AMA | August 2023

0:00:00.0 Sean Carroll: Hello everyone. Welcome to the August, 2023. Ask me anything edition to the Mindscape podcast. I'm your host, Sean Carroll. This is the week after I released my solo episode on the Crisis in Physics. I just wanted to say that I'm quite gratified at the reception that it has gotten. There are, if you ask what I mean by the reception, there are various places where people can talk about the podcast episodes. Of course, Patreon supporters who are the one asking the questions for these, ask me anything episodes can do so on Patreon, we have a somewhat active group there. There was a Sean Carroll subreddit, S-E-A-N C-A-R-R-O-L-L, where people could talk about podcast episodes, but also other things that I do and so forth. There's also comments that come up on Twitter and YouTube and what have you.

0:00:50.7 SC: And people seem to like the solo episode and they took it in the spirit I think that it is offered, which is, it's not primarily polemical, right? It's not primarily there, it wasn't primarily there to persuade you to have a certain point of view. I have a point of view, and I'm trying to explain to you what my point of view is, but also it's mostly to help you understand why physicists do the things they do. You're allowed to disagree with how physicists go about their business and what problems they choose to tackle and how they choose to do so. But you should do so. Everyone should do so from a situation, from a standpoint of understanding why they're doing the things they do. And that was really the primary point of the episode and why it was so long 'cause I got to sort of expound on the whole history of 20th century and now 21st century physics.

0:01:39.3 SC: There are a couple of people, of course, who just snarked about it, but if you look carefully at their snark, it was also completely clear they didn't listen to the episode. So the lesson I'm taking from this is that all of my episodes should be four hours long, and then the people who wanna snark about them will clearly be not very relevant to the actual discussion. Probably not gonna happen. Four hours is a lot of time. I don't know if it's clear that it was not a single sitting that I did that episode in, but sometimes you have a lot to say and you gotta keep talking about it. I also wanted to express appreciation because the recent episode with Katie Elliott was also very well received, and you never know, it's a many episodes are slightly different from each other.

0:02:22.4 SC: Katie was kind enough to come on and rather than just talk about her specific work, help us give a general chat about metaphysics and what it is and what it's for. And people really liked it, which I thought was great. And so kudos to Katie for that. And also she was very good at asking me questions and having it really be a conversation that's hard to do for many guests depending on what their thing is that they're doing for a living. Why would they have any questions for me? But hopefully it came through why I think that the physics, science, I should say slash philosophy boundary is such an incredibly fertile area to think about things. Philosophy does have something to offer, and physics absolutely has some questions that they have some answers to that will help the philosophers but they have some questions that they can use philosophical help with.

0:03:13.6 SC: I should also say that I've been on a couple of other podcasts lately. Most notably a couple of episodes of Robinson's podcast. Robinson Erhardt does a very nice podcast where he gets a lot of philosophers and other thinkers on. So, recently I was on with David Albert talking about quantum mechanics and Boltzmann brain and things like that. I was also on episode of his podcast with Slavoj ŽIžek the famous, in certain circles, extremely well-known psychoanalytic philosopher Slovenian, and quite a character also. And Slavoj apparently is thinking very hard about quantum mechanics these days. So poor Robinson got to say very little in the podcast 'cause it was basically, Slavoj was the host and he was giving his opinion and then asking me questions. So you can look that one up on the internet sits on YouTube and elsewhere.

0:04:05.2 SC: Otherwise remember these Ask me anything Episodes are sponsored by Patreon supporters of the Mindscape podcast. And you too could be a Patreon supporter. Go to patreon.com/seanmcarroll and pledge your $1 per episode or whatever you want per episode. No one has yet pledged a $1,000 per episode, but I don't discourage it. You're allowed to. I hope that there's no one out there who's really been wanting to pledge a $1,000 per episode and thought that I would be insulted. I would not be pledge whatever you want or pledge nothing at all. We love all Mindscape listeners equally, but we appreciate the support from Patreons. So with that, let's go.

[music]

0:05:00.5 SC: As I'm doing this recording, there's a few hot topics out there in the sciencey Physicsy discourse going on. So naturally I get some questions about them. And so I'm gonna start by grouping together some questions on these various topics and sort of giving you a collective answer. I don't have anything very deep to say about any of them, but I'll explain why I don't have deep things to say about them. So the first one is two questions I'm gonna group together. One is from Anonymous who says, can you weigh in on the plausibility of the room temperature, atmospheric pressure, superconductor papers released to archive last week. I realize it's paper reading, but I would be surprised if you hadn't brought it up before. If you pe... If you just have judgment without reading, it'd be interesting as well. And so, Pendu Harsh says, are you satisfied with the evidence for room temperature superconductor LK-99? Or do you think extraordinary claims require extraordinary proof?

0:05:55.6 SC: So for those of you who have a very different social media feed than I do, there have been a couple papers. There were a couple papers, I don't know, a couple weeks ago now, maybe from a group in Korea, it's a little South Korea. There's a little bit of confusion about why there are two papers and they don't have exactly the same author lists. And there's one person who's on one paper but not the other. And one of the papers has six authors, and one of the papers has three authors. And some people are saying, well, the three author paper was trying to get their names out there so that they could be the three Nobel Prize winners. If this is true, I have no idea. So I've not actually read the papers. No, I've glanced at the papers, but there's another level of reading if you want to really understand things at a physics level, and I have not done that.

0:06:39.4 SC: But the claim is that they have a room temperature, atmospheric pressure, superconductor, for those of you who are not super familiar here, superconductors are just materials that have no electrical resistance. You can send a current down them without losing any of that current or without even really putting any effort into it at all. It would be a very big deal technologically, if you had easily makeable room temperature, atmospheric pressure, superconductors, you have to say atmospheric pressure there because you can create higher temperature superconductors by putting enormous amount of pressure on them. But that kind of removes the whole benefit of just having something that you can put into your computer, no problem. Right? We have high temperature superconductors, which are very, very big deal in physics, but high temperature just means high compared to absolute zero kelvin.

0:07:29.7 SC: These are still quite low temperature compared to room temperature. And the more this new claim is that you actually have room temperature superconductors, which are things that people have hoped for and looked for for quite a while. So I will say that I'm a little skeptical. I am not on the bandwagon, but bandwagon saying that this is probably going to be right. If I had to bet at even money, I would say it's going to go away. There's various pieces of evidence that are given here. One of them is that you can sort of float a little piece of material above the superconductor, and that is evidence that the purported superconductor is expelling magnetic field lines, the Meissner effect. And this is evidence for superconductivity. But there's also, as it turns out, other ways to get that kind of behavior that don't necessarily rely on ordinary superconductivity.

0:08:23.2 SC: And the other thing about these papers, and all I know is from reading people who are actually experts. I'm a physicist, but I'm not a condensed matter physicist, not a superconducting expert. As I've said before, though, I'm an expert on sort of knowing how reasonable physics claims sound up initial or a priori or whatever you want to call it. So it's just a little shaky, the plot in the paper as I understand it, that purports to establish superconductivity number one, doesn't look like superconductivity in the sense that when you have a superconducting transition, it's a phase transition and you plot the electrical resistance as a function of temperature. And it tends to have a discontinuity near the superconducting phase transition. And in these papers, it doesn't, it sort of gradually goes down to the lower level. And of course, you can't measure a number that is exactly zero.

0:09:21.8 SC: So you have some measurement uncertainty when you're measuring the superconductivity or the conductivity or resistivity, which is one over the conductivity. And in, as I understand it, again, from what other people are saying, and reproducing plots and things like that, there's basically two different samples that the original papers looked at. One is a relatively thick slab of the stuff, and it doesn't even conduct as well as copper does [laughter] It doesn't, it's definitely no evidence for superconducting in this thing. The other sample is a thinner film and it conducts better 'cause maybe 'cause it's thinner, right? So it doesn't smell real to me. I am not averse to their being room temperature superconductors, there's no law of physics. It says that can't be true. There's nothing special about room temperature, right? It's just what, how, where we happen to live.

0:10:12.1 SC: So I'm very open-minded about the possibility of room temperatures, superconductors being discovered. And maybe there's something interesting going on with this particular object, LK-99, this particular substance. But it is hard doing physics at a very precise high level is just hard. So it's very, very easy to trick yourself if all the bases are not covered, et cetera. And the people who are experts who have looked at this are like, well, maybe it's true, but it doesn't kind of fit together. It doesn't kind of seem like what you would expect. So I would say that it's absolutely possible. I certainly, lemme put it this way, hope that it is, it would be awesome if it is. So I'm kind of rooting for it. But there's a whole nother aspect to talk about, which is the fact that it's actually not that hard to synthesize this material LK-99, following the instructions in the papers.

0:11:10.5 SC: So this is not like searching for supersymmetry at the Large Hadron Collider, where you need a $10 billion particle accelerator. If you have various lab equipment pieces already in place, you can just do it yourself. And what this has led to is a whole bunch of people, some of them are at established labs, others are, it's a little unclear where they are trying in real time to reproduce these results and see whether or not they can check it out. I have strongly mixed feelings about this whole thing. On the one hand, I feel very positively about the fact that if you can do it, if you have the technology to participate in the science search, then great. The more people the better. That is wonderful. On the other hand, it does, the fact that so much of it is being done in real time in public might be fun for the spectators, but it does not lead to the best science.

0:12:07.0 SC: Science has as its product something that should be widely shared and understood by everybody. But there is a moment in the doing of science when you need to be careful and quiet and by yourself, and really concentrate on getting it right and having people following your every move and cheering you on while you're doing it, that is not conducive to the most careful scientific practice. I think that in the modern world, it's so easy to put things out there in public, right? To put various work that you're doing or ideas that you have. And overall, that is a good thing, I think. But I do notice that, when I talk about a workshop I was at, or a talk I went to or I'm planning for the future, people will always ask, is it online? Is it being streamed so we can see it? And sometimes the answer's yes, and sometimes the answer's no it's perfectly legitimate question.

0:13:07.1 SC: Is it going to be online? But then sometimes when the answer is no, I will get responses like, well, that's not how science should be done. You should put everything online. And I just don't think that's true. I don't think you should put everything online. There has to be a place for people just to talk without the whole world looking over their shoulder. And that's just for theorists who are relatively low stakes in this game, right? For experimentalists who are trying to do very, very careful testing and measuring of quantities, and there's expectations, and it would be world shattering if this were true. Well, not shattering, but changing it would have an effect on the world if you had a room temperature superconductor that was easily synthesized.

0:13:48.9 SC: So the pressure is on, and that's not the kind of situation that under which we do our best science, let's put it that way. So I have very mixed feelings about the public nature of people trying to reproduce this. And also just the impatience, like the day to day, oh, this person did this, this person did that. That's also not really conducive to good scientific practice. It's kind of like if you are very good at playing the piano, you can have a concert and people can come and listen to your concert, and that's great. But you have to practice [laughter] There's some parts of your mastery of playing a musical instrument that are private by construction. And likewise for science, there should be a public aspect and a private aspect. And I worry that this kind of thing is a little bit too messy and out there, and it's getting people too excited too quickly.

0:14:45.9 SC: Like sometimes science has to be careful and slow, and I'm not sure whether this is going to be the best example of that. Okay. The next set of questions is about Oppenheimer. So what did you think of AJ says, so what did you think of Oppenheimer, the movie Carlos Nunez says, what is your opinion of the new movie about a Robert Oppenheimer by Christopher Nolan? Do you think that the physics is represented accurately? And if not, what would you have done differently? I did see Oppenheimer also saw Barbie, which is the other part of the pair of movies people are very excited about these days. I thought Oppenheimer was brilliant, honestly, I am, I was pleasantly surprised at how much I liked it. I am generally a fan of Christopher Nolan, but I think that he's uneven.

0:15:33.6 SC: I felt that Memento, his very first movie was genius it's one of my favorite movies of all time. His subsequent movies I've seen good bits in. But every one of them kind of had its ups and downs. I think Inception was really, really good. Some other movies were not as good and so Oppenheimer just blew me away. It was a three hour long biopic of J. Robert Oppenheimer. And I think that biopics are just intrinsically very difficult to pull off because human beings, life stories are not structured like a three act Hollywood movie, right? Or not structured dramatically more generally there are ups and downs and there's no culmination, et cetera. And so Nolan, who wrote the script based on a book called American Prometheus, which was a biography of Oppenheimer, but he pulled out all the stops in terms of storytelling. He had two parallel tracks going, one of which was sort of up to, from Oppenheimer's Young Days as a physics graduate student through the Los Alamos Manhattan Project.

0:16:42.8 SC: And then up to his trial, not really trial I should say, but if you know Oppenheimer's story, he was at one point the head of the Manhattan Project. And then not long after his security clearance was taken away. That's a very controversial moment in the history of American politics and its relationship to science because Oppenheimer would hang out with known communists, his brother, and I think his wife also were pretty much admitted communists. And Oppenheimer was a complicated guy who, on the one hand, he built the atomic bomb. He was in charge of that project and he was very proud of that project, and he was very much in favor of it. He wasn't ambiguous about that. But then he also kind of felt bad about the fact that, oh, it killed hundreds of thousands of people and he was partly responsible for that.

0:17:32.3 SC: And you can argue back and forth whether or not, well, he shouldn't have felt bad because he helped end World War II, or he should have felt way more than bad 'cause he killed a lot of people, right? And other people will say, well, he didn't kill a lot of people, the government. So there's a lot to be said about that. It's a complicated thing. So that's one story. And then Nolan inserts another story, which he sort of tells in parallel. It's a more minor story, but it's about Louis Strauss, who was a trustee of the Institute for Advanced Studies, who helped hire Oppenheimer as the head of the IAS, but then also had his own political issues. He was nominated ultimately for Secretary of Commerce, I think. And Congress refused to approve him for that, in part because of his betrayal of Oppenheimer in a very real sense.

0:18:25.6 SC: And so, I about the physics in the movie, it was fine. Like, I don't look at movies like this as physics lectures myself. I'm not looking for the physics to be a 100% accurate. I'm not even looking for the history to be a 100% accurate in the following sense. I don't want them to dramatically misrepresent history. I don't want them to show things in a movie based on historical events that are really just completely opposite to the sense of what actually happened. But people complaining that this or that figure who was really playing a role at Los Alamos but didn't appear in the movie, in whatever, or Oppenheimer was shown talking to Einstein when he's really talking to somebody else, I don't care about that, make a good movie. And I thought that Oppenheimer was a good movie, and it was very much as far as I could tell, honest and careful about the spirit of what was happening.

0:19:21.3 SC: It was not really dramatically misrepresenting people or events. It streamlines things to tell a story and I think that's great. So I encourage people to see it, it is a long movie. I'm getting, as I get older, it's harder for me to watch three hour movies and keep riveted the whole time. But this did a pretty darn good job. I was very impressed. And I didn't think that it would be that cinematic, honestly. So I was very happy to see it come out that well. There's also Barbie, and I love Barbie too. Barbie is a whole nother movie, it was very hilarious that it came out at the same time and people got excited about it. Like a couple months ago, someone on Twitter said, "Yeah, these movies are coming out the same day, and I can't imagine there's a very big group that's gonna wanna see them both."

0:20:06.5 SC: And I instantly replied, like, okay, I guess I'm in a very unusual group 'cause I definitely wanna see them both. As it turns out, lots of people wanted to see them both. And I loved Barbie too. It was a great movie. I think Oppenheimer was better. It was a higher level achievement. It's really just a memorable movie for, in terms of my whole history of watching movies as far as this year is concerned, I think it's Barbie, Oppenheimer and Into the Spider Verse, which were my favorite movies this year. And Barbie was just unapologetically feminist and talks about the patriarchy and everything, but it does so in such a lighthearted loving way. And there's no easy resolution to the problem that Barbie is a toy and it starts in Barbie land and the toys don't really have the kind of agency that you would want to have if you were a functioning human being.

0:21:02.6 SC: And that's a structural problem when you wanna make movies about toys, right? I think it, toy Story has some philosophical issues with it, for example. But yeah, it was just fun and super well acted. And again a little bit long Barbie, but it was threading a needle of okay, you wanna get, you wanna support Barbie in her desire to have a more fulfilling life, but you don't wanna do that at the expense of anyone else. And Ryan Gosling, who's playing, Ken does an amazing job as he starts the movie as basically the only character in Barbie land who is not satisfied, right? Who's not happy living in Barbie land. And that's telling, because Barbie land is a matriarchy and Matriarchies aren't any better than patriarchies, right? He's just a secondary character, and that's a, it's completely fair for him to be a little bit dissatisfied.

0:22:01.4 SC: And how do you resolve that? It's all very tricky. I encourage you to go see the movie. It was pretty awesome. Okay, final big topic of the last couple weeks. I'm gonna group together several questions on it. Robert Rux Andresque says, what do you make of the recent meeting in Congress on the topic of UAPs/UFOs? For those of you who don't know, UFOs, of course, unidentified flying objects, but UAPs are the rebranding effort to try to make them seem more respectable, unidentified aerial phenomena. Okay, Stevie CPW says, in a cocktail infused debate with friends over the recent congressional inquiries about UFOs, I proclaimed that the more reports of extra terrestrial events there are, the less likely they were to be true. My argument was based on the incredible odds in space and time against even one visit from an extra terrestrial, let alone hundreds, is my logic flawed?

0:22:54.8 SC: Also, what is your opinion on this topic? And do you think that it could be a subject of a congressional hearing should be a subject of a congressional hearing? And then Ollie Wright says, I am a natural skeptic, but the recent congressional hearing on UAPs and in particular the UAP amendment to the National Defense Authorization Act contained stunning claims. My question is, how would you suggest approaching this situation? What is a rational path that avoids both conspiracy theory woo but also remaining open-minded in a scientific sense? The great deceiver finally says, first off, did you watch any of the hearings into UFOs even for fun? And if yes, what are your thoughts? So I'll give you my general thoughts, and I feel bad about giving you my general thoughts because they do not line up with some of the people asking the questions, sorry about that.

0:23:38.6 SC: But I think it's all complete nonsense. Those are my general thoughts, [laughter], I do not think that there's any evidence to be taken seriously presented in those hearings that should lead you to believe that aliens are visiting the earth in their little spaceships and crashing and being captured by the government and kept from us in secret. Okay? Again, it's possible. And again I kind of hope that I'm wrong, like that would be intriguing, but weird, but kind of awesome. But I would put that as way lower probability than the superconducting stuff. I suppose in the superconducting question, I should have been more clear in the answer to Sabendu's question, do you think extraordinary claims require extraordinary proof? Yeah, exactly. Absolutely. I think that, or, but the, it's not that extraordinary claims in the sense of it would be really important if this is true, require extraordinary proof, extraordinary in the sense that your prior is really low, right?

0:24:40.3 SC: As a good Bayesian, you start with priors and all these propositions, and in order to be persuaded that there is substantial credence in them, you need the data to come in, more information to come in, that you judge the likelihood of that data to be really, really big if this extraordinary claim is true and really, really small, if it's not true, okay? That's the kind of extraordinary proof that you need. And for the superconducting stuff, the proof is not really that extraordinary right now, but who knows, maybe it will firm up, for the UFO stuff, [laughter] the proof, the evidence is just laughable. It is true that this dude, I forget what his name is went and made claims to a congressional committee that were quite astonishing claims, let's put it that way. It's also true that he has all of the vibes of a complete crackpot.

0:25:26.3 SC: I mean, at one point he was talking about the holographic principle and extra dimensions and stuff like that. If that didn't set up off your alarm bells, I'm not sure what would, so the two things to say are that, first my prior on the idea that there are aliens buzzing us in UFOs and crashing and being captured by the government is extremely, extremely low. That whole scenario makes no sense for many reasons. I've talked about it many times before. The whole idea that aliens are so sufficiently technologically advanced that they can easily fly across interstellar distances and yet can't and try to avoid detection, right? Like, they're not just landing on the lawn of the White House and saying, take me to your leader, right? They're trying to be cagey, but they're not very good at it. So we get these fuzzy pictures of them, right?

0:26:21.7 SC: And then you probably have seen these maps of sightings of UFOs worldwide, when they take a map of the world and they put little dots where there are sightings of UFOs and they're basically all in the United States. That would be weird, right? Why would the aliens so much care about the United States? That doesn't make a lot of sense. Maybe there's something else going on. So I think that it's just not the way that it would happen if there were really technologically advanced aliens out there that they would buzz us in their spacecraft and then occasionally crash and be captured by the government. That's just as very, very low prior probability on my chart. And then you ask, okay, well what is the evidence here? Well, what is the evidence? It's a guy making claims. That's the evidence, right?

