very real way. It's also not the best way to learn general relativity for the first time for many people, including myself and I say that as someone who took a course that used that book to teach. So it's... Everything in is in there and it's all correct, but it can be hard to extract sometimes. So my own book, it's... This is not exactly correct, but my own book can be thought of as Wald translated into English for people who are not thinking in the pristine, mathematical way that Bob Wald thinks. And it's not just that I put other things in there as well, but different books are gonna be useful for different people, and I think that's the least surprising thing in the world, really.

1:46:43.3 SC: Anthony Nault, N-A-U-L-T says, "What grounds spacetime structure. I like being Humean about the laws of nature and wonder if this intuition should extend to the laws of geometry. In other words, why is distant simultaneity a thing in Newtonian spacetime, but not in Minkowskian spacetime? Are we just at ontological bedrock?" So I'm not at all sure that I understand this question. I like it. I like the words that appear in the question. I'm not sure that I'm getting what is being asked in the sense of what grounds, spacetime structure. When you say, why is distance simultaneity a thing in a Newtonian spacetime, but not in Minkowski spacetime? Part of me wants to say, well, because it is, that's part of how we constructed it. What Newtonian spacetime is, it's literally an absolute space with absolute time. Distant simultaneity is just there, built into what you mean by Newtonian spacetime. Whereas in Minkowski spacetime, it's just not.

1:47:47.3 SC: If what you're asking rather is why is the world more like Minkowski spacetime than Newtonian spacetime, that's a different question and that, I don't know what the answer is. There is one nice feature that relativistic spacetime has that Newtonian spacetime doesn't, which is the speed of light limit. Those of us who imagine hopping on a spaceship and flying across the galaxy might think that the speed of light is annoying, it's very, very slow. But in terms of physics, it has this extremely nice feature that it limits what you can do in the universe.

1:48:20.0 SC: If you clap your hands right here, right now in the room, you're in, the impulse, the impact of that on the universe cannot travel outward faster than the speed of light. In fact, it will travel out slower than the speed of light, as far as the sound is concerned but the visible impact will travel out at the speed of light. But that means that you cannot instantly affect something a billion light years away and then instantly be affected back by it. And that's kind of a convenient thing when it comes to physics. It sort of provides a little bit of a limitation on what you can do. And as you know in physics, very often, we write down a theory that looks pretty good, but it has some weird implications. And I think that Newtonian spacetime would have weird implications because of this infinite velocity that we would in principle be able to have that we don't need to worry about in the real world because we live in something more like Minkowski space. I mean it's not exactly Minkowski space 'cause the universe is curved, the spacetime, but it's locally Minkowskian.

1:49:19.0 SC: James Heath says, "Is the Sleeping Beauty problem the same as self-locating uncertainty in many-worlds and my credence about the coin flip should be 50/50." The Sleeping Beauty problem, for those of you who don't know, is a thought experiment invented by philosophers. I think that Adam Elga popularized it, but I don't think that he invented it. So the puzzle is this. So Sleeping Beauty volunteers to appear in a philosophy experiment. Okay? And the philosophy experiment is the following thing. You're going to give Sleeping Beauty, Sleeping Beauty is going to go to sleep. That's what she does. That's what she's good at. And when she's asleep, you're going to flip a coin. If the coin comes up heads, she will wake up the next morning. Call it Monday. Okay? If the coin... And that's it. That's all it happens. If the coin comes up tails, then she will wake up the next morning, Monday, and then she will be put to sleep again with a memory erasing drug so that she does not remember having woken up Monday and then she will wake up again on Tuesday. Okay, so coin heads, you wake up once on Monday. Coin tails, you wake up twice, once on Monday, once on Tuesday. In both cases, of course, after that, later on, she wakes up and she's fine.

1:50:31.7 SC: She knows ahead of time what the experimental protocol is going to be. Okay? And the question that she's asked when she wakes up is what is your estimate? What is your value that you place on the probability that the coin came up heads? And there are two very plausible sounding answers. One is well there's three possible things I could be. I could be waking up Monday in the coin is heads universe. I could be waking up Monday in the coin is tails universe or I could be waking up Tuesday in the coin is tails universe. Those are three possibilities. I have no way of telling. There's no difference to any of them when I wake up and therefore I would give a one third one third one third probability. So I'm a thirder for the coin coming up heads.

1:51:20.4 SC: The other argument says before I went to sleep I would have said it's a 50/50 chance that the coin was heads or tails. After I go to sleep and wake up, I'm a good Bayesian, I collect more data but the data tells me nothing about whether the coin was heads or tails. Therefore Bayes' theorem says it's still 50/50. There's a 50% chance, a halfer position on the coin being heads to begin with. So yes, this problem is very similar to many-worlds but it's not exactly the same because the worlds are different. In many-worlds there are multiple worlds but they're simultaneous with each other. They exist at the same time.

1:52:02.2 SC: In Sleeping Beauty problem, in any one realization of the experiment, there's only a single world. So maybe you're comparing different possible worlds or something like that. I think that it is plausible but far from obvious that we should treat different possible worlds that you can imagine in the same way as you treat different actual worlds in the wave function of many-worlds. I will be very honest. My views on the original Sleeping Beauty problem go back and forth. As of the current moment, I am a halfer. I think that the Bayesian argument that I gave you makes more sense. And partly this is because I taught a course last fall on topics in philosophy of physics and we talked about the Sleeping Beauty problem. And one of my students wrote really good paper that convinced me I should be a halfer. But of course, maybe next year, another student writes another paper that points me the other way. I've not actually sat down... I really do hope to sit down and think about it and be able to come up with an answer all of my own. So they're related is the short answer, but they're not exactly the same problem.

1:53:14.6 SC: Danny says, "How is the bass playing going and what songs are you attempting to learn?" So I'm only including this just to say sorry that I haven't updated anyone on my bass playing or anything else that I'm doing in my life because I've not been doing it. I've not had time. It's been quite a year, let me tell you. And I think that as fun as it would be to share some stories about bass playing out there, it's more important for me to finish writing this book. So I got to do that. I have to prioritize that. Sorry about that. The bass playing is not going well because it's not going at all right now.

1:53:46.5 SC: Justin Proctor says, "While I understand that WIMPs, weakly interacting massive particles, and axions are considered the two leading candidates for dark matter, what exactly are the differences between the two?" They're very, very different. It's sort of an interesting feature that particles that are so different from each other can both be very viable candidates for cold dark matter. So by the way, dark matter, one of the things we know about dark matter is that it's cold. What that means is that it's the particles themselves were moving very, very slow compared to the speed of light when they sort of first formed in the very early universe. And that's important, the fact that they're moving slowly compared to the speed of light because if they're moving fast compared to the speed of light, that would have a big effect on the formation of large-scale structure in the universe and we would know it. So there were ideas early on about hot dark matter and so forth those have been ruled out by experiment.

1:54:42.1 SC: For very different reasons, WIMPs and axions are both cold dark matter even though they're very different kinds of particles. Weakly interacting massive particles are you know they're kind of like the Z boson. You know the Z boson is one of the force-carrying particles of the weak interactions. It interacts weakly, it has zero electric charge. The only difference is the Z boson is not stable. It decays away very very quickly so it's not going to be a good candidate for dark matter. But if you somehow had a model of particle physics in which there was a thing like the Z boson but it was perfectly stable, that would make a very good dark matter candidate.

