SEAN CARROLL | The Problem With Quantum Mechanics | Modern Wisdom Podcast 126

1. 0:00 – 0:59

   Why quantum foundations stalled: war, distance, and a focus on applications

   1. SCSean Carroll0:00

      In the 1920s, you know, Niels Bohr and Albert Einstein or Erwin Schrödinger and Werner Heisenberg could get together in the same room and talk about these things. Ten years later, they're all on different continents, right, or different countries, and they can't travel back and forth. And they didn't have the internet. So just the pace of progress was enormously slowed down. The focus of physicists shifted to very practical things, right? Building bombs and stuff like that. And then even if they did more impractical things like understanding particle physics and quantum field theory and all these other very pressing questions. So quantum mechanics, measurement problem kinds of questions were put on the back burner. And for that matter, it's not clear how to make progress on these questions, right? If you have a question about particle physics, you can take two particles and smash them together and do the experiment and see what happens. Eh, for the interpretations of quantum mechanics, as, as they used to be called, uh, it wasn't clear how you would ever know what the right answer was.

2. 0:59 – 3:59

   Mindscape, writing goals, and why Carroll wrote 'Something Deeply Hidden'

   1. CWChris Williamson0:59

      (wind blowing) I'm joined by Sean Carroll, theoretical physicist specializing in quantum mechanics, gravity and cosmology, research professor at the Department of Physics, the California Institute of Technology, and also the man behind the Mindscape podcast. Sean, welcome to the show.

   2. SCSean Carroll1:18

      Thanks very much for having me.

   3. CWChris Williamson1:19

      Super excited to have you on. Been a little while since we delved into physics on this show, so we're gonna have to get over a little bit of inertia, some audience and my inertia as well. And, uh, we'll, we'll kickstart everything back off again, right?

   4. SCSean Carroll1:32

      Inertia's part of physics. That's okay. We understand it.

   5. CWChris Williamson1:34

      You are the man. If there was ever a man to get over some inertia-

   6. SCSean Carroll1:38

      (laughs)

   7. CWChris Williamson1:38

      ... it's you right there. Um, before we, before we even start talking about physics, Mindscape podcast that you... It's not been going super long, right? But you've had some insane guests, like Max Tegmark, Seth MacFarlane, the guy, the-

   8. SCSean Carroll1:51

      Yeah.

   9. CWChris Williamson1:51

      ... the Family Guy creator-

   10. SCSean Carroll1:52

       (laughs)

   11. CWChris Williamson1:52

       ... like the voice of Brian and Stewie Griffin. Like, that's insane.

   12. SCSean Carroll1:55

       Yeah. Yeah. No, it's been great. It's been like a year and a half. Uh, I've been very, very lucky in people saying it. Not everyone says yes. Um, but, uh, you know, I always wanted to be one, one of the things why I wanted to do the podcast was just so that I could talk to intelligent people about things other than physics. And, uh, I've gotten some, uh, great guests and all over the spectrum. So yeah, it's been fun.

   13. CWChris Williamson2:19

       Is that informing the direction of any sort of more writing that you're looking at? Are you p- are you tempted to branch out into anything after having these conversations?

   14. SCSean Carroll2:27

       I've always been tempted. You know, I've al- and I always do. So, um, you know, my previous book, The Big Picture, was very broad, a lot of, uh, philosophy, a lot of biology was in there. And my last book was pure physics. So the next book is not gonna be pure physics again. I wanna, you know, keep it, keep it mixing it up.

   15. CWChris Williamson2:47

       Oh, got you. So tell us about your last book, Something Deeply Hidden. Why'd you write it?

   16. SCSean Carroll2:53

       Well, it's a book about quantum mechanics, and the world does not need another book about quantum mechanics, uh, for its own sake. There's lots of books out there of varying degrees of usefulness. Uh, one of the things I did for the book, you can see the results right in chapter one, is I went to Amazon and typed in the word "quantum" into the search bar and looked at all of the books that had quantum in the title. Quantum healing and quantum leadership and quantum yoga.

   17. CWChris Williamson3:19

       (laughs)

   18. SCSean Carroll3:19

       And it just goes on. And none of them have, uh, the Schrödinger equation or any differential equations at all in them. So I didn't want to, you know, just be negative, but I think that a lot of even the good books on quantum mechanics have this attitude of saying like, "Quantum mechanics is really, really bizarre. It's mysterious. We'll never understand it. Look at how bizarre it is." And I wanted to kind of be the opposite. I wanted to kind of say, "Look, this is just science. It's perfectly understandable. Even if we don't understand it yet, there's no reason to think we can't understand it." And even though I have a favorite way of understanding it, what I really care about is the fact that it can be understood.

3. 3:59 – 5:06

   ‘Understandable vs. understood’: quantum’s unusual taboo against deep questions

   1. CWChris Williamson3:59

      What does it mean for something to not be understood, but be understandable?

   2. SCSean Carroll4:04

      Well, you know, we think that there's plenty of puzzles in science, right? There are things we don't yet know. We don't know exactly how the moon came to be, right? Was it something colliding with the Earth or was it sort of a, the mutual formation of the moon and the Earth at the same time? But no one thinks this is impossible to understand, right? I mean, we'll, we'll, we'll get there. It's just a matter of, you know, getting the evidence and doing the theory, etce- et cetera. And I think that quantum mechanics has this weird unique status where it was put together in the 1920s with sort of a, a wall around it saying, "Don't ask questions about the deep things going on here." The way, what I like to say it is, physicists are extremely good at using quantum mechanics without understanding it. It's exactly like I'm good at using my smartphone to send text messages or take pictures, but I couldn't build one, right?

   3. CWChris Williamson4:52

      Mm-hmm.

   4. SCSean Carroll4:52

      And that's how quantum mechanics is for physicists. And I don't think that, uh, it needs to be. And I think that this is a, a huge, huge, uh, wrong turn that physics took, and we're trying to sort of correct the course right about now.

4. 5:06 – 6:05

   The measurement problem: two rulebooks for one universe

   1. CWChris Williamson5:06

      Hmm. So what, what is it about quantum mechanics that makes it so slippery?

   2. SCSean Carroll5:13

      It's that in the rules that we developed in the 1920s, and we still teach to our undergraduates today, quantum mechanics is unique in the following way. There's a set of rules for what physical systems are and how they evolve, just like every other theory of physics, when you're not looking at them. And then there's another set of rules that apply when you look at something, when you measure it, when you observe it. And no other theory of physics has anything like that. I mean, there's general relativity, Maxwell's equations for electromagnetism, Newtonian mechanics. Like, there's no set of rules that say what you see and what happens to a system when you measure it. And so why does quantum mechanics seem to need these extra rules? And this is what's called the measurement problem of quantum mechanics, and it's just been hanging around unanswered since the 1920s.

5. 6:05 – 8:39

   Wave vs. particle behavior: electrons, atoms, and collapse as a patch

   1. CWChris Williamson6:05

      Could you break down the measurement problem? Could you explain it with, uh, some experimental examples?

   2. SCSean Carroll6:11

      Sure. You know what we teach our students is that if you take an electron, for example, right, y- you imagine the standard picture of a, an atom that we've all seen that has a little nucleus at the center and electrons are orbiting around it. So we know that can't be right, because electrons orbiting like that would give off light, would lose energy, would spiral in, and all atoms would collapse into a point if that picture were correct.