0:27:07.1 SC: To change my prior about that, to update my credences. Show me a spaceship. Show me an alien, let us go touch it and do science on it and stuff like that. Then I'll be convinced this is nowhere close. This is just some dude who likes attention, is a bit of a conspiracy theorist. Again, as I always say, I could be wrong about this, but if I have to bet, if I have to think about what I'm gonna do with my life, I'm not putting a lot of credence on these particular claims. And by the way, these claims happen all the time. It's like every six months or every year there's some new idea that, okay, this time we're gonna get the evidence, we're gonna find out and you'll be sorry, you being all of us skeptics.

0:27:51.7 SC: And I'm like, okay, call me up after it happens [laughter] and I'll look forward to being sorry. And by the way, I will look forward to being sorry. I will absolutely very, very quickly repent and say not. And I will not claim if we actually find out that the government does have alien crash relics, okay? I will not pretend that I was supportive of that idea all along, I will be completely wrong if that turns out to be true, and I'll be very, very quick to admit that I was wrong. Let's just get that in the record right now. Meanwhile though, let's move on to other AMA questions. Michael Lesniak says, basketball games. Okay, now we're talking basketball games tend to drag out with all the timeouts. Curious what your take would be on handling them the way curling does the team calling timeout can talk to coaches and strategize, but the other team cannot unless they also burn a timeout or do you think that the game is fine just the way it is?

0:28:50.6 SC: I think that there's two things going on here. One is that the [laughter] level of physical exertion involved in basketball and curling are not really comparable. Okay? Basketball's really tiring, even though you're only the clock is only running for 48 minutes, which is not that long. You're running very fast up and down the court that whole time, and you're being smashed into by other players and there's a high probability of injury. The number of injuries in the NBA in recent years is just astonishingly high and very frustrating to me. I don't remember it being like that back in my day, and I'm not quite sure why they seem to be more prevalent these days. But basketball players the timeouts are not just to strategize, they're to catch your breath.

0:29:34.8 SC: That's kind of an important thing. So I think that you shouldn't change the general feeling of timeouts and what they are. Having said that, there's just an obvious problem with NBA games and how they end, right? When the games are close, there are various ways for coaches to drag out the ending of the game to give themselves some small chance of maybe pulling out a miracle win. You foul the other player, you send them to the free throw line, and then you get the ball back and they can only shoot twice if you foul them on a two shot foul. And then you can try to make a three pointer. So maybe you can climb back into the game. There have been suggestions for ways to do it better. There's something called the Elam ending, or Elam, I don't know how to pronounce it.

0:30:20.3 SC: But there's also just ways of punishing the team more harshly if they foul in the last two minutes of a game, right? So I would be entirely in favor of fixing the rules so that the very last couple minutes of an NBA game don't drag out that long. But the general strategy, the general tenor and tone of timeouts in the NBA is fine with me. We all need a commercial break for various reasons, right? Nick C says, what would you say is the best or most convincing explanation of the Copenhagen interpretation that you've come across? And what is the interpretation of quantum mechanics that you find most compelling besides Copenhagen in many worlds? So let me be clear. I've tried to be clear before, the Copenhagen interpretation is not an interpretation. It's not a theory, it's not at all respectable.

0:31:06.7 SC: It can't be right [laughter], there's just, it's just not on the table as a competitor, okay? It is what most physicists go along with, but that's just 'cause they're not trying to understand it very hard. And the reason why is not because the Copenhagen interpretation is wrong, it's just because it's incomplete, it's fuzzy, it's vague, it's not a theory, it doesn't tell you what happens. You say, I measure a quantum system and it's wave function collapses, but it doesn't say what a measurement is. It doesn't say when things collapse. It doesn't say how closely you have to measure any of those things. So it's just not one of the things that are taken seriously when it comes to formulations of quantum mechanics.

0:31:48.5 SC: Now, there is a sort of post Copenhagen School of Epistemic approaches to quantum mechanics, which is to say, we don't treat the wave function as representing reality, we treat it as a way to make predictions. And the fundamental real things are the agents, the observers making those predictions that's more respectable than Copenhagen. It's sort of a gussied up, more respectable looking version of Copenhagen 'cause Copenhagen also tries to make some of those moves, but I still don't think they make any sense in their current state because you haven't told me what an agent is like, what does an agent that has experiences, how do I model that mathematically? That's just not what I do in physics or in science more generally. I need to be a little bit more specific, a little bit more rigorous, et cetera. So I don't think the Copenhagen is in the running, but the epistemic approaches are, even though I think they have very, very deep problems themselves in terms of other interpretations, probably something like Bohmian mechanics is the next one after many worlds.

0:32:51.3 SC: But honestly, it's so far behind the, in my personal ranking that I don't spend any time thinking about the alternatives to be honest. Bobs and Elli asks a priority question. So remember that AMA questioners, that is to say Patreon supporters have in their pocket as soon as they join Patreon, one opportunity in their life to ask a priority question. And a priority question is one that I will do my best to answer the idea that you can only do it once in your life is not very harshly enforced. [laughter] If you want to sneak in another one, it's hard for me to notice 'cause I'm not really keeping track. So I'm relying on the honor of the questioners to only ask one question in their lives. But we're at the point now where I cannot answer every question and I feel bad 'cause some of these priority questions, people clearly very much wanted to ask the question.

0:33:49.1 SC: Maybe they did ask it before and I didn't answer it. And usually when I don't answer a question, it's because I don't have anything interesting to say and then they ask it as a priority question. And it still remains true that I don't have anything interesting to say [laughter] So I feel sorry about that. Sorry. Anyway, Bob's question is, assuming an extremal black holes is possible in very general terms, how would its gravity field be described? Yeah, not that much differently than a regular black hole, to be perfectly honest. There are, the idea of an extremal black holes has to do with the fact that black holes are characterized by their mass charge and spin, right? And in some sense, not a very exact sense, but in some sense, the effect of charge and spin counteracts the effect of mass on the gravitational field of the black hole.

0:34:38.6 SC: So it's easiest if we think about electrically charged black holes. Okay? So spin is a whole another thing because then you're violating spherical symmetry and you have to be careful and you can be, but let's think about electrical charge. Okay? So the thing that happens when you have an extreme old electrically charged black hole is imagine that you have two electrically charged black holes and they're sitting at some distance away from each other, right? So they have a gravitational pull because they're black holes. So they have gravitational pull, but they also have electrostatic repulsion. If they're both positively charged for an extremal black holes, those two forces exactly cancel. The two black holes can just sit there at a constant distance from each other because they're pulling each other gravitationally, pushing each other apart electromagnetically, and therefore they don't move. But if you're an uncharged particle, if you are not yourself electrically charged, then you will just fall into the black hole.

0:35:39.2 SC: It's just a black hole as far as you are concerned. You're not being pushed away from it by any force if you're just not electrically charged yourself. There's a whole very, very interesting discourse about extremal black holes because the interior space, time structure of extremal black holes is quite different. I once wrote a paper about this with Matt Johnson and Lisa Randall because there's a sort of discontinuity in the internal space time structure as you go from a regular black hole to an extremal one. Basically what you're trying to do is imagine taking a regular black hole and then throwing electrical charges into it to charge it up, right? But the thing is, when you're throwing electrical charges into the black hole, those electrical charges are massive particles. There's no massless electrical charges. So you're adding both mass and charge to the black hole.

0:36:28.4 SC: And as far as we know, you can't make an extremal black hole because you are always adding more mass than charge in some sense in which you can generally compare them. So if you were able to go super extremal, if you were able to imagine a black hole that in some units, once again had more charge than mass, then you could have a naked singularity out there in the universe. And so that's a super extremal black hole. So extremal black hole is kind of on the boundary. It's an unstable thing. It's right, barely at the point where there might be a naked singularity, but there isn't really. So it's thought that in nature you're not gonna get any extremal black holes for that reason. But they're interesting things for theorists to think about. In particular, if you're doing super gravity or super string theory, there are configurations of super symmetric extremal black holes where you can really talk about some of their properties analytically very nicely.

0:37:29.4 SC: So they're very fun things for a theorist to think about. They're not really out there in nature as far as we know. And if you were just standing next to one, you'd fall into it and then you could not come out 'cause it's a black hole. Leon B says, loved the conversation with Katie Elliot in the last half an hour or so of your discussion, it became clear to me that she was fishing for something she found unclear in your philosophy and was probing to uncover exactly what it was after her parting shot. That's the saddest thing I've ever heard. It occurred to me that maybe what's missing is humanity. You have a very well-defined philosophy of particles, but doesn't philosophy have to say something about people? Well, yeah, sure. Philosophy has something to say about people.

0:38:08.7 SC: If you read my book, the Big Picture, I talk about people an awful lot. It's a big part of things. I think it's gotta be compatible with your philosophy of particles because people are made of particles at some simple level, right? And I actually don't think that that I agree with your diagnosis that there is something that Katie found unclear or unsatisfying in my perspective, but I don't think it's that people were not there. I think it's more metaphysical, ontological and it's a very common difference, like on the some spectrum of philosophical views. There's various different questions you can ask yourself philosophically, right? So there's a particular question you can ask yourself about what is the world made of? How many things, how many kinds of things are there out there in the world? How rich is your ontology versus how sparse and barren is your ontology? There's one point of view which says like, there's lots of different kinds of things in the world that have some reality and they all interact together in certain ways to make up the world we know.

0:39:10.1 SC: There's another point of view that says the world is basically one kind of thing. And that for our own personal convenience, we subdivide it into pieces and talk about those pieces in that particular continuum. I am far on one side where I think that the world is just one simple kind of thing, and all the subdivision into pieces, et cetera, is apparent and effective and useful to us at a higher level. It is emergent, it is not fundamental. And I think that that is what I think we'd have to ask Katie, but I think that that is the difference that she was trying to pinpoint because I'm really in favor of removing almost everything from the ontology and then showing how the stuff that we see in the world, whether it's causes and effects or tables and chairs or free will or choice or anything like that emerges at some higher level.

0:40:02.8 SC: And I like to say it doesn't mean it's not real, it's still real tables and chairs are real, so is free will, so are causes and purposes, but they're approximate higher level things that we explain in terms of some deeper level unified thing. I think that's what she was aiming at. But we'd already gone on long enough so maybe she'll write a paper about it and then refute all of my opinions. Liam McCarty says, I saw you on Robinson's podcast with David Albert. David disagreed with you about the many world's interpretation saying it wasn't clear how probability could apply to branching of the wave function since each branching event happens only once. You said he needed to get to let go of the frequent, his view of probability. After all, we speak of the probability of a candidate winning an election even though the election will only happen once.

0:40:54.3 SC: But I've never understood how such a probability say of a candidate winning an election makes any sense. Sure, it expresses a degree of confidence in the election result, but there's no way to ever know if that confidence was appropriate since the election happens only once. Can you explain how a non frequentist view of probability makes sense? Yes, I can, or at least I can try. And then you can decide for yourself whether it works or not. I think that your attitude or your perspective on this, Liam, is a common one. I think that a lot of people come across the idea of probability. It gets explained in terms of a frequency, what do you mean when you say the coin is 50% chance of being heads, 50% chance of being tails. What you mean is if you did it more and more times it would converge on 50-50, right?

0:41:41.8 SC: And that is the objective frequentist view of probability. The subjectivist says something that is very, very different than that. And you can agree with it or disagree with it. It's going to completely match the coin example and the simple examples. But it is fundamentally a different kind of thing. And so what I would just say is that you can't just secretly or not secretly assume that what is meant by probability is an objective frequency. That is one possible thing. It could be, but other people mean something else by it. What they mean is a degree of confidence that something is going to happen. When I have a coin, and I'm gonna flip it, I don't know anything more than to say that it's 50-50 heads and tails, okay? And I can't flip it an infinite number of times. So the Subjectivist says, look, all this talk about infinite number of times is just something you made up to make yourself feel better.

0:42:42.9 SC: In the real world, you're gonna flip the coin, it's going to be either heads or tails. And the statement I give it a 50% chance of being heads and a 50% chance of being tails is nothing more than a statement of your ignorance as to which the actual future is going to be. It fits well with the sort of Eternalist perspective on time as Katie and I talked about a little bit. If you think there is actually some fact about the future and you just don't yet know it, that would be the relevant feature of the world. In the case of the flipping coin, and a subjectivist says, what I mean by saying is a 50-50 chance is that I will give equal confidence, equal credence to either one of those two things happening. And I think that the thing to pinpoint is where you say where is it?

0:43:33.1 SC: Oh yeah, this probability expresses a degree of confidence in the election result, but there's no way to ever know if that confidence was appropriate. Aha. So basically you're showing us that you have a preexisting view of probability where it must be objective. That's what appropriate is doing here. Like what do you mean appropriate? You mean the right probability, but a subjectivist about probability doesn't believe that there's no such thing as the appropriate confidence, right? There's the best we can do. That's all there is.

0:44:07.7 SC: There is a fact that in some way the probability of the coin being heads is either one or zero, but we don't know which one [laughter], that's the only appropriate thing to do. So we subjectively say it's 50,50 and the subjectivist is going to say, that's no different than saying who's gonna win the election? Who's gonna win the NBA championship? Which branch of the wave function I'm going to end up on? There's a bunch of things that happen and I, this is why I like the, which branch of the wave function I'm going to end up on, right? Because that's clearly a misstated idea 'cause I'm not gonna end up on one branch of the wave function or another. There will be a version of me that ends up on one and a version of me that ends up on a...

0:44:52.0 SC: The other. So, there's no objective probability, there's no frequentist probability that you can assign to that. All you can do is say, once I'm in that state that I measured a particles it's either spin up or spin down, I don't know which one it is yet. What is the probability? What is the credence that I put on being one branch or the other, that's an intrinsically subjective thing? And I can make arguments based on rational pushing around of those credences that says, I'd better use the Born rule to do it. But this is a fundamental psychological barrier, people who think that probabilities must be objective are not gonna be happy with the many worlds interpretation of quantum mechanics.

0:45:32.2 SC: All I'm saying is there's a whole bunch of things that they're not gonna be happy with, in which we use probabilities all the time. Like go to the internet, you can find the odds about who's gonna win the Super Bowl, that's a statement of probability, even though it's only gonna happen once. Paul Theroux says... Oh, another one about the same podcast. "When you and David Albert went on Robinson's podcast, David mentioned self-locating uncertainty as a guide to quantum probabilities. He gave a thought experiment involving Captain Kirk using a transporter that splits him into two people, one wearing green and one blue. And before Kirk can see his clothing, he attaches some probability to green."

0:46:09.8 SC: You then talked about bending odds and wound up supposing that the two resulting Kirk's share one bank account. Do they have to share some reward like a bank account? Isn't the point rather that the one Kirk anticipating having transported and the two after must all share the same probabilities since the brain processes that lead to their credences are identical, and the only probability that makes sense in that context is 50-50."

0:46:36.7 SC: Well, I think yeah, you stated the issue clearly and correctly, but I think that there's two different issues going on here, okay? One is the one that you're getting at is, what could the probability possibly be other than 50-50? And there's no sensible thing that it could be other than 50-50. There's no rational decision, procedure or way of thinking that says, in this experiment by the way. So, the experiment is Captain Kirk is put in the transporter machine, but it's a malfunctioning transporter machine that makes two copies of you. So, it's just the duplication machine. And in one copy, Kirk is wearing a blue shirt, the other one, some green shirt, and so what should the probabilities be? Like you say, it's exactly identical, it should be 50-50, but there's no possible way that you would ever get to 90-10 and really defend. Oh yes, it's definitely 90% green. That makes no sense.

0:47:32.3 SC: But that's not David Albert's objection. His objection is not that it should be 90-10 rather than 50-50, his objection is that you shouldn't assign probabilities at all. You should just remain silent, you should follow Professor Vitkeinstein's advice. This is something about which we cannot speak, so we should just remain silent about it, okay? I don't agree with that at all. I think that that's not a good way to go through life. If you are going to wake up after this experiment and either have the green shirt or the blue shirt or whatever, it is part of your attempt to cope with the world around you, to assign credences to the different possibilities.

0:48:12.7 SC: I can say if I climb out the window and jump, I just don't know whether I will fall up or float in the air. Who knows that's asking me a question about how the fundamental laws of physics work, and I'm not sure, so I can't possibly be expected to assign a credence. No one believes that, by the way, no one actually thinks that way. My point is, you should think that way if you logically are bound to think that way, if you think that you cannot assign probabilities and these duplication processes. It's in both cases a subjective probability about a case where there's objectively something's gonna happen, you don't know what it is, for sure, you are basically required as a functioning human being to have a credence in the different possibilities.

0:49:03.6 SC: You have a credence and whether or not you're gonna float in the air or fall down, if you jump out of window. That credence is not frequency, we're not gonna change the laws of physics many, many times. Laws of physics are what they are, that particular event only happens once. But you can get evidence for it by doing other experiments, it's a tricky thing. I don't wanna... I disagree with David about this, this is a central point of our disagreements, even though we agree with many, many things. But it's not a silly argument, his argument makes perfect sense. It's one that I don't agree with, but I can see why someone would feel that way because the situation is quite different than what we're used to.

0:49:48.6 SC: We're used to situations, we feel comfortable in situations where what we mean by probability is some borrowed version of a frequency. When you're flipping coins or playing dice or playing cards, then you can really imagine doing the experiment an infinite number of times, get a frequency. These kinds of things where you can't imagine doing the experiment a few number times to get a frequency, therefore bother people. My response is that you never really were doing it an infinite number of times, you were always attaching a credence to it, you got to do your best to come up with a credence, it makes logical sense to you. Anonymous says, if we look at the global economy is a complex system, can we see it functioning so that each individual person could spend continually less working time to meet his or her needs, say according to Maslow?

0:50:37.9 SC: Well, this sounds like a very complicated question about the function of the global economy, but I do think personally that... Yeah, it's pretty darn clear that we are in a situation where we generate a lot of wealth in the world, and it is not spread anything like equally or evenly. We could lift a whole bunch of people out of poverty, the question is... Sorry, by spreading around the wealth more equally, I think that's perfectly clear. The question is whether or not that is a net good thing to do? Because the argument, the counter-argument is two-fold. Number one, there is a sort of result-oriented argument that says that the existences of inequality are important for making progress, yes, not everyone benefits, but the people who do benefit benefit so much, and there's so many of them that it's still worth it. That's one attitude.

0:51:31.6 SC: The other is just a more moral-based argument that says that people have the right to keep what they earn and things like that. But anyway, I do this... You're generating answers to a different question than what you asked, so I should come back to the question you're asking. I do think that it makes no sense that we have so many people in the world who are so poor. Let's put it that way. I think we could do much better than we actually do. Could each individual person continually spend less working time to meet his or her needs? I think that just depends a lot on what you mean by the needs, right?

0:52:05.4 SC: I didn't need to have the internet 100 years ago, do I need to have the internet now? Well, I don't need it, you don't need to be listening to this podcast. What do you mean by need? As the world changes, the things that we want and the things that we might reasonably aspire to change with them. Life expectancies were a lot shorter 100 years ago, and now we might reasonably expect to get decent healthcare that carries us through ordinary medical circumstances. We don't always achieve that. I do think that we're just building computers, building robots, building manufacturing, that lets us produce a lot more stuff, so if you keep the amount of stuff everyone has constant, the amount of work everyone has to do, will go down.

0:52:52.0 SC: But almost no one wants to do that. Maybe some people do, but most people want more stuff. [laughter] And that doesn't sound... That sounds worse than it is. Some of the these stuff we want is just like healthy food and long life and health care, and the ability to talk to our friends and have fun and stuff like that. These are good things to want. Let me put it this way. I think that the world is changing very rapidly. And our cognitive set up the way that we think as human beings is trained on conditions from a thousand years ago or 5000 years ago. And I don't think that we have done a very good job in setting up the global economy in the best possible way, given our current capacities.

0:53:39.8 SC: I don't know necessarily what the best way to do it is, but I think that this is something where it's just hard to do the right thing. Because a lot of people don't know what the right thing is, they don't even agree on what it is if they do think that they know it, they can't figure out ways to work together to make it happen. Some people don't want the right thing to happen, 'cause they benefit from the wrong thing happening. It's a complicated mess. So I would like to move toward... I did a... We did a podcast with John Danaher about this. The possibility of an automated utopia where people could just stop working and enjoy life and be creative and so forth.

0:54:14.3 SC: And I don't think that's what will happen in the medium-term future, but it's something that we can at least talk about now in a way that we couldn't talk about 200 years ago. Brent Meeker says, Roy Kerr claims that Penrose's singularity Theorem for black holes doesn't actually apply to the real world. He says that there's a spinning object in the center of a black hole, he points out to the solutions that describe black holes are vacuum solutions, and so a vacuum is just assumed. They are not solutions for collapse of real bodies. Are there any solutions, even computational ones for the collapse of matter into a spinning black hole, do they treat the matter quantum mechanically?