1:55:20.6 SC: So it's very heavy, you know, a hundred times the mass of the proton or something like that. So that's a lot. So mass of the proton is almost a billion electron volts. So there's be a hundred billion or even a thousand electron volts for a WIMP. Whereas axions are very light particles, less than a single electron volt, less than a thousandth of an electron volt in fact. And what you might expect is that they're so light it doesn't take a lot of energy to make them move so you might expect them to be hot dark matter. But the axion has a very unusual way that it is formed from spontaneous symmetry breaking in the early universe. I'm not going to go through the whole thing right now but the upshot is that that even though the axion particles are extremely light, they move very very slowly when they are formed, essentially zero velocity.

1:56:08.9 SC: So even though the mass range here is very different, what is that 10 to the 16 difference between the mass of a typical axion and the mass of a typical WIMP, they both count as cold dark matter. They also interact very differently with ordinary matter. WIMPs interact via the weak interactions, thus the name. Axions interact primarily electromagnetically with known matter in terms of at least the ways that we are trying to look for the axions. That's the interaction that we're looking for. But it's a very, very tiny interaction because when I say interact electromagnetically, the axions are not electrically charged. They're neutral. So there's zero charge.

1:56:52.4 SC: You might think they don't interact with electromagnetism at all, but because of quantum field theory there's always indirect interactions. The axion interacts with something that interacts with photons. So ultimately you see an effective interaction with an axion and two photons or something like that. So there... We've been very good at building experiments looking for WIMPs. We've been less good at building experiments looking for axions. It's a little bit harder to look for and find axions but we're trying so who knows maybe we'll do it. You know, WIMPs could have been found by now. The searches are that good. It's not that we should have found them by now, but we had a chance and we haven't yet. So as a good Bayesian, your credence that the dark matter is axions should have gone up over the last 10 years or so.

1:57:40.1 SC: Jason says, "In some of your podcasts, you refer to the ongoing decades of learning where we shifted our view of energy density in a space vacuum from being zero to being significantly greater than zero and ultimately validating the cosmological constant. My confusion from an outsider perspective is was there a legitimate belief at one point that the energy density was exactly zero?" Well I think so. I had that belief so I think it was legitimate but to be completely fair this is a tough game where the rules are very unclear. Okay so what we're talking about here is the cosmological constant the energy density in empty space itself which by the way is not significantly greater than zero.

1:58:22.0 SC: Even if it is the thing that is causing the universe to accelerate, it's still a very tiny number. What 10 to the minus 8th ergs per cubic centimeter or something like that is the energy density in the vacuum. We didn't know. We never had a theory ahead of time for how big the cosmological constant should be. We had a sort of hand-wavy way of thinking about it which said the following. If you start with some classical value of the cosmological constant and then you turn on quantum mechanics, so you live in a world where there are quantum fields and the quantum fields vibrate in their vacuum state, etcetera. It turns out that there are quantum corrections to that classical value and they tend to make the classical value bigger and bigger and bigger.

1:59:04.1 SC: In fact, naively, they make it infinitely big. So that's a problem. The cosmological constant has divergent quantum corrections in our ordinary understanding of quantum field theory. Of course, that's a little bit of a cheat because to get those divergences, you have to include contributions at arbitrarily small distances, arbitrarily large energies, and those probably aren't there. So if you put a cutoff on those contributions at the Planck scale, that's where you get the idea that the prediction is 10 to the 120 times bigger than the actual value.

1:59:39.9 SC: And so the logic was the following. People knew that their naive prediction, and everyone agreed that it was naive, but it was the only thing we had. The naive prediction was that the vacuum energy should be 10 to the 120 times bigger than the limit we had on how big it actually could be. And they said, look, we don't know why. It's clearly much smaller than that. We would have noticed if it were that big. We don't know why it's smaller, but it's smaller by at least a factor of 10 to the minus 120 from what we expect it to be. And here's where the magic happens. In the space of all possible theories that we haven't yet thought of, it is easier for us to imagine a theory that takes a big number all the way down to zero than theories that take a big number down to 10 to the minus 120, that big number.

2:00:29.5 SC: Because zero has a symmetry, right? Zero is a nice round number and even if we don't know why, it's plausible to imagine theories that would make the vacuum energy exactly zero. And In fact, there were plenty of candidates. Sidney Coleman came up with the famous quantum gravity argument that the cosmological constant should be precisely zero. And there was no good reason. There was no theory on the market that said it should be precisely 10 to the minus 120 times the Planck scale except for the anthropic principle. Except for the idea that if the vacuum energy took on different values in different parts of the multiverse, we would only be here if it were small enough to allow us to exist.

2:01:12.1 SC: And Steven Weinberg and others pointed out that that would naturally give rise to a vacuum energy of order 10 to the minus 120, what the Planck scale prediction really was. So I think that that argument was fine. I think that was a good argument that the cosmological constant is probably zero. It certainly was never an airtight argument. It's not a statement along the lines of, here is the correct theory of everything. Here is the correct set of laws of physics that I'm going to use to predict the cosmological constant and get you a definite answer. That was never it. It was always and supposed to be a way of reasoning about what would seem natural to us given that we don't know the final answer. It turned out to be wrong. I think it was wrong.

2:02:02.3 SC: It's still possible the cosmological constant is zero and the actual dark energy is something else but I think the smart money is the cosmological constant is not zero. So that reasoning was wrong. So that's good. That's a clue. That's helping us move in the direction of something that is more correct, but we're not there yet. We don't have the final answer to those kinds of questions. James Swift says, "You've mentioned writing a little bespoke software yourself in the past to help with some research problems. And I've heard other academics talk about it as a necessity on occasion. But I also get the impression that it's frustrating and too time consuming, especially since they are not often trained or very experienced in software development. My question is, given the modern culture of open source development, do you think it would useful for academics to put out calls for software to the wider community. Perhaps they do this already and I am unaware."

2:02:50.4 SC: Yeah, they absolutely do do this. Now I think that there's... Don't get the wrong impression, there's plenty of academics, plenty of physics professors who write software all the time at a very deep sophisticated level. I just happen not to be one of them, I'm more of a pencil and paper person myself. And for better or for worse, the following thing happens. The people who know enough about open source development and how to use GitHub and things like that are also the people who are perfectly competent at just doing it themselves.

2:03:22.8 SC: So they will always be happy to outsource certain subroutines or whatever, certain tasks that are relatively common and they can plug into their software. But for the most part, really hardcore computational physicists are going to have to write it themselves or at least their team of grad students and postdocs and so forth will write it along with them. So it's a give and take. What the open source community would be useful for is people who almost never write software, but those people also don't know how to take advantage of the open source community to find what they need. And maybe they don't need to. Like if they don't use the software that much, it's just easier to write the little bit themselves when the need arises.