   3. CWChris Williamson6:37

      Okay.

   4. SCSean Carroll6:38

      So they came up with the idea, well, maybe the electron is not a particle. Maybe it's a wave, maybe it's spread out and the wave sort of has a minimum wavelength, just like when you pluck a violin string or something like that, there's different allowed wavelengths. And that's why the electron can't collapse because it's actually spread out. That's a great idea. Then we have an equation, the Schrodinger equation given to us by Erwin Schrodinger which says how the electron behaves, et cetera. Except that when we look at the electron, when we shoot an electron through a detector, it doesn't look like a big fuzzy wave, like, you know, an electromagnetic wave would look like. It looks like there's a track, like there's a trajectory, like it's a particle, like it's located somewhere. So that seems to imply just from, on the one hand we need the electron to be a wave so that atoms don't collapse, but on the other hand when we look at it, it looks like a particle, that we need two separate sets of rules. And in what's called the Copenhagen Interpretation, the standard interpretation of quantum mechanics put together in the 1920s, they sort of just leaned into that. They said, "Yes, there's a separate set of rules for what happens when you measure a quantum system." But what they didn't do is tell you what it means to measure a quantum system. Like does it have to be a person? Could it be a video camera? Could it be a cat?

   5. CWChris Williamson7:56

      Mm-hmm. Mm-hmm.

   6. SCSean Carroll7:56

      You know, does it require consciousness?

   7. CWChris Williamson7:56

      Does someone get a ruler out? Yeah.

   8. SCSean Carroll7:58

      Yeah. How quickly does it happen? What if I just glance at it and don't measure it very accurately, does that count?

   9. CWChris Williamson8:04

      Out the corner of my eye. Yeah.

   10. SCSean Carroll8:06

       Yeah. And, you know, what we teach our students is that when you measure the system, its wave function dramatically changes. It, it collapses as we say. It was all spread out, and then it collapses to being at one point, that's where you see it. And it... you can't even predict where it's going to collapse to. You can only say the probability of getting different measurement outcomes.

   11. CWChris Williamson8:25

       Mm-hmm.

   12. SCSean Carroll8:25

       So the questions of, you know, what counts as a measurement? When does it happen? How quickly does it happen? Why is it a probability rather than a definite outcome? All of these are bundled together in what you call the measurement problem of quantum mechanics.

6. 8:39 – 10:39

   Who should fix it: experiments are fine; theory is behind

   1. CWChris Williamson8:39

      How much of that is, uh, an experimentalist's problem, and how much of that is a theoretician's problem?

   2. SCSean Carroll8:46

      Zero of it is an experimentalist's problem. The experimentalists have done the experiments, you know? I mean, it was an experimentalist problem in the 1920s, but those experiments have been done. There's extra experiments to be done, but the fact that when we look at the quantum systems, they behave differently than we don't look at them, it's not because the experiments aren't very good.

   3. CWChris Williamson9:04

      Mm-hmm.

   4. SCSean Carroll9:04

      The experiments are fine. It's because the theoretical prediction is wildly at odds with what you observe if you don't think hard about what's going on.

   5. CWChris Williamson9:12

      Got you. So there's no... I- in your view, is there any more work to be done from the theoreticians at this point? Or is it that the... uh, sorry, from the experimentalists at this point? Or is it that the theoreticians kind of need to get their chalk on the blackboard and do a little bit more?

   6. SCSean Carroll9:25

      Yeah, I would say the theorists are way behind right now, because they, uh, stopped thinking about this in the 1930s. You know, people like Einstein and Schrodinger, so not exactly lightweights in the history of physics, they were outraged by the state of quantum mechanics. They're like, "We're not done. Come on. Obviously we need to think harder, we need to do better than what we have now." But people like Niels Bohr and Werner Heisenberg and their friends said, they go, "No, no, no, we're basically done. Let's put this to work and, you know, do particle physics and do nuclear physics, and do condensed matter physics and so forth." And as a result, for whatever reasons, you can... there's a long list of reasons, it's an interesting topic to talk about, the physics community went on the side of Bohr and Heisenberg against Einstein and Schrodinger. And they said, "Let's just not think about this very hard." And they said this very explicitly. Like when people started to try to think about it, they got kicked out or ignored or abandoned or whatever. So I would say yeah, the theorists are way behind right now. We're just not anywhere near, uh, where we should be in terms of thinking through things like, you know, um, what counts as a measurement, are there different ways to experimentally distinguish different theories, how do you get the classical world out of the quantum world? All these questions.

7. 10:39 – 15:51

   Where does the classical world come from? The ‘cheat’ and why quantum is universal

   1. CWChris Williamson10:39

      Is that one of the reasons that there is a, a discrimination towards the classical world that the real world applications appear to be... the, the classical world is emerging out of the quantum world, we know enough to then be able to build a rocket or make a building work or create an iPhone, and it's like, well, we'll look at that, what, at some point in the future? Is that why?

   2. SCSean Carroll11:06

      Well, a lot of it is, you know, the, the story that we were told back in the 1920s is you're an observer. The thing you're observing quantum mechanically is a particle. Okay? Or maybe a couple of particles. An atom or an electron or something like that. A photon. And you're big and the particle's small. And forget about in between. Like, you're so big compared to the particle that you're basically classical. Right? So you can have a classical observer, according to this story, which I think is a bad story, but this is what we tell ourselves, the observer can be treated classically. The system that they're looking at is treated quantum mechanically. And you can get away with that as long as you are, you know, in the 20s or 30s and just observing particles. These days, we're... our technology is moving forward to the extent where we can create quantum mechanical effects in much bigger systems. So the dividing line between quantum and classical is not nearly as clear-

   3. CWChris Williamson12:04

      Mm.

   4. SCSean Carroll12:04

      ... as it used to be. And so that's part of what... I mean, if you wanna build a quantum computer, for example, or all these other quantum technologies, you actually have to take quantum mechanics seriously in a way that you didn't have to if all you wanted to do was bump two particles into each other and see what happened at the end of the day.

   5. CWChris Williamson12:21

      Hmm. Where does quantum mechanics become classical mechanics?... is there a, is there a line?

   6. SCSean Carroll12:26

      I would like to know this question.

   7. CWChris Williamson12:28

      Okay.

   8. SCSean Carroll12:28

      So, there's a whole nother long... you know, you... Don't get me started, is what I wanna say.

   9. CWChris Williamson12:32

      Go ahead.

   10. SCSean Carroll12:32

       Um, we are so bad at quantum mechanics that one of the ways that we're bad is that y- we take the classical world for granted. So the classical world, just to be clear, is the world that you're used to, right? There are tables and chairs and baseballs and soccer balls and they move in trajectories, and you can look at them and see where they are. What you need to know is where they are and how fast they're moving, and you can predict everything else from that. The quantum world, as we learned to describe it in the 1920s, is a completely different place. You know, there are no positions or velocities. That's just not... It's not like we don't know what they are. That's not what exists. What exists is this wave of entangled particles all throughout the universe. It's an entirely different way of talking. And you might think that there is this huge challenge of starting with this abstract wave that quantum mechanics talks about and explaining how it becomes to look like a classical world-

   11. CWChris Williamson13:31

       Mm-hmm. Mm-hmm.