0:54:49.7 SC: I think that Penrose's singularity theorems are the wrong thing to talk about in this context. The whole point of the singularity theorem is they don't assume spherical symmetry, they don't assume vacuum or anything like that. They're quite generic, they assume what are called Energy conditions, which means that there are certain kinds of energy and momentum that you're not allowed to consider. Basically negative energy energy. Negative mass, repulsive gravity, if you could have repulsive gravity negative masses, then you would not need to have a singularity. You could just throw a bunch of negative energy into the black hole and prevent the singularity from happening.

0:55:27.0 SC: But the whole point of the singularity theorems otherwise is that they are otherwise completely generic, they do not assume any symmetries or anything like that. Before the singularity theorems came along, you could very well have worried that singularities you predict and gravitational collapse. By the way, Robert Oppenheimer's most famous physics work was exactly on this, but anyway, those gravitational collapses that we could understand analytically, were all spherically symmetric, and maybe the singularities were just an artifact of that. The whole point in Singularity theorems is to say no, even if you look at more general situations, different things happen.

0:56:06.7 SC: Now, if you want to know exactly what does happen, that's harder, okay? So, the singularity theorems say that as long as you have certain energy conditions being obeyed, then you will get a singularity. They don't tell you how you get a singularity, they don't tell you what kind of singularity you have. The Kerr solution, Roy Kerr for those who don't know, was the guy who wrote down the first exact solution to a spinning black hole, the Kerr Metric. And outside of the spinning black hole, the Kerr Metric is very close to being exactly right. Inside it predicts singularities, but weird singularities. It also predicts like worm holes that go to other universes and things like that, things that almost no physicists think actually are there, because they're not realistic solutions describing the formation of a black hole.

0:56:52.1 SC: There are solutions describing a black hole that was always there with no matter in it, okay? When you do have spinning matter, matter with non-zeroing the momentum that collapses to a black hole, the situation is way more complicated. There are computational attempts at understanding it, but you don't know exactly. Things go crazy and it's hard to know how reliable they are. Do they treat the matter quantum mechanically? Well, what do you mean like quantum mechanically? Do you mean you have a wave function or you're doing quantum gravity, or do you just mean that the energy and pressure are compatible with more quantum mechanical constraints? I'm not sure.

0:57:32.0 SC: I'm not a super expert on this, I do know that the quest to understand exactly what happens when realistic things collapse to make black holes is an ongoing one. It's not a settled field right now, there is a sort of consensus as to what happens, which is that you collapse to a singularity. [laughter] In a real world spinning black hole. But until you know for sure, you don't know for sure. Paul Conte says, assuming that university level textbooks in general, reach a smaller number of readers, do you think it would be better to write more popular science books, which I imagine reach a larger audience/readership. Do you prefer writing popular science or textbooks for students?

0:58:15.6 SC: You're not gonna be surprised. And my answer here, which is that you should do both. I am always a believer in a wide, pluralistic ecosystem of attempts to talk about science, to teach science, to educate people about science. There is absolutely a very obvious good reason to write textbooks, which is that even if you reach a smaller readership, that readership is very important. It's the readership that will grow up to become professional physicists and do new physics. So, that's a important thing you can do for the world, I'm always very, very tickled when I see people who are now doing great things, finding gravitational waves or something like that. And they say they learned the general relativity from my text book that tells me I did something worthwhile for the world.

0:59:00.8 SC: But it's also very, very important to write popular level things like you say you reach a larger readership. That readership might be mostly interested in simply understanding things or increasing the knowledge about the universe for their personal reasons rather than for future contributions to science. But that's entirely okay, I'm very much in favor of that. I think that the same thing goes for different media, whether it's books or podcasts, or blog posts, or lectures or TV shows or movies or whatever. Do it every single possible way that you can do it, I think they're all different ways of reaching different kinds of audiences.

0:59:42.8 SC: In terms of preference, I don't know about preference, you get different rewards from writing a good textbook versus writing a good popular book. So it's a tie. Let me cop out by saying that. Kevin Harrang says, I learned a lot from your recent discussion about artificial intelligence with Raphael Milliere. Thank you, but I'm still wondering where you personally come down on the question of whether current or future iterations of AI are something to be worried about, like Einstein warned about nuclear technology, either intrinsically or just in the wrong hands?

1:00:13.3 SC: Yeah, sure. You should definitely be worried. [chuckle] The question is, what is the form that that worry takes? I thought that the open letter that said, "Stop doing research on AI for six months while we figure out what to do." Was as a practical matter, completely unrealistic for various reasons. Number one, how are you gonna get everyone in the world to agree to stop doing that? What's to stop people from actually continuing AI research? Just not tell anybody about it. Especially people in other countries or whatever.

1:00:45.4 SC: Number two, what in the world makes you think that in six months, you'll figure out what to do about it? I think it's a misunderstanding of how these things work. These things are processes that are ongoing, both the process of developing AI and the process of developing safeguards and figuring out what to do about it. The open letter did a lot of good in the sense that it sort of got people talking about the issue, which is great. But I don't think it actually, what it was asking for was the right thing to ask for, the right thing to ask for is very accelerated hard thinking and policy implementation of safeguards that would protect us from the worst aspects of AI. What those aspects could be, there's obvious, many choices, like if you turn over important technological or industrial features to control by computers rather than humans, you open up the possibility of disastrous errors.

1:01:43.9 SC: Humans using AI to spread misinformation and fake news and so forth is an obvious big problem. So, I think that it's very, very important to put safeguards and to think about what the safeguards are that we need. I didn't really like the idea of just pausing research for six months, I also don't like the idea of emphasizing the possibility of existential risk where you kill every human being on earth. That is absolutely possible, but it's sufficiently low probability that I don't think that it's the right way to think about it. I think that if you will actually increase the chances of bad things happening, if that's the kind of risk you worry about rather than the real, very, very imminent risks that we have from AI.

1:02:26.6 SC: And furthermore, if you worry about the imminent risks, you're much more likely to also ameliorate the existential risks. So, that's what I would be in favor of doing. I think I wanna group two questions together. One is from Miasfia Nagua saying, leaving aside the provocative name, does Conway's Free Will theorem prove anything stronger about hidden variable theories than Bell's? And then Kyle Hicks says, I've heard Roger Penrose state something to the effect of The superposition of the Wave Function makes the geometry of space time unstable, which causes collapse of the wave function. Even proposing at the time of collapse can be predicted by the formula T equals H Planck's constant divided by the gravitational energy.

1:03:08.7 SC: Could you explain why he would believe that space time could become unstable and your general thoughts on the Diósi-Penrose model? You might be wondering why I'm grouping these two questions together, they're both about quantum mechanics, but otherwise they're very different. It's 'cause I wanted to say something, I've said it before, but maybe I got to keep saying it. Different people are listening. I am not an expert on other theories of quantum mechanics and the foundations of quantum mechanics in general. The foundations of quantum mechanics is one of my specialties, if you look up my CV, it's one of the things I do for a living. But what I mean by that is I investigate the many world's interpretation of quantum mechanics, and in particular, what that has to say about other areas of physics. Whether it's quantum gravity or field theory or whatever.

1:03:54.0 SC: Because I personally think that many worlds is much more likely to be true than any of the other ones. As I always say, I would love to be wrong, it'd be cool, and then I would change my mind. That would be great, but yeah, only have a finite number of heartbeats. Every human being has on average three billion heart beats in their life, you have to allocate your heart beats, you have to allocate the time you have on Earth to think about these questions. If you're a scientist, if you're a researcher, there are literally an infinite number of research questions that you could devote yourself to, you have to pick the ones you think are going to be most productive.

1:04:31.8 SC: So, I actually don't spend any serious time thinking about other approaches to quantum mechanics. And this is all by way of preamble of saying that I'm not gonna give very satisfying answers to these two questions. So [1:04:45.6] ____ questions is about Conway's Free Will theorem. The one thing I can say about this is a terrible name, everyone agrees is a terrible name, has nothing to do with Free Will. If you're not... If you don't know what this is, this a theorem that was proved a couple of years ago, perfectly valid result in quantum foundations, and it has to do with the fact that it's one of these theorems, like Bell's theorem that tries to make statements that are independent of a particular picture of quantum foundations. So, rather than saying, let's work within this picture or some other picture, it says, imagine the following axioms hold.

1:05:23.2 SC: Then what can we conclude about that. So, Bell's theorem assumes that you have definite measurement outcomes, various other kinds of things. The Free Will theorem is basically that says under certain very reasonable assumptions, there will be a case where you cannot predict the spin of the electron. It is in deterministic, it is not deterministic. Okay? Under certain very reasonable sounding assumptions, no signals traveling faster than the speed of light and so forth. So, this seems very, very unsurprising, if you know about quantum mechanics. Quantum mechanics already says that you can't predict the outcome of an experiment completely deterministically, the Free Will Theorem, which is Conway and also Kochen, I think. They say that, "Well, okay, we don't need to assume quantum mechanics, we're assuming these weaker principles and you can derive the fact that physics will not be deterministic."

1:06:19.5 SC: They called it Free Will, rather than in determinism just for PR purposes. It has literally nothing to do with free will, it's just saying that certain aspects of physics are indeterministic, which if you believe in quantum mechanics already, you already knew is true. So, just like Bell's theorem, these kinds of theorems are constraints on alternatives you could maybe imagine someday developing two conventional quantum mechanics. But for me, and I'm not looking for an alternative to conventional quantum mechanics. I like conventional quantum mechanics, I'm sticking with that.

1:06:54.4 SC: Likewise for Kyle's question about Penrose's model, Penrose violates conventional quantum mechanics. He has an objective collapse model. The most popular objective collapse models in quantum foundation circles are the spontaneous collapse models, where wave functions just have a certain probability of collapsing all by themselves for no good reason. Whereas Penrose's model is in the vein of what you might call triggered or induced collapses. When a certain condition is met, the way function collapses and that condition in Penrose's version has something do with gravity. That's all I know about it. I don't know anything else about it.

1:07:31.0 SC: For all the reasons that are conventionally put forward, I don't love the violence being done to the Schrödinger equation by saying that wave functions truly collapse for occasional purposes. Nor do I think that gravity should play a role in the understanding of quantum mechanics at a deep level. And now I understand, I don't hold that latter point of view quite as strongly as some of my friends, 'cause some people are like, "Well, look, when I talk about the simple harmonic oscillator or two spins or whatever, gravity is not involved. I don't need gravity to talk about quantum mechanics. And I do think it's perfectly fair for Penrose to respond to that by saying, "What I care about is the real world, and in the real world, there is both gravity and quantum mechanics." So I'm perfectly allowed to use gravity to help me understand the mysteries of quantum mechanics.

1:08:21.7 SC: I'm sympathetic to that perspective argument that Penrose could put forward, but I don't think that as a matter of fact, gravity has anything to do with quantum mechanics at the fundamental level. Allan Bolton says, I found Joe Silks moon science argument compelling. Would you go to the moon if you had an opportunity to work there for say a six-month mission? I think this depends a lot on what my credence was that it would be safe to go to the moon. I would love to go, that will be a lot of fun. Just for a touristy mission not for a one-way trip, if I thought I was gonna come back. The question right now is that space travel is certainly not safe, there's a certain fraction of the time when you have a disaster and people die, and that fraction is uncomfortably large. I think one way to put it is, if I thought that the chance of a fatal accident was 10 to the minus four, then I would go. If I thought it was 10 to the minus one then I would not. [chuckle]

1:09:17.6 SC: Somewhere in between, it would be a difficult decision, but of course it would depend a lot, therefore, on exactly who was flying, what their track record was and so forth. Oleg Ravinski says, "I've recently read your new paper on discretizing quantum mechanics." Yeah, so by the way, I put a couple of papers on the archive lately, one was one on... They're very, completely different. One was with my old grad school advisor, George Field, on making primordial magnetic fields from Axion dynamics. And the other one was from just me on discretizing quantum mechanics. So the idea here is that we talk a lot about discrete space time, but even if space time is discrete, if you believe in quantum mechanics, the theory, the fundamental scientific theory that you're working with is not discreet because it's quantum mechanics.

1:10:04.2 SC: Quantum mechanics is not a discrete theory, the elements of the theory are vectors in Hilbert space and vector spaces like Hilbert space are smooth, they are not discrete. The space of states is in no sense discrete in quantum mechanics. And furthermore, there a smooth time evolution according to the Schrödinger equation. So, I ask myself the question, is it possible to discretize Hilbert space in a way that is naturally compatible with the Schrödinger equation? A couple of other people have already asked about whether you could discretize Hilbert space at all, and they said yes. But I specifically said, "Could you do it in a way compatible with the Schrödinger equation?" And then I said, "Yes, you can, but there's problems with doing it, it does get rid of infinity in the sense of infinite number of states, but the universe still lasts eternally even if time steps are discrete according to the evolution, and so you get recurrences.

1:11:00.8 SC: There's only a finite number of things that can happen, but they happen infinitely often, and so you get all the problems that you normally get with Boltzmann brains and things like that. So anyway, you can look it up on archive if you want. So Oleg asks, "I also recently got another paper, The Borde-Guth-Vilenkin theorem in extended de sitter spaces, which if I understand correctly, states that the universe is geodesically complete I.e., had to have a beginning. I wonder whether there's a contradiction here between these models or are they compatible? So, I'm using this, I have not read this paper that Oleg, is pointing to The Borde-Guth-Vilenkin theorem in extended de sitter spaces, but I'll use it as an excuse to talk about The Borde-Guth-Vilenkin Theorem in general.

1:11:42.6 SC: Borde-Guth and Vilenkin are top notch theoretical cosmologists, you should believe what they say. But what they do is they prove a theorem about certain kinds of classical spacetime, and all the words here matter, they make some assumptions. They say, if the following things are true, then the following things follow. And the thing that they're proving their theorem about is classical spacetime. Now, some people want to pretend that they're not proving the theorem about classical spacetime, because when you think about the difference between quantum mechanics and classical mechanics. There's two differences, there's many differences, but two are relevant here.

1:12:20.3 SC: One is that you are talking about a different kind of thing, if you have a point particle in classical mechanics that has a location and a momentum. And that's a state and you can predict it's future. If you're doing quantum mechanics, you're talking about a wave function of that particle, that's a completely different kind of thing. Like wise, if you're doing gravity, you have classical spacetime, if you do quantum gravity, you have a wave function of spacetime, a completely different kind of thing. That second ingredient is you have specific equations of motion that tell us how those things that you're studying evolve with time.

1:13:00.2 SC: Newton's equations or Hamilton's equations, or the Euler-Lagrange Equation has a separate set of equivalent ways of talking about classical equations of motion. And likewise there is the Schrödinger Equation or The Heisenberg Equation of motion or the Feynman Path Integral. These are distinct equivalent ways of talking about the quantum motion. And the Borde-Guth-Vilenkin do not assume classical equations of motion, so some people say, "Well, they're not really doing classical spacetime." But the spacetime is completely classical. They've changed their... They throwing out the equations, we're not assuming any certain equations about it, but they're still looking at classical spacetime. And what they're saying is, certain kinds of classical spacetime under certain assumptions have a singularity in the past.

1:13:44.6 S2: So, what do we learn from saying that? Can we say that the universe therefore had a beginning, no, of course not. There's no version of the Borde-Guth-Vilenkin Theorem that implies the universe had a beginning. I'm not sure how to say this more clearly, for two obvious reasons. Number one, their assumptions might be wrong. [laughter] There are plenty of examples on the market of universes that don't have a beginning. Models of universes that don't have a beginning, they're all completely compatible with the Borde-Guth-Vilenkin Theorem because they don't fit the assumptions that it makes.

1:14:18.8 SC: But the second and more conceptual important thing is, spacetime is not a classical thing. We have quantum gravity, so you cannot possibly learn anything fundamentally necessarily true about the real world by looking at the Borde-Guth-Vilenkin Theorem or any of its generalizations. What you learn when you look at a theorem, whether it's Penrose's singularity Theorems or the BGV Theorem, is that there is a singularity in this classical spacetime, and how to interpret that is to say that our understanding gives out our ability to think of spacetime as classical ends at this point. It says nothing at all about the universe ending or coming into existence, or having a beginning or anything like that. It's just about the applicability of the classical approximation.

1:15:17.5 SC: So, therefore, no, there's no incompatibility whatsoever. My paper on quantum mechanics was about quantum states that evolve eternally with time, that is completely compatible with anything that you might want to argue about classical spacetimes. The question is, does a certain model of quantum mechanics that is to say a certain Hamiltonian, et cetera, reproduce this particular behavior or not, that I don't know. But there's no in principle incompatibility there. By the way, I said the BGV Theorem is perfectly respectable scientific theorem. If you look at who cites it in the literature, it's almost all religious people. It's almost all people arguing for the beginning of the universe, 'cause they wanna prove the existence of God.

1:16:03.6 SC: Scientists know that it's an interesting theorem, but just not that central, because the universe is fundamentally quantum, not classical. Craig Hooper says, "What do people mean when they say the universe is infinite? Do you think it's infinite?" I'm with Jan 11, and we've never encountered any physical quantity that is infinite camp, so it seems unlikely to me that there is one quantity like diameter of volume that is the exception." Well, I don't know about that particular argument, we've never encountered any physical quantity that is infinite. How would you know whether you did, on the real number line, there's an infinite number of points between zero and one, but if you actually draw a line segment on a piece of paper in front of you, you can't tell whether there's some discretization, and there's really only a finite number of points or whether it's an infinite number of points.

1:16:50.8 SC: So, my attitude is, let it... I don't know. We let it be either way, we don't have any way of telling right now. There are... I don't wanna under-play the importance of the question, that's why I wrote this paper I was just talking about, because maybe you can help resolve some conceptual mathematical, philosophical puzzles if you don't need to invoke infinity when you talk about the physical universe. And in quantum mechanics, standard quantum mechanics, you do need to invoke infinity. So, I wrote a little paper saying, "Here's a version of quantum mechanics where you don't." But again, it's all toward the purpose of keeping an open mind about whether infinity really does exist or not, I just don't know.

1:17:32.2 SC: Georgio says, "The professor I work for to achieve my Masters degree has high ambitions and a broad spectrum of knowledge. He chooses his students based on potential and their motivation and he gives us complete freedom in our research. Do you also prefer this long leash approach with your students or do you prefer a sort of more hand-holdy approach?" Well, I think something in between, I'm not very hand-holdy, that is true, just because I'm stretched too thinly. I'm doing too many things, I'm working with several different students, I have a podcast that you might know about, I write books, I teach, I do various different things. So, I am not the kind of advisor who is working on the student's problem just as hard as they are. I know that there are such advisors and I admire them, but I just don't have time to do that. And I try to tell prospective students exactly that, that I'm very good at coming up with ideas.

1:18:24.8 SC: The ideas aren't always very good ones, I'm very good at coming up with them, so they'll ever be a point in our relationship where I say, "Yeah, geez, I don't know, I'm not sure what to work on. I have no good ideas." I have too many things I want to work on, and I'm very happy to share them with students, but when I do, the student is gonna be much better off if they can take some initiative. We'll talk regularly and we'll try to bounce ideas back and forth, and we'll both do calculations, but the student is gonna do much of the heavy lifting. If a student wants to do just work on their own ideas, that's great, I'm perfectly happy to do that. But most students don't wanna do that. At least they don't wanna exclusively do that, or they don't wanna do that in the beginning.

1:19:05.9 SC: When you just start out as a graduate student, one of the most useful things that an advisor can do for you is not just give you ideas to work on, but help you understand which ideas are promising, worth following up on, et cetera. It's a real art form, choosing not just to do some projects, some calculation or whatever, but also to do the right ones, do the ones that are worth doing.

1:19:30.1 SC: So generally speaking, graduate students appreciate this either implicitly or explicitly, so they want to talk to the advisor about what are good things to work on, and I'm very happy to do that. I'm still new here at Johns Hopkins, so I haven't built up a group like I used to have at Chicago, and then in Caltech where I was having four or five grad students at once or more, and they could work together and talk to each other, but we're gonna get there. We're working toward that goal. Jan or Yan, I'm not sure M says, "In the past, you gave several talks about religious beliefs and even had public debates with apologists, which I find very interesting. However, in the last few years, talks and debates from you about this topic became quite rare as far as I can tell. Do you still think that engaging in these debates is a worthwhile activity for you, and if not, what has changed your mind?"