2:04:07.2 SC: Taroon asks, "As a Humean constructivist, does the criticism that this point of view leads to a form of moral relativism concern you?" No, it does not concern me. There are actually moral relativists. It doesn't concern them either. I think that it doesn't... Who cares whether it concerns you or not is one way of stating my response to this. If it's true, you have to deal with it. The thing that I am most... Put the highest credence on is that there are no objective moral truths out there in the world. That I'm pretty confident in. Again, not 100%, never 100%, always willing to change my mind, but pretty confident in. Less confident in what we should do about it.

2:04:53.0 SC: I do think that there are ways of moral reasoning as a constructivist and I can talk about why we reason in one way rather than another, etcetera, but I think that's the best we can do. There's just no choice. I don't take seriously the option of pretending that there are real moral structures out there in the universe that I should listen to just because I worry that someone else is not going to agree with my arguments for taking a certain moral stance or another one. Constructivists remember just take seriously the idea that we're human beings who are constructing our moral systems and that's okay. That's all we have. So there's nothing that we should regret or feel bad about or be concerned about just because of that. It's just accepting the real world for what it is.

2:05:41.3 SC: Laurent Delamere says, "Is it possible to know how different our universe would be if physics were simpler, I.e., none of the complexity of quantum mechanics exists, only the classical physics and standard model of particles?" Well, I think that you put the word simpler in quotes, which is a good thing to do, but maybe you should be even more impressed with how ill-defined that is in a context like this. I don't think it's at all obvious. Maybe in fact it's obviously false to say that quantum mechanics is more complex than classical mechanics. It is less intuitive to us and quantum mechanical structures lead to complex systems, but I'm not quite sure what the objective measurements of complexity or simplicity would be according to which quantum mechanics is more complex than classical mechanics.

2:06:30.3 SC: Furthermore, and more down to earth level, you cannot say only classical physics and the standard model of particle physics because the standard model really, really, really needs quantum mechanics to make sense. You need quantum mechanics to make sense of atoms, right? Without quantum mechanics, electrons would spiral into atomic nuclei and matter would be dramatically unstable. So there's no such thing as a model of the world that is essentially classical but looks more or less like our world. The macroscopic world that we live in looks essentially classical but we know that when we put a microscope on it, it begins to look quantum and we don't know how to not do that.

2:07:15.4 SC: We don't know how to get a purely classical world unless you really just didn't have atoms and molecules, you just had fundamental substances, right? Steel and wood and water and air were all just separate fundamental substances with their different properties. That would be a possible world where you could be classical. And I say that because that was the world that people thought they lived in 200 years ago, right? But it's not a simpler world. It's a world where you have a lot of fundamental materials and substances. That's way more complex than the quantum world we think we live in.

2:07:49.9 SC: Andrew Goldstein says, "It seems like the ultimate conundrum when we humans try to study our own psyche. How can we arrive at objective conclusions when we are both the subject and the observer? I've heard Richard Feynman was skeptical about the science of psychology and psychoanalysis. You know, I think that it's okay to be skeptical about psychology and psychoanalysis, but I mean in a very short question I feel like there's a lot to say here. One, there's a huge difference between psychology, which is the study of human beings and how they think and things like that, versus psychoanalysis, which is one school of therapy founded by Sigmund Freud within the psychological tradition. So those are very different things.

2:08:32.6 SC: Two, you can be skeptical about the amount of progress that has been made in psychology because human beings are hard. They're complicated. It's difficult to study them and we have a desire within academia to get surprising results and things like that. This leads to replication crises and so forth. So that doesn't mean we shouldn't do it or that it's impossible. It just means that it's hard and we should take that difficulty seriously. Third, who cares whether Richard Feynman was skeptical about it? I mean either there are reasons to be skeptical or not. I don't think that Feynman is a special expert on the reliability of psychology. So I wouldn't put a lot of weight on that thing.

2:09:12.8 SC: The final thing is you're pointing to a specific reason why you might be skeptical about the ability of psychology to make progress, namely that we are the subjects that we are studying, right? We are both the subject and the observer. And I think that that's absolutely something to take seriously and keep in mind. You know, we have intuitive feelings about how people act, how they think and so forth, because we are people who act and think and so forth. And so we never come into this particular set of experiments without any opinions about what is going on and that can get in the way. There's no question about that. But guess what? We can also just study other human beings. We can look at what other human beings do. We can make hypotheses. We can test them against data. All that is possible. So I don't think there's any fundamental barrier sort of epistemologically to making sense in psychology. There's just a practical difficulty that human beings are kind of complicated.

2:10:11.6 SC: Astro Nobel says, "Is there any particular reason why we call neutrinos matter and antineutrinos anti-matter or could they have been named the other way around just as well?" There actually is a reason. If you think about the decay of the neutron, okay, beta decay as it's called because it gives off a beta ray and back in the early days and now we know it's just an electron. So a neutron is slightly heavier than a proton and will decay after a few minutes if it's left alone into a proton, an electron so the charges cancel out, but also an antineutrino. Those three particles, you need all of them. The experiments have verified that picture very well. Why an antineutrino? Well if you think about lepton number, okay, the number of leptons in the universe. Number of leptons in the universe in every experiment that we've ever done is conserved. It's a constant. We have speculative ideas about how maybe it's not a constant in nature, dealing with the matter anti-matter asymmetry and so forth, but we've never done an experiment where lepton number was not conserved.

2:11:18.0 SC: But that doesn't mean the total number of leptons plus anti-leptons. It means the total number of leptons minus the total number of anti-leptons is conserved. So when you have a neutron, how many leptons do you have? Zero, because leptons are electrons and muons and neutrinos and so forth. The neutron is a baryon, not a lepton. So you have zero leptons lying around. If you said that the neutron decayed into a proton and electron and a neutrino, a real neutrino, not an antineutrino, then you would have that lepton zero initial state go into a lepton number two final state because you have an electron and a neutrino. So you violated lepton number.

2:12:02.7 SC: Whereas, if you say that I am saying it's an antineutrino with lepton number minus one that is created during neutron decay then the number of leptons minus the number of antileptons is zero both before and after and you can keep that conservation law intact. Now there's reasons why there's that conservation law and there's interactions in [2:12:25.5] ____ model etcetera. You can go through all that. Furthermore you could just choose to use intentionally dumb notation, right? To call what we call the antineutrino the neutrino and then just nevertheless assign it a different lepton number but this way does make a lot of sense. There really are differences between neutrinos and antineutrinos in how they interact with other kinds particles.

2:12:51.7 SC: Casey Mahone says, "The eternalist view of time makes perfect sense to me on an intellectual level but I find that it betrays my intuitive sense of the world in such a deep way that I struggle to really believe it. This is also true of many things in physics such as the branching of the wave function. Have you gotten to the point where you can effortlessly see these concepts playing out in the world around you and make them feel real or is it simply enough for them to fit together on an intellectual and conceptual level?" I think that they mostly feel real to me. I'm not going to say that I have a complete intuitive grasp of various high-level concepts in physics and certainly not in mathematics. But I think that what we call our intuition changes. We can train it over time.