   12. SCSean Carroll13:31

       ... with baseballs and tables and stuff like that. But basically, we cheat. We say like, "Well, we know that the world does look classical at the end of the day, so we're gonna put in that answer into all of our analysis." I'm gonna t- treat me and the planets as classical and not worry about anything until we get down to elementary particles. So this question of how you go from just a purely quantum description to the classical world, I have opinions about it, but it's one of the things we haven't explored as theorists nearly as well...

   13. CWChris Williamson13:59

       Mm-hmm.

   14. SCSean Carroll13:59

       ... as we should have. Some of the very basic questions are completely unanswered at the current state of the art.

   15. CWChris Williamson14:04

       It's like, uh, you know when you go to a theme park, and it's, "You must be this tall to ride the ride." It's like, "You must be this big to be classical mechanics." Like, where's that?

   16. SCSean Carroll14:12

       Yeah.

   17. CWChris Williamson14:13

       How high is that sign?

   18. SCSean Carroll14:13

       Where does that come from? Right. That's right.

   19. CWChris Williamson14:15

       Is- is it strange... Again, coming from someone who has no idea about physics, to me, it would seem natural for physics to be constant, for there to be rules and laws that govern everything, and for there to not be some sort of resolution at which, if you look below that in terms of how closely you look at something, that those laws change. Is that surprising? S-

   20. SCSean Carroll14:42

       Well, I think it's not... I think it's not the situation we're really faced with. Um, it's... Quantum mechanics, it's not something that becomes true when you look at small systems. That is what we were telling ourselves back in the 1920s. But again, that's kind of nonsense. (laughs)

   21. CWChris Williamson14:59

       Mm-hmm.

   22. SCSean Carroll14:59

       Quantum mechanics is true at all scales. You and I are perfectly quantum mechanical, and there's a limit where things become big enough that you don't need quantum mechanics to describe what's going on.

   23. CWChris Williamson15:10

       Mm-hmm.

   24. SCSean Carroll15:10

       There's a good approximation given to us by classical mechanics. So it's not that... It would be indeed very, very weird if the rules of nature were different for big things than small things. And, uh, I'm sure we'll get to, at some point in the conversation, the many worlds, uh, formulation of quantum mechanics, which was invented by Hugh Everett, who was a graduate student in the '50s. And this is exactly one of the things that he complained about. You know, he was writing letters to Niels Bohr, et cetera, saying, "H- why in the world can you treat the observer as classical?" You know, we're made of atoms and things like that. We're made of things that obey the laws of quantum mechanics. Why don't we? We should take that seriously. And that's exactly what he showed how we could do.

8. 15:51 – 26:24

   Sociology of ideas: reputation, gatekeeping, and foundations getting sidelined

   1. CWChris Williamson15:51

      So just before we get onto- t- t- to that, I'm right in thinking that there was a few people in your book that you quote as having been dissuaded from working on quantum mechanics, yourself included, right? Could you tell us some stories about that? I had, um, Sabine Hossenfelder on talking about politics in physics. And for the listeners who didn't catch that episode, it's- it's a while ago. It's back in the 30s, I think. The number 30, not in the 1930s.

   2. SCSean Carroll16:16

      Not the 1930s. That would be good.

   3. CWChris Williamson16:17

      That would be a sick podcast.

   4. SCSean Carroll16:18

      (laughs)

   5. CWChris Williamson16:19

      And, um, but, uh, and I- I totally didn't understand just how political and, um, subject to social bias and reputation and, you know, all of that stuff, uh, that the world of physics was. And the synopsis that Sabine sort of came up with that she finished was, "Well, if nothing else, today you have learned that politicians are p- that, uh, physicists are people too." And I was like, "Yeah. Yeah, I have."

   6. SCSean Carroll16:50

      (laughs)

   7. CWChris Williamson16:50

      So could you talk us through some of the- the sort of the politics of- of quantum physics?

   8. SCSean Carroll16:56

      Yeah. Um, you know, physicists are people too, and there's a lot of, uh, politics and reputation. All that stuff is very true. I don't like to, uh, use the word politics in this context because it has ramifications, like it has connotations elsewhere. Um, and furthermore, you know, it's very hard to get this right, because especially in theoretical physics, where we're saying, well, we know that our current theories are pretty good, but not perfect. We wanna get to better theories. Everyone has a different opinion about which theory might be better. We don't know the right answer yet. And so if someone's best favorite theory is not the one that is accepted by other people, they're gonna say, "Ah, those people just don't appreciate my theory. They're just being political-"

   9. CWChris Williamson17:44

      Mm-hmm. Mm-hmm.

   10. SCSean Carroll17:44

       "... and driven by, you know, groupthink and whatever." And maybe that's true, or maybe your theory is just not as good, right?

   11. CWChris Williamson17:53

       Mm-hmm. Mm-hmm.

   12. SCSean Carroll17:53

       So it's... Uh, I'm- I'm literally... I'm not saying that rhetorically. Maybe your theory is better, maybe it's not. And that's very, very hard to pick that out in an unbiased way, uh, especially in a situation like we're in right now or like we have been in quantum mechanics for many decades when the experiments don't clearly distinguish between one way of doing things and another way. So putting aside, like, politics and bias and stuff like that, it's certainly true that the course of physics is heavily influenced by influential people, right? It's, you know, someone who is a brand new graduate student and has a good idea, uh, an idea that is not in accord with the prevailing standards of the field is gonna have a much harder time than a famous Nobel Prize winner who everyone respects.And that's not crazy because there are m- a lot more people who are young graduate students who have really bad ideas, right?

   13. NANarrator18:50

       (laughs)

   14. CWChris Williamson18:51

       (laughs)

   15. SCSean Carroll18:51

       So (laughs) it's not just... Y- just by being unknown doesn't make you right. (laughs)

   16. CWChris Williamson18:56

       Yeah. Yeah.

   17. SCSean Carroll18:56

       So there's a filter. You know, physicists have a way of dealing with new ideas and just like everyone else in the world, they're, they're like, they take, pay attention to different ideas depending on where they're coming from and how plausible they sound and how well-articulated they are and so forth. Okay. Having said all of that, I do think that there, as I've said, there was this massive abandonment of the right set of questions to ask in the whole 20th century, um, by physicists. And part of it was... So there's so many different things going on. Part of it was just World War II happened, right? I mean, think about when this is and think about who it was. A lot of these smart people were Germans. A lot of them were Jewish. So e- in the 1920s, you know, Niels Bohr and Albert Einstein or Erwin Schrödinger and Werner Heisenberg could get together in the same room and talk about these things. 10 years later, they're all on different continents, right, or different countries and they can't travel back and forth and they didn't have the internet. So just the pace of progress was enormously slowed down. The focus of physicists shifted to very practical things, right? Building bombs and stuff like that. And then even if they did weren't practical, things like understanding particle physics and quantum field theory and all these other very pressing questions. So quantum mechanics, measurement problem kinds of questions were put on the back burner. And for that matter, it's not clear how to make progress on these questions, right? If you have a question about particle physics, you can take two particles and smash them together and do the experiment and see what happens. Eh, for the interpretations of quantum mechanics, as, as they used to be called, uh, it wasn't clear how you would ever know what the right answer was. And part of-

   18. CWChris Williamson20:36

       The barriers to entry are quite high.