1:20:19.3 SC: Well, I think two things have changed. Number one, I've kind of said most of what I have to say, there's a segment of people who enjoy and participate in either debates or public speaking more broadly for the intrinsic thrill of it. Because they actually enjoy that. They enjoy getting up there and matching wits with someone they disagree with. That's really not what I'm about, to be honest. That's why on the podcast, I had people who I disagree with sometimes, but I'm really about learning from them. I'm not really about debating them, it's just a personal style thing. Sometimes debates are useful. I know there's other people on the other side that I don't quite agree with who say like, "Ah, debates are a waste of time, because that's not how real intellectual work gets done." And it's certainly true that real intellectual work is not getting done in the debates, they're more about entertainment, but also about inspiration and information.

1:21:14.3 SC: You can reach audiences by doing a debate that maybe you couldn't reach otherwise, but I've done it. I've said what I have to say, they can find those debates on YouTube. I think I would just be repeating myself. But the other thing is there are just not that many of them. I have never been the person to suggest doing a debate with a religious person or about some topic like that, they've always come to me or some intermediary has come to me to set it up. And that doesn't happen that much anymore. I think just because that whole genre has faded away a little bit. It's not just me, most people have said what they wanted to say. It doesn't seem to be a lot of hankering as far as I can tell for new material along those lines. Perfectly okay with me, I have plenty of other things to talk about. I'm not running out anytime soon.

1:22:05.8 SC: Yoganathan Paretes says, "We know we don't live in anti de sitter space or so I understand. Assuming that's correct, can you help shed light on what we hope to gain from AdS/CFT theory? Sure, very briefly, it's a lamppost. Do you know the old joke about looking for your keys under the lamppost, even though it's not where why you dropped them. It's supposed to be a joke because of course, if you drop your keys some place, you should look for your keys there, but if you have a lamppost, do you at least have a chance of finding your keys? [laughter] 'Cause you can see where they are. So, it's not a completely insane strategy, there's something to be said for the drunk guy looking for his keys under the lamppost.

1:22:44.8 SC: AdS/CFT is not the real world, but it is a model that we understand much better than any model that does approximate the real world. So, I personally don't do a lot on AdS/CFT, written a couple of papers along those lines, but I prefer to spend my time working on ideas that are closer to the real world, even if they are less well-defined, more speculative and so forth. Other people are gonna say, "No, let me work on the model that I understand, maybe I will gain some insight to that down the road will be of applicability to the real world." This does intersect with something I said in the Solo episode on the Crisis in Physics, one of the very legitimate critiques of modern theoretical physics, especially in the string theory side of things, is they're too willing to say, some day this will be relevant to the real world, even though not now.

1:23:40.7 SC: That's a perfectly good status, but tell me when you're gonna get there, or at least remember that getting there is the important thing. It's the real world that ultimately matters, and I think that it's kind of sad that you can get as far as you can and make a whole career without making much contact with the real world. So I get... I think it's a perfectly legitimate reason to work on AdS/CFT, I just think that it should be part of a broader portfolio, it can't be what everyone focuses on.

1:24:13.8 SC: Girolamo, sorry, Girolamo, I'm not pronouncing your name very well. Girolamo Castaldo says, "I heard you mention at least a couple of times, the Geiger counter as an example of quantum effects influencing the macroscopic world. Would you mind elaborating on that?" Sure. The Geiger counter is a quantum measurement device because it measures the decay of a radioactive particle. It clicks when a particle has been emitted by a radioactive substance, that nucleus or whatever the particle was that was doing the decaying has a wave way function, that wave function is typically in a superposition of I've decayed and I've not decayed, and the Geiger counter performs a quantum measurement. So, it collapses that wave function or at least apparently collapses it.

1:24:54.8 SC: And so, if you do anything in response to what the Geiger counter is doing, then in some very real sense, you are responding to a measurement of a quantum state. You have amplified quantum superposition to a real life superposition. The real life superposition is the Geiger counter clicked and then it is not clicked. So, that's why Schrödinger, when he invented the Schrödinger cat thought experiment had a Geiger counter like thing as part of it. It's just a way of measuring a quantum system in a very straightforward, familiar way. Andrew Goldstein says, "Consequences of climate change continue outpacing computer-generated weather forecasts. Wouldn't computer models be more accurate if fudge factors were included to account for consistent underestimates?"

1:25:38.6 SC: Well, I wanna say yes and no here. The first thing I wanna say the zero thing is, I'm not a climate scientist, I'm not gonna second guess what they're doing, it's a very, very hard problem to work on. Very non-linear, many, many aspects coming in and the measurements are how they do, et cetera, et cetera. Okay? So you should really talk to a real climate scientist, not to me about this. But there's two things going on, you're right, that in the last few decades, the amount of temperature variation that we've seen, et cetera, has generally been more than was predicted by typical models. So, there's two things that you can try to do, one is you can just say, okay, if every time that I try to make a prediction, the real result is 1.2 times the prediction that I'm going to make. I'm just gonna multiply my prediction by 1.2 and make that my prediction.

1:26:27.6 SC: I get why you would wanna do that, the problem is that these models aren't just best guesses. They are models, they're trying to actually model the underlying dynamics. You can't just multiply by some fudge factor and declare a victory, you have to understand why you released your goals to understand why the actual readings are coming out on top of your predictions ahead of time. So, it will be much more respectable to dig into the simulations and figure out why the effects are bigger than what you predicted. The other thing is, though, you might want to, as a policy maker rather than as a scientist, you might want to plan for the future, and then you might want to take into account that over the last several years or whatever the models have been underestimating the effect.

1:27:17.7 SC: In that case, it's perfectly legit to say the models have been traditionally under predicting the size of the effect, therefore, I'm going to play it safe and boost a little bit. So, I think it's the difference between being a modeler who's doing their best to understanding what all the effects are, and to put them into their computer code or whatever, and make the best prediction they can. Versus a policy maker or a predictor of the future where you can be more phenomenological about it and say, "Well, given the track record so far, I'm gonna predict the following thing in the future" There's no reason why you couldn't do both depending on what hat you were wearing at any one particular time.

1:27:57.5 SC: Bits plus Adams says, was it a difficult decision to go on to the Joe Rogan show? Do you worry that having a reputable scientist like yourself on the program, Casa halo over other less reputable guests. Is the audience reach too good to pass up?

1:28:12.2 SC: It's a complicated question. I was on the Joe Rogan show. You have to remember, it was years ago. It was pre-pandemic. Okay? I have not been on in a long time, and I do think that the show has steered much more towards even more, let's put it that way, towards conspiracy theories and things like that since the pandemic has hit. So, I think it will be a much more difficult decision now, but in general, even if that's true, it's still a difficult decision. It's a little bit easy to be overly moralistic about it, but I think that there's a lot of factors that should go into thinking about something like this.

1:28:52.2 SC: One is, it's really important to reach audiences that you can't otherwise reach. It's not just the audience is big, it's that it's a different audience. And if you say, well, they're just hearing from all these crack pots is, if you're really on the side of these people who are listening, getting the best information, shouldn't some good people go on too? [laughter] I don't know, I'm just raising the issue. I'm not giving you the answer because I do think that, yeah, you can also lend your credibility to an operation if it's not clear what is going on. But maybe you can combat that by being very clear where you stand. We just... When I was on Rogan, we talked about pretty straightforward questions about quantum mechanics and things like that, nothing controversial or a conspiracy-centered.

1:29:40.0 SC: If I ever went on again, I think that I would, which I don't know, I've not been invited. I'm not contemplating it anytime soon, so it's completely hypothetical. But I would absolutely want to talk about what I consider to be an anti-science attitude in the conspiracy theories, the anti-vaccination stances, things like that that we've seen on the show since then. Having said that, I'm not a listener of the show, so maybe there's a whole bunch of things going on that I'm just not aware of. Mosher Feder says, imagine that your novel, definitive proof of many worlds has won a Nobel Prize. You decide to indulge yourself by using a fraction of the new spare cash for three time pieces, a dress watch, a sports watch, and an over the top weird or technically dazzling watch. Which three watches do you pick and why? Oh, this is a good question because it's never gonna happen [laughter]

1:30:28.8 SC: I am neither the Nobel Prize nor the spending all that money on watches. I do, I am a fan of mechanical wrist watches. This is an outgrowth of the first trade book that I wrote From Eternity to Here about the arrow of time where when you write your first book you never know whether you're gonna write another one. So you try to pack everything in. So I tried to talk a little bit about everything having to do with the idea of time in one book, which that's a lot of stuff. So there's a lot of stuff in the book, and I discovered I hadn't really followed the whole thing. Many people haven't. But the idea of making a functioning wristwatch is quite a technological challenge, right? There's the whole longitude issue. That was the subject of prizes back in the day of sailing and so forth.

1:31:12.2 SC: And John Harrison eventually won the prize. So a mechanical wristwatch that is to say, not a quartz battery powered wristwatch, but a little machine with gears and springs and things like that, that accurately keeps time is quite a marvelous kind of thing. And when you dig into it, you realize there's a lot of history and artisanship going on. And plus there is this whole historical event called the Quartz Crisis, where, like in the 1950s, the only watches were mechanical watches, and they figured out how to make quartz wrist watches. And basically the whole Swiss mechanical watch industry essentially collapsed. And it's not mostly, I mean, to some extent it is, but most of these watches are not like mass produced, there's a lot of actual human effort that goes into them and knowhow that is easily lost.

1:32:12.0 SC: Like we used to be able to put people on the moon, and now we've forgotten how to do that. A lot of this wristwatch making knowledge was not really passed down from generation to generation. So there's a famous story, for example Zenith, I guess it looks like Zenith, but it's a Swiss company, so pronounce it differently. They had a very famous movement, as it is called for a chronograph, the El Primero movement, the first automatic that is to say self-winding mass produced or popularly produced chronograph. And they had a particular machine that was clearly necessary in the construction of these chronographs, which were famous and historic and so forth. And in the '70s, the word came down from the owners of the company. "Yeah, just throw all that stuff out. We don't use that anymore. We're making Quartz watches from now on."

1:33:01.1 SC: And one of the old timers basically hid the plans and the machine in a room where they couldn't be found. And then what, 10, 20 years later, whenever it was, the whole Swiss watch industry realized, oh my goodness, we can rebrand ourselves as a luxury good. And now we want to recover all this knowledge we had. And they were able to pull the machines out of the mothballs. So I love all that stuff. I think it's great. And I love wearing real wristwatches. I don't wear like a smartwatch or anything like that. The bad news is, and I should say it's like quite a hobby, right? So there are people, they get together, they have message boards on the internet and magazines and whatever. Okay?

1:33:46.5 SC: So it's fun, and I like it. The problem is it costs money [laughter], and you quickly realize that it costs too much money. And so, like, I'm not gonna be a player in this game, really. It's much more sensible to look at pictures of wristwatches on the internet than to actually buy them all. So I have a collection of several different kinds, but they're not like the super duper fancy kinds 'cause those are beyond my budget. So I like this idea that I would get the Nobel Prize and blow all the money on Wristwatches, but I don't have good answers to, well, I don't have a good answer to, the question is dress watch, a sports watch, an over the top weird or technically dazzling watch. I don't have a good answer for sports watch. Well, actually, maybe, no, there is a famous Patek Philippe, which is a super duper hoity-toity watch company.

1:34:38.4 SC: They make what's it called, the Aquanaut, which is like a very kind of weird, it was the '70s. It's a very, you would never, if you were not a watch person, you would not see it on someone's wrist and think it was anything special. But it's like the good watch companies are able to like, just do all the details exactly right. And so it's a very functional, fun sports watch. For dress watch, I'm a huge fan of Lange & Söhne, which is this German company, or the whole German watchmaking industry was devastated not just because of the Quartz crisis, but because of the Iron Curtain and the Berlin Wall, et cetera. But now Glashütte Germany has become second to only Switzerland in terms of watchmaking capabilities. They've come back to life and Lange & Söhne is a German company that makes like some of the most beautiful dress watches in the world.

1:35:25.0 SC: I don't have a specific one that I would want, but I think that company way outside of my price range now. So yes, my future Nobel Prize winning self might splurge on that. In fact, I would not buy three watches. I would just buy one. And probably that would be it for technically dazzling watches. Again, I don't have a particular favorite, but there are some amazing ones out there. People really have fun 'cause it's purely, what can I say? It's purely frivolous [laughter] I think that's the thing to say. There's no, nothing about, I remember speaking of religious debates, when I debated William Lane Craig one of the comparisons he made was to say that something was he compared a Rolex to a Timex. And the Rolex being much fancier and therefore more accurate, and the Timex being cheaper and therefore less accurate.

1:36:21.0 SC: I forget what the purpose of the analogy was, but I pointed out, no, A Timex is a quartz watch. Those are gonna be much more accurate than even the best mechanical watches if you do a halfway decent jobs. A Rolex is a perfectly decent mechanical watch, but it's not gonna be as accurate as a good cheap quartz watch. So the mechanical watches are not made for any technical purpose except to kind of have fun with it. And I kind of like that. I like the search for having fun. I'm in favor of that. So anyway I don't remember any like the companies or anything that do this, but there are some astronomical watches that almost look like a little miniature solar system on your wrist that you can see all the planets moving around. Like you have a little ory down there.

1:37:05.0 SC: I have no idea whether that would be practical to wear but again, since it's not happening, I'm happy to say that would be my choice. Robert Antonucci says a priority question, once again, is dark energy the only thing that affects other things without being itself affected? No back reaction. Well, I can see why you would ask that, but I think that the, if you're bothered there's an implicit thing here that it bugs you, [laughter] that dark energy affects other things without itself being affected. So I think there's two ways to ameliorate that bothersomeness. One is, in some sense it is affected because as space-time expands, there's more and more dark energy, right? There's the same amount of dark energy per cubic centimeter. There are more cubic centimeters, therefore there's more dark energy. So in some sense, it is definitely being affected.

1:37:57.5 SC: But the other one is the amount of dark energy per cubic centimeter is just a constant of nature. It's like the mass of the electron or the fine structure constant, et cetera. It's a parameter, it's a value. So the fine structure constant is not affected by things around it, but it does affect things around it in some sense. I think you have to be a little bit more precise about what is the thing that does affecting and so forth. This is one of those questions where the completely technically correct, but unsatisfying answer is, all the equations work out fine [laughter] If you write down the theory of quantum gravity in the weak energy regime with the standard model of particle physics, with some cosmological constant, with some dark energy, everything works out fine.

1:38:50.3 SC: Energy conservation is obeyed in the covariance sense. That is to say in the sense of expanding space, time, et cetera. There's nothing weird or special about the dark energy. It's just the same kind of thing you see elsewhere in the equations of motion of the universe, except it's a constant rather than a variable. Dave Whipp says, I recently heard of Brown and Susskind's proposal for the second law of quantum complexity, intentionally evocative of entropy, but it continues to grow even past thermodynamic equilibrium. If this understanding is correct, would that make it a more fundamental arrow of time than entropy? Or is there some characteristic of entropy is, or that is lost? I'm not gonna give you a satisfying answer to this one. Sorry. There is an, this idea, the second law of quantum complexity that Leonard Susskind, Adam Brown, his collaborator and others have talked about.

1:39:38.0 SC: But there's two things about it. Number one is it's not supposed to be a general principle, like the second law is it's supposed to have something to do with black holes, or at least horizons or quantum gravitational systems looked at in a certain way. And the second thing is, I'm not really sure, I haven't looked at it very closely. I've read the abstract, but I've not dug into the paper and tried to reproduce the results myself. But I'm a little skeptical of things like this because what they do is they define complexity by imagining you have a bunch of qubits, okay? Two-dimensional quantum states that can be zeros or ones or superpositions thereof. And you think of the universe as a quantum computer, or at least some part of the universe as a quantum computer.

1:40:25.7 SC: So you take your qubits defining the state of the universe, and you run them through some gates, AND gates and NOT gates and stuff like that, just like a regular computer except the quantum mechanical equivalent of it. And then you can define the complexity of a state as some measure of the number of gates you have to run it through to go from an unentangled state to whatever state you're looking at. I'm very leery about the choice of gates that you have to do that you can always take any state and turn it into any other state in one step, okay? But that one step might be non-local or something like that. So I'm not quite sure where the justification comes from for using some gates and not others. There might very well be on page 38 of the paper, a perfectly good justification that I just haven't seen yet.

1:41:12.9 SC: So I don't think that I'm not completely sold on the proposal yet, but I haven't tried to be sold on it. So I'm still open-minded about that. I'm gonna group together a bunch of questions. I'm sorry, they're long questions. Hopefully we can keep them in mind. But you'll see they're vaguely along the same topic. So David Rabinovitz says, when you interviewed Judea Pearl, he seemed to argue he saw macroscopic states such as billiard ball arrangements as a fundamentally subjective concept. You have generally leaned against this idea with a counter that macro states are not arbitrary, but rather are constrained by the laws of physics. But when Steven Wolfram claimed that broad conceptions of possible agents leave an enormous scope for what a macro state might be, you seem to sympathize in part. So the question is, do you have more precise views on what constitutes the most relevant and useful physical constraints upon observers choosing micro states?

1:41:58.6 SC: Maybe you meant macro states, I'm not sure. And how important is it to understand these constraints to understand entropy and emergence? Then Henry Jacobs says, I recently listened to the Judea Pearl interview on causality, and he asked you about time reversibility and billiards. You mentioned coarse graining and Stat Mac to illustrate how an arrow of time emerges. Pearl felt that the way coarse graining was done introduces a choice and is therefore not fundamental. However you held yourself back from responding other than saying you felt as if the arrow was fundamental. Can you elaborate here? Okay, so I think these are more or less the same question, but then there's another question from Jim Murphy. I was listening to old AMA episodes and you had a good response to my question about complexity and cellular automata. You explained that the automata would need to start with some low entropy state in order to see complexity emerge.

1:42:47.4 SC: What's interesting is that for any particular set of rules, the states that constitute low entropy may look very different. Has anyone done research into determining the entropy of different states or various CA rule sets, or should I start this research afresh? So, okay, maybe the third question is sufficiently different. I should address the first two first. The general idea, in case you're lost by the questions, is that when we talk about entropy, it's always a macroscopic. I'm thinking classical entropy, not quantum mechanical entanglement entropy or anything like that. But classically entropy is a macroscopic coarse-grained idea. On Boltzmann's tombstone, it says S equals K log W, what W means is the number of micro states within a certain macro state and a macro state is defined as the set of all micro states that are observationally macroscopically the same looking to you. So in a box of gas, if you have macroscopic access to the temperature and density and velocity of the gas at every point, how many different arrangements do you have of the microscopic atoms that have that set of macroscopic variables?

1:43:58.8 SC: That's the macro state. And from that understanding, if you start in a small entropy macro state where there's not that many states that look like that, it's very natural for entropy to increase. And you go to larger entropy states. So once you have defined your macro states, the fact that you have entropy for any particular state of the universe is an objective fact. It's conditionalized on their particular choice of macro states that you have, but it's not something that has anything to do with your knowledge of the system. There's a whole nother way of thinking about entropy, which is more Gibsean or Shenenean in spirit, where you say you have some distribution over possibilities, and the width of the distribution is roughly telling you the entropy. This is not that the Boltzmanian way of doing it is how many other ways are there counterfactually that I could rearrange my micro states to look the same macroscopically.

1:44:55.0 SC: So it does depend absolutely on how you coarse-grain into those macro states. And there are various people, including Judea Pearl, who have sort of objected to that saying who says that I should coarse-grain in that particular way? But I think that those objections are misplaced. I think that people are imagining incorrectly that it's kind of innocent and okay to imagine that we have perfect information about the system, the reason why that's not okay, not only that we don't ever have perfect information about the system, but if we did, the world would be an entirely different seeming place. There wouldn't be an arrow of time. If you had perfect information about the system, you would just say what the system does. Again, we're thinking classically here, you would be Laplace's demon. There's no entropy for Laplace's demon. So this introduction of subjectivity in the definition of your macro states, or for that matter, if you wanna be more Gibsean information theoretic about it in your distribution over possible micro states, these are both subjective quantities that's absolutely necessary.

1:46:07.4 SC: That's not optional when you're talking about entropy and the arrow of time, the good news is that even though it's subjective, it's not arbitrary. We basically agree on what the right way to coarse-grain systems is. Or at least we agree enough that the differences don't make any difference to how you define entropy or the fact that it's going up. So I might coarse-grain by taking a box of gas and dividing up into one cubic millimeter cubes and then averaging over the number of atoms and so forth. And you might divide it up into one cubic centimeter cubes and do the same thing. So we have a different course graining, but they're completely compatible with each other, right? Why do we all agree on how to do it? Because again, we define our macro states by what we can observe about the system.