2:13:34.8 SC: So the eternalist view of time, for example, I have no trouble at all thinking about it. It just makes perfect sense to me. I honestly do have trouble not thinking of time in an eternalist perspective. I have even trouble articulating what a non-eternalist is trying to say. So I think there's a journey you come through. You start with some folk, naive, manifest image kind of picture of the world. You learn new things. You struggle to internalize them and at the end of the day, you think about them very differently so that you actually have trouble remembering what it was that you originally thought about things.

2:14:10.6 SC: Kyle Stevens says, "The Sixers have again failed to make it out of the second round of the playoffs." I don't know why all you Patreon supporters are just riling me up about the Sixers like this, but I get that we're all in it together, so that's okay. Anyway, Kyle continues on, "Subsequently, Doc Rivers has been fired and the team seems to be due for a shakeup. The coaches for the Bucks and the Suns, two other playoff teams with championship aspirations who fell short this year, have also been fired. Coaches in today's age seem to have a very short leash. Do you think that this is the right approach to take in the quest for a championship?"

2:14:45.4 SC: Well, I think that's a very hard question actually. I'm glad that I don't have Daryl Morey's job as president of basketball operations for the Philadelphia 76ers. Trying to judge the track record, the empirical success of a coach in team sports is just a really hard thing to do because guess what the players matter more than the coach overall. And also crazy things matter like random luck and you know various situations that you can find yourself in and not really be in control over. So there are coaches who have had good records and coaches who had bad records even though they are good coaches and bad coaches respectively. Nevertheless it's your job as the general manager or the president to try to find the best coach for your team.

2:15:34.9 SC: I think that there are absolutely situations where a coach can be objectively good and yet have run their course for the team. At some point the players start tuning out the coach almost no matter how good they are or maybe they're just not a good fit. In the case of the Sixers, Darryl Morey who is the boss was hired after Doc Rivers was hired as the coach. So Morey did not hire Doc Rivers. He has now managed to hire his own choice for the coach. And Darrell was always very, very supportive of Doc Rivers. I would say that Doc Rivers gets a bad rap in Philly. I think he was a good coach, but I don't think he was great. I think the specific shortcomings he had showed up year after year.

2:16:22.9 SC: So I think it made perfect sense to let him go as long as the owners want to keep paying him because he had a contract, so they have to keep paying him even though he's no longer employed. As far as the other teams, the Bucks and the Suns, I really don't know. I was very surprised that Milwaukee Bucks fired their coach. I thought they had had a lot of success, but people get impatient and likewise for the Suns. But maybe those people have inside scoop. Maybe the owners or the general manager of those teams knows something about the locker room or the practices or the strategies that were used on the court that I don't know about, right?

2:17:03.3 SC: So I simultaneously think that the implication of your question is probably true that there are owners and management structures that are too impatient and are not willing enough to let coaches work through some occasional rough patches. And I think that it makes sense sometimes to fire your coach, even if you've been doing pretty well. So again, that's why I'm glad I don't have that job.

2:17:32.0 SC: Peter Blankenheim says, "A few weeks ago your guest Thomas Hertog said, the cosmic inflation created a universe in a fraction... A big universe rather, in a fraction of a second. I've read elsewhere that at the end of the inflationary epoch, what is now our observable universe was about the size of a watermelon. If inflationary theory is correct, what is your best estimate for the size of our universe at the end of it?" I think the important here is you really have to take seriously the distinction between the size of the universe and the size of what is now our observable universe, right? Because the universe, we have no idea how big it is. It could be infinitely big for all we know. It could be some finite size. So the relevant thing is to not care about the size of the universe, but we can care about the relative size of the universe at different moments in time.

2:18:18.7 SC: The important thing for inflation is it expands by a factor of something like e, the number e to the power 60 or more. It could expand by much much more than that but we don't know. I've written papers about it. There are different people who have different thoughts about it and so forth. As far as our observable universe on the other hand, yeah when it was at very very high energy scales, if you go backwards, so it's nothing to do with inflation. You just take the post inflationary universe and you can ask how, how big was it when it was a certain temperature, the observable universe today.

2:18:52.6 SC: I think I remember, I don't know, I haven't done this lately, but I think that our observable universe, if you push it all the way back to the Planck scale, was something like a centimeter across. Of course, if you have inflation, the thing about inflation is we don't know that much about it, including when it ends and what the temperature was. But it might not be that much smaller than the Planck energy. Therefore, it might be a little bit bigger than a centimeter. So watermelon size, that's completely reasonable. But honestly, we don't know for sure because we don't know when inflation ended, even if inflation did happen.

2:19:29.2 SC: Jeff B says, "I'm very fascinated by the idea of aphantasia, the inability to visualize things in one's mind. I believe that my own ability to visualize is probably quite weak, but I'm able to do it. I tend to think more in words and impressions than in images. There is a test that is often given for visualization ability. You're asked to visualize an object in your mind, such as an apple, then you'll be asked questions about the apple you visualized, such as its color or size. If you have a strong ability to visualize, then all of these details should be obvious to you without you needing to fill them in all after the fact. Do you know where your ability to visualize may fall on this scale and how important do you find it when doing work in physics?"

2:20:10.6 SC: Yeah, I took one of these tests a while ago, years ago, and I think that my ability to visualize is pretty high, although not super-duper high. My artistic skills are nearly nonexistent, so I can't visualize well enough to then reproduce anything with my hands. But I do have, if I think about an image of an apple that is more than just a cartoon image or something like that. So it's pretty good, not super-duper great. How important is it to doing physics? I think not much, but I'm not sure because of course in physics we often deal with things that can't be visualized at all and it's more important to understand the equations that generalize from simple cases, you know, the two-dimensional plane or whatever where you can visualize things to 10-dimensional spacetime or 10 to the 122-dimensional Hilbert space where you simply can't. You just have to understand how to deal with the equations.

2:21:03.1 SC: On the other hand, there's absolutely cases in physics where visualization is helpful. I think that intuition is very, very helpful, but I think that different people have different kinds of intuition. Some kinds of intuition are going to be quite visual. Some are going to be quite symbolic. You know, I had a friend in grad school who thought of tensors as multilinear maps, which is the technical definition of tensors, but even just starting out, he had a much easier time thinking of a formula with little slots in it for various arguments rather than a picture. That was his style of thinking of things and whatever works for you is what I'm in favor of.

2:21:41.6 SC: Jim Murphy says, "A few months ago I asked about whether you have any starstruck students or if other professors treat you differently due to being a public personality. You essentially said that you may view me as famous but it largely goes unnoticed in my day-to-day life, which I think is true. Despite this, if I were to meet you I would definitely be a little bit starstruck and nervous to say hello. Since you're the only famous person I have a chance to communicate with regularly, I wanted to ask how does does it feel to have this starstruck power over other people and how would you prefer a fan to approach you in public if at all?"

2:22:12.6 SC: I don't think it's really any power over people at all to be perfectly honest, not trying to be falsely modest there. Some people are fans, I get that, that's great, I love it, I appreciate it. And if someone sees me and recognizes me and wants to just politely say hi and they like the podcast or the books or whatever, that's wonderful. I'm all in favor of it. If I'm in the middle of a conversation or having dinner or whatever, then don't stick around for too long. You know, let me get back on with my life. But I don't think... I just think it's ordinary politeness going on here. I don't think there's anything very special about it. So I wouldn't, I wouldn't sweat it too much. Just be an ordinary, considerate, polite person and you're fine.