   19. SCSean Carroll20:38

       Well, it's... The, the state of the art was so bad that the ideas that were being thrown around did sound more like literary interpretations than good old physical theories. The state of the art has gotten a lot better. Now we're actually talking about real, distinct, well-defined rigorous physical theories that we can distinguish between. But in the '40s and '50s, that just wasn't the case.

   20. CWChris Williamson21:04

       It's a shame. It's a, it seems to me a surprise as well that Einstein's side of the fence didn't win.

   21. SCSean Carroll21:14

       Yeah, exactly. So that is a surprise. You might think that Einstein has some kind of-

   22. CWChris Williamson21:18

       Kind of thought he would have been like the, you know, the, the Deontay Wilder of the, the big heavyweight champion of the world when it comes to, like, directing the physics community.

   23. SCSean Carroll21:27

       But he was also kind of a loner, right? He didn't collaborate a lot. Uh, he didn't have students very much. He moved to the Institute for Advanced Study in Princeton where he didn't teach classes. Uh, whereas someone like Niels Bohr was endlessly gregarious, worked with everyone, had an institute of his own where he invited everyone to hang out and so forth and would badger them and hector them into doing things. Uh, so there's a personality. This, I think this goes to Sabine's point. There... Just the fact that Niels Bohr and Albert Einstein were very different personalities affected the course of physics. And it became so bad that, you know, at some point the major journal within the physics community just stopped looking at, uh, papers on the foundations of quantum mechanics. Um, David Bohm, who was one of the best people working on this stuff, uh, was hounded by the House Un-American Activities Communi- uh, Committee for being a communist in the 1950s and was eventually... He had to flee to Brazil. Um, I mean, he had letters of recommendation from Einstein and Oppenheimer and he couldn't get a job in the United States, right?

   24. CWChris Williamson22:33

       Wow.

   25. SCSean Carroll22:33

       Uh, John Bell, who was a brilliant theorist who worked at CERN, uh, was a particle physicist by day and worked on the foundations of quantum mechanics at night and didn't tell anyone that he also worked on them.

   26. CWChris Williamson22:44

       (laughs)

   27. SCSean Carroll22:44

       His colleagues, you know, in the next door office didn't know that's what he was doing. And now his theorem is very famous and we all know about it. Uh, even today, I tell the story in my book, you know, I do a bunch of different things. Some fundo- foundations of quantum mechanics, some gravity and particle physics and stuff like that. And I'm told that when we're doing our grant renewals, don't talk about quantum mechanics, talk about cosmology or gravity. That's the serious stuff. That's what's gonna get us funded.

   28. CWChris Williamson23:10

       That's the sexy stuff. It's interesting that you've got, you've got someone like Albert Einstein that still needs to play the game, I think. I was having a discussion, uh, yesterday with, with someone on an Instagram live, uh, and we were talking about exactly that. That you can have the most virtuous, beautiful message in the world, which is full of integrity and adding value, but you still gotta play the game to some-

   29. SCSean Carroll23:31

       Yep.

   30. CWChris Williamson23:31

       ... degree. And, you know, if Albert Einstein p- perhaps fell afoul of that a little bit, and he's, you know, one of the, if not the best known physicist of all time, I think there's, there's probably a lesson to be learned that I don't know... It's probably a difficult pill to swallow, isn't it? Someone who's got so much talent and, and, and had raw ability and understanding w- like, was just not playing the social game correctly. It's a interesting, really interesting point.
9. 26:24 – 30:00

   Entanglement explained via spin and Higgs decay

   1. SCSean Carroll26:24

      Well, let me explain entanglement a little bit, 'cause that'll help us understand many-worlds.

   2. CWChris Williamson26:28

      Perfect.

   3. SCSean Carroll26:28

      And again, this is Einstein's, uh, point in his 1935 paper. Um, so I, I, I like to d- to describe it using a, um, the Higgs boson, which Einstein didn't know anything about, right? So elementary particles like electrons and Higgs bosons and photons can spin, and, and they have a, a feature that they're always spinning by the same amount. They don't speed up or slow down. But an electron, for example, can spin clockwise or counterclockwise. Uh, the Higgs boson doesn't spin at all. It's spinless, spin zero. So the Higgs boson can decay. It can... You know, in fact, it does decay very, very quickly into, let's say, an electron and a positron. Positron is the anti-electron. And we know the total amount of spin in the universe is constant. It's not being created or destroyed. Angular momentum is not being created or destroyed. So how in the world can a spin-zero particle decay into two particles that have spin?

   4. CWChris Williamson27:25

      Mm-hmm. Mm-hmm.

   5. SCSean Carroll27:25

      The answer is, they better be spinning in opposite directions so it cancels out, right?

   6. CWChris Williamson27:29

      Okay. Yep, yep.

   7. SCSean Carroll27:30

      And that's... That makes perfect sense. But-

   8. CWChris Williamson27:32

      S- sorry to interject there.

   9. SCSean Carroll27:33

      Yeah.

   10. CWChris Williamson27:33

       How do you know it's spin zero? Uh, sorry. How do you know-

   11. SCSean Carroll27:36

       Well-

   12. CWChris Williamson27:36

       ... that the total amount of spin in the universe is the same?

   13. SCSean Carroll27:39

       Oh, how do you know it's the same? I mean, it's, uh, it's one of the famous conservation laws of physics. So energy is conserved, momentum is conserved, angular momentum is also conserved, and, and spin is a form of angular momentum.

   14. CWChris Williamson27:52

       Even down at the quantum level, there's no special rule down there?

   15. SCSean Carroll27:56

       Nope, there's no special rule down there. It's, uh, this... It's an absolute... You know, quantum mechanics doesn't say anything goes. It doesn't say anything can happen, or even any, everything has a probability of happening.

   16. CWChris Williamson28:05

       Mm-hmm.

   17. SCSean Carroll28:05

       There are some things that just don't happen in quantum mechanics.

   18. CWChris Williamson28:09

       Or do happen. Or all of them happen. Yeah, yeah.

   19. SCSean Carroll28:10

       Or there's also things that do happen-

   20. CWChris Williamson28:12

       Yeah.

   21. SCSean Carroll28:12

       ... but, you know, there are some things that don't happen.

   22. CWChris Williamson28:14

       Got you.

   23. SCSean Carroll28:14

       So a spin-zero particle converting into a spin one-half particle is not ever gonna happen.

   24. CWChris Williamson28:19

       Got you.

   25. SCSean Carroll28:19

       It's just never gonna happen.

   26. CWChris Williamson28:20

       Okay. Cool.

   27. SCSean Carroll28:21

       So when... So it's... So the Higgs splits into two particles, an electron and a positron. (phone ringing) Oh, sorry. That's my phone. And, uh-

   28. CWChris Williamson28:29

       That's a good ringtone.

