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Sean Carroll

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2024-04-22
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2024-04-22
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  1. “The classic experiment to explain quantum mechanics to people is called the Stern Gurlock experiment. You're measuring the spin of a particle, okay? And in quantum mechanics, the spin is just a spin. It's the rate at which something is rotating around in a very down-to-earth sense. The difference being is that it's quantized. So for something like a single electron or a single neutron, it's either spinning clockwise or counterclockwise. Those are the only two, let's put it this way. Those are the only two measurement outcomes you will ever get. There's no, it's spinning faster or slower. It's either spinning one direction or the other, that's it. Two choices, okay?”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  2. “It's a monist theory in classical mechanics. I have a particle here, particle there. I describe them separately. I can tell you what this particle is doing, what that particle is doing. In quantum mechanics, we have entanglement, right? As Einstein pointed out to us in 1935. And what that means is there is a single state For these two particles. There's not one state for this particle, one state for the other particle. And indeed, there's a single state for the whole universe called the wave function of the universe, if you want to call it that. And it obeys one equation and it is our job then to sort of chop it up, to carve it up, to figure out how to...”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  3. “Right, that boggles my mind. It's much more comprehensive. General relativity is about gravity, and that's great. Quantum mechanics about everything and seems to be up to the task. And so I don't know, is that beauty or not? But it's certainly impressive.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  4. “Quantum mechanics is a harder one. You know, I wrote a textbook on general relativity and I started it by saying general relativity is most beautiful physical theory ever invented. I will stand by that. It is less fundamental than quantum mechanics, but quantum mechanics is a little more mysterious. It's a little bit cludgy right now. If you think about how we teach quantum mechanics to our students, the Copenhagen interpretation, it's a god-awful mess. Like no one's going to accuse that of being very beautiful. I'm a fan of the many worlds interpretation of quantum mechanics, and that is very beautiful in the sense that fewer ingredients, just one equation, and it could cover everything in the world. It depends what you mean by beauty, but I think that the answer to your question is quantum mechanics can start with extraordinarily austere, tiny ingredients, and in principle lead to the world.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  5. “The first paper was called Limits on the Lorentz and Parity Violating Modification of Electromagnetism or Electrodynamics. So we figured out how to violate Lorentz invariance, which is the symmetry underlying relativity. And the important thing is we figure out a way to do it that didn't violate anything else and was experimentally testable. people love that. The second paper was called Quintessence and the Rest of the World. So Quintessence is this dynamical dark energy field. The rest of the world was because I was talking about how the quintessence field would interact with other particles and fields and how to avoid the interactions you don't want. And the third paper was called Ismic Speed Up Due to Gravitational Physics, something like that. So you see the common theme. I'm taking what we know, the standard model of particle physics, general relativity, tweaking them.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  6. “Well, by the way, this is not awesomeness. This is impact. Right, there's no correlation between awesomeness and impact. Some of my best papers fell without a stone, vice versa, right? Yeah.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  7. “Yeah, it does. I agree. Again, that's why it is a little bit, I tear my hair out when people who are not physicists think accuse physicists, like you say, of sort of losing the plot because they need dark matter and dark energy. I don't want dark matter and dark energy. I want something much cooler than that. I've tried, but you got to listen to the equations and to the data.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  8. “Great thing about physics is there are equations, right? I mean, you can come up with the words and you can wave your hands, but then you got to write down the equations, and I did, and I figured out that it could help with the dark energy, the acceleration of the universe. It doesn't help with dark matter at all.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  9. “And we've already said what gravity is. What is gravity? It's the curvature of spacetime. So there are mathematical quantities that measure the curvature of spacetime. And generally, you would say like I have an understanding, Einstein's equation, which I explained to the readers in the book, relates the curvature of spacetime to matter and energy. The more matter and energy, the more curvature. So I'm saying what if you add a new term in there that says the less matter and energy, the more curvature. No reason to do that. Except to fit the data, right? So I tried to unify the need for dark matter and the need for dark end.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  10. “Of galaxies, you don't need dark matter. Not so surprising because the density of stars and gas is very high there, and the dark matter is just subdominant. But there's generally a radius inside of which you don't need dark matter to fit the data, outside of which you do need dark matter to fit the data. So that's again, when gravity is weak, right? So I asked myself, of course we know in field theory, new effects should show up when fields are strong, not weak, but let's throw that out of the window. Can I write down a theory where gravity alters when it is weak?”