2:22:48.2 SC: Tim Allman says, "I've been reading Leonard Susskind's book, The Cosmic Landscape, which contains much discussion about the anthropic principle. He defines it as the principle that requires that the laws of nature be consistent with the existence of intelligent life. The statement seems to me to be devoid of predictive or explanatory power simply because the existence of a friendly universe does not imply that intelligent life must develop. I seem to be missing something. Would you please comment on the anthropic principle and why some people take it seriously?"

2:23:15.7 SC: Well, I have to confess, I think that Lenny's book, Leonard Susskind, former Mindscape guest, his book, The Cosmic Landscape, I was not a fan of that book, honestly. I think that he kind of dashed it off. It had that feeling about it. Whereas his follow-up book to that, which was The Black Hole Wars, I think is one of the best books ever written about science, about physics at a high level for a broad audience. So he just wasn't inspired when he was writing The Cosmic Landscape. And I do think that these issues of the anthropic principle are precisely places where philosophy is useful and important. And not every physicist takes it as important. And I think that therefore you get some sloppy statements about what it says.

2:23:55.4 SC: There's different versions of the anthropic principle. I think the most straightforward one is that it's a selection effect. If you have a situation where there are many different environments in the universe, then intelligent life will only arise in those regions where the conditions allow for the existence of intelligent life. That sounds tautological, but it's the conditional that is doing all the work here. If you have a universe with many different kinds of conditions from place to place. You know, by the way we do in some sense. Here in the solar system conditions are very different on the surface of the earth versus the interior of the Sun versus interplanetary space, right. Guess what? Life has arisen in those regions here in the solar system which are hospitable to the existence of life.

2:24:42.7 SC: Now what does that get you? Why is that a useful thing to say? Well it means that you shouldn't necessarily generalize from just what you see to the rest of the whole picture. Back in the day before people knew that planets were just like the Earth and the Sun was a star and things like that, you might very well have thought that most of the universe was hospitable to life, right? Most of the universe was kind of like the earth that we live on. Now we know better. That was a mistake to have thought that because of course you're subject to the selection effect, that you are seeing the part of the universe that allows you to exist.

2:25:23.5 SC: If the universe is just single. If... Going now to the cosmic universe, so we have the multiverse with possibly different regions with different laws of physics and things like that. There's a comparison you wanna do between two different kinds of scenarios. One is that multiverse scenario and one is just a single universe where things like the cosmological constant and other parameters of particle physics do allow for the existence of life. And the point is it affects scientific practice in a very clearcut way. If you knew there were a multiverse where the cosmological constant took on different values from place to place, certainly you would not spend your time trying to think of a theory that predicted the value of the cosmological constant, uniquely once and for all, because it wouldn't be right 'cause you know, the cosmological constant is different in different places, but we don't know whether that's our universe or not.

2:26:19.0 SC: So there is still some motivation for looking for theories where you predict the cosmological constant once and for all because maybe there is only the cosmological constant we observe and we're trying to explain why. But we keep in mind that maybe that's not the explanation. Maybe the explanation is just it takes on very different values from place to place. And the anthropic principle tells us we live in the part of the universe where it is small and allows for life to develop.

2:26:40.8 SC: Dennis Briggs says, "I have your, you are here t-shirt. It shows the low complexity with both low and high entropy. I cannot explain to curious observers that what may appear to be a contradiction. You've said before is because of gravity or lack thereof. Could you expand yet again on that?" So I don't think it's because of gravity actually. The thing that is because of gravity is the statement that in our actual universe, the early universe looks both thermal, right? Like a black body at a constant temperature and very smooth and yet is low entropy. That to explain those facts, to reconcile those facts, you need gravity.

2:27:15.7 SC: But the t-shirt you're talking about shows two graphs. One where entropy is just increasing with time. And the other is complexity is first increasing and then decreasing. And that's gonna be true whether or not gravity is involved. And by the way, it's not necessarily true. There are absolutely possible evolutions of the universe where complexity never evolves. Okay? Sometimes with the right conditions, the right laws of physics, right dynamics, complexity can evolve, but it doesn't have to. So the general form of the curve where it goes up and goes down is going to be true, but maybe it goes up almost invisibly and, or maybe it goes up by a lot. Now why is complexity low on the far left and the far right?

2:28:01.1 SC: Well think about what it means to be very low entropy. Think about a box of gas, right? Low entropy means that the gas is confined to some really tiny region. It's not taking up all the phase space that it could take up. And that means there just aren't a lot of states. Low entropy means there aren't a lot of states that you can be in. So there's not a lot of room for things to be complex. There's not a lot of specification of what's going on. Whereas at high entropy, then everything is in equilibrium, right? Everything is as smooth as it could possibly be when you're at high entropy. And that is also not very complex. In order for complexity to exist, you need enough room to wiggle around, enough room for the things to arrange themselves in interesting ways without being completely smoothed out into high entropy equilibrium. So without knowing, as a matter of fact, how complexity does evolve in any particular system, we can say with pretty much confidence that in ultra low entropy situations it will be simple. And in ultra high entropy situations, things will also be simple.

2:29:01.4 SC: Andrew Jaffe says, "I enjoyed the Andy Clark episode. It got me thinking if the brain utilizes a predictive algorithm to create our reality and presumably improve our survival, is it not also creating the sense of time passing. Psychedelics appear to break the brain's ability to utilize past experience to seamlessly produce a smooth changing landscape, while also altering one's sense of time? Does time then not exist outside of the mind?" So I was with you right there until the end, Andrew [laughter] The... You have to distinguish between time and our experience of time, right? I mean, those are very... They're related, but they're obviously very different things.

2:29:39.2 SC: When you're asleep, you're not experiencing time that much at all if you're not dreaming or whatever, but time is still passing. After you're dead time will still be passing, but you will not be experiencing it. So time passes, time is real, time exists outside of the mind, but under what circumstances and exactly how do we appreciate it passing? Do we experience it passing? Do we feel the passage of time? And I do think that it's a subtle question. I mean, ultimately the answer is because entropy is increasing, but that's always the beginning of the answer. That's not the end of the answer. Entropy is increasing, and the particular way in which it is increasing leads to certain phenomena, whether it's mixing of cream into coffee or having a memory of the past or predicting the future. You know, the way I like to think about the passage of time is, at any one moment of time, your brain carries with it images of the immediate past and the immediate future.

2:30:34.7 SC: The immediate past being its most recent memory, what it has just saw or heard out there in the universe. And the immediate future, it's predicting what's going to happen next. And it is constantly updating both of these as more experiences come in as you live in the world and breathe, etcetera. You're constantly making new predictions for what happens next. Constantly updating your picture of what the universe was just like. And it's that constant sense of updating that gives us the sense of time flowing.

2:31:02.4 SC: Lewis B says, "I have been really moved by your series of arguments on the physics of consciousness, that makes the point, the proponents of nonphysical theories of consciousness really have to actually choose between their non-physical components, either having no effect or interacting in a way that violates in profound ways, the laws of physics in our brain. What I haven't heard is if you've heard any responses to this, are there any popular replies, any that you find interesting?"