   29. SCSean Carroll28:31

       Uh, well, you know, it's a, it's a tune. Yeah. Uh, Madesky, Martin, and Wood, for anyone interested out there-

   30. CWChris Williamson28:36

       Fun tune.
10. 30:00 – 32:05

    Einstein’s ‘spooky action’ and Bell’s theorem: why hidden certainty fails

    1. CWChris Williamson30:00

       Could there not be the case that one is up and one is down, it's not a case of them being either way, and when you observe, it just happens to be that?

    2. SCSean Carroll30:08

       Could be, but the quantum mechanics makes a prediction, right? It makes a prediction, uh, for when you see the particles come out of Higgs boson decay, there'll be 50/50 up and down for the electron, 50/50 up and down for the positron.

    3. CWChris Williamson30:22

       Okay. So it's not a case that it is one of those two and you just look at it and it is the one that it is. It's a case that it could be either of them, and the observation causes it to choose.

    4. SCSean Carroll30:33

       Well, you know, this is exactly the question that Einstein was worrying about, you know? So, so he makes the point that I can make these two particles... He was using different examples, but the, the underlying point is the same. I could take one of them and just let it fly off to another star system light years away, right? And then I could measure-... my particle here, and the entanglement doesn't fade away or get less and less as the particles become separated. So if you take quantum mechanics seriously-

    5. CWChris Williamson31:03

       Mm-hmm.

    6. SCSean Carroll31:03

       ... says Einstein, "You're telling me that when I measure my particle here to be spin up or spin down, instantly, light-years away, the other particle changes to be oppositely oriented." That's his spooky action at a distance. That's what he's like, "How does it know fo- four light-years away at Alpha Centauri that I just measured the spin right here?" And he's like, and again, you know, his, his, his thing is, "Surely you don't believe-"

    7. CWChris Williamson31:29

       (laughs)

    8. SCSean Carroll31:29

       "... that's what it was." So he thinks, Einstein is convinced, that there was some fact of the matter about when you were gonna measure this electron, were you gonna get spin up or spin down? And therefore, there's some fact of the matter for the other one too. There, that there's something, something deeply hidden, right, that would predict with 100% probability what you're going to eventually observe. And what John Bell proved is that that can't happen unless there is some other kind of spooky action at a distance all along. So the spooky action at a distance is just absolutely part of quantum mechanics, like it or not.

11. 32:05 – 33:38

    Faster-than-light influence—but no faster-than-light messaging

    1. CWChris Williamson32:05

       Hmm. Am I right in thinking as well that that, the measurement of one versus another that was four light-years away would happen instantaneously? So that would-

    2. SCSean Carroll32:15

       That's the, that's the idea, yeah. That's right.

    3. CWChris Williamson32:17

       That's the prediction. Which also would ... I mean, does that break the lo- the maximum speed of something? Is it, is it-

    4. SCSean Carroll32:25

       S-

    5. CWChris Williamson32:25

       ... actually a speed that something's happening-

    6. SCSean Carroll32:27

       Yeah.

    7. CWChris Williamson32:27

       ... at the same time?

    8. SCSean Carroll32:28

       That's, that's a great question. You know, it's, uh, the answer seems to be that it violates the spirit of-

    9. CWChris Williamson32:34

       (laughs)

    10. SCSean Carroll32:35

        ... the speed limit, but not-

    11. CWChris Williamson32:36

        I love the idea (laughs) of the spirit of it. It's like-

    12. SCSean Carroll32:39

        Right.

    13. CWChris Williamson32:39

        ... this, this might be true, but it's unfair and we really shouldn't sanction it.

    14. SCSean Carroll32:43

        So, when you ... The thing is, when you measure the particle here, you know that if you get spin up, that's gonna be spin down. If you get spin down, that's gonna be spin up. So the state, the way that we describe the particle over there, does change instantly right away.

    15. CWChris Williamson32:58

        Outside of your light cone, right?

    16. SCSean Carroll33:00

        The thing is, outside of your light cone, faster than the speed of light. The thing is that the person over there doesn't know what answer you got.

    17. CWChris Williamson33:07

        (laughs) Until-

    18. SCSean Carroll33:08

        You measure it here.

    19. CWChris Williamson33:08

        ... until, at the very least-

    20. SCSean Carroll33:09

        So as far as they're concerned-

    21. CWChris Williamson33:10

        (laughs)

    22. SCSean Carroll33:10

        ... it's still 50/50. So you can't actually use it to send any information.

    23. CWChris Williamson33:17

        (sighs) This, the speed limit, the speed of light's a ... an idea, isn't it? Just an idea.

    24. SCSean Carroll33:21

        Well, you know, Einstein invented that, so of course he was very offended by this idea.

    25. CWChris Williamson33:25

        So it's his fault.

    26. SCSean Carroll33:25

        Yeah. That the quantum mechanics was somehow sneaking around the law somehow. He didn't like that at all.

    27. CWChris Williamson33:31

        Yeah, that is funny. Okay, so we've got entanglement.

    28. SCSean Carroll33:34

        Yeah.

    29. CWChris Williamson33:35

        And now let's go many worlds.

12. 33:38 – 36:47

    Many-Worlds: Everett removes collapse by taking Schrödinger evolution literally

    1. SCSean Carroll33:38

       Good. So Hugh Everett in the 1950s, um, looks upon quantum mechanics and just says, "Oh my God, this is (laughs) this is not good enough." And he says basically what you folks ... You've forgotten two things. One is the observer, as we've already said, is a quantum mechanical system themselves. So they're not really classical. So when you observe, let's say when you observe at a single electron, right? Um, what does it mean to do an observation? Well, if the electron has a wave function and can be in a superposition of different things, then in principle, you can have a wave function too and you could be in a superposition. And the other thing that he says everyone forgot is entanglement. So not only can you have your own wave function, but in fact really there's one wave function that describes you and the electron at the same time. And he says, "Forget about all these dumb rules, about measurement and observation. What if we didn't have those rules? What if we just had the equation, the Schrodinger equation, which says what happens when you, the quantum system that is you, interacts with the electron that's in a superposition of spin up and spin down?" And he says, "The answer is 100% perfectly clear. You and the electron evolve into an entangled superposition where there's one part of the entanglement which says the electron was spin up and I observed it to be spin up, and there's another part of the entanglement which says the electron was spin down and I observed it to be spin down." And everyone agrees that that's what Schrodinger's equation predicts. That's the nice thing about having equations. They, they predict things and they're not really up for debate. That's, that's what it predicts.

    2. CWChris Williamson35:18

       Mm-hmm.

    3. SCSean Carroll35:19

       But where they are upset is nobody in history has ever said, you know, "I observe that electron and now I feel like I'm in a superposition-"

    4. CWChris Williamson35:31

       Yeah.

    5. SCSean Carroll35:31

       "... of having observed it spin up and spin down." And Everett says, "I know why. Because there are now two copies of you. There's one copy that observed it spin up and one copy that observed it spin down. And what happens to one of these copies of you will not affect or be influenced by or interact with the other copy ever again. They go their own ways. It's as if they are separate worlds." So the many worlds interpretation of quantum mechanics did not come about because Hugh Everett said, "What quantum mechanics needs is an infinite number of extra worlds."