1:46:54.1 SC: Things like temperature and velocity and pressure and density are things that we can observe with a measuring apparatus. Even if we don't actually do it, we could do it. Whereas the position and velocity of every individual atom is not something that we can observe even in principle. So I think that this is something that people who want to help themselves to perfect microscopic information about the world have to get over [laughter] You can't do that. You don't have that information, and it's crucially important that you don't have that information. That is really where the arrow of time comes from. So I think that answers at least as much as I can do for the first two questions. For the third question, maybe I shouldn't have lumped this question in. Sorry about that, Jim. But the question is has anyone done research into determining the entropy of different states or various cellular automata rule sets?

1:47:50.3 SC: So I think this is a different thing because now we're talking about cellular automata rather than statistical mechanics. And there is a huge, huge difference between the two because the whole thing about statistical mechanics and classical physics is the microscopic rules of engagement are perfectly reversible. That is what allows for the existence of Laplace's demon, et cetera, who from the current information about the world can perceive both the future and the past. The typical cellular automata that people talk about aren't irreversible, even at the microscopic level. So you don't need to start in a special state necessarily to see complexity emerge. It's an open question. It depends on the kind of cellular automaton you're working on. So in the microscopically reversible case, because information is conserved at the micro level, no new complexity can ever arise that wasn't implicit in the state all along. Whereas in the cellular automata irreversible case, you can start with just not only low entropy, but almost no information in the initial state. And it can evolve into a very high information density state. That can't happen in classical mechanics, but it can happen in cellular automata. So it's just a very different question. I'm not giving you the answer to the question 'cause I'm not quite sure what it is, but it's a very different kind of setup with therefore different approaches are gonna be necessary.

1:49:21.5 SC: John Stout says, I recently bought David Albert's book Time and Chance, I also bought the Probability Map of the Universe essays on David Albert's Time and Chance. The latter makes reference to your book from Eternity to Here. Yes, and in fact, I think I have an essay in that book, so I hope that there's more than just a, my book appearing there. I think my name appears in there. Anyway, John goes on the latter makes reference to your, sorry. Can you provide some context around this issue of the arrow of time and what the disagreements are or what the alternative theories are in terms of the arrow of time, if not driven by entropy? In principle one could imagine that our current best theories of fundamental physics are very wrong. I say that because our current best theories of fundamental physics are fundamentally reversible at the microscopic level.

1:50:10.7 SC: As we just talked about Macroscopically, we see a world where there's lots of irreversible things happening. Ice cubes melting into water, breaking eggs, what have you, mixing cream into coffee. And we have a very good reconciliation of those two facts based on the fact that we're coarse graining and we have entropy and the entropy started low, et cetera. There's a set of people, and Tim Malden might actually count himself among them, who think that actually the fundamental laws are just not reversible, that there just is an arrow of time built in. But I'll tell you, that's a very minority point of view these days. I think the majority of physicists fall into two camps. One, what you might call the conventional picture that I just sketched out, where you have reversible fundamental laws, but some boundary condition, the past hypothesis that says entropy grows, and the arrow of time stems from that.

1:51:05.4 SC: And two people who haven't thought about it that much, [laughter], there's gonna be plenty of those people bless their hearts, but they just haven't really worried about this question. I think that the overwhelming number of people who have really thought deeply about the arrow of time think it comes down in one way or another to entropy. And the reason why the alternative is not that attractive is just because it's really putting a huge amount of effort into pushing against a completely open door. Because the fact is there is entropy, right? There is micro states, we can coarse-grain, we can calculate the entropy that entropy was low, whether or not you have fundamentally reversible or irreversible laws, and that entropy will go up. So whatever you think are the fundamental dynamics of the world, we have an arrow of time from increasing entropy, and there's a project that is ongoing and I can play my little part in it and so do other people.

1:52:03.7 SC: But connecting the growth of entropy to other manifestations of the arrow of time, causality, memory, aging, whatever. But I mean, we already have an arrow. Let's just work on understanding that arrow versus inventing an entirely new arrow and then trying to give that credit for everything. That's what I think the others are trying to do. So it's not that attractive a proposition to me. Jameson says gravity is sometimes described as the bending of space-time and other times described by as a force from gravitons. I've even heard that in string theory, if we are living in a three ring that gravity is the only force that could escape into extra dimensions if it's just carried by gravitons. Why is gravity still so different than the other three forces? Well, there's particles and there are particles. Particles are different. Is one very simple answer, but there's a sort of more subtle answer.

1:53:00.3 SC: So let me do the first answer first, gravity is special because it's universal. And what that means is that the particles, the gravitons couple to everything, they couple to all of energy and momentum in the universe, photons and gluons and W and Z bosons interact with some kinds of particles. They don't interact directly with other kinds of particles. Gravitons interact with everything that's fundamentally a consequence of the principle of equivalence, as Einstein dawned on Einstein back in the 1910s or whatever. So the particles interact differently, is the short answer to the question. The other answer is that when you say that gravity is due to graviton particles, that's supposed to be, there's all sorts of hidden footnotes there, [laughter] and caveats that maybe you're not aware of. It's really a field, right? It's really the gravitational metric, tensor field, just like photons come from the electromagnetic field and so forth.

1:54:00.4 SC: Gluons come from Chromodynamic field and there is a regime in which the excitation of that field look like particles. So in particular, when you're very close to the vacuum state, empty space, nothing else going on, and you just perturb the field a little bit. Those perturbations look like particles, but in other regimes, they don't look very particle-like at all, right? Inside a proton, the gluon fields do not look like individual gluons, because they're not small perturbations of the vacuum and maybe in a black hole near the singularity, there's really no regime in which the gravitational field looks like a collection of gravitons. So the particle language is a little bit less general than the field language and the particular way that the fields work in generalativity, of course, is the very shape of space-time itself. That's the explanation in Einstein's way of thinking about it for why gravity is universal and couples to everything as opposed to other forces, which are fields living within space-time rather than fields defining a feature of space-time itself.

1:55:13.3 SC: So the gluon fields, electromagnetic fields and so forth are allowed to interact with some particles and not others. Gravity has to interact with everything. Frederick Apollo says, "Why do particles decay into other particles? Is it really just the second law?" Well, it's not just the second law, but the second law is absolutely involved in particle physics, any interaction, like let's say you have a muon and muons can decay into electrons and a neutrino, antineutrino pair, that is an allowed interaction in particle physics. The reverse interaction is always allowed. You can always imagine taking an electron, a neutrino and an antineutrino pointing them exactly at each other in exactly the right way to make a muon, at least temporarily. But as you might guess, it's harder to do that. And in fact, it is just a little toy version of unmixing the cream from the coffee or unbreaking the egg.

1:56:13.6 SC: There's a smaller number of ways to be a single muon than there are ways to be an electron, a neutrino, and an antineutrino. So the second law is not the explanation for why particles decay into other particles period, but it's the explanation for why that's a much more common event than several particles coming together to make a small number of particles. David Maxwell says, my favorite modern sci-fi is called Bobiverse. It tells of a near future IT engineer who pays to have his brain frozen on death, but wakes up much later as the sentient of a von Neumann probe, able to harvest resources with drones, 3D-print anything and replicate himself. He cannot travel or communicate faster than light, but he does have fusion engines being artificial. He's also able to live at any speed he chooses and meet with other Bobs in real time, et cetera, et cetera.

1:57:06.4 SC: His choices are around time and resources, replicate civilize or explore. Would you choose this option for life after death? And how do you think you might prioritize your choices very broadly? Would you be more an explorer, a world builder, or a multitude? So I'm once again not gonna give a very satisfying answer to this question because I kind of question one of the implicit assumptions of even asking the question where you, it's not implicit, it's very explicit. You say, would you choose this option for life after death? So I think that I would no longer exist, there's no sense in which I can continue my life as A von Neumann probe. And this is, there's a philosophical point here that we talked about with Katie Elliot a little bit. I think that, and we talked about with David Albert also when I talked with him about, Captain Kirk and things like that.

1:58:10.7 SC: Strictly speaking why do you think you're the same person that you were five minutes or five years ago? Well, you're not exactly the same person, right? You're a little bit different. You've aged a little bit. You have a memory that you didn't have back then. Even your constituents, atoms, constituent atoms are a little bit different now than they were back then. But there are reasons to psychologically as well as physically identify some continuity over time between yourself now and yourself some time ago. And we can sit down and debate how accurate or how fundamental that identification is. But it is convenient. It serves some purpose to us because there is continuity there. This kind of thought experiment, which I'm not arguing against this kind of thought experiment. It's a lot of fun, by all means, let's contemplate it. But it is doing something that is enormously far outside our experience, and it's asking us to make decisions based on our experience [laughter], which is very hard to do.

1:59:11.4 SC: It might even be fundamentally no rational way to do it, which is something we talked about with Laurie Paul back on the podcast some time ago about transformative experiences. In other words, unlike me now and me five minutes ago, or me five years ago, there's relatively little continuity between me now and some uploaded version of me in some interstellar probe [laughter] that is completely artificial, right? I mean, maybe what you mean is that there are some patterns in my neurons that you try to replicate inside some silicon substrate or something like that and do your best, but it's not gonna be me anymore. It'll have some relationship to me. But the direct relationship that I have with my five-year-old self, five years ago self is just not there. It's something much more tenuous than that. In particular, I would imagine that all of my motivations and drives and so forth would be completely different in that context than they are now.

2:00:14.1 SC: So I don't think that the question, would I choose that option? And how would I behave if that option were chosen? Makes any sense? You can ask a more specific question. If someone takes this and that neuronal pathway pattern from your brain and reproduces it in a machine and launches it into space, what do you predict that thing would do? I have no idea what the answer is, but I don't think it's me in any real sense. John Campbell says, of the many ways I could buy or borrow your books, which would you recommend? I'd like to vote with my wallet and I have my own thoughts about the relative merits of borrowing from the library, buying a book used, buying an ebook, buying a book from the publisher, et cetera. But I'd like to hear yours. Well, I appreciate the question. Thank you, John.

2:01:00.1 SC: But honestly, it doesn't matter that much in terms of me making money. I think that you buying it from a bookstore or from Amazon is probably the best. Maybe buying from the publisher. I honestly don't know. I haven't really kept track. I don't think it matters from those three options. There's an absolutely sentimental part of me which would like to see people buy from bookstores from good old independent bookstores. Or you can go to bookshop.org and that's a internet way of buying from independent bookstores. That's usually how I try to buy books if I can. But if you want to buy the ebook or if you want to buy, get it from the library, I don't make any money if you get it from the library, but I have no objections to that whatsoever. I'm a huge fan of libraries, a huge fan of people taking my books out in libraries.

2:01:47.7 SC: That's great. Whatever's most convenient for you, whatever makes you most likely to get the book and read it, do that. That would be my advice. Masterwork Tools says I was explaining the new gravity wave results to my father, and he has a question for me that I couldn't answer. What other types of wave are there and can we expect to be able to eventually detect them? As I understand it, we have mechanical waves, electromagnetic waves, and gravity waves. Am I correct on these? And are there any others we know of that we can expect will be useful for investigating the universe? Well, I think there's not quite a parallel between some of the examples you're giving. Mechanical waves, electromagnetic waves and gravity waves. Those are all waves but mechanical waves is a huge category. I mean, there are sound waves, right?

2:02:35.8 SC: That's a kind of mechanical wave. But there's also vibrations there's waves on water. These are all kinds of waves that are propagating in some particular medium, and there's potentially huge number of different kinds of them. I think maybe what you're thinking about are more fundamental particle Physicsy notions of waves. So there are electromagnetic waves and gravity waves right there in the basic laws of physics, unlike sound waves, which are higher level emergent kinds of things. So if what you're asking is are there other kinds of fundamental waves that we might measure astrophysically? The short answer is no. It does depend on whether or not you count matter particles as well as force-carrying particles, right? Like in some sense because of quantum mechanics, when you detect cosmic rays or neutrinos or high energy electrons or whatever from space, those are kind of wavy in some sense because of quantum mechanics, but they're what we call fermions in physics.

2:03:43.0 SC: So they're actually particle-like waves. You cannot pile them on top of each other to make bigger and bigger waves like you can with electromagnetic waves and gravitational waves. There are also other forces in nature. There's the weak nuclear force and the strong nuclear force, but they're both short range. So electromagnetism and gravity are the only two fundamental force-carrying waves that are long range and can therefore propagate over astrophysical distances. At least those are the only ones we know of. We have zero reason to expect there are any others besides that, but they could be there, right? That's the job of theoretical physicists to invent models in which that's the case. I've done it myself. So I think it's a respectable thing to do. But short answer to your question, in terms of wave-like things that we might find by doing astrophysics, it's one version or another of electromagnetism and gravity, Chris Murray says, it is said that barring proton decay, quantum tunneling will eventually turn black dwarfs into iron stars, then into black holes.

2:04:47.6 SC: It's been pointed out such a black hole would come from a small fraction of the iron star, which then swallows the rest. Since quantum tunneling is overwhelmingly more likely to form a smaller black hole before a larger one. But doesn't this mean there would be an abundance of the very smallest black holes first, some being on the surface whose instant evaporation would gradually eat away the hole star before any long lasting black hole could be formed? I'm not sure who is saying these things. You keep saying it is said that I'm not sure who is saying that and pointing it out.

2:05:14.2 SC: This is a true-ish kind of scenario. But weirdly specific it's weirdly specific because it's a thing that can happen, but other things can happen also. It's much more likely that stars or planets fall into big black holes than that they quantum tunnel into being black holes in their own right. But it could happen. But what, when it happens, if you, so you're correct. I think that when there's a quantum tunneling event that makes a new black hole, that black hole is going to be very small, but it will eat right. It will accrete other things or it will just evaporate away right away. It's very possible it evaporates away right away, in which case it's just not there anymore. But if it's big enough that it can survive by growing, by eating, then it will generally eat up whatever star or planet it's in, long, long before that star or planet has a chance to quantum tunnel into another black hole.

2:06:15.4 SC: So there's no regime that I know about where you will accumulate many small quantum tunnel black holes before the whole thing just collapses to one big black hole. Gregory Kuznick says, if you were Education Emperor, how would you reform college admissions policies? Yeah, I'm not gonna be education emperor anytime soon. I'm not quite sure who counts as that. It's not something that I put a tremendous amount of thought into very honestly. I do have like vague feelings about it. I think that we stress it too much. College admissions I think that the undergraduate college experience is just not nearly as affected, well, sorry, I should say this. The educational side of the undergraduate college experience is not nearly as affected by where you go as parents and high school students tend to think it is. I was a grad student at Harvard.

2:07:13.3 SC: I know plenty of undergraduates who did not get a great education while they were there at Harvard 'cause they chose to focus on doing other things. I was an undergraduate at Villanova and I know that there were students there who got a wonderful education. As long as you're at a relatively good school and you put in the effort as the student, you can absolutely get a wonderful education almost anywhere. There are slight differences that can be very important. For example, I definitely suffered as a Villanova undergraduate from not having a graduate program in physics that I could sort of sneak into most undergraduates who want to grow up to go to grad school. If they're at a place that already has a graduate program, then they will take some classes ahead of time. It helps preparation and things like that. But that's a very specific thing.

2:07:58.8 SC: If you want to eventually go to grad school, it helps you go to an undergraduate school that has a grad school there, it's not necessary. If you're at a smaller school, you can get more attention. So there's pluses and minuses, but the idea that you have to go to the top school to get a good education, I think is entirely wrong. In fact, it's very often the case that the top schools are the ones where, well, again, this is not necessarily a correlation, but there's plenty of professors at top schools who are not really interested in undergraduate education. What makes the top schools, top schools is their research, right? Their scholarship, their intellectual output. It makes a big difference for graduate school where you go because there you're working with an advisor and that advisor really has a huge effect on your life.

2:08:42.4 SC: For undergraduate education it's honestly probably the biggest effect is who your peers are, who your other students are going there. And the benefit of going to a place like Harvard or Princeton or Stanford is that you're hanging out with people who are gonna go on to rule the world and that can be beneficial to you. So I don't know how to reform it, but I would lower the temperature on discussions about it, honestly, I could see an argument for just making it random having some lower cutoff. Like if you're okay, you're good enough to get into the basic schools and then we'll just randomize who gets in what? I think that would be fine, in many ways, not necessarily advocating that, but I might not be worse [laughter] than the current system. Let's put it that way. There is of course a lot of discussion about this issue right now because of affirmative action and the Supreme Court coming down against it, which is just, I think the reaction against affirmative action is just weird and bad, I think, and it's also intellectually misplaced.

2:09:49.5 SC: Affirmative action is not a great system. For those of you who are not in the US. Affirmative action is a system according to which black people and other underrepresented minorities get preferential treatment when being accepted into colleges or other things. You could define the term affirmative action differently. It's not a great system because you would like in principle to very carefully judge each individual case. And if someone really was discriminated against personally, then okay, you can make up for that in your admissions decisions because they're fighting against this extra optical, et cetera, et cetera. But all that takes effort, [laughter] and time and resources that places don't have. The thing about affirmative action is it's cheap and easy. Just say, okay, we're gonna let in more members from this particular applicant pool to increase the diversity of the school.

2:10:45.3 SC: That's it, right? It's just not a big deal. It's cheap and easy. And yet super effective studies have shown that affirmative action has absolutely changed the demographics of the United States by contributing greatly to the growth of a black middle class. Because people go to these schools they wouldn't otherwise have gotten into. They get to know friends and whatever. They're exposed to a whole different world and that helps them later in life. So I think that there's various things to be said. I don't think that schools should discriminate against Asian Americans like they sometimes do 'cause otherwise Asian Americans disproportionately do very well on tests and would get into schools. On the other hand, I don't think that test scores should be the overwhelming thing either. I think that it's perfectly okay to take plenty of different considerations under your advisement when you're letting people into schools.

2:11:38.3 SC: So overall, I don't care that much, but I'm absolutely in favor of doing things to make a diverse and interesting student body. Peter Cain says, are there any fun and interesting ideas about dark matter that could be used for science fiction novels, TV shows or films? It's very hard because one of the things we know about dark matter is it doesn't interact with itself very strongly. If it did, then it would be more like ordinary matter and the distribution of dark matter would be very similar to that ordinary matter, but that's not true. Dark matter halos in a galaxy or cluster tend to be big and puffy and diffuse. Whereas the ordinary matter, because it interacts, electromagnetically, can give off, photons, lose energy, dissipate and fall to the center of halos, as we discussed in the solo episode recently. So the zeroth approximation, your first guess is that dark matter just doesn't interact with anything else, at least not very much, and therefore not very interesting for science fiction novels, et cetera.

2:12:44.2 SC: Maybe you can try to be more creative. I did write a paper with Larry Ackerman, Matt Buckley, Marc Minkowski about dark radiation. We imagine that there could be a version of electromagnetism that only interacted with dark matter. And once that's true and we show that it is allowed, at least it seems to be allowed. So you can then imagine dark atoms, dark chemistry, dark molecules, et cetera. But it has to be different than ordinary matter because of just what I said, because the dynamics of dark matter are different. So I'm not sure if there's a regime where you can do interesting things like that.

2:13:17.0 SC: But even that would still be the dynamics of things going on in the dark matter by itself, not interacting with ordinary matter. If dark matter interacted noticeably with ordinary matter, we would have detected it a long time ago. So in terms of being useful to we people made of ordinary matter, I don't really see how dark matter can be very helpful. Simon Carter says "I was beginning to be convinced by Tim Maudlin's argument for pilot waves but in the reflections, you mentioned that making it compatible with quantum field theory is a hurdle too far. Could you expand on that, please?

2:13:52.6 SC: I can't expand very much for the reasons that I've said earlier, that I have not actually dug into the details about these alternatives to Everettian quantum mechanics, but there's a difference in philosophy here. In Bohmian mechanics, pilot wave theories, they were first developed in the context of nonrelativistic quantum mechanics of point particles. And the motto was, "Why do you see wave-like behavior sometime and particle-like behavior sometimes in quantum mechanics?" In pilot wave theories the answer is because there are both waves and particles. And that seems good, but then you realize, well, actually in modern physics, the world isn't made of particles, the particles are just quantized excitations of fields. So now what do you do? And the Bohmian people don't agree yet on what to do. There are different strategies going on. The point is that one way or another, you need quantum wave functions and something else and some hidden variables.

2:14:54.6 SC: So constructing a quantum theory in Bohmian mechanics is more difficult than doing so in Everettian quantum mechanics because it's more complicated. There are more ingredients and more equations. So there are choices you need to make, things you need to think about, and quantum field theory doesn't naturally fit in. Maybe you can squeeze it in some way. Whereas with Everettian you just tell me what the Hamiltonian is, which runs the dynamics and you're done. That's all you got to do. Everything else is supposed to follow from that underlying equation. And the same thing goes true, but even more, when you get to something like quantum gravity where you're not gonna assume ahead of time, there is anything called the metric tensor, it might be emergent and so forth. So again, for Everett, this is all plug and play, it's all very easy.