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  11. “So here is what we know about dark matter and dark energy. They become important in regimes where gravity is very, very, very weak. That's kind of the opposite from what you would expect if you actually were modifying gravity. Like there's a rule of thumb in quantum field theory, et cetera, that new effects show up when the effects are strong, right? We understand weak fields. We don't understand strong fields. But okay, maybe this is different, right? So what do I mean by when gravity is weak? The dark energy shows up late in the history of the universe, early in the history of the universe, the dark energy is irrelevant. Remember, the density of dark energy stays constant. The density of matter and radiation go down. So at early times, the dark energy was completely irrelevant compared to matter and radiation. At late times, it becomes important. That's also when the universe is dilute and gravity is relatively weak. Now think about galaxies. A galaxy is more dense in the middle, less dense on the outside. And there is a phenomenological fact about galaxies that in the interior”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  12. “Away the dark energy, but it would not explain away the dark matter. And so I thought it was not that interesting, actually. And then two different collaborators of mine said, has anyone thought of this idea? Like they thought of exactly the same idea, completely independently of me. I said, well, if three different people found the same idea, maybe it is interesting. And so we wrote the paper. And yeah, it was very interesting. People are very interested.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  13. “The super cool explanation would be modifying gravity rather than inventing a new particle. Sadly, that doesn't really work. We've tried. I've tried. That's my third paper that was very successful. I tried to unify dark matter and dark energy together. That was my idea. That was my aspiration, not even idea. I tried to do it. It failed even before we wrote the paper. I realized that my idea did not help. It helps, it could possibly explain”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  14. “Bigger than the galaxy, sadly. We think that in the galaxy, dark matter is lumpy, but it's just weaker, its effects are weaker. But of the scale of large-scale structure and clusters of galaxies and things like that, yes, we can show you where the dark matter is.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  15. “Today, the evidence for dark matter is both much better than it was in the 1980s and from different sources. It is mostly from observations of the cosmic background radiation or of large scale structure. So we have multiple independent lines of evidence, also gravitational lensing and things like that, many, many pieces of evidence that say that dark matter is there, and also that say that the effects of dark matter are different than if we modified gravity. So that was my first answer to your question is dark matter, we have a lot of evidence for. But the other one is, of course, we would love it if it weren't dark matter. Our vested interest is 100% aligned with it being something more cool and interesting than dark matter, because dark matter is just a particle. That's the most boring thing in the world.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  16. “So 100% constant works, right? And it's also very robust, it's just there, it's not doing anything, it doesn't interact with any other particles. It makes perfect sense. Probably the dark energy is the cosmological constant. The dark matter, super important to emphasize here, you know, it was hypothesized at first in the 70s and 80s, mostly to explain the rotation of galaxies.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  17. “The tune A difference between dark matter and dark energy, right? Dark matter, as far as we are hypothesizing it, is a particle of some sort. It's just a particle that interacts with us very weakly. So we know how much of it there is. We know more or less where it is. We know some of its properties. We don't know specifically what it is. It's not anything fundamentally mysterious. It's a particle. Dark energy is a different story. So dark energy is indeed uniformly spread throughout space and has this very weird property that it doesn't seem to evolve as far as we can tell. It's the same amount of energy in every cubic centimeter of space from moment to moment in time. That's why far and away the leading candidate for dark energy is Einstein's cosmological constant. The cosmological constant is strictly constant, 100% constant. The data say it had better be 98% constant or better.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  18. “Well, two ways. One way is those people would have had to say the same thing when we discovered the planet Neptune. Because it's exactly analogous, where we have a very good theory, in that case, Newtonian gravity in the solar system. We made predictions. The predictions were slightly off for the motion of the outer planets. You found that you could explain that motion by positing something very simple, one more planet, in a very, very particular place, and you went looked for it, and there it was, right? That was the first successful example of finding dark matter in the universe”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  19. “And you can kind of run the numbers and say, you know, you can't make a very precise prediction because we're just making up this model. But if you want to roughly fit the data, you can predict how much polarization rotation there should be. A couple of degrees, okay? Not that much. So that's very hard to detect. People have been trying to do it. Right now, literally, we're on the edge of either being able to detect it or rule it out using the cosmic microwave background. And there is just, you know, truth in advertising. There is a claim on the market that it's been detected, that it's there. It's not very statistically significant if I were to bet, I think it would probably go away. It's very hard thing to observe. But maybe as you get better and better data, cleaner and cleaner analysis, it will persist and we will have directly detected the dark energy.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  20. “Detection yet because asymmetry could stop it from interacting with all these other fields and therefore makes it harder to detect. And just by luck, I realized, because it was actually based on my first ever paper, there's one loophole. If you impose these symmetries, so you protect the dark energy field from interacting with any other fields, there's one interaction that is still allowed that you can't rule out. And it is a very specific interaction between your dark energy field and photons, which are very common. And it has the following effect. As a photon travels through the dark energy, the photon has a polarization up, down, left, right, whatever it happens to be, and as it travels through the dark energy, that photon will rotate its polarization. This is called birefringence.