    6. CWChris Williamson36:04

       (laughs)

    7. SCSean Carroll36:05

       It came about because he said, "What quantum mechanics needs is getting rid of all these dumb rules about measurement and probability and collapse. Just take seriously what the equations are trying to tell you, and it says that you will branch into separate worlds and all you have to do is deal with it. If you're willing to deal with it, all of your problems go away." If you, in other words, if you think that an electron can be in a superposition of spin up and spin down, then you should be able to believe that you can be in a superposition and you should be able to believe that the universe can be in a superposition of all different things. And if you treat those different parts of the superposition as different worlds, you solve all the problems of quantum mechanics.

13. 36:47 – 44:16

    Branching, decoherence, and why you don’t see other worlds

    1. CWChris Williamson36:47

       When you say interact with or o- observe the electron, what does that mean? Does that mean, uh, uh, like how high of a, uh, fidelity are we looking at this? Is it every-... plank length amount of time that happens of every single movement and every single potential movement? How, how does that work? Do you know what I mean? Like, what, what, what's going on?

    2. SCSean Carroll37:09

       Yeah. No, it, it's a very specific criterion. When a quantum mechanical system that is in a super position of different possibilities-

    3. CWChris Williamson37:18

       Yeah.

    4. SCSean Carroll37:18

       ... becomes entangled with the wider outside world, the wave function branches. So there's an electron sitting in every atom in your body right now, but mostly they're not becoming entangled with anything. They're just staying in their atoms, right? That's just what they're doing. Um, but when you take an electron and r- and remove it from an atom, and then send it through a magnetic field, it'll be deflected one way if it's spin up, be deflected another way if it's spin down, and then you let it hit a screen and you observe a dot either there or there. And that process, observe a dot-

    5. CWChris Williamson37:50

       Okay.

    6. SCSean Carroll37:51

       ... it angles the electron with the wider world because there's now a dot on the screen.

    7. CWChris Williamson37:55

       Okay.

    8. SCSean Carroll37:56

       And that's why the world branches.

    9. CWChris Williamson37:57

       So it only happens during specific sorts of experiments, or is this happening when I-

    10. SCSean Carroll38:02

        It happens when-

    11. CWChris Williamson38:02

        ... sit down in my chair?

    12. SCSean Carroll38:04

        It happens when quantum systems become entangled with the outside world.

    13. CWChris Williamson38:08

        Okay.

    14. SCSean Carroll38:08

        So for, it's happening all the time in your body right now.

    15. CWChris Williamson38:12

        Mm-hmm.

    16. SCSean Carroll38:12

        Because there are, there's a certain number of radioactive nuclei in your body. There's about 5,000 radioactive decays per second in a typical human body. And so though that radioactive decay means that some nucleus emitted a particle, and that particle bumped into things and became entangled with the outside world. So 5,000 times a second, or actually much more than that, but at least that many times, uh, you're branching the wave function just by sitting there in your chair.

    17. CWChris Williamson38:39

        Multiplied by however many particles there are in the universe that are also potentially-

    18. SCSean Carroll38:45

        Yeah, a lot.

    19. CWChris Williamson38:45

        ... doing this-

    20. SCSean Carroll38:46

        That's right. Yes.

    21. CWChris Williamson38:46

        ... per second?

    22. SCSean Carroll38:47

        Again, most, you know, like a typical photon or whatever, it just spreads out throughout the universe. It doesn't become entangled with anything until it hits a telescope or a planet or something like that. So, uh, you know, radioactive elements are rare, you know, in the universe. So particles can become, can go a long way without being entangled with anything, um, but they often are also, so both things are happening. Yeah.

    23. CWChris Williamson39:09

        How, how is there two worlds that exist at the same time? Like, how does it, how do I not just see another chair or another atom, or like there be an extra little bit on the edge of the chair? What, what, what's going on there?

    24. SCSean Carroll39:22

        Well, because they have truly become separate, and this is the process which in quantum mechanics we call decoherence. Think of, I like to think of, um, uh, Schrödinger's cat, right? So in Schrödinger's cat, there's the box, and there's a quantum event, which with 50/50 probability either, in Schrödinger's telling, either kills the cat or keeps it alive. In, in my book, I made it put the cat to sleep or let it stay awake because it could just be sleeping gas. You had no reason to kill the cat. But the point is that in the conventional telling, the cat is literally neither awake or asleep. It is at a super position of both, until you open the box and you observe it. And again, Schrödinger is saying, "Surely you don't believe that. Surely you don't believe that the state of the cat dramatically changed." So here's what a modern Everettian would say, uh, "If the cat's asleep, lying on the floor of the box, versus being awake, it's up and walking around trying to get out of the box, all of the stuff in the box, all the air molecules, all the photons of light and so forth, may or may not bump into the cat depending on whether it's lying on the floor or walking around." So what that means is that these two different parts of the wave function of the cat, awake and asleep, interact with the environment around it in different ways, and that means the cat becomes entangled with the environment around it, like it or not. So the w- the universe branches into two different copies long before you open the box.

    25. CWChris Williamson40:52

        Mm-hmm.

    26. SCSean Carroll40:53

        And to, to get to the answer to your question, because there's so many photons hitting the cat or not hitting it, and because, you know, one will get absorbed if it hits the cat or just go right on by if it doesn't, those two parts of the wave function of the universe, cat awake and cat asleep, become completely separated from each other. So you, if, if, if it's you in one branch with the cat awake, you can't see any result from the branch of the wave function where the cat is asleep. The particles in that branch of the wave function just don't interact with you. They're, they're perpendicular to you literally in the space of all possible wave functions.

    27. CWChris Williamson41:32

        (sighs) It's so challenging to wrap-

    28. SCSean Carroll41:36

        Yeah.

    29. CWChris Williamson41:36

        ... your head around. For someone-

    30. SCSean Carroll41:37

        I recommend buying my book. (laughs)
14. 44:16 – 51:13

    Quantum woo, observers, and why consciousness isn’t needed

    1. CWChris Williamson44:16

       I get you. So moving on to the, the spooky stuff. Uh, I'm sure that a lot of the people that are listening, and, you know, you said yourself, you do your, your Amazon search, you're thinking about, "What am I gonna call my new book?" It's all about quantum... Ah, quantum yoga. Ugh.

    2. SCSean Carroll44:33

       (laughs)

    3. CWChris Williamson44:33

       Quant-

    4. SCSean Carroll44:33

       Quantum leadership.

    5. CWChris Williamson44:34

       Quantum leadership. I bet quantum accounting, uh, I could probably do with-

    6. SCSean Carroll44:38

       Quant touch. (laughs)

    7. CWChris Williamson44:38

       ... some quan- quantum accounting. Um, but yeah, it's, it's used to crowbar all sorts of stuff in, but one of the things is kind of more spooky, spiritual stuff into reality.

    8. SCSean Carroll44:51

       Yep.

    9. CWChris Williamson44:51

       W- why do you think that is?

    10. SCSean Carroll44:54

        Well, I think there's two things going on. One is, um, wishful thinking, right? Uh, quantum mechanics seems mysterious and there's a lot of other things that seem mysterious, so maybe they're related somehow.

    11. CWChris Williamson45:05

        Mm-hmm.