2:31:29.9 SC: Short answer is no, I have not had any responses that I find interesting. Obviously this kind of argument that I make, it's not unique to me. Other people have made it. The thing that is slightly unusual or different about the way that I make it is really emphasizing the laws of physics side of things. You know, I want to not talk too much about the details of consciousness. I want to put the burden of proof where it lies. If you think you need to violate the laws of physics or change them or alter them or whatever, tell me exactly how, and tell me exactly how that's supposed to be compatible with these enormous amount of evidence that we have. And people are very... Some people are very eager to open their minds to changing the laws of physics without doing the work, doing the homework of being very precise about how those changes should happen.

2:32:14.6 SC: And I don't even mean write down the theory of everything that includes consciousness, I mean at all. Tell me how you are literally changing the laws of physics rather than just saying, oh, I have a thing, a new picture of new kind of stuff. Tell me how that new kind of stuff interacts. Or even not new stuff, but new properties or new origins or new natures. Tell me how it changes the ordinary behavior predicted by physics and people don't do that. So I'm gonna keep talking about it. Philip Goff and I are having a little debate in September, hopefully it will be recorded and put online and maybe that will spread the word a little bit more. You know, it's one thing to have ideas and to put them out there, it's another thing to get people to listen to them, right? I'm not a consciousness expert. So I'm not the first person people turn to in that regime, but my paper on consciousness and the laws of physics was published, people have read it, so hopefully it'll have some impact.

2:33:10.2 SC: Stephen Lord says, "From Andy, my 7-year-old who listens to your podcast with wrapped attention." Hi Andy, how's it going? The quote is, "What if the multiverse contains groups of universes that share the same laws of physics, but whose evolution differs? What possible differences could there be between universes within the same group?" Yeah, that is absolutely a hundred percent possible. In fact, it's easier, right? Maybe different regions of spacetime that we are tempted to label as different universes have different local laws of physics. Maybe they don't, that's not something that we really know for sure.

2:33:50.4 SC: But it's very easy to imagine different regions of the universe with the same laws of physics, but slightly different initial conditions. Especially because we think the leading candidate for the origin of the fluctuations and the perturbations in the early universe that led to galaxies and the stars and all the large scale structure in the universe eventually evolving was a fundamentally quantum fluctuation driven process. So when you look at the pattern of galaxies spread across the sky, as seen in some galaxy survey or something like that, the idea is that that's a particular observation of a quantum fluctuating initial condition. And if you believe many-worlds version of quantum mechanics, that means that there are many, many other branches of the wave function where the galaxies are in different places. So I think that the physics being the same means that the evolution, if from roughly similar initial conditions will be roughly the same.

2:34:47.6 SC: You know, the number of galaxies, the type of galaxies, the type of stars will all be statistically the same. But the specific examples of which stars and galaxies there are could of course be very different. Not to mention things like the evolution of life could be very, very different because that maybe depends on some details we don't really know yet.

2:35:05.4 SC: Lewis McCartan asks a priority question saying, "In special relativity, there's no objective notion of now there's no fact of the matter about distant simultaneity. If this is true, how do we think about distant observers? If my buddy is on Alpha Centauri, how do I think about their conscious experience? Does the relativity of simultaneity change anything about this? Does it impact how we should think of each other? Or do I just regard them as conscious, like normal, they move through their world line, through their perspective and I do the same?"

2:35:40.4 SC: Yeah, the latter. It's completely normal. Like living on Alpha Centauri, you wouldn't... If there was a nice earth-like planet around Alpha Centauri, you wouldn't feel anything different just 'cause you're four light years away from the earth, right? From their perspective, everything is completely normal. The only new thing in special relativity where like you say there is no objective notion of now is there is no objective notion of now. So there is a set of experiences experienced by your friend on the planet around Alpha Centauri. It's just that none of them, none of those moments in their lifespan correspond to something you can identify as what's going on now. So their life is completely normal, but you have to think of the four dimensional span of their life.

2:36:18.5 SC: This is why eternalism... This is one of the reasons, the big reason why eternalism, the idea that all moments in time are created equal, are equally real, both past, present and future, became much more popular after relativity came along. Because you can't even define what you mean by this present that is supposed to be special and existent. There's no such thing as the present moment stretching throughout the universe. Individual people at every moment call that moment now but they're gonna keep calling it now. A year from now, they'll still say yes, this is now. So you just have to get used to not attributing nowness to people who are very far away.

2:36:56.0 SC: Paul Hess says, "My son and his friend in advanced 10th grade chemistry class posed this question to their teacher who was stumped. Can you help? In quantum physics when a photon with a precise energy required to transition an electron from the n=1 shell to the n=2 shell interacts with a neon atom where the n=2 shell is already fully occupied, what would be the expected outcome? Considering the principles of quantum mechanics, particularly the Pauli's Exclusion Principle, would the electron and the n=1 shell still be able to absorb this photon and attempt to transition to the n=2 shell. If not, what alternative interactions might occur?"

2:37:35.1 SC: So just to sort of rephrase what the question is going on here you have the Pauli's Exclusion Principle, which says that there's only one quantum... One particle, one fermionic particle like an electron allowed per quantum state. And so you have shells around atoms and maybe all those shells are filled up with electrons, so there's no more electrons you can put there. And you could have a situation where you could have had an electron transition from one shell to the other 'cause it was hit by a photon and it bounced upward. But if the new shell that it wants to go to is full, what happens?

2:38:11.9 SC: The answer is it doesn't go there. It doesn't matter that a photon passes by, in quantum mechanics, what you're gonna do is you are gonna calculate the probability that that photon interacts with the electron in such a way as to send it to some new state. And if all the new states that it might want to go to are full, then you calculate that probability and it turns out to be zero. And that's fine. You know, there are such a thing as photons passing by atoms not interacting with them at all. That's what would happen in this case.

2:38:39.0 SC: Qubit says, "Let's assume that I have the wave function of the universe and I want to divide it up into different [2:38:45.0] ____ each of which corresponds to a different world and cannot interact with any other [2:38:49.7] ____." I think we would ordinarily say components or branches, but I get what you're talking about. "I believe there are many possible ways to do that. How do I know the correct way which actually corresponds to the real worlds that I could potentially experience? Is it also possible to derive this correct way of splitting from the Schrödinger equation or do we have to add it from the outside as an additional postulate?" Well, you can't... You're right that once you have a vector, I mean forget about Hilbert space and complicated things, think about a vector. Okay? I can put a vector in a vector space, just a two-dimensional vector. I can have x and y coordinates and I can talk about the x component of that vector and the y component to that vector. But I can also rotate my coordinate system and have x' and y' coordinates. And with the original vector, I can now talk about the x' component and the y' component.