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  21. “You can impose asymmetry that protects your new field from talking to any other fields. And this is good for two reasons. Number one, it can keep the dynamics slow. So if you just, you can't tell me why it's slow. You just made that up. But at least it can protect it from speeding up because it's not talking to any other particles. And the other is it makes it harder to detect. Naively experiments looking for fifth forces or time changes of fundamental constants of nature like the charge of the electron, these experiments should have been able to detect these dark energy fields. And I was able to propose a way to stop that from happening.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  22. “The fact that dark energy pervades the whole universe and is slowly changing, that should immediately set off alarm bells because particle physics is a story of length scales and time scales that are generally guess what small, right? Particles are small, they vibrate quickly, and you're telling me now I have a new field and it's typical rate of change is once every billion years, right? Like that's just not natural. And indeed, you can formalize that and say, you know, look, even if you wrote down a particle that evolved slowly over billions of years, if you let it interact with other particles at all, that would make it move faster. Its dynamics would be faster, its mass would be higher, et cetera, et cetera. So there's a whole story. Things need to be robust and they all talk to each other in quantum field theory. So how do you stop that from happening? And the answer is symmetry.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  23. “I since I like to talk to people in different areas, I was sort of more familiar than average with what a respectable working particle physicist would think about these things. And what I immediately thought was, you know, you guys are throwing around these theories. These theories are wildly unnatural. They're super finely tuned. Like any particle physicist would just be embarrassed, just be talking about this. But rather than just scoffing at them, I sat down and asked myself, okay, is there a respectable version? Is there a way to keep the particle physicist happy, but also make the universe accelerate? And I realize that there is some very specific set of models that is relatively natural. And guess what? You can make a new experimental prediction on the basis of those. And so I did that. People were very happy about that.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  24. “Try to take things other people were talking about and ask myself whether or not it really fit together. My two, so I guess I have three papers that I've ever written that have done super well in terms of getting cited and things like that. One was my first ever paper that I get very little credit for. That was my advisor and his collaborator set that up. The other two were basically my idea. One was right after we discovered that the universe was accelerating. So in 1998, observations showed that not only is universe expanding, but it's expanding faster and faster. So that's attributed to either Einstein's cosmological constant or some more complicated form of dark energy, some mysterious thing that fills the universe. And people were throwing around ideas about this dark energy stuff. What could it be and so forth? Most of the people throwing around these ideas were cosmologists. They work on cosmology. They think about the universe all at once.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  25. “What is the best match between my interests, my abilities, and what is actually interesting? And honestly, I've not been very good at that over my career. I have my process traditionally was I was working in this general area of particle physics, field theory, general relativity, cosmology. And I would sort of”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  26. “There's a very interesting thing that happens once you're a theoretical physicist, once you become trained, you're a graduate student, you've written some papers and whatever, suddenly you are the world's expert in a really infinitesimally tiny area of knowledge, right? And you know not that much about other areas. There's an overwhelming temptation to just drill deep, right? Just keep doing basically the thing that you started doing. But maybe that thing you started doing is not the most interesting thing to the world or to you or whatever. So you need to separately develop the capability of stepping back and going, okay, now that I can write papers in that area, now that I'm sort of trained enough in the general procedure,”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  27. “And let me just mention the name of Oliver Friedrich, who was a postdoc who led this. He deserves the credit for doing this. I was a co-author and a collaborator. I did some work, but he really gets the lion's share.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  28. “Oh, just because there are fewer, fewer high energy neutrinos. So there's a spectrum, and it goes down. But what we're plotting here is number of neutrinos versus energy, it's fading away. They just get very, very few”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  29. “You can go and you can get a plot from the Ice Cube experiment. How many neutrinos there are that they've detected with very high energies? And we predict in our weird little holographic guessing game that there should be a cutoff. You should see neutrinos as you get to higher and higher energies and then they should disappear. If you look at the data, their data gives out exactly where our cutoff is. That doesn't mean that our cutoff is right. It means they lose the ability to do the experiment exactly where we predict the cutoff should be.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  30. “That's all you need. There's not that much, right? Yeah. There's more ice in the Arctic than anywhere else. Right. So anyway”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  31. “Yeah, so the whole point is most cosmic rays are protons. Because why? Because protons exist and they're massive enough that you can accelerate them to very high energies. So high energy cosmic rays tend to be protons. They also tend to hit the Earth's atmosphere and decay into other particles. So neutrinos, on the other hand, punch right through, at least usually, right, to a great extent. So not just Antarctica, but the whole Earth. Occasionally, a neutrino will interact with a particle here on Earth, and his neutrino is going through your body all the time, from the Sun, from the universe, etc. And so if you're patient enough and you have a big enough part of the Antarctic ice sheet to look at, it's the nice thing about ice is it's transparent. So you've built yourself nature has built you a neutrino detector.