    12. SCSean Carroll45:06

        Uh, and, you know, other theories of physics are, are put to bad uses also. So it's just sort of a bit of sloppiness that people necess- or inevitably, uh, give into. But the other thing is that physicists have done a bad job of trying to understand quantum mechanics. And for a long time, it... they've made it seem like the role of a human observer is somehow crucial to explaining what happens in quantum mechanics. Someone measuring things, right? Someone actually looking at systems. And that lets you insinuate that somehow you are bringing the world into existence just by looking at it. And that is very close to saying that you influence the world, that the ways in which you look at the world and the ways in which you interact with it can somehow change the world out there.

    13. CWChris Williamson45:57

        Mm-hmm.

    14. SCSean Carroll45:57

        So you, not only do you bring the world into existence by looking at it, but you can choose what kind of world to bring into existence.

    15. CWChris Williamson46:03

        Mm-hmm.

    16. SCSean Carroll46:04

        Now, none of that has anything to do with quantum mechanics. That's all just crazy talk, wishful thinking, woo-woo kind of nonsense. But the physicists should ser- share, uh, part of the blame for letting people talk that way because they've not been at all clear about what quantum mechanics really says.

    17. CWChris Williamson46:21

        People talk about m- manifesting realities and, uh, uh, and I think they use the measurement problem or I think they would call it like the observer effect or whatever, right?

    18. SCSean Carroll46:32

        Yeah.

    19. CWChris Williamson46:32

        To, to explain that stuff. There was (clears throat) another conversation I was having recently about online coaches, so online fitness and diet and nutrition coaches, and I was asking the... these guys, I was saying, "Why is the industry filled with so much kind of misinformation and so many charlatans and people like that?" And they said that it's because of, uh, a lack of obvious causality between-

    20. SCSean Carroll46:59

        Mm-hmm.

    21. CWChris Williamson46:59

        ... what, what happens and then the results on the other side of it. And where you have this vacuum of a lack of information, it just allows people to just throw speculation in there, right?

    22. SCSean Carroll47:09

        Yeah.

    23. CWChris Williamson47:09

        It's like, "This sounds like a plausible narrative." And, uh, you know, if, if there is anything that I hope that the listeners can take away from today's conversation, it is that plausible narratives are probably not the world in which quantum physics operates.

    24. SCSean Carroll47:25

        (laughs)

    25. CWChris Williamson47:25

        Like, it's not because there's, like, a good guy and a bad guy. There's like... it's not that-

    26. SCSean Carroll47:30

        Yeah.

    27. CWChris Williamson47:30

        ... like the Higgs boson was like the referee or something like that. Like, we, we, we love to personify these sorts of stories, right? Because it, it brings them onto our level, it brings them onto the level of a social being, but that's not what's happening.

    28. SCSean Carroll47:45

        Yeah. No, it's absolutely right. And, uh, as David Albert, who's a philosopher of physics once said, you know, when, when it comes to trying to understand the deepest mysteries of nature, if they don't make you uncomfortable, you're not doing it right.

    29. CWChris Williamson47:57

        (laughs)

    30. SCSean Carroll47:57

        Because, you know, of course, the world's most fundamental level is something very, very different than we experience in our everyday lives. So, uh, this is one of the things that, that, uh, is an important consideration when you compare many worlds to other possible interpretations of quantum mechanics. Many worlds is the simplest. It has the fewest equations, the fewest ideas, it generates what you see in a very natural way. But it's very far away from what we observe, right? It's so different in structure and language than, uh, the world of our everyday experience that it is perfectly legitimate to say, "I just don't believe that this very, very simple lean and mean theory really gets us to the world we see." So other alternatives to many worlds generally bring in a lot more extra stuff that is somehow latching on to the classical world that we experience, whether it's extra hidden variables or ways that particles behave or whatever. And it's not crazy to think that that might be the way to go, that there might... there might be something extra other than the pure bare bones formalism of quantum mechanics that gives us this classical world we see around us. But...... the, I would, for people like me, the Many Worlds version is just so simple and so compelling that it's worth taking that extra effort to map it on to the world we see. So we actually see it as sort of a, a feature, not a bug that Many Worlds is so alien and so different, because that's what the nature should be like at its deepest level.
15. 51:13 – 53:27

    Determinism, Laplace’s demon, and what ‘predicting everything’ means in Many-Worlds

    1. CWChris Williamson51:13

       Got you. Um, I remember reading, I, I might have even heard you say it yourself, talking about how if we knew... is it if we knew the position and the velocity of every particle that we could work time back infinitely?

    2. SCSean Carroll51:27

       Yeah, in classical mechanics-

    3. CWChris Williamson51:28

       We could work out where everything had been in the... Is that, is that correct?

    4. SCSean Carroll51:31

       Classical mechanics, that would have been true. This is a famous thought experiment from Pierre-Simon Laplace around the year 1800. So Isaac Newton gave us the rules of classical mechanics in the 1600s, um, but it still, it took some time, even though classical mechanics we think of now as pretty trivial compared to quantum mechanics-

    5. CWChris Williamson51:48

       (laughs) Yeah.

    6. SCSean Carroll51:48

       ... it still took some time for it to sink in. You know, this idea that left to its own devices, an object will keep moving in a straight line at a constant velocity is actually pretty radical when you think about it. No one has ever seen that, right?

    7. CWChris Williamson52:02

       Mm-hmm.

    8. SCSean Carroll52:02

       You know, and everyone see things that you don't push on them, they come to a rest, right? As Aristotle would have predicted. So it did take some time for people to catch on to Newton's way of thinking and Laplace is one of the first people to really get it at a deep level in his bones. And he pointed out that, uh, Newton gave us a bit of... a set of laws, set of laws of motion, equations of motion, with the property that if you tell me the position and velocity of everything in the universe, and I have infinite calculational capacity, uh, the future and the past of the universe are now set. They are completely deterministically predicted by those laws and by that current configuration of stuff. The quantum version of this depends on whether or not... depends on what your favorite version of quantum mechanics is-

    9. CWChris Williamson52:47

       (laughs)

    10. SCSean Carroll52:48

        ... uh, uh, which we don't agree on. So for a Many Worlds person, there is a version of this which says that, uh, the wave function of all the branches, all the worlds at once, uh, evolves completely deterministically. So it evolves in the way the Laplace would have thought. So you can wind it backward or forward, but you, you need more than just what the world is doing in your branch right now. You need what's going on in all the branches of all the different parts of the wave function of the universe.

    11. CWChris Williamson53:18

        Okay. So you need a fair bit of computing.

    12. SCSean Carroll53:20

        You're not going to get any one of these things.

    13. CWChris Williamson53:21

        (laughs)

    14. SCSean Carroll53:21

        This is purely a thought experiment. Let's not even aspire to try and do it.

16. 53:27 – 1:01:30

    What Carroll works on now: classical emergence, and why quantum gravity is hard

    1. CWChris Williamson53:27

       Wow. Yeah. Um, so to round up, can you tell us what you're working on at the moment or what's been sort of occupying your time right now?