2:15:43.9 SC: Every time you invent a new kind of quantum theory, you have to reinvent your pilot wave theory, but you don't have to do that in many worlds, one of the reasons why it's so much more attractive. Proitos says I read Thomas Hertog's book thanks to your recent podcast. Thomas admits in the book that Stephen Hawking's favorite technique of rotating time into an imaginary direction to create Euclidean quantum gravity has long been dismissed as a Cambridge eccentricity. There's a difference between mathematical calculational tools like wick rotation and Stevens believe that imaginary times in some sense physically real. How do other professionals view Euclidean quantum gravity is nothing more than a dubious parlor trick."

2:16:26.3 SC: Well, it's an interesting question. So for those of you who don't know the background here, when you learn relativity, when you learn special relativity first and then general relativity, you learn that basically the idea that space-time is a unified thing means that you can calculate distances or intervals in space-time just like you can in regular space. In regular space, if you have X, Y, Z coordinates, you use Pythagoras theorem to do that. X^2 + Y^2 + Z^2 equals the distance squared. The difference is, in space-time, that you do the same thing with not only space, but also time, so it's T X, Y, Z. The difference is that there's a minus sign in front of the new version of Pythagoras theorem for the time direction. This is all explained in grizzly detail in "The Biggest Ideas in the Universe volume 1 Space, Time, and Motion." So it's -T^2 + X^2 + Y^2 + Z^2.

2:17:20.8 SC: And that's fine, it fits the data and you can test it, et cetera, but it's a little bit mathematically inconvenient. So what you can do is, if you want a little bit more mathematical convenience, make a change of variables from T, the time coordinate to τ where the relationship is T = I x τ. So I^2 -1. We're just defining a variable. No one can stop you from doing that, but in terms of τ, which is -I x T, if you wanna put it that way. -T^2 is +τ^2. So in terms of the τ coordinate, the metric, the distance is +τ^2 + X^2 + Y^2 + Z^2, that's a Euclidean kind of metric, which is why we call it Euclidean space or when we do gravity, Euclidean quantum gravity.

2:18:14.9 SC: Now, this trick turns out to be super useful in ordinary quantum field theory. It's just a trick in quantum field theory, that's the way that we usually think about it. Wick rotation is sometimes the word that is used to describe it, because we're thinking of time as being a complex number T + Iτ and rotating from the T axis is to the I axis, to the τ axis is rotating from a real number to an imaginary number. So we also call it imaginary time. And it doesn't make that much of a conceptual difference in quantum field theory, it makes a big conceptual difference in gravity because you're integrating over the space of all possible metrics, and now these metrics are Euclidean metrics rather than space-time metrics. So you guess or pause it that the answer is the same, that it doesn't matter whether you integrate over Euclidean metrics or Lorentzian metrics, as we say space-time metrics. Nobody really knows for sure, but it's certainly easier to deal with the Euclidean calculation, so you cross your fingers and you hope it works.

2:19:20.0 SC: That is kind of half of the particular program that Hawking and others have called Euclidean quantum gravity. Half of it is this rotation from real time to imaginary time, but the other half is in the context of quantum cosmology, dramatically cutting down on the number of degrees of freedom that you're considering by just assuming that the universe is homogeneous and isotropic to start, so-called minisuperspace models and maybe then you perturbed a little bit by adding some perturbations on top of that. But you're still treating the metric as the fundamental thing, and it's not very holographic or emergent or anything like that. So I think that people respect this tool very, very much as a tool for doing calculations, the Hartle-Hawking vacuum state in the quantum field theory version of this really just is the vacuum state.

2:20:15.5 SC: It does calculate the vacuum state, which is an interesting, useful thing to do, so I think that most people think that it has some use, I think that most people think that it's not gonna be the definitive final way to construct a full theory of quantum gravity. Paul Hess says, "If you could meet a well-versed philosopher from 100 years in our future, what would you ask them about, and do you think their answer would be any more valuable than asking well-versed contemporary philosopher? I am trying to get at whether you feel that philosophy is an area that actually advances like physics does."

2:20:46.9 SC: Well, I absolutely think that philosophy advances. Whether advances like physics does is a harder question, 'cause it's a very different field. Physics is about creating models and fitting them to the data, and philosophy is not about that. Partly, and sometimes that happens, at the boundaries of course, between physics and philosophy, but philosophy is about the conceptual logical analysis of the underlying arguments, not about creating models and fitting them to the data. So it's a different kind of advancement. Obviously, in my mind, philosophy advances.

2:21:20.5 SC: You know what? If you've ever heard me say, thinking about morality, for example, well, you can consider consequentialist theories or deontological theories or virtue ethics theories. So there's a kind of a road map for different possibilities, as soon as you say those words, you are borrowing advances that were made by previous philosophers. Even if the advance is not of the form, here's the once and for all, final answer, understanding the space of possibilities is absolutely an advance. Some areas of philosophy are more rigorous and logical, and you prove theorems in mathematical logic, for example. There's no question that once you've proven some theorems, you've made a little bit of in advance.

2:22:01.1 SC: What I would wanna ask a future philosopher, there's lots of questions I would like to ask. I would like to ask what they're thinking these days about morality and ethics, metaethics and the grounding for ethics, the role of consciousness and AI and emergence and things like that. There's a whole bunch of things I would like to ask. Not even to mention, what I do for a living, the philosophy of cosmology and quantum mechanics and the arrow of time. All these. A whole bunch of things I'd be very interested to see what philosophers have learned over the course of a century.

2:22:32.4 SC: Nate Grady says, "In your most recent solo podcast, I heard gravity is nonlocal along with some quick examples of the non-locality e.g., black hole information. That's the first I've heard of it after listening to plenty of your podcast, can you explain in any more detail the examples and their consequences" Well, this is not something that is very well understood, let's be very clear. And it's a purely quantum gravity statement, that gravity is nonlocal. Classically gravity is perfectly local, it's a local field theory, and what that means is you have an equation of motion, Einstein's equation that holds separately at every point in space-time.

2:23:09.7 SC: That's what it means. What happens at one point in space-time is being affected by its derivatives or it's very, very nearest neighbors, not affected directly or immediately by things that are happening far away. In quantum mechanics, there's my personal point of view here, and there are things that are well-recognized by others. In my personal point of view, locality is something that we need to explain why it's a good approximation at all. Because quantum mechanics doesn't look local, it doesn't have space built into it much less locality. You have to get that as an emerging thing. So to me, it's completely unsurprising that there might be some non-locality, the question is what exactly is the kind of non-locality that arises?

2:23:51.6 SC: The evidence that we have that this is an important thing comes like you say, from black hole information, but also just from the holographic principle, if you think about AdS/CFT, the claim is that there are two theories that are equivalent to each other, one is defined on the boundary, it's a theory without gravity, and one is defined in the bulk, it's a theory with gravity in one higher dimension. So the relationship between those theories is gonna have to be non-local, they are not even in the same number of spatial dimensions.

2:24:21.8 SC: So holography, if you believe that it's a good approximation or valid in any case at all, is going to be a very vivid example of non-locality and quantum gravity. The black hole information puzzle is also seemingly such an example, because the whole reason it's a puzzle is because once you send the information into the black hole, it's behind a horizon and it's hard to see how it gets out. How does it get out? Well, through something non-local, but very, very subtle. We don't know the details about that. I don't know the details. There are people who are working on it and have made a lot of progress, I don't think the situation is quite settled yet but there's evidence that the quantum mechanical level, there's little tiny influences that share entanglement over non-local distances, but it's not something that is perfectly well understood right now.

2:25:12.7 SC: Brian says, "I find your work in discussions of position and momentum as emergent properties from the state vector in Hilbert space fascinating. Do you have similar lines of thinking about the emergence of fields and the internal spaces they inhabit?" Well, not really. I mean, I have some aspirations, some vague ideas, some scribbles, maybe some notes in a notebook somewhere. I have not put a lot of effort into thinking about that. In part because it's supposed to be easier getting fields or whatever to emerge from quantum mechanics already shouldn't be that hard, but then it becomes hard when you wanted to get the right fields, the fields that we actually have in the standard model of particle physics, et cetera. And also when you want that to play nicely with the idea of space and maybe space time themselves emerging. So there are people who thought about that, but I haven't put a lot of effort into it myself quite yet.

2:26:06.6 SC: Christopher Humble says, "My question concerns Cormac McCarthy. Did your paths ever crossed at the Santa Fe Institute? And if so, what were those conversations like? Also more generally, can you talk about the interdisciplinary collaboration of the institute and the kinds of conversations that go on? I almost didn't answer this question because the answer is no, I never ran across Cormac McCarthy. The time that I started spending more time at the Santa Fe Institute was about the time he had started spending less time just 'cause he was not feeling as well. But when I was just there most recently, there was a little memorial celebration for Cormac McCarthy. He was, and so I was there for that, and it did make an impression. He was a very big presence at the institute.

2:26:49.9 SC: He had a lot to do with the physical design of the place. He had opinions, he helped redo the entire library, he donated wood for the stairs from his farm and things like that. What was claimed, and I have no reason to doubt it is that he never really liked Santa Fe. He was not a New Mexican, he was a Texan originally. And the reason why he moved to Santa Fe was to be close to the Santa Fe Institute and to interact with people there. If people don't know his two most recent novels, his last two novels, "The passenger" and "Stella Maris" are very much based on conversations that Cormac McCarthy had with physicists, especially at the Santa Fe Institute.

2:27:36.5 SC: He helped SFI rewrite their publicity materials, things like that. He would help people edit their books. And so you get a feeling that he had a certain grumpiness about him in some cases, but also a really touching warmth about him in other cases. So I'm sorry that I didn't get to interact with him much more. In terms of the interdisciplinary collaborations and conversations that go on, a lot of the effort or a lot of the secret to success at a place like SFI is just making sure the people you invite are into the spirit of the place right away. So the thing about SFI is you go to lunch and everyone goes to lunch, it served and everyone sits in the same place and you can sit down next to whoever you want. And there might be an architect and a historian and a linguist and a computer scientist, but the thing is, they're not randomly chosen architects and historians, et cetera. They are ones who want to come to the Santa Fe Institute. They are the ones who are interested in talking to physicists and to biologists and to whoever.

2:28:41.7 SC: And so that's what makes it great. And there's plenty of people out there who are interested in this kind of interdisciplinary collaboration. It's a matter of finding them and getting them into the right place. Not every physicist wants to talk to a historian or an economist or et cetera, but SFI attracts those people.

2:29:00.4 SC: Nalida S. Says, "I really enjoyed listening to your episode with Katie Elliot. Would you kindly expand on the connection you postulated in Newcomb 's paradox between eternalism and the many world's interpretation of quantum mechanics." I'm gonna have to apologize for not remembering exactly what I said. I was thinking about in the case of Newcomb's paradox, if you believe in quantum mechanics, then the existence of this kind of spooky being who knows exactly what the future is going to hold is much harder to understand in quantum mechanics, where there's randomness in the world.

2:29:37.8 SC: So, if you think that there's some quantum fluctuation-ness that will prevent you from getting the right reward then it's harder to even set up Newcomb's paradox. I don't think that many worlds either implies eternalism or is against it or anything like that. I do think that there's an overall philosophy of physics that says the physical laws describe what happens in reality as convenient summaries of the actual motion dynamics of the physical world. And the physical world comes first, and the world includes various moments of time and described by different states of the wave function.

2:30:20.4 SC: So it all kind of fits together into a neat package that includes many worlds and eternalism. But I wouldn't wanna say that either one of them nudges you in the direction of the other. You can be classical eternalist perfectly well, or about a Bohemian eternalist or whatever. Alright. I'm gonna group two questions together.

2:30:38.6 SC: Doorbell Jeff says, "I'm a PhD student in observational cosmology and I don't get the cosmological constant problem. To my limited understanding of quantum field theory, at a certain energy scale, we measure the normalized values of a parameter of the theory and thanks to renormalization, we can then predict how it scales with energy. That is correct. Cosmologists measure the renormalization value of the cosmological constant to be small. And this is often claimed to be a mystery, but we can obtain that value through a renormalization procedure. To me, this seems to solve the problem, like it solves the problem for any other parameter we measure in physics. What am I missing?

2:31:13.6 SC: Whereas Bill quirk, as I enjoyed episode 245, "Crisis in Physics." You mentioned the difficulty of understanding the small but non-zero number for the cosmological constant. What are some of the ways that people have tried to explain why the number is so small, but not zero. I believe the calculation depends on the Planck scale. What would the Planck scale have to be for the number to be as small as the observed cosmological constant.

2:31:35.8 SC: So it is completely true that the cosmological constant at the end of the day is something you'd go out and measure. And you can always fit that into your theory as a renormalized parameter that you measure, just like you can fit the mass of the Higgs boson to the fine structure constant, or whatever. That's not the mystery. There's no mathematical obstacle to just dealing with whatever cosmological constant you measure.

2:32:02.4 SC: The mystery is that the value of it seems unnatural to us. This whole procedure of renormalization is not just a way to fit the final answer into your calculations, but it also offers a natural scale for these answers to be act. Especially when you have numbers that are dimensionful, they are compared to other numbers in some way, and there's a set of contributions that you add up. I think that the real impact of the cosmological constant problem comes from imagining that you treat it as the sum of many different contributions. So, there's a vacuum energy in a different quantum field, but there's a separate contribution to the vacuum energy from the electromagnetic field and from the neutrino fields and from the quark fields and all these different fields.

2:32:49.9 SC: Some of these contributions are positive, some of them are negative, there could even just be a classical bear contribution. And the mystery is, why in the world do all these positive and negative numbers add up to give you such a tiny number? That's the mystery. And it's very important to emphasize that it is clearly a misunderstanding. So you can't be too adamant about what I just said, adding up all these contributions and getting the tiny number, as the right thing to do. It seems to be the right thing to do from our current understanding of effective quantum field theory, but it doesn't give you the right answer. So clearly something's wrong. That's easy. The hard part is, what is wrong? So to Bill's question, I don't have any good theories to explain why the cosmological constant is small. I proposed a couple, but they're not great.

[laughter]

2:33:46.8 SC: I wouldn't say that they're on the right track. I will mention two ideas that are out there that may or may not develop into the right answer. The best idea we have right now is the anthropic principle. There are small numbers in physics, and the cosmological constant has the special property that it is small, but if it were bigger, we wouldn't be here to talk about it. It would not allow for the existence of galaxies and stars, et cetera. So therefore, if you have a multiverse with different values of the cosmological constant, you can account for its smallness by saying that we only see those regions in which the value is small. This was argued by Linde and Weinberg and others all the way back in the '80s maybe even before that. Who knows?

2:34:32.7 SC: So that's absolutely a possibility. It would be a sad possibility in the sense that it would remove the ability to create a clever theory that actually predicted the value of the cosmological constant. The other perspective which Tom Banks and others had pushed for a while, and I'm actually quite fond of is in the case of the cosmological constant, the entire paradigm of effective field theory is just not the right way to think about it.

2:35:00.3 SC: He has a paper, Tom has a paper, the cosmological constant maybe with Willie Fisher, the cosmological constant has a boundary condition. And the idea is the following, that if you live in De Sitter space, which is empty space solution to general relativity with a positive cosmological constant, then you have a horizon around us in all directions, it's sort of like we're in the center of the horizon rather than outside the horizon as we would be in the case of a black hole.

2:35:28.4 SC: And there's an entropy associated with that horizon, and it's the area of the horizon in blank units, just like the entropy of a black hole calculated by Hawking way back when. And you can make the case that it's a high entropy state, so the dimensionality of Hilbert space is E to that entropy, so E to the 10 to the 122, roughly speaking. So it's a very big number, the dimensionality of Hilbert space, but the point is, what Tom would say, and what I would completely agree with is if that's the right way to think about it, that's not something you get by renormalizing of parameter and your effective field theory, that's the dimensionality of Hilbert space, that's just a fixed number that's just given to you by God. It's not the sum of various contributions, and then so this doesn't solve the cosmological constant problem, it just says you're thinking about it in the wrong way. The cosmological constant is not supposed to be thought of under the rubric of effective field theory.

2:36:26.9 SC: It doesn't tell you why it's so small, but it tells you that it's not the kind of thing you should think of as being the sum of many different contributions, some positive and some negative. Thomas Prante says, "I've heard you describe gauge theories a couple of times, like in the Crisis in Physics episode, and while I think I understand the words, I'm still missing something. If all field configurations related by the gauge transformation are physically the same, then how can that transformation also generate a force which does not have real effects. It seems like a symmetry should keep things the same rather than generate something new.

2:36:58.9 SC: Yeah, so I think that there's an ingredient missing here, which is that the gauge transformation itself does not generate a force. What happens is you would like your theory to be invariant under gauge transformations, and that means you have to be able to compare what is going on at different locations in space. How have you rotated your axis in some charge space or color space or whatever, at one point compared to another one. What that means as you crank through the math and figure that out means that you have to introduce another field. A field in addition to the electron or the quark and whatever you had, there has to be another field that helps you compare what is going on at one point in space to what's going on at another point in space.

2:37:46.9 SC: That field is called the Connection field or the gauge potential, and how that field changes from place to place, the field is constant, that the connection is just constant everywhere. Then there is no force. Then there's no thing that is physically pulling particles together or pushing them apart. But that field contains energy, the connection field, if it has twists and turns in it, those cost energy to make, and we call those twists and turns the electric field and the magnetic field.

2:38:17.4 SC: And so the force-carrying fields are related to the changes through space-time of the connection field that helps you implement those gauge transformations. So it's not that the gauge symmetry itself gives rise to a force, but mathematically making it all work out introduces another field which gives rise to the force.

2:38:37.3 SC: Sandro Stukey says, "I love the episode with Katie Elliott" During your discussion of the principal's vision, reason, I picked up on this statement of yours, which resonated with me. How confident are we that when we're reasoning about things that are very different from our universe that we can say things that are not overly tainted by our real world experience. I feel like that's a big issue that goes right down to the foundations of philosophy and logic itself. But maybe that's not what you meant. Can you elaborate a bit on that thought and share some more of your ideas on the topic. Well, I don't think I can give a once and for all definitive answer to it, but I do agree with you, I think, that this is a central concern when you're doing philosophy, or for that matter, when you're doing science. We have certain intuitions feelings about how the world works.

2:39:21.8 SC: We are not blank slates. We human beings. So when we do the practice of science, which is to propose theoretical models that might explain the world and then experimentally test them against what the world is actually doing, we naturally tend to gravitate to things that make sense to us, that make intuitive sense to us. Sometimes the data or some other feature of the world is just so unmistakable, so impossible to miss that it pushes us away from our intuitions like in relativity and quantum mechanics and so forth, even in natural selection in some ways.

2:40:01.0 SC: But other times it doesn't, or other times its influence is a little bit weaker, so we have to work harder, and there's no magic pill, there's no algorithm that says. "Here is how to avoid being tainted by your everyday experience of the single universe that you find yourself in." You have to work at it. So it's just a matter of practice and concentration to be able to take a step back and say like, "Okay, am I trying to force the universe into my preferences or am I taking the universe on its own terms?

2:40:36.3 SC: Dirk Schmidt-Hofer says, What is your sleep schedule, roughly hours per day. Do you take naps or are you one of those people who only need three hours a night. I am not one of those people who only need three hours a night. I would love to get eight hours a night, probably it's more like six usually. Big fan of naps when I can get them. I can't always get them, especially after a big meal or something like that, but the same schedule changes around depending on what I'm doing how I'm traveling, things like that. I'm just not a very organized person when it comes right down to it. I don't know whether that's useful information or not.

2:41:12.8 SC: Dan Inch says, "Do you have any plans to incorporate the cats into your reflections videos?" The spectacle of cats calling over a famous physicists would be fun even if they are not in a super position. I would love it if the cats wanted to participate, Ariel and Caliban, but I think that they don't know. I've noticed they don't like it. When I'm sitting alone in my office talking into the microphone, they think that's weird.

2:41:35.3 SC: I know other cats are different, every cat is different, they have their own personalities, that's part of the charm. So they're very happy to climb over me if I'm just sitting and reading, but if I'm talking into the computer, they are not interested in that, they will go search for other fun. Maybe just 'cause they can tell that I'm concentrating on something else. They're not as interested in bugging me, they're like most cats if you have a book, oh, they love that. They love sitting on the book, but if I'm at the computer, they're less interested. I'm gonna group two questions together, Aaron says Shouldn't Boltzmann brains become less likely over enormous amounts of time as particles become more spread out in space and interact less often. And then Yu Misiara says, "How can a Boltzmann brain pop up in the very, very distant future, if in this very distant future, there will be nothing to interact with the quantum fields and produce particles.