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  32. “They should disappear. Like if you see a whole bunch of nearby neutrinos but then further away, you should see fewer. And there is an experiment called Ice Cube, which is this amazing testament to the ingenuity of human beings, where they go to Antarctica and they drill holes and they Is their detector And they're looking for flashes when a cosmic ray or neutrino or whatever hits a ice molecule, water molecule in the ice.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  33. “Weird overlap of states would show up in the equations of motion for particles like neutrinos. And then we made predictions on how the neutrinos would behave on the basis of those wild guesses. And then we compared them with data. And what we found is we're pretty close but haven't yet reached the detectability of the effect that we are predicting. In other words, basically one way of saying what we predict is if a neutrino and there's reasons why it's neutrinos, we can go into if you want, but it's not that interesting. If a neutrino comes to us from across the universe, from some galaxy very, very far away, there is a probability as it's traveling that it will dissolve into other neutrinos because they're not really perpendicular to each other as vectors as they would ordinarily be in quantum field theory. And that means that if you look at neutrinos coming from far enough away with high enough energy,”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  34. “In a region of space is correctly described by holography, it is somewhat overcounted by quantum field theory, but that's because the quantum field theory states are not exactly perpendicular to each other. I should have mentioned that in quantum mechanics, states are given by vectors in some huge dimensional vector space. Very, very, very, very large dimensional vector space. So maybe the quantum field theory states are not quite perpendicular to each other. If that is true, that's a speculation already, but if that's true, how would you know? What is the experimental deviation? And it would have been completely respectable if we had gone through and made some guesses and found that there is no noticeable experimental difference because, again, these things are in regimes very, very far away. We stuck our necks out. We made some very, very specific guesses as to how this”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  35. “Vector space, then the dimensionality of the vector space. Can I make them almost perpendicular to each other? And the mathematical answer is, as the number of dimensions gets very, very large, you can fit a huge extra number of vectors in that are almost perpendicular to each other. So in this case, what we're suggesting is the number of things that can happen”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  36. “Predict. How do you reconcile these two things? So there's one way of doing it that had been suggested previously, which is to say that in the quantum field theory way of talking, it implies there's a whole bunch more states, a whole bunch more ways the system could be than there really are. And just I'll do a little bit of math just because there might be some people in the audience who like the math. If I draw two axes on a two-dimensional geometry, like the surface of the table, right? You know that the whole point of it being two-dimensional is I can draw two vectors that are perpendicular to each other. I can't draw three vectors that are all perpendicular to each other, right? They need to overlap a little bit. That's true for any numbers of dimensions. But I can ask, okay, how much do they have to overlap if I try to put more vectors into a”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  37. “Well defined enough to make a prediction, right? Like it's kind of, I know it's going to happen in some cases. I don't know what's going to happen in other cases. So we did the following thing. As I've already mentioned, the holographic principle, which is meant to reflect the information contained in black holes, seems to be telling us that information, there's less information, less stuff that can go on than you might naively expect. So let's upgrade naively expect to predict using quantum field theory. Quantum field theory is our best theory of fundamental physics right now. Unlike this holographic black hole stuff, quantum field theory is entirely local in every point of space, something can go on and then you add up all the different points in space. Not holographic at all. So there's a mismatch between the expectation for what is happening even in empty space in quantum field theory versus what the holographic principle would be.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  38. “Yeah, you know, I'm always interested in since my first published paper taking these wild speculative ideas and trying to test them against data. And the problem is when you're dealing with wild speculative ideas, they're usually not”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  39. “Yeah, you know. Would be less surprising to me because I've already written papers about that. We don't have again strong reason to think that the interior of a black hole leads to another universe, but it is possible, and it's also very possible that that's true for some black holes and not others. This is stuff we don't know. It's easy to ask questions. We don't know the answer to. The problem is the questions that are easy to ask that we don't know the answer to are super hard to answer.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  40. “I mean, maybe the point is that black holes are mysterious for various reasons. So, yeah, if our best theory of the universe is wrong, that might help explain why.