2:39:42.9 SC: And when I have many, many, many, many more dimensions, there's correspondingly a lot more ways to do that. So the math itself, just in the Hilbert space description or the Schrödinger equation, does not tell you how to divide the wave function up into worlds. And this is called the preferred basis problem in quantum mechanics, but we kind of know how to solve it. And the answer is something called pointer states and decoherence. So you know that decoherence happens when some quantum system becomes entangled with its broader environment. And the question is why? I mean, think of it in terms of Schrödinger's cat. Okay? Schrödinger's cat is in a super position of awake and asleep. You open the box, you always see it either awake or asleep.

2:40:23.7 SC: And as far as the quantum mechanical description of the cat goes, there's absolutely a possible basis for the cat's wave function, which is 1 over the square root of 2 cat awake plus cat asleep. That's one basis vector. And the other basis vector is 1 over square root of 2 cat awake minus cat asleep. Okay? Those are two good basis vectors just as awake and asleep individually are good basis vectors.

2:40:48.4 SC: But when we open the box, we never see the cat in a state of 1 over square root of 2 awake plus asleep or minus asleep. We see either awake or asleep all by itself. Why? Because those are the pointer states. Those are the states that are robust under being monitored by the environment. Think about it this way, if the cat was in a super position of two different macroscopic positions, immediately it would become more entangled with the environment because the photons in the box would hit the cat differently depending on whether it was in one position or another. It's only when you're in the pointer state, which is sort of a macroscopically coherent ensemble, like a cat that is awake or a cat that is asleep. Then the photons in the box interact with the cat, they get absorbed by it or they reflect off it or what have you, but they don't entangle with it, as we already talked about earlier in the AMA because they don't interact with it differently depending on different parts of its wave function.

2:41:45.8 SC: So there is a whole discourse Wojciech Zurek is the guy who is... His name is generally associated with this, understanding why we see certain pointer states in the world and we don't. It's not from the Schrödinger equation, it's from this additional requirement that we sort of form a coherent classical thing that does not keep entangling with the rest of the world.

2:42:09.4 SC: Mark Schoen says, "Do you see chatGPT and other AI tools as being useful in your work, perhaps either for research or for teaching?" Yeah, I think they will absolutely be useful. I mean, for research it's very easy to sort of use it as a search engine, right? You just have to be super duper careful because it will lie to you, but you should always be super duper careful because anyone can lie to you. Wikipedia can lie to you, Google can lie to you, etcetera. But if I say like, "I just have forgotten what is the notation that people usually use for this constant," right? I can ask chatGPT and it will tell me, and then I will either go, "Oh yeah, I knew that." Or I could double check it by looking somewhere else, right? So that's very useful and a very straightforward way. Also, you can use it for programming, right? You can use GPT tools to help you write computer programs I found that very useful. For teaching, I haven't been teaching since the revolution happened, so I'll be doing that in the fall for the first time. And I think the right strategy is just to lean in, like not to pretend that you can force your students not to use it, right?

2:43:18.9 SC: Students are going to use these tools to write papers. So I've seen various ideas about what to do. For example, you can assign an assignment rather than saying write a paper on this topic. You can say, let chatGPT write a paper on this topic and then critique it. And the critique is what you're graded on because chatGPT is not very insightful. It really... It is very much like a student sort of just core dumping a whole bunch of sentences that sound good in the hopes that one of them is the magic one that the professor will give a good grade to, right? So hopefully you can train your students to do better than the current generation of AIs are able to do.

2:43:52.6 SC: Max says, "Do you have any advice for meeting people from other academic disciplines at Caltech or just in general? I'll be starting my physics PhD there this fall, but I'm coming from a large state school where I was able to make friends who studied philosophy, art history, etcetera. I wanna try my best to stay well-rounded. Any tips?" I don't think there are any specific tips about Caltech or anything like that. I think that it's almost all in your attitude, right? Like the very fact that you want to do it is the most important thing. You can go to seminars in other departments and you know, if you see people over and over again, you can strike up conversations, you can go to classes taught by people... Can sit in on a philosophy class or whatever. I don't know where people live in the... In grad school at Caltech. I mean, typically what I did when I was a grad student was lived in the dorms in the first year and then moved into an apartment after that and I met people in the dorms and I made friends with them for a very long time.

2:44:54.8 SC: So I think it's just up to you. Grad school is hard. Grad school is a lot of work. You certainly will hang out with people who are in your department because they're gonna be the ones who are helping you and and commiserating with you along the way. But I think that your life will be richer if you go outside a little bit. I mean, Caltech as a particular place is less amenable to that than most places. Caltech is about getting your work done, not about stretching yourself outside of your discipline, but if you wanna do it, you absolutely can.

2:45:24.0 SC: Nelita S says, "Do you personally believe that we are both a physical system and a spirit or just a physical system? If the first, then how do those two systems communicate the way they do? If the second, then how does a physical system alone give rise to a spirit?" Good. You know, this is the kind of thing that I have thought about and I have a very strong opinion that it's the latter. We are a physical system, and any temptation you might want to talk about us in terms of a soul or a spirit or anything like that, is just a slightly loosey goosey way of talking about emergent properties of that physical system.

2:45:57.4 SC: I won't go in the details because I did write a book where I went into great details. That's The Big Picture, so not The Biggest Ideas, that's a different book. But in The Big Picture, I talk exactly about this issue and I bring up the example of Princess Elisabeth of Bohemia, who was a brilliant thinker back in the day of Descartes. And she corresponded with Descartes and gave him a hard time about exactly the idea that he had, that we are both a physical system and a mental system, both as... Both a body and a mind separately. And Elisabeth pointed out how difficult that is in practice to really pull it off. It goes back to the question we were just answering a little bit ago, how in the world could non-physical things interact with physical things? Maybe they could, but tell me exactly how they're gonna do it. And no one is actually able to tell me that.

2:46:47.3 SC: Chris Gunter says, "Can you help me understand a question on black holes and time reversibility. For a normal irreversible event, like a glass shattering on the ground, the bound up energies dissipate into sound and motion and heat. We could imagine precisely that same form of energy conspiring to arrive at just the right moment to reinvigorate the glass and put it back on the table. Not impossible, just discountable unlikely. Can such a thing be true for a book falling into a black hole? Can some conspiracy minded forces arrive at just the right time to let us see the books spontaneously arise from the event horizon? This gets described as impossible in popular literature. Not merely unlikely. Do we need Hawking radiation to make this happen?"

2:47:26.4 SC: Well, you could have Hawking radiation spit out a book. It's super duper unlikely, but you know, there's a probability that's gonna happen and you could calculate the probability, but that wouldn't be what you're thinking of. The issue is that in a black hole, as you make the black hole, there is already a time orientation to the event horizon. The event... The outside of the black hole is in the past of the event horizon and the future of the black hole is in the future. So it's not like a glass shattering on the ground. There really is a boundary, a barrier that separates the past from the future in the black hole.

2:48:02.1 SC: What you can do is imagine time reversing the whole shebang, and then you get a white hole. And a white hole is also an event horizon where the interior is in the past and the exterior is in the future, and it would spit things out. What... Would those things being spit out be very, very low entropy, etcetera? That is harder to say because in the real world there aren't any white holes out there. But you know, the Big Bang is kind of like that. The Big Bang is a low entropy singularity in the past. So there's some commonalities there. But I think that the short answer to what you're asking is that the very idea of a black hole is by itself irreversible.