    2. SCSean Carroll53:37

       Well, this question that we talked about, about, you know, really taking quantum mechanics seriously and trying to derive the classical world from that rather than putting it in, sneaking it in by hand, uh, is really beginning to seem more and more important to me. So I'm working on that both sort of at the most fundamental level, just saying, you know, okay, quantum mechanics, how do we go from quantum mechanics to a classical world at all? And then more specifically, we had this longstanding worry about gravity. Einstein explained that gravity is the curvature of spacetime. Uh, that's Einstein's general theory of relativity. And ever since then, we've been struggling to reconcile gravity with quantum mechanics. So my conviction is that one of the things holding us back has been this insistence on starting with a classical theory and quantizing it. So you can start with general relativity and try to quantize it, and it doesn't work. So now people start with something like string theory and try to quantize that. I think that maybe you'll get some insight by just starting quantum from the start. And if you understand in general how the classical world can come out of the quantum world, then a classical world featuring curved spacetime and gravity and things like that should hopefully be part of it.

    3. CWChris Williamson54:52

       How easy is it for you to describe why quantum theory and gravity don't agree?

    4. SCSean Carroll55:00

       Ah, uh, it's not easy, but, uh, I like to say there's sort of two different sets of puzzles, and either one of them alone would probably be enough, but there's two of them. One is just that there's... are what we call technical puzzles, right? Um, if you take, uh, X plus Y and X is a finite number and Y is a finite number, and you add them together and get an infinite number, something has gone terribly wrong, right?

    5. CWChris Williamson55:21

       (laughs)

    6. SCSean Carroll55:22

       And so, uh, this happens when you start with a classical theory and try to quantize it, often things that should be finite blow up.... you get infinities. And Richard Feynman and others back in the '50s won Nobel Prizes showing how to tame these infinities in quantum field theory. Uh, we've not been able to tame them in general relativity, and so that's the biggest single, uh, nice thing about superstring theory is that these infinities go away. Okay? So there are technical problems, um, that string theory seems to help us with. It inter- it, it... At the cost of raising an entirely new technical problem, namely that string theory really only makes sense if space-time is ten-dimensional. And we look around, and space-time looks like it's four-dimensional, three dimensions of space, one dimension of time. So string theorists need to be very, very clever about getting rid or hiding those extra six dimensions, and that's another technical problem we haven't quite solved yet. There's also the conceptual problems. So when you talk about electromagnetism or particles, other things that we typically do in quantum mechanics, you can at least say things like, "Okay, I don't know where the electron is. It has a wave function, but I have two particles that are gonna come in. They both have their wave functions. They're gonna interact when they're overlapping, or at the same point in space," right? That's when two things overlap. That's the principle of locality, that's what Einstein was very, very, uh, um, protective of. He wanted locality to be very, very important. But here's the thing. In quantum gravity, if you're gonna take space-time and let it be curved, and you're gonna quantize the whole kit and caboodle, then just like an electron can be in a superposition of spin up and spin down-

    7. CWChris Williamson57:08

       Mm-hmm.

    8. SCSean Carroll57:08

       ... the geometry of space-time itself should be able to be in a superposition of all different kinds of geometries. And when that happens, you can't even point to a location in space and say, "Here." (laughs) Because in these different parts of the superposition, in these different geometries of space-time, there's no way to associate one point in one space-time with another point in the same space-time with a different geometry. The idea of a fixed point in space seems to have no meaningfulness in quantum gravity. So what are we gonna do about that? How are we gonna make sense of the idea that fields and particles only interact when they're located the same place when there's no such thing as being located in the same place in quantum gravity? So that's a conceptual problem. It's not like things blew up and we got infinity. It's just like, what do you do with this? Like, what's going on? How do we make sense of this? And, uh, that's the kind of thing people are still struggling with.

    9. CWChris Williamson58:04

       To finish up, one thing that I've always been thinking about is, to me, again, as a, a total noob to physics, it feels like theories should reduce down to something that's very simple. I don't know why. I- i- i- it just seems like there should be, um, universal rules which don't require an awful lot of complexity to get back to.

    10. SCSean Carroll58:31

        Right.

    11. CWChris Williamson58:32

        Do you think that as we roll forward over several hundred thousand years that we are going to see these, these theories become more complex or more simple?

    12. SCSean Carroll58:45

        Well, I think, uh, I'm pretty optimistic that things will become more simple. Uh, that's certainly been the way physics has been going for a very long time. Many worlds is very, very simple, just like general relativity is very simple. They're both very alien to us. They're not our everyday experience, so they seem difficult. But no one denies that writing the theory down is really, really simple. It is li- the algorithm is very, very short. Um, there's no guarantee that it continues like that. And of course, like, like we said, there's a whole extra work to be done at mapping these fundamental theories to the everyday life, you know? I don't think that the theory of psychology is ever gonna become simple. (laughs)

    13. CWChris Williamson59:23

        (laughs)

    14. SCSean Carroll59:23

        I think that there's some ineluctable complexity about a human being that's not gonna go away. But the fundamental laws should be very simple. That might not be true. It's an empirical question. We'll have to find out. But I would, I would definitely say that's the way to bet.

    15. CWChris Williamson59:38

        I hope so. Uh, for the listeners who want to find out a little bit more, where should they head, Sean?

    16. SCSean Carroll59:44

        I have a website, preposterousuniverse.com, uh, where you can see the books I've written, bunch of videos, and of course I have my own podcast called Mindscape where I'm talking to all these cool people, not mostly about physics. Some physicists, uh, do speak on, but, you know, t- today I released an episode with a philosopher, and I've had musicians and poker players and biologists, uh, and it's a lot, been a lot of fun for me.

    17. CWChris Williamson1:00:08

        That's awesome. Also, you're, you're pretty prolific on Twitter ali- as well, right?

    18. SCSean Carroll1:00:13

        Twitter, yep, SeanMCarroll. And again, that's, that's a link from my home page, but, uh, yeah, I really like Twitter. It's, uh, it's, it's for better or for worse, I used to be a prolific blogger on my website, and I still do that sometimes, but, uh, it's easier to get my point across in a quick tweet and then move on to do real work, uh, rather than write a respectable blog post. So I- I'm letting this side down, but I do try to say some interesting things on Twitter, yeah.

    19. CWChris Williamson1:00:37

        I like it. I like it. If you're a physicist who's able to get his message across in less than 360 characters or whatever it is, I think, yeah, you're doing a good job.

    20. SCSean Carroll1:00:46

        Yeah. (laughs)

    21. CWChris Williamson1:00:46

        Uh, ladies and gentlemen, it's been an absolute pleasure. You know what to do. The links to all of Sean's books plus his website and his Twitter and everything else will be linked in the show notes below. If you've got any questions, comments, or feedback, get at me @chriswilex on all social media, leave a comment in the YouTube channel, or just hassle Sean on Twitter and he'll, he'll give you a reply maybe if he's-

    22. SCSean Carroll1:01:07

        Maybe.

    23. CWChris Williamson1:01:08

        ... if he's online.

    24. SCSean Carroll1:01:08

        You never know. Yeah. (laughs)

    25. CWChris Williamson1:01:10

        (laughs) Sean, thank you so much for your time. It's been great.

    26. SCSean Carroll1:01:13

        Thanks very much, Chris.

    27. NANarrator1:01:17

        (instrumental music)

Episode duration: 1:01:30