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Sean Carroll is a cosmologist and physics professor specializing in dark energy and general relativity. He is a research professor in the Department of Physics at the California Institute of Technology. His new book "Something Deeply Hidden" is now available and also look for “Sean Carroll’s Mindscape" podcast available on Spotify.

Black holes, wormholes & other things I'll never understand

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The J.Rogan experience. I remember there's a woman who came to the comedy store after the last podcast that we did and she apparently is also working on it and she was trying to explain it to me, her version of it, you know, after hearing your version of it, it was very similar, but I believe she was from Romania, so she was struggling a little bit with English, but she was so excited to discuss it. It's so fascinating when you see someone who's like for the limited number of how many of you guys there are and gals there are out there, I mean, whatever the number is, when that spark gets ignited and other people start tuning into it, she was so excited that this was being discussed on a podcast and she wanted to talk to me about it to say, you know, please have more people on, please talk about this more, you know, we need support, we need, it's, It is, it's, it does baffle me a little bit how difficult it is swimming uphill to get more support for this kind of thing because it is just an enormous privilege to be able to call your job thinking about the fundamental nature of reality, right? Like, you know, I gave, you know, my first book tour talk was last Tuesday and I had dinner the night before with, you know, several philosophers of physics in the New York area, you know, from Columbia and NYU and ever. And, and, you know, we're all friends and we could talk about, you know, our cats in our cars, but every single word discussed at the table all night long was about the philosophy of physics. Is it because you guys work in isolation, essentially? And then when you get together, you just you're so pumped up to be discussing these things with like minded souls? You know, in part, yeah, I mean, I there's no one else in the physics department at Caltech who cares about these issues. I mean, some of them care in the sense that they are happy that I'm doing it, but no one does it themselves. Yeah. Well, there's a couple other people in the philosophy department who care about these. And a lot of folks you were saying get pushed into philosophy. And why is that? I mean, is it just because it's so complex that it's so esoteric? There's so many people that just they don't, the support for it's not there, but the support for philosophy is more common and mainstream. Yeah, you know, there's different kinds of support. One kind of support in academia is who do you hire? Right? What what areas do you want? Like a physics department will generally say, yeah, we should have some people doing particle physics, some people doing astrophysics, some people doing condensed matter and solid state physics. And then and then it becomes hard. Do we need people doing biophysics? Do we need people doing this? And by the time they get to the foundations of quantum mechanics, there's there's usually very little support. Philosophers, their their job is being patient and clarifying difficult conceptual questions. And so they get that quantum mechanics is fertile territory for philosophy. Like it, you know, one of the big problems in philosophy compared to science is that many of the questions that they're asking cannot be tested experimentally. Right? What is infinity? Well, you know, okay, it's hard to do an experiment there, but it's an important question. Right? And so you need patience, but also, it's harder to make progress because it's easy to be trapped by your intuition. Right? Like when it's just you thinking and trying to think hard and be rational and so forth, it's easy to fall into a trap of, well, this looks reasonable to me. And quantum mechanics doesn't look reasonable to anybody. So it's a wonderful corrective. It's a wonderful reality check when you think, well, reality has to be this way. And then someone can say, well, look at quantum mechanics, that's different than what you said. So philosophy and quantum mechanics, they sort of, they share some sort of a border. Yeah. Oh, yeah, absolutely. I mean, the things, so I was always a big fan of philosophy ever since I was an undergraduate and I discovered it for the first time. But when I was an undergraduate and my favorite philosophy classes were like the philosophy of morality or political philosophy. Right? I took philosophy of science classes, but they seem to be kind of dry to me because they were all about how scientific theories are constructed and chosen. You know, the structure of scientific revolutions is the famous book that everyone reads, people like Thomas Kuhn and Paul Feilroppend and so forth. And okay, that's interesting, but it's sort of meta science, right? It's like how science is done, not how the world works. And it wasn't until, you know, circa 2000 that I discovered that there are philosophers of physics who are kind of really doing physics, you know, that they're not asking how physics works. They're asking how the world works, but they're asking in a way that is comfortably located in philosophy departments and right now not so much in physics departments. There was a part of the book that shocked me because I had a ridiculous idea once and this idea was not my idea. Apparently, Laplace had a very similar idea as a thought experiment. I had an idea once that if one day there was a computer that was so powerful that it could accurately describe every single object on earth that we would be able to figure out the past. And Laplace was saying that not only that, but he proposed for the entire universe, like every single object, electron, everything in the atom in the entire universe that you would not only be able to show the past, but also predict the future. That's right. So this is called Laplace's demon, although he never called it that. Pierre Simone Laplace was a brilliant guy. He deserves to be much more well known. So I think I've mentioned him to his name in every book that I've ever written for totally different reasons. He helped invent probability as we currently understand it, for example. But yeah, so Isaac Newton came up with the rules of classical mechanics in the 1600s, but it wasn't until Laplace around the year 1800 that this implication of classical mechanics was realized. It's a clockwork universe that the way classical mechanics works is if you tell me the state of a