2:48:44.8 SC: Nick B says, "How differently would an intelligent alien perceive the universe and what effect would that have on their physics? I really enjoyed your conversation with Thomas Hertog, especially the section where he talked about moving from a god's eye view to a worm's eye view. The idea that any perspective of cosmology depends on who's looking, got me thinking about an alien's eye view in your session with Arik Kershenbaum. How different could an alien physics or cosmological theory be given that they are looking at the same universe but perceiving it very differently, eyes that see a very different frequency of light or so forth?"

2:49:18.9 SC: Well, I don't know is the short answer because I'm not an alien. I've never met any to talk to them. I do have a suspicion. I mean, of course aliens could be very, very different than we are. They could have very different senses, not just Arik Kershenbaum, but also the conversation with Ed Yong is about how the sensorium of different species right here on earth are actually very, very different from one to the other.

2:49:49.5 SC: But at the same time, those sensoria are putting us in touch with the same external reality, right? I am perceiving a desk right in front of me on which my computer and my microphone are on. No sensible alien would deny the existence of this desk just because they have different sensory apparatuses. So my strong suspicion is that the central pieces of physics as developed by aliens are going to be the same as the central pieces of physics developed by us. Could be wrong about that, like I said. But I do think that it's... It's important to recognize that the ways aliens work could be very different, but we still live in the same universe with the same underlying physical things happening.

2:50:28.0 SC: Maxine says, "I need help reconciling something based on my naive understanding of quantum mechanics. Specifically, what intuitively prevents the uncertainty principle from allowing a particle to travel faster than light. My understanding is that if we observe the position of a particle more and more precisely, the velocity becomes more and more uncertain. If we measure the position approaching exactly in principle, what prevents the particle from traveling outside of the light cone in the next instant?"

2:50:54.5 SC: Well it's... There are details here. Okay. I hate to say it, but sometimes the description of things like the uncertainty principle in words don't quite measure up to what you need here. I mean, it's not actually position and velocity that are limited by the uncertainty principle, it's position and momentum, okay? And you can get momentum to be bigger and bigger and bigger without ever going faster than the speed of light. In relativity, momentum approaches infinity as you approach the speed of light. So there's no reason just to say, I'm gonna measure the position so accurately that I have to include velocities greater than light.

2:51:37.0 SC: All you have to do is include momenta that are very, very big in your uncertainty, and you can do that. So there's no problem there with violating the speed of light. But the other thing to do, just because I have the opportunity to do it, is to say, as I like to say, and I will go on about at some length in volume two of The Biggest Ideas in the Universe. The uncertainty principle should not be interpreted as saying that if you observe the position of a particle, the velocity becomes more and more uncertain. Ultimately, the uncertainty principle is not about observations. And ultimately there's no such thing as the position or the velocity or the momentum of the particle. Those are possible observational outcomes. Those are not things that exist when you're not observing it.

2:52:21.1 SC: The better way to think about the uncertainty principle is there are wave functions, there are quantum states, and they have predicted spreads or uncertainties or standard deviations in the possible measurement outcomes for momentum and for position. And there is no wave function for which the possible measurement outcomes are simultaneously zero in both position and momentum at the same time. That's what... That's one way of thinking about the uncertainty principle. There are other ways of thinking about it, but that's the basic idea.

2:52:55.1 SC: The Great Deceiver asks a priority question saying, "Given your excellent communication skills, I'm super curious to know if you are familiar with or have taken any courses in general semantics. General semantics is defined as concerned with how events translate to perceptions, how they're further modified by the names and labels we apply to them, and how we might gain a measure of control over our cognitive, emotional and behavioral responses. Frankly, I believe that the limits of language lie at the heart of many problems in physics and philosophy."

2:53:24.5 SC: So I've heard of general semantics, but no, I've not taken any courses, read any books in it or anything like that. I do think that both physicists and philosophers have their own techniques for trying to get things right as far as definitions and meanings and semantics is generally concerned. I'm very happy to imagine that they fail, that they don't do a good job in particular specific circumstances. But I don't wanna tar them with the brush of failing in some general way. I think you need to be careful about what exact questions you're talking about there.

2:53:58.5 SC: Darren Rogers says, "How would we ever know if we arrived at a brute fact? What is the difference between a brute fact and something that we can't yet figure out an explanation for?" Well, a brute fact is one where there is no explanation. That's the difference. The important question you asked is the first one, how would we ever know if we arrived at a brute fact? And the answer is, we wouldn't. That's okay. You know, we never know lots of things. We never know for certain if our theories of physics are correct, right? We do the best we can. We try to understand the universe. We say, "All right, here's my best current idea, law of gravity," or whatever it is.

2:54:33.9 SC: Likewise, I say, "My current best understanding is that the following thing is a brute fact, but maybe it's not. Maybe we can keep looking." good. You can keep looking. The only thing to keep in mind is it might be a brute fact. I can't promise you that this or this... This or that fact about the universe is brute, has no further deeper explanation. But that doesn't mean that I can't believe that it could be. That I must be driven to find some deeper explanation. I had to keep in mind the possibility that no such deeper explanation exists.

2:55:05.8 SC: Okay, the last question for today comes from David Dubrow, who says, "A previous AMA question about living a thousand years made me think about the possible limits of our episodic memory. If our sense of identity is dependent on our episodic memories, and we lived many hundreds of years, would we lose so much of our episodic memories from when we were younger that we would not be anything close to the same person we were in previous centuries?" Well, the honest answer here is, I don't know, because again, just like I've never met aliens, I have also not lived for hundreds of years or thousands of years or whatever. It does seem very plausible to me. In fact, I would be shocked if someone thought that you could live for thousands of years and not be a very different person.

2:55:51.3 SC: I mean, I'm a pretty different person than I was when I was 10 years old, which is, believe me, not hundreds of years ago. It's a while ago. So I would like to think that it's very natural if you live for hundreds of years further, you would turn into a different person. Part of it would be that your memories would be different. Of course, if we actually get technology to extend our lifespans that long, we might also have technology to improve our memories, right? And maybe that's not just improving our brains. Maybe that's in, as Andy Clark talked about with the extended mind hypothesis. Maybe that's offloading some memories to our hard drive or something like that, right? Very plausible in the far future where we're imagining living for thousands of years.

2:56:33.6 SC: So I think it's gonna be interesting because we have so much intuition, so much feeling for what it means to be human, to be a person, to be a self that has been trained over all of human history. And now we are on the cusp of changing those things in dramatic interesting ways, right? Through technology and brain computer interfaces like we talked with Nita Farahany about. But also possibly longevity, gene editing, things like that. All sorts of biological advances that we haven't even yet foreseen. So it's gonna be very, very interesting how people want to hold on to what we used to be and how we adapt to what we are going to become. It's very fun to think about, very hard to get it right. I think that the goal is less to try to figure out what actually things will be like and to try to rather scope out the space of possibilities so that we are aware of whatever might happen next, and maybe we plan for it just a little bit. That's all we got for this month. Thanks for tuning in. Thanks as always for your support of the Mindscape Podcast. Bye-bye.

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