2:42:24.6 SC: Well, this is actually a contentious issue. Let me tell you the conventional wisdom, and then I'll tell you a little spin on it. The conventional wisdom is that we just said a little bit ago, if you have a positive cosmological constant and you approach a De Sitter phase of the universe, then you have a horizon around you, and that horizon has an entropy. And as Hawking showed for black holes a long time ago, having a horizon with an entropy also implies a temperature. So this is a feature of quantum field theory in curved space-time that in the future, even though everything empties out classically, quantum mechanically, there is still a non-zero temperature. And so naively, you expect that at that temperature and the temperature does not go to zero. It goes to a certain number that depends on the value of the cosmological constant. The higher the cosmological constant, the higher the temperature. So it does not asymptotes to zero, it asymptotes them to some finite non-zero number. It's a very, very low number, but much lower than the microwave background temperature today, but still not zero. So the conventional a thinking is that if you have a non-zero temperature, you have thermal fluctuations, they are very, very rare, very, very mild, but you have infinitely long to wait, so eventually you will get some dramatic large fluctuations.

2:43:40.6 SC: I think that that's a mistake. I think the conventional wisdom is wrong about that, at least... Let's put it this way, it's not necessary, it depends on details of quantum gravity and things like that, in fact, it depends on the dimensionality of Hilbert space and whether we can consider our observable universe as a closed system or open system, et cetera.

2:44:00.0 SC: So I wrote a paper with Kim Boddy and Jason Polik a while ago, making this point that. The word temperature means something a little bit different in quantum mechanics and classical mechanics. In classical mechanics, the temperature of a box of gas or something like that, is the average kinetic energy of all the molecules up to some constant factor. In quantum mechanics, it's a little bit different because in the classical box of gas, even if the macrostate is constant, even if you're in equilibrium, so all of your gas is perfectly smooth everywhere, the microstate is not constant at a constant temperature, at a fixed temperature, the molecules are moving around. So the fact that the state looks like its static is just a trick because you're not seeing the microstate, you're only seeing the macrostate.

2:44:52.5 SC: Whereas in quantum mechanics, a state that is in thermal equilibrium, what we call a thermal state in quantum mechanics is actually truly static, microscopically as well as macroscopically. So there are no thermal fluctuations that dynamically bring things into existence like they would in a moving around box of gas. This is a subtle difference, but an obvious one, once you think about it, between what we mean by a thermal state in quantum mechanics and a thermal state, in classical mechanics. In quantum mechanics, there's no actual single unique micro-state where the particles are moving. There is a unique quantum density matrix or, yes a density matrix, if you have a thermal state, but it's static, 'cause nothing is happening. So Kim and Jason and I argue that in fact, if that's the kind of state that the universe asymptotes to in the future, which it arguably is, then you will not get fluctuations in to Boltzmann brains.

2:45:52.5 SC: So I would say, and not everyone agrees with us, even though they should. So I would say that the question about whether or not Boltzmann brains actually do fluctuate into existence in the real world is an open one. We don't know whether or not that's actually gonna be what happens. Not to mention we don't know whether the cosmological constant will last forever, and things like that. Those are separate questions.

2:46:15.0 SC: Mark says, "Would your views on religion and atheism be substantively different had you not studied and being trained in astronomy and astrophysics and were instead a non-science professional or tradesman?" Well, as we just discussed a little bit ago, I would not be me if I were not someone who studied what I study. I'd be a different person. So you're not asking what I would think, you're asking what some completely different person would think, and that's impossible to tell. If what you mean is, do I think that my studies in astronomy astrophysics had a large impact on my opinions about theology and religion, not a large impact. No.

2:46:55.2 SC: I think vaguely maybe thinking like a scientist has had a large impact, but I don't know whether if I were a completely different profession, so and this is why it's a difficult hypothetical question. I don't know why I would be a different kind of profession, whether I would still have the love of science and the interest in science that I have. There's plenty of people out there, of course, who are not in any way experts or knowledgeable about science, who are still atheists, so I'm not quite sure if there's gonna be a strong connection there.

2:47:26.0 SC: O or OA says, it's been stated several times in the Podcast and elsewhere that an event horizon is not a thing, and you wouldn't notice passing through it if the tidal forces are sufficiently small. But once inside the black hole, all future paths must be closer to the singularity. What does that mean for biological functions like pumping blood or sending signals over nerves. Yeah, there's two things going on here. One is the aventerizing, which is a location in space, time, and again, you're right, it's not a thing. And I therefore say, When I'm careful, I say, if the black hole is big enough, you would not notice when you cause the aventurizing.

2:48:05.3 SC: So what is the size of the black hole have to do with anything? And the answer is that there are tidal gravitational effects, tidal as in T-I-D-A-L. In other words, when you're near a black hole, depending on its size again, and the smaller, the more dramatic the title effects are, gravity is pulling you in some directions and pushing you in others, just like the tides here on earth are pulled toward the moon, but they are also squeezed in the direction perpendicular to the line of sight pointing toward the moon, that's what tides are. So gravity doesn't just pull overall, it also stretches differentially in different directions. That's a very noticeable effect. And a small black hole where the differences from place to place are large, the noticeability is bigger. This is what leads to the phenomenon known as spaghettification. If you fall into a black hole, eventually you're going to be stretched in one direction and squeezed into other directions and turn into a little piece of spaghetti.

2:49:06.3 SC: So you will very definitely notice the gravitational field of the black hole, it's just that there's no sign post precisely at the location of what we call the aventurizing. Redmann says, "With the unambiguous detection of polarized B-mode swirls in the cosmic microwave background constitute case closed evidence of cosmic inflation." So for those of you who don't know, when you have inflation, inflation in the early universe, there's still quantum fluctuations in the early universe, so inflation tries to make the universe as smooth as possible, but it can't make it perfectly smooth because of these quantum fluctuations. Quantum uncertainties, if you like. And that's what, if inflation is right, gives rise to the density perturbations we see in the cosmic migrate background.

2:49:53.0 S2: That's a very nice picture in inflation, and it's one of the reasons why people are so positive about the idea of inflation. But it's not knock down evidence for inflation because maybe there's some other mechanism that we haven't invented yet that accounts for those fluctuations. So people who think about inflation have pointed out that there is another thing that inflation does. It doesn't just lead to fluctuations in densities, it leads to fluctuations in the gravitational field as well, which show up as gravitational waves, and those have a very specific unique signature in the cosmic microwave background, a certain kind of polarization signature that is called the B-mode as opposed to the E-mode. The E and B are the signals for electric field and magnetic field, but they don't mean that in this case, they just mean different mathematical characterizations of the polarization pattern in the CMB.

2:50:48.0 SC: And the E-mode is predicted to be there, whether or not there are gravitational waves, and it has already been observed. The B-mode is predicted to be a special thing predicted by inflation, and it has not been observed yet to be a little bit more careful about that, a primordial B-mode has not been observed. A B-mode from the very early universe, there are other things that can give rise to B-modes in the later universe that don't give us evidence for inflation or against it.

2:51:15.9 SC: So the question is, this is a prediction of inflation, would it be case closed evidence? No, because there is never a case closed evidence in science that's just not how things work, especially when the science is something as speculative and hard to constrain as the super-duper early universe. The question again would be, is there something else that could possibly give rise to the same signature, is there another model or another theory, and we don't know of one right now, but what you would do is just you would be a good bazian. You would say, "What is the chance that I would see the signature if inflation were right versus if it were not right, and I would update my credence appropriately."

2:51:54.4 S2: I would say that it would be strong evidence, a lot of people's minds would be noticeably shifted by observing B-modes, but it's not case-closed evidence in any real sense.

2:52:05.8 SC: Emile Roha says, "Are particles in an atom or molecule quantum entangled or in the nucleus for that matter. Yes, they are, but it's a subtle kind of thing, and I'm gonna try to say things that are all completely true. Let's see if I can succeed here. The reason why they are is because the kind of particles, let's say electrons, let's stick to a specific example, electrons in an atom. Electrons in an atom are identical particles. There's no difference between one electron and another one. You can't say, "Well, I noticed electron A over here, and electron B over there" other than to say, where you found them.

2:52:45.9 SC: So if you have, let's say, two electrons that are in two different orbitals in an atom. And the same, it doesn't matter. The same atom could be a different atom, very far away, whatever, two different electrons. And you measure one of them, you can't say which one you've measured, there's no such thing, and that is a feature of them being entangled with each other. The reason why I have to be very cautious about this is because it doesn't quite have the same implications that you're used to entanglement having.

2:53:14.1 SC: When I talk about entanglement, when you talk about spins being up and down, and Alison, Bob and an EPR experiment, then there's a feature that says, If you know the particles are entangled so their spins are oppositely aligned, then we say when you measure one and it's spin up, you instantly know the other one is spin down. So you've learned something about the particle over there.

2:53:36.0 SC: In the case of electrons entangled in an atom and maybe their spins are also entangled in such a way, but we're ignoring that for right now, we're just talking about the necessary entanglement because they are identical particles, you don't learn anything about the other electron by measuring one. All you do is that you learn that the other one is the other place. So if you have two different orbitals and you know exactly what they are, you have exactly two electrons in them, they are in some entangled wave function, but all you learn by measuring one is that the other one is in the other one.

2:54:06.5 SC: So you don't actually get any new information by measuring them, but strictly speaking, they are entangled, yes. But because of this observational implication, you can very well treat them as if they are not. So when you take your chemistry class, you don't need to learn about entanglement or anything, because there's two electrons and two different orbitals, but they're identical, it doesn't matter which one is in which one. So if you just wanna talk about the electron that is in this orbital and the electron that is in that orbital, you're fine. All the physics is the same, all the observational consequences are the same, et cetera.

2:54:42.7 SC: Danny Avidan asks a priority question. "Last time I asked you who constitutes a moral subject, that is how should we treat morality. A follow-up please. Do you think we can understand, analyze, critique, argue, and expand our moral system without a definition of who is and who should be included in it?" No, I think that obviously, if you're gonna talk about morality, you better talk about who it applies to. I can imagine different moral systems having different answers to that question, some would be for all conscious creatures or ascension creatures.

2:55:15.6 SC: Some would be for human beings. Some would have a nuanced in-between kind of view where sufficiently intelligent creatures would count under your moral system and others wouldn't, some would say that some moral strictures apply to some creature than others, so I think all of these are on the table as possible things to think about. The other thing to think about is the when I think about morality, I don't think that there are sharp bright lines anywhere. I think that morality is kind of a fuzzy thing, and then we should be better at embracing that and accepting it, or at least not living in denial about it. That doesn't mean it's not super important and you shouldn't work hard to be moral, but you should realize that it's not like a mathematical proof or something like that. It's a little bit more subjective, more than a little bit more subjective than that.

2:56:02.6 SC: Egor Pushkin says, "In quantum field theory, why is it? Why is the invariance of the Lagrangian that we require, and not the equations of motion. How do we know our theory requires this exact invariance. So I don't think it's true that we require invariance of Lagrangian rather than the equations of motion. I mean, maybe there's a subtlety because sometimes you have an equation of motion and a symmetry, and the thing is that the symmetry acts to change the equation, but it changes every term in exactly the same way. So the underlying solutions to the equations are not changed. Whereas you might have a situation where the Lagrangian is simply unchanged.

2:56:45.7 SC: So again, for those of you who don't know, the nice thing about a Lagrangian, a Lagrangian is an expression from which you can derive many different equations of motion at one. So you might have a theory that has different kinds of particles and fields and whatever in good old fashioned Newtonian mechanics, classically, you would have to separately have equations of motion for all of those pieces of your theory, they might involve interactions with each other, but they would have separate equations. The Lagrangian point of view lets you combine all of the things in your theory into one expression, the Lagrangian, and you integrate that overall space time and you minimize that, and those are your equations of motion. And the great thing about this single expression.

2:57:28.0 SC: Is that it not only has equations of motion for everything, but that it's very easy to implement the symmetries, you just look at the Lagrangian as a whole rather than all the separate equations of motion, okay? So there's various conveniences involved in doing things the Lagrangian way, but at the classical level there's no deep difference between Lagrangian and the equations of motion, you just use one to get the other. Quantum mechanically, there's something extra going on which is not mentioned in this question, which is that you're not classical anymore, you're quantum mechanical and the... Neither the Lagrangian nor the equations of motion are the whole story. One nice way of thinking about that is to think about the path integral, when you have a regular integral of a function F of X, you write integral F of X DX, right? And F of X is the function you're integrating, DX is the measure, it tells you how much counts from each little interval DX. The same thing is true for the path integral formulation of quantum mechanics, you're summing over all the different possible paths or histories or field configurations, and what your summing is E to the IS where S is the action that you get from the Lagrangian. But there's also that measure there in the path integral and you need to separately have in variants of Lagrangian end of the measure.

2:58:52.0 SC: If you have a theory and you can very well have the theories, as I remember I talked in the solo episode about anomalies, that was one of the reasons why String Theory had a bandwagon that got launched in the 1980s, 'cause people realized that anomalies could cancel in String Theory. And an anomaly is precisely a situation where there's an invariance or a symmetry that is there in the Lagrangian, but is not there in the measure that you put in the path integral. So it is a classical symmetry that is violated by quantum mechanics. So anyway, all of that is to say it's not the Lagrangian that matters, it's the whole theory that matters, some ways of thinking about the theory might be more convenient than others. Shambles says, "After listening to all your podcasts and reading your books, it could be said there was a kind of emergent Sean Carroll that exists in my brain, perhaps to the point where I can imagine your response to certain AMA questions or words or phrases you might use with a fairly poor degree of accuracy. I can imagine for your spouse or a close friend their version of you will be much more real." In what ways is the emergent Sean Carroll in their brains the same or different to the emergent Sean Carroll in Sean Carroll's brain." I don't... So I presumed it by the emergent Sean Carroll in Sean Carroll's brain you mean myself?

[laughter]

3:00:08.7 SC: Not just me having the emergent person in my brain, I think the point is that you do model other things in the universe in your own mind, and that includes the physical universe, it also includes people, right? So you have different ideas about how people will act, what they will do, they might be probabilistic or approximate and so forth, they might be more nuanced once you get to know people more. Some people might not need a lot of information to model them very effectively, they're very predictable, other people will need a lot of information to think about them, and they're less predictable. So I think that it is, I'm not exactly sure what the question is to be honest or how best to answer it? I think that whenever you try to imagine what another person is going to do, you will inevitably over-simplify, right? You don't know all the details about what is going on in someone's brain to get them from point A to point B, sometimes you don't have to, right? Sometimes the answer is just obvious or you know it or you've heard it before, other times the answers can be very tricky, this is what makes human beings so delightful, that they're not quite so predictable. So you don't have enough room in your brain to accurately model many other people, because their brains are also pretty complicated, but it will differ in the details, absolutely, from person to person.

3:01:34.0 SC: I'm not sure if that's in any way a satisfactory answer, but it's the best I can do. P. Walder says, "You have provided an explanation for how the Core Theory can fully account for the matter we engage with in our everyday lives, including neurons in brains. Is this now the accepted view in the physics community or are there gaps in the explanation which would allow for yet to be discovered energy fields to play an explanatory role without having to create a whole new physics paradigm." I think it is overwhelmingly the accepted view among people who have thought about it that much, which is not that many people, right? It's the kind of thing where when you tell a professional physicist what this claim is, they will think about and go, "Yeah, okay, that sounds right." There are some well-known counter-examples, there are absolutely professional physicists who want to think that the fundamental laws of physics as we currently understand them will need to be updated even to take account for things that we see in the everyday world. So they're out there, but I think that they are a dramatic minority, I think that the more important thing is that most people haven't even asked themselves the question.

3:02:40.4 SC: Chris V says, "Do you have any guidance for folks who work in intellectual fields? Curiosity, learning and iterating on ideas is wonderfully fun, however, I often find myself lost in thought or living in my head. Have you experienced this yourself, and if so, do you have any advice for separating that aspect of your life in order to be more present in others?" So I'm not exactly sure whether you're focusing in on being a productive intellectual or being a productive or maybe productive isn't the right word, being a successful person who is also an intellectual but working, but appearing sometimes in non-intellectual contexts. So there's two issues when it comes to sort of being lost in thought, one is that your job is an intellectual job to come up with ideas and talk about them and test them and so forth, there are absolutely people who like the part where they get to think and don't like the part where they got to work to explain their ideas to somebody else or to develop them fully, or to write them down or whatever. If that's the issue, then there's one thing which is being intellectual in thinking, there's another thing which is earning a living at it, and earning a living at it absolutely means you gotta do the work, you gotta really put in the effort to turn your thinking into something that is productive for other human beings.

3:04:05.0 SC: There's nothing bad about that, by all means spend your time thinking, but if you want that to be your job, then your job involves also talking to other people. If what you're getting at is you love thinking and you get lost inside your head and then the person you're having dinner with says, "Hey, you're not paying attention to me." That's a very different kind of issue to worry about, I think that's just a matter of politeness, you have to be able to balance the fact that it is fun to think about things, to get lost in your head, to dream about things, with the fact that there's a time and place for doing that. And we all know charming people who are kind of absent-minded, there's famous stories about intellectually accomplished people who are kind of unable to get through the day, tying their shoes and so forth. The charm only goes so far, [laughter] and I think that most people, if you're smart enough to be good at intellectual work, you're also smart enough to tone it down in circumstances where it is not appropriate. Josh Charles says, "When looking through a telescope at a galaxy, is it fair to say that those photons are branching their wave functions by interacting with my retina for the first time since being admitted, or would that have happened when the first lens it encountered." Well, it happens both.

3:05:26.7 SC: So very often a photon that is emitted from an atom in some astrophysical event has a wave function that moves in more or less a spherical pattern away from its source. And what will happen is it will bump into that spherical wave function, has a probability of bumping into various things along the way. So if you have lots of little atoms around the event, and maybe some of those atoms are in the form of telescopes, somewhere in the form of eyeballs, but some were just in the forms of rocks and things like that. For each thing that the wave function could bump into, there's a branch where it does, and then in the wave function disappears in that branch from all the other directions in which it was moving. But then there's the other branch where it didn't hit that thing you're talking about, and it continues to move on in all of these different directions, and then it could be absorbed by something else. So that's just to say that the ultimate set of all the branches of the wave functions are ones in which that photon has been absorbed by many different things throughout the universe. So there's the first time it gets absorbed, but in the rest of the wave function it keeps going and it will be absorbed and branch further down the road, depending on what it hits.

3:06:44.3 SC: Rams Seshadri says, "If I was a lowly patent clerk and came into possession of earth-shattering new physics, how could I go about attributing credit to myself and is there any chance I can pull it off without getting rumbled?" I think that this very question really is completely upside down, it shows that you don't... Not really familiar with how academia works, it is absolutely possible to have your ideas stolen, okay, in academia, but the way that that happens is precisely that you don't tell anybody about your idea except for maybe one or two people and those one or two people go and scoop you, okay? That's how it happens. Once you put a paper into the public domain, whether it's on archive or in a publication or even just on your website, then you have priority if there's a date on it that is verifiable, then someone else can try their best to write a paper and say, "Hey, I thought of this." But the whole rest of the world can just say, "No, no, it appeared here earlier, right there." There's no issue when you put a paper out into public, that's when you get to claim it, that's it, that's when you get the idea. The idea that by putting a paper, a result into public it will get stolen is precisely backwards, it is by not putting the idea out there into public that it can get stolen, okay?

3:08:09.7 SC: Especially if it's a true idea, if you actually come up with something correct, you're not gonna be the only person who comes up with that idea, someone else is gonna hit on it before too long, so all of the incentive is to get it out there into public as soon as possible. So in terms of if you're not a credentialed scientists, where do you put it, et cetera, the best thing is to put it on the archives but you need to get some endorsement from some person who knows you and who has talked to you, who already has the ability to put it on the archive and then can endorse you to also put it on there. If you find yourself never having talked to a professional scientist about your work, then I think that the chances that your work is really earth-shattering are probably very small, certainly, that is not the condition that Albert Einstein, when he was a patent clerk, was in in any sense of the imagination. Rue Phillips says, "I am writing a book on prediction, I spent a couple of years and got a solid foundation, but eventually got stuck. I write in a robotic voice, not super interesting and I'm not especially skilled with grammar language, et cetera, enter ChatGPT, I'm doing an experiment where I revise my chapters using GPT4 with a particular tone. It is making great improvements along with better structure and even providing additional examples to include."

