Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation | Lex Fridman Podcast #47

The following is a conversation with Sean Carroll, part two, the second time we've spoken on the podcast. You can get the link to the first time in the description. This time, we focus on quantum mechanics and the many worlds interpretation that he details elegantly in his new book titled Something Deeply Hidden. I own and enjoy both the e-book and audiobook versions of it. Listening to Sean read about entanglement, complementarity, and the emergence of space time, it reminds me of Bob Ross teaching the world how to paint on his old television show. If you don't know who Bob Ross is, you're truly missing out. Look him up. He'll make you fall in love with painting. Sean Carroll is the Bob Ross of theoretical physics. He's the author of several popular books, a host of a great podcast called Mindscape, and is a theoretical physicist at Caltech and the Santa Fe Institute, specializing in quantum mechanics, arrow of time, cosmology, and gravitation. This is the Artificial Intelligence Podcast. If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon, or simply connect with me on Twitter, @LexFridman, spelled F-R-I-D-M-A-N. And now, here's my conversation with Sean Carroll. Isaac Newton developed what we now call classical mechanics, that you describe very nicely in your new book, as you do with a lot of basic concepts in physics. So, with classical mechanics, I can throw a rock and can predict the trajectory of that rock's flight. But if we could put ourselves back into Newton's time, his theories work to predict things, but as I understand, he himself thought that they were, the interpretations of those predictions were absurd. Uh, perhaps he just said it for religious reasons and so on.

(laughs)

But in particular, sort of a world of interaction without contact. So action at a distance. It didn't make sense to him on a sort of a human interpretation level. Does it make sense to you that things can affect other things at a distance?

It does, but, you know, that, so that was one of Newton's worries. You're actually right in a slightly different way about the religious worries. He, um, he was smart enough... This is off the topic, but still fascinating. Newton almost invented chaos theory as soon as he invented, uh, classical mechanics. He realized that in the solar system... So he was able to explain how planets move around the sun, but typically, you would describe the orbit of the Earth ignoring the effects of Jupiter and Saturn and so forth, just doing the Earth and the Sun. He, he kind of knew, even though he couldn't do the math, that if you included the effects of Jupiter and Saturn, the other planets, the solar system would be unstable. Like, the orbits of the planets would get out of whack. So he thought that God would intervene occasionally to sort of move the planets back into orbit, which is how you could, only way you could explain how they were there presumably forever. Um, but the worries about classical mechanics were a little bit different. The worry about gravity in particular. It wasn't a worry about classical mechanics, it was a worry about gravity. How in the world does the Earth know that there's something called the Sun 93 million miles away that is exerting a gravitational force on it? And he said, he literally said, "You know, I leave that for future generations to think about, because I don't know what the answer is." And in fact, people underemphasize this, but future generations figured it out. Pierre-Simon Laplace in, uh, circa 1800 showed that you could rewrite Newtonian gravity as a field theory. So instead of just talking about the force due to gravity, you can talk about the gravitational field or the gravitational potential field, and then there's no action at a distance. It's exactly the same theory empirically. It makes exactly the same predictions. But what's happening is instead of the Sun just reaching out across the void, there is a gravitational field in between the Sun and the Earth that obeys an equation, Laplace's equation, cleverly enough, and, uh, that tells us exactly what the field does. So, even in Newtonian gravity, you don't need action at a distance. Now, what many people say is that Einstein solved this problem because he invented general relativity, and in general relativity, there's certainly a field in between the Earth and the Sun, but also there's the speed of light as a limit. In Laplace's theory, which was exactly Newton's theory, just in a different mathematical language, there could still be instantaneous action across the universe. Whereas in general relativity, if you shake something here, its gravitational impulse radiates out at the speed of light, and we call that a gravitational wave, and we can detect those. So, but I, I really... It rubs me the wrong way to think that we should presume the answer should look one way or the other. Like, if it turned out that there was action at a distance in physics and that was the best way to describe things, then I would do it that way. It's, uh, actually a very deep question because when we don't know what the right laws of physics are, when we're guessing at them, when we're hypothesizing at what they might be, we are often guided by our intuitions about what they should be. I mean, Einstein famously was very guided by his intuitions, and he did not like the idea of action at a distance. We don't know whether he was right or not. It depends on your interpretation of quantum mechanics and it depends on, uh, even how you talk about quantum mechanics within any one interpretation.