system right now at one moment, by which in classical mechanics you would mean the position and the velocity of every part and you knew the laws of physics and you had arbitrarily large computational capacity, Laplace said a vast intelligence, okay, then to that vast intelligence, the past and future would be as determined and known as the present was, that's the clockwork universe. It's deterministic. Everything is fixed once you know the present moment. Now, quantum mechanics comes along and throws a spanner into the works a little bit. If you're a many worlds person, Laplace's demon is still possible. So if you know the wave function of the universe exactly, and you have infinite calculational capacity, you could predict the past and the future with perfect accuracy. But what you're predicting is all of the branches of the wave function. So any individual person inside the wave function still experiences apparently random events, right? So you can't predict what will happen to you even if you can predict what will happen to the entire universe. Oh, Sean Caro. My goodness, there's a lot of people pausing this podcast right now just shaking their head like, you know, I wrote a little article that just appeared in Quantum Magazine, which by the way, if anyone here is a science fan, Quantum Magazine is the best online magazine for science these days, they have really, really good high level articles, but all sorts of things. And so I wrote an article called What is probability? Because you know, again, this is a philosopher's kind of question, like, you know, physicists will just put it to use and get on with their lives. Philosophers will say, well, what do you really mean by probability? The traditional answer is if you're flipping a coin, and you say it's 5050, what you mean by that is that if you flipped it an infinite number of times, half the time it would be heads, half the time it would be tails. That's what you mean. It's called the frequentist idea of probability. But then what do you say like, well, what is the probability that Donald Trump wins reelection? That's not going to happen infinite number of times, you're not going to do the experiment or even better, what was the probability that Lee Harvey Oswald actually was the lone shooter of JFK? That already happened. That's in the past, right? But we can easily say, well, I think it was an 80% chance that that's true. Right? So this is called Bayesian probability, where rather than thinking of an infinite number of things going on, you're assigning a degree of confidence to your lack of perfect knowledge, right? Like, I don't know exactly, there's something, yeah, there's something going on, I don't know what it is. So I assign a probability and that just like a frequency that you know, there's the credence, as we say, that you assign to these different ideas, is a positive number, then all the credences add up to one because something happened. So in quantum mechanics, is probability more like frequentist probability or is it more like Bayesian probability? The answer is it depends on what your favorite version of quantum mechanics is. In one of these spontaneous collapse theories, it's very much like a frequency, like, you know, you just things happen randomly, and it's purely objective. In something like many world, well, sorry, I should say in something like hidden variables, it's Laplace's demon all over again. So Laplace's demon doesn't work in a spontaneous collapse theory, because you the laws of physics are not deterministic, you don't know when things are going to collapse all by themselves. In a hidden variable theory, the hidden variables and the wave function evolve deterministically, but you don't know what the hidden variables are. So you can assign some probability to having them be different things. So there's some ignorance involved. Many worlds is the coolest idea, because it's it's kind of then this is what is kind of hard to wrap your mind around. On the one hand, there is only the wave function, it describes the universe exactly. But imagine that I measure the spin of an electron, okay, so I actually do know what the wave function is going to evolve into, it's going to evolve into a 5050 split of I observed that spinning up and I observed it spinning down. And then but I only ever find myself in one side or the other. So there is always a moment in between when the wave function splits. And when I know about it, it splits much faster than I can know about it. The rate, the speed of a wave function branching is some incredibly tiny number 10 to the minus 20 seconds or something like that. And timescale of things happening in my brain is like 10 to the minus three seconds at best, okay. So there will always be a time when there are two copies of me. One on the branch where the spin was up, one on the branch where the spin was down, but they're both identical, they don't know which branch they're on yet. So they need to be good Bayesians and say, well, what probability should I assign that I'm on one branch or the other. And it turns out that the probabilities work exactly like the textbook quantum mechanics tells you the probability should work out. How's that? The wave function squared is the probabilities. The wave function squared. Yeah, this is a rule called the Born rule after Max Born who was the physicist who invented it. So I mean, read the book, of course, but like you said it to the very start, the history of quantum mechanics is just so fascinating and hilarious. Schrodinger, Erwin Schrodinger of Schrodinger's cat fame, invented the idea of the wave function and wrote down the equation that it obeys. But what he hoped was that if you had the wave function of electron all by itself, if you solved his equation, it would sort of show that the wave function becomes localized, peaked at one location, the electron kind of acts like a point particle. And that's why we see particles. That was his hope. What actually happens when you solve the equation is that the electron spreads out all throughout the universe. So his hope was dashed. And then he's like, all right, I have this equation. What is it? Like what does the wave function do? And it was Max Born, a whole other guy who said what the wave function does is you square it. And that's the probability of seeing something somewhere. Like if the wave function looks like this, it's some spread out thing. There's very small probability over here and large probability over there because the probability is the wave function squared. And Schrodinger said like, Oh my God, that's awful. I'm sad I had anything to do with it. He regretted being involved with this whole idea of probabilities and collapses and all that stuff.