3:09:29.3 SC: It has unblocked me, however, I'm now worried that I'm a fraud or the no lit agent or publisher would want to touch it" What are your feelings about the situation I'm in, or what advice can you offer?" I think this is a great question, this is a question that is gonna become very, very important down the road, and I don't know really what the right thing to do here is, my... I'll give you an impression, which is sort of, you should think of it as my first impression, subject to being updated later, as I think about it more and we learn how this goes. Which is that ChatGPT or GPT4, whatever, these are tools that you're very, very welcome to use if they help you write then that's great, but maybe you wanna give credit to them, maybe you wanna say in the acknowledgements of your book, "I used this tool to help me write or whatever." I don't think that there's anything today about using this tool any more than using a dictionary or a calculator or whatever, having said that there's one huge footnote here, which is very, very important. Which is that unlike a dictionary where some people sit down and consciously compile it and then offer it out there into the world for you to buy, these large language models have been trained on other people's writings, so it's completely legit to worry the things that are written by ChatGPT, et cetera, are just plagiarized in some sense.

3:10:56.3 SC: They're not literally plagiarized because ChatGPT remixes them to be completely unrecognizable in some way, but they were trained on something and those somethings were written by other people. So I would tread very, very carefully, I would use it. I have a friend who uses the I Ching to make difficult decisions, [chuckle] you cast some stones or some sticks and then you look up in a book what it's advising you, he doesn't actually think that the I Ching is predicting the future or anything like that. The point of using the tool is to shake yourself out of your rut, right? To roll some random numbers, get a suggestion and then think about the suggestion, you're like, "Do I like that suggestion? Do I not like it or whatever? I think that's the best way right now to use these AI tools, not to do your work, but to inspire you to do better work, if you can come up with a better way of writing a better metaphor or whatever that is inspired by playing around with ChatGPT, I think that's okay. That's no different than being inspired when you're writing a novel by reading other people's novels, right? You're not rewriting the same novel, but you can absolutely get stylistic points or inspiration or things like that, I think that's how you should treat AI models. But again, all of this is very new and changing rapidly and subject to update without notice.

3:12:24.0 SC: Anonymous says, "There is renewed interest in psychedelics from mainstream media and institutions like Johns Hopkins. Have you any thoughts on this that you want to share, for either the renewed interest or the compounds effects themselves?" Yeah, I'm very much all in favor of it, we did a podcast back a while ago with Robin Carhart-Harris who is one of the world's leaders in this field, I think that research into psychedelics got a bad wrap back in the '60s for cultural reasons, not for scientific reasons. And you can use them for pleasure, but you can also absolutely use them for therapeutic reasons, and I'm not an expert, but my vague impression about how to think of it is that there's various syndromes in the human mind that come about because you get stuck, you get stuck in a vicious cycle or a rut or whatever. It can be PTSD or depression or addiction, there's various ways in which the brain kind of gets stuck in a bad place, and this kind of thing is exactly what psychedelics are pretty good apparently in the terms of the tentative research that's been done at breaking us out of. So I think that there's potentially enormous possibility for using psychedelics for good therapeutic reasons, we need to understand what the side effects are, e etcetera, and like I said, and not an expert so I don't.

3:13:46.9 SC: But I'm pretty optimistic that this is something that's gonna be very useful going forward. Emerge Holographic says, "Atheist often take the stands that reincarnation is impossible, that we get one life, and that's it. But it would seem to me that placing such importance on one life is actually granting exceptions to consciousness, we used to be dead before we came to life so why can't that happen again? What is the functional difference between pre-life death and post-life death that makes them mutually exclusive." I think that there is an actual simple answer to this, which is that it is true that I can take the raw materials of a person, the atoms, the hydrogen and carbon and oxygen and what have you, that has died and their body ceases to be in existence et cetera, but I can take all of their atoms and I can imagine reassembling them into a different person. The important thing is that there's zero sense in which that person is the same person that you started with, because they don't have any continuity of memory or of experience or anything like that. So there's nothing special about the state of your atoms pre-death versus post-death, but there is a cessation of your consciousness that is irretrievable once you're dead in that sense, you're not coming back.

3:15:08.5 SC: Sid Huff says, "The idea of the multiverse seems to be having it's day in the sun, the June 24th issue of The Economist included an editorial discussing how popular multiverse concept has become in modern cinema. The article suggests that the popularity of the multiverse might be responding to some deeper yearning, perhaps that reality might be more complicated than previously thought. Theoretical physics aside, do you sense that the growing awareness of the multiverse idea is emerging from a deeper yearning in humanity." Short answer, no, [chuckle] I think that there's nothing new about this deep yearning, right? I think that there are ideas that sort of catch fire in popular imagination, and if someone uses the idea of a multiverse in a movie or in a novel or whatever, it's probably because they saw it in somebody else's movie or a novel or something like that. I don't think it's because humanity has developed a new deep yearning, and also I don't quite agree with the diagnosis that there is a yearning that reality may be more complicated than previously thought, maybe that's true but I think there's a much more straightforward reason why the multiverse is so fascinating from a narrative point of view. It's that you and I have in our heads, different alternative histories, right? Have you ever done something in the past that you regret?

3:16:34.6 SC: Have you ever thought of how things would be different had you done them differently, or had things happened differently? I think that for 99.999 et cetera, percent of humanity, the answer is yes, and the multiverse in the narrative sense allows you to explore those possibilities. How your life could have been different, that people care about their own lives, basically, right? That's really when it comes down to it what they're interested in. So, of course, there's alternative histories and you can tell stories of alternate histories without imagining that they're literally there in the multiverse, but imagining that they are there in the multiverse puts a little bit more oomph to them, right? Imagining that there's a version out you out there who did ask that person to senior prom or whatever. I'm not sure if it's valid to think that way, and you just think of these things as completely hypothetical rather than caring about whether or not they're actually out there in physical reality would be my personal advice. Michael Lacy says, "Tim Maudlin mentioned the quantum entanglement might enable faster than light communication under certain conditions. What are your thoughts of the likelihood of this being possible, it seems that it would reek havoc with relativity and lead to paradoxes such as information traveling backward in time and arriving before it was sent."

3:17:50.7 SC: Well, I think it's super duper unlikely, honestly, Tim was very vague about what exactly the claim was and it seemed to be in the context of Bohmian mechanics in particular, which I don't think is the right version of quantum mechanics. So one of those things where very unlikely to come true if it does come true it'll be big. I wouldn't rule it out a priori, right? Physics got along pretty well for hundreds of years without realizing that the speed of light was a limit on anything, so we could go back to that just 'cause you can go fast than the speed of light doesn't necessarily mean you can go back in time. To be super duper careful about it, if relativity is true and you can go faster than the speed of light, then you can go back in time. But under these theories may be relatively just isn't true, maybe there's some other way of slicing space time so that you can't go backwards in it. So I don't know exactly what the idea is, I would be open to the possibility of it, but my credence on it ahead of time it would be very, very small indeed. Love feels best says, "If I crafted a glass sphere with a perfectly reflective interior mirror surface, and I inserted light into it along with a tiny camera, would we see light forever from a device receiving the camera's signal?"

3:19:11.7 SC: Nope, you would not and the reason is because when the camera detects the light, that photon or whatever it was, stops existing, so you can't say both that you have a glass sphere with a perfectly reflective interior mirror and that there's a camera in there. You can say one or the other, if you did just have a perfectly reflective interior and we forget about details like the whole glass sphere collapsing into a black hole or whatever, then the light would stay inside bouncing around forever, but once you start observing it, you start attenuating it and it would go away. Sam Hartog says, "Outside of the obvious difference in rate, how does cosmic inflation in the early universe differ from the expansion we observe today? If I had a dial that let me manipulate the value of the cosmological constant and I cranked it up from 10 to the -52 to 11, do any phenomenological distinctions remain.

3:20:04.9 SC: Given the naive similarities between the two, I was surprised that my perfunctory Googling didn't turn up any relevant results." Well, I'm not quite sure what kind of differences you want, in terms of the difficulty in solving Einstein's equation it doesn't matter what the size of the cosmological constant is, you just plug in, you get the solution, et cetera. But the cosmological constant is not a dimensionless number, it's a dimensionful number, it has a scale. So one way of thinking about that is if you are in De Sitter space, so again, you have no... If you idealized your space time by saying you have nothing in the universe other than the cosmological constant, then you would have a horizon and that horizon would have a size, okay? So you have a physical length scale associated with the value of the cosmological constant, if the cosmological constant is large, that length scale is small, they are inversely proportional to each other. So you can compare that length scale to other things, to the sizes of atoms or stars or whatever, if you're length scale, your Hubble radius or your De Sitter radius is small compared to the size of a star, then you're not gonna have any stars in your universe and so forth. So there will absolutely be quantitative differences, but it will still at the end of the day, be the same kind of solution to Einstein's equation underlying the whole thing.

3:21:26.3 SC: Joy Callback says, "Do gravitational waves from separate black hole neutron star mergers constructively and destructively interfere?" Yeah, they do, but not that much, [laughter] usually, when you think about observable interference patterns, like in a double-slit experiment, it's kind of important that the slits are nearby each other, once you move the two sources far away from each other then it just becomes harder to see any interference patterns. So the short answer is yes, longer answer is yes, but that it has no really noticeable effects on anything that we see in these events. Colleen Edwards says, "In your academic journey, was there one or maybe a few particular concepts or ideas in quantum mechanics that really challenged you when you were trying to wrap your head around it?" I don't know about that one, I think that's a good question because it's very hard for me to remember what was going through my mind when I was in undergraduate learning quantum mechanics for the first time. Honestly, when you typically and certainly for me, when you first learn quantum mechanics, it's mostly about doing the problem sets, right? Which mostly involves solving the Schrödinger equation for some potential matching at some boundary, calculating some rate, doing the WKB Approximation, thinking about harmonics of and Clebsch-Gordan coefficients and atomic orbitals.

3:22:55.0 SC: All that detailed technical stuff, it's really not about deep conceptual things, these days I'm learning more about quantum mechanics in a different context, when you have quantum information and measurement theory, there's a whole new world out there of different kinds of measurements you can do and different results you can get from them and information being shared between different quantum sub-systems. And all that is fun, but hard. So it's hard, just 'cause it's hard, it's not conceptually difficult, I can't get it, my brain is not big enough, it's just work [chuckle] it's work and it's rewarding to do the work and get the answer. So I taught a course at Caltech several years ago where I taught some of that stuff, it was very rewarding to do it, and I'm gonna try to figure out that I can some... Sorry that I can fit some of that stuff into a quantum mechanical textbook, but not other stuff, I don't know I have to... Whenever you're writing a textbook you have to take into consideration that you can't do everything, right? You have to kill some of your darlings and that's gonna be something I need to keep my mind on and then decide very carefully what deserves to be in and what doesn't. Anonymous says, "I really liked the conversation with Brian Lowry on the social deconstructed nature of the self. Here's a question related to that, it's possible to socially construct an identity that's detached from any physical or tangible living thing."

3:24:18.5 SC: "For example, a fictional character or personified concept like Lady Liberty or sports mascot. Intuitively, I think that as a human being my identity has a certain secret sauce that Spider Man will never have, no matter how many movies or comics or fan works people make about Spider Man, is this secret sauce and innate self that I have after all or is my intuition wrong. Am I just another Spider Man?" I think it's a great question and I think that my favorite answer is that neither one of those is quite right, I think that Brian is right to say there is no secret sauce there, there is no separate immaterial immortal soul, or even mortal soul or even immaterial anything. You are a bundle and jumble of different impulses and thoughts and social relations in various ways. And so that gives you some freedom, that gives you some agency to shape your notion of self, especially in response to the social world around you. But I don't think it's arbitrary or unaffected by physical reality, I think that there are also constraints, like I cannot identify as a 10 foot tall person, no matter how much I want to, I can claim that I am, but no one's gonna take me seriously, but I could identify as all sorts of different things that do plausibly comport with the physical reality around me. So I think there's a give and take and interplay, I think that Brian's point about the social construction of the self is a good one, but it's not the only thing to say.

3:25:49.8 SC: I do think the physical reality also has a say, and you need to be compatible at the end of the day, I don't have any great ideas about working that out in full, I'm gonna leave that to the psychologists. Jeffrey Siegel says, "I enjoyed your conversation with Katie Elliot, regarding the conversation about traveling back in time before the coin is flipped, if the Everettian view is correct, couldn't you have seen the coin comes up heads, but then travel back in time and then end up on a branch of the wave function where the coin is tails instead of heads." Well, the only honest answer to a question like this is who knows, nobody has a good theory of time travel and Everettian quantum mechanics, as we discussed, I think a little bit in that episode or certainly I have mentioned elsewhere. There are theories of how to construct consistent quantum states in the presence of closed time like curves, David Deutsch, for example, has put such a theory together, but it's not the whole Everettian apparatus of branching in multiple worlds or anything like that, it's just the wave function as a whole that he's talking about. So nobody has a sensible physical picture in which you both have branches of the wave function with different outcomes and time travel that starts you in one branch and goes back to a different one.

3:27:10.0 SC: So you would have to make such a setup in order to answer these questions, I thought about doing it, I thought about putting effort into trying to make that make sense, and then I realize that I have a job, [laughter] I have real work to do and inventing the time travel scenarios is probably not the most productive thing I can do with my time. But, I don't think that you can claim right now that there is a once and for all a sensible answer to that until you have some specific scenario in mind. Tara Lumaghi says, "Considering the new NANOGrav results, do you think we may be able to detect a frequency that actually tells us what space time is made of? Any experiments that can help us rule out quantum gravity versus String Theory, maybe even detecting echo of the Big Bang." Mostly, no, is what I wanna say here, by the way, it's not quantum gravity versus String Theory, String theory is an example of a quantum gravity theory, one that also involves other things as well, but quantum gravity is part of it. But that's just not what NANOGrav is doing, NANOGrav is like all other currently available windows on gravitational waves, very, very deep into the classical regime. It's gravity, sure, it's telling us something about gravity and so far what it's telling us about gravity is that General Relativity is doing fine, but it's not telling us about quantum gravity.

3:28:35.7 SC: The whole reason why quantum gravity is hard is because the scales at which you can detect gravitational effects are just different than the scales of which you can detect quantum effects, and that doesn't change with NANOGrav or anything like that. You're not looking at gravitational waves made by single atoms, et cetera, but there is the one... There's a couple of caveats there that might be relevant, maybe we do continue data collection and experiments, and we find that we no longer fit the data with classical general relativity. Then you open up a can of worms, but a very delightful can of worms where you can start saying, well, maybe it's quantum gravity or maybe it's some other classic theory or who knows what's going on, this hasn't happened yet, so we don't know what the answer would be. And you can imagine detecting something that you might call the echo of the Big Bang, but it's only a poetic kind of phrase, that is to say, you can imagine primordial gravitational waves, we've already talked about them generated by inflation. My impression is that the predicted amplitude of primordial gravitational waves generated by inflation is nowhere close enough to be detected by NANOGrav, but maybe I'm wrong about that, or maybe everyone is wrong, and maybe there is some particular feature that you could some day detect or a phase transition in the early universe or something like that, I just don't know.

3:30:01.4 SC: If we did, it would be indirect evidence, it would not be immediately very useful, but it would be a clue to something and that will be very, very exciting indeed. Jonathan Byrd says, "Besides calculus, what branches of mathematics do you use the most in your work? What would you recommend for a physics undergrad now, that might not be obvious, is there some little sub-field that has been surprisingly helpful for you?" I think it depends a lot on what kind of physics you do, I think that maybe I'm being [3:30:32.0] ____ about this, but I have the following thought in mind. There's a lot of physics students who catch on pretty quickly, that math, [chuckle] is kind of important in this game, right? In doing physics, and so they get the idea that they can sort of advance their physics knowledge by learning more math, by taking math classes, et cetera and you know what, they're right, it's true. Learning more math, whether it's Analysis or Group Theory or Abstract Algebra or Topology or whatever, can be very useful to your future physics career. However, all else being equal, it is less useful than learning more Physics [laughter] before worrying too much about learning more math don't forget to learn Physics, learn E and M, and Stat Mech, and classical mechanics, and quantum mechanics, and quantum field theory, and general relativity, and condensed matter physics and all of those things.

3:31:25.7 SC: There's a lot going on in the world of physics, and much of it doesn't require super advanced math, it requires calculus, maybe some complex analysis, if you're in the quantum mechanics side of the world, there's a lot of linear algebra, okay, that's maybe the one thing that they don't teach you enough of. 'Cause everyone learns Calculus and differential equations and things like that. Some sub-fields, you're gonna have to spend a lot of your time doing intergrals or doing series expansions or what have you, various approximations schemes, WKB and so forth. Others, you're gonna be getting exact results or you're gonna be doing linear algebra, diagnosing matrices, it depends on what kind of physics you wanna do. But almost always the kind of physics that you are learning, will teach you the math along the way, or at least will indicate to you what kind of math it is you need to know. Every good general relativity book teaches you enough about differential geometry and Tensors to get by, there's always more to learn, you can learn the math for its own sake and that's fun, and maybe you end up being in that small minority of physicists for whom this extra bit of mathematical knowledge is super helpful.

3:32:37.6 SC: But typically, honestly, as a physicist, you pick it up as you need it, as you go along, you don't spend your time guessing that maybe this is gonna be useful to you down the road. Tyler Whitmer says, "Do you think there is any specific area of inquiry in physics or philosophy that are being held back by institutional barriers to interdisciplinary work that could be advancing dramatically if more institutions took the SFI approach." Well, I don't know about specific areas, but I think that the whole boundary of physics and philosophy falls into that category, likewise, the boundaries of psychology and philosophy, or computer science and philosophy or biology in philosophy, et cetera, or for that matter, history and economics, or economics and literature. I think that there are various aspects of academia that for reasons that makes sense, but are imperfect, concentrate on specific disciplines to the fail... Not failure, but to the detriment of interdisciplinary work, people wanna know not just that the work is good, but which area it's in, which department in particular it should be in. And so you have to target. If you wanna do interdisciplinary work, you have to come up with some justification for this particular kind of interdisciplinary work, the interdisciplinary work of physics and philosophy and the foundations of physics and so forth.

3:34:05.7 SC: That's a kind of work that is very important, we're trying to build up that effort here at Johns Hopkins, SFI is doing a different kind of thing, right? It's looking at complex systems, which is a different area, which I also think is interesting, but it's not exactly the same kind of thing. So I think that you have to have a particular area in mind, every area is gonna be different, and then you have to figure out how to make it work, right? Where it could work, who, what kind of people, remember when I was talking about Cormac McCarthy and SFI, I said the individual people matter a lot, the barrier to physics and philosophy getting along better, is partly that there are institutional barriers to it, but also partly that the individual physicists and philosophers aren't interested, right? Some of them are, many of them have been on this podcast, but most of them are not, and that's sad in some sense, it makes me a little bit sad, that they don't see the beauty of it and the intrigue of it that I do, but that's more work for me, more things for me to do.

3:35:12.3 SC: So I don't have an overall theory of how to do this, I think you have to do it on a case by case basis, we're trying to do it in foundations of physics, philosophy of quantum mechanics, space time, arrow of time, statistical mechanics, cosmology, all of those things, there's plenty of room for great work to be done at those interfaces. Okay, the last question of this overly long AMA is from the Nikita Lozovoy, Nope, there's no N there, Lozovoy, "Do you sometimes think about the future of humanity specifically whether we are going to reach other habitable worlds, and if yes, how, or if we are going to be confined within Earth and solar system quarters until the humanity is extinguished due to the environment becoming uninhabitable or due to lack of natural resources, et cetera." Yeah, I think about it. I don't want humanity to become extinguished, I think eventually it will happen, right? I don't know if it matters to our current planning strategies, but we're not gonna be immortal, we are going to equilibrate, we're headed toward thermal equilibrium, but that's a long way off, so we have plenty of time. I suspect that humanity will survive and it will expand beyond earth, that we'll expand to the solar system and beyond. Some people worry that the stars are too far away and that it's a science fictiony scenario to imagine traveling to them, but in reality if your hard-nosed about it, it would take too long.

3:36:39.4 SC: I think that's wrong because I think it's easy, it's not possible to go faster than the speed of light, but it is easy to live a very long time, it's much easier to solve the problem of longevity than it is the problem of FTL travel. So the galaxy is some tens of thousands of light years across, give us a few million years there's no reason at all that we shouldn't be able to fill the galaxy, it's the word we there is doing a lot of work, millions of years is a very long time on human evolutionary time scales, especially now that we're entering a technological era where we have not only computers and AI, but gene editing and synthetic biology and things like that. So whatever it is that ends up traveling to other parts of the galaxy might end up looking very different than we humans look now, but I'm overall optimistic that we will not kill ourselves, and I suspect strongly that if we don't kill ourselves we will not just stay here on earth, we are going to travel to the stars, that's an optimistic place to end. Thanks as always for supporting Mindscape on Patreon and elsewhere, I hope you have a good month. Talk to you next week bye.

[music]