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  41. “Does depend on what they would say, you know. I think that There are colleagues of mine who think that we're pretty close to figuring out how information gets out of black holes, how to quantize gravity, things like that. I'm more skeptical that we are pretty close. I think that there's room for a bunch of surprises to come. So in that sense, I suspect I would be surprised. The biggest and most interesting surprise to me would be if quantum mechanics itself were somehow superseded by something better. As far as I know, There's no empirical evidence based reason to think that quantum mechanics is not 100% correct. But it might not be, that's always possible, and there are, again, respectable friends of mine who speculate about it. So that's something I would, that's the first thing I'd want to know.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  42. “As far as we know, nothing dramatic happens. We're not anywhere close to being confident that we know what's going on here yet. So there are good unanswered questions about whether time is fundamental, whether time is emergent, whether it has something to do with quantum entanglement, whether time really exists at all, different theories, different proponents of different things. But there's nothing specifically about holography that would make us change our opinions about time, whatever they happen to be.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  43. “Also, you know, by the way, just in relativity, special relativity, forget about general relativity. It's enormously tempting to say, okay, here's what's happening to me right now. I want to know what's happening far away right now. The whole point of relativity is to say there's no such thing as right now when you're far away. And that is doubly true for what's inside a black hole. So you're tempted to say, well, how fast is their clock ticking? Or how old are they now? Not allowed to say that according to relativity.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  44. “It depends on what you might preserved. It's there in the microscopic configuration of the universe. It's exactly as if I took a regular book, made a paper, and I burned it. The laws of physics say that all the information in the book is still there in the heat and light and ashes. You're never going to get it. Matter of practice, but in principle, it's still there”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  45. “Time always ticks by one second per second. That's all it can ever do. Time can tick by differently for different people. And so you have things like the twin paradox, where two people initially are the same age, one goes off near the speed of light and comes back. Now they're not. You can even work out that the one who goes out and comes back will be younger because they did not take the shortest distance path. But locally, as far as you and your wristwatch are concerned, time is not funny. Your neurological signals in your brain and your heartbeat and your wristwatch, whatever's happening to them is happening to all of them at the same time. So time always seems to be ticking along at the same rate.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  46. “They came back to tell. This is a theoretical prediction. But, you know, I'll say one super crucial feature of the black holes that we know and love, the kind that Schwartzshield first invented. There's a singularity, but it's not at the middle of the black hole. Remember, space and time are parts of two different spacetime. The location of the singularity in the black hole is not the middle of space, but our future. It is a moment of time. It is like a big crunch. You know, the Big Bang was an expansion from a singularity in the past. Big crunch probably doesn't exist, but if it did, it would be a collapse to a singularity in the future. That's what the interiors of black holes are like. You can be fine in the interior, but things are becoming more and more crowded. Space time is becoming more and more warped, and eventually you hit a limit. And that's the singularity in your future.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  47. “The black hole, I said that the black hole is the highest density of information, but it's not the highest amount of information because the black hole can evaporate. And when it evaporates, and people have done the equations for this, when it evaporates, the entropy that it turns into is actually higher than the entropy of the black hole was, which is good because entropy is supposed to go up. But it's much more dilute, right? It's spread across a huge volume of spacetime. So in principle, All that you made the black hole out of the information that it took, is still there, we think, in that information, but it's scattered to the four winds.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  48. “You do, you do, but I will point out one other thing. It's information dense, but it's also very, very high entropy. So a black hole is kind of like a very, very, very specific random number, right? It takes a lot of digits to specify it, but the digits don't tell you anything. They don't give you anything useful to work on. So it takes a lot of information, but it's not of a form that we can learn a lot from.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  49. “We like to do better, but we're trying. I mean, that's why we have an information loss puzzle because we haven't completely solved it. So here's just one thing to keep in mind. Once space-time becomes flexible, which it does according to general relativity, and you have quantum mechanics, which has fluctuations in virtual particles and things like that, the very idea of a location in spacetime becomes a little bit fuzzy, right? Because it's flexible and quantum mechanics says you can't even pin it down. Information can propagate in ways that you might not have expected. And that's easy to say, and it's true, but we haven't yet come up with the right way to talk about it that is perfectly rigorous”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source

  50. “And so we are not information dense, right? The density of information in us or in a book or a CD or whatever, a computer RAM is indeed encoded by volume. Like there's different bits located at different points in space, but that density of information is super duper low. So we're just like the speed of light or just like the Big Bang for the information in a black hole, we are far away in our everyday experience from the regime where these questions become relevant. So it's very far away from our intuition. We don't really know how to think about these things. We can do the math, but we don't feel it in our bones.”

    2024-04-22 · Lex Fridman Podcast · #428 – Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · IDENTIFIED FROM THE TRANSCRIPT · source