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What if the universe is a machine, and every moment in our past, present, and future is already encoded in the positions of its particles?

Physicist Sean Carroll explores the unsettling implications of classical mechanics, from Newton’s laws to Laplace’s thought experiment, showing how determinism challenges the very idea of free will.

SEAN CARROLL: The structure of classical mechanics implies that if you knew the position and velocity, not just of one particle, but of every particle in the universe, then the laws of physics would determine what happens next, at the next moment and the next moment, and infinitely far into the future, and for that matter, indefinitely far into the past. Now, this might bother you a little bit if you wanna think, well, wait a minute, I’m a person, I’m a human being, I have the ability to make choices. I’m not determined by the laws of physics. And both scientists and philosophers thought about that. They still don’t agree on what the right way to think about it is. I’m Sean Carroll. I’m a physicist and philosopher at Johns Hopkins University, host of the “Mindscape Podcast,” and also author of a bunch of books. Most recently, “The Biggest Ideas in the Universe” series, including “Space, Time, and Motion” and “Quanta and Fields.”

– [Narrator] Is reality just a clockwork machine? I like to say that physics is hard because physics is easy, by which I mean we actually think about physics as students. You know, we took classes, we read books, and it was hard because there’s all this new stuff, all these ideas, all these equations that we don’t come across in our everyday lives. But the reason those ideas are hard and those equations are there, is because physicists have a technique that has been amazingly successful, which is to take all the messy world around us with all its peculiarities and specificities, and to boil it down to really, really simple systems. You might imagine or remember when you were taking physics courses, there were frictionless surfaces, there were pendula that rocked back and forth perfectly. We’re always idealizing, we’re always imagining there are no complications, and then we’re putting them back in. That’s a strategy that would completely fail if you tried to do it for psychology or biology or political science. But for physics, it turns out to work incredibly well. There’s a joke that physicists like to tell each other. They don’t like to tell it to others ’cause it’s not very funny. But the idea is that a dairy farmer wants to improve his yield, more milk out of the cows, and for some reason, he approaches a physicist and says, “Could you look at my farm and tell me how to get more milk out of the cows?” And the physicist thinks about it, and he comes back with a sheaf of calculations and he says, “Okay, first imagine a spherical cow.” The idea, I know it’s not that funny, but the idea is that the first thing the physicist is gonna do is try to imagine a simpler situation. Real cows are not spherical. It would be a very different dairy farm if the cows were spherical, but you can calculate the volume of the cow and the metabolic rate of the cow much more easily if it were spherical. And the joke, of course, is that that doesn’t work in dairy farming, but it works really well when you’re considering a spherical universe or a spherical solar system or a spherical atom. The first really huge revolution in physics was the existence of classical mechanics handed down by Isaac Newton and others. It took a while. Newton was building on the shoulders of giants. But before Newton, there was Aristotle. And Aristotle says that things have natural places they wanna be, natural ways they want to move. And Newton says something completely different. He says, if something is not acted on by a force, it’s gonna continue in a straight line at a constant velocity forever. And if it is acted on by a force, I can tell you how it’ll move. I have an equation to do that. Physicists like to simplify things a great deal. But billiards, you know, the pool game, is pretty close to being simple. It’s not exactly, because you know, when you hear those balls click against each other, that sound is giving off energy, and it’s kind of wasteful. But in principle, if you had no friction, no sound, no air resistance, the balls bouncing around the pool table, let’s also imagine there’s no pockets, so the balls can just bounce off the edges of the table forever. They would go forever. They wouldn’t stop, right? The energy contained in the system remains constant. And the laws of physics, as Laplace points out, suffice it to predict exactly what the balls are gonna do at every moment, given what they’re doing right now. So to the extent that it’s okay to ignore friction and noise and things like that, not only is it true that if you imagine hitting the balls and watching them move, you could predict exactly what’s gonna happen on the basis of the laws of physics. If somehow you could take a snapshot later in their motion so you know both where they are and how fast they’re moving, so maybe a little clip of a movie, then the laws of physics would let you go backwards and reverse engineer what exact configuration the balls were in. We don’t perceive that in our everyday world because the world is full of noise and dissipation and air resistance and things like that. But in the pristine, perfect world of imagined classical mechanics, the past and future work equally well. You can go from any one moment to any other moment. It’s interesting that Newton came up with the framework of classical mechanics in the 1600s, and people were very excited, you know, physicists, mathematicians, philosophers. They didn’t really have physicists at the time. They were all considered to be natural philosophers, but they worked on it, you know, they thought about the motions of the planets and things like that. And the implications of this idea are profound for how we think about what physics is, what physics tells us. Because it wasn’t realized until Pierre-Simon Laplace over a hundred years after Newton. But the structure of classical mechanics implies that if you knew the position and velocity, not just of one particle, but of every particle in the universe, and you knew the laws of physics and you had infinite calculational abilities, none of these are at all plausible, but we’re imagining right now. Then the laws of physics would determine what happens next at the next and the next moment and infinitely far into the future, and for that matter, indefinitely far into the past. So you can take any one moment in the history of the universe according to classical mechanics, and the information contained in what is going on at that moment is sufficient to fix what will happen at every other moment in history. And Laplace, who is quite imaginative about these things, put it in terms of a metaphor. He says, “Imagine a vast intelligence.” Later, commentators dubbed it Laplace’s demon. He didn’t call it that, he was famously an atheist. He didn’t like to talk about demons. But the demon, the vast intelligence who could know everything about the universe at any one moment, Laplace says to that vast intelligence, the past and future are an open book. You would know everything because what happens now fixes the entirety of space and time. The idea that the laws of physics fix what’s going to happen in principle precisely and exactly if you know what’s happening right now. So this became known as the clockwork universe paradigm. The universe clicks along in perfect accord with the laws of physics forever. Now, this might bother you a little bit if you wanna think, well, wait a minute, I’m a person, I’m a human being, I have the ability to make choices. I’m not determined by the laws of physics. And both scientists and philosophers thought about that. They still don’t agree on what the right way to think about it is. But the favorite way to think about it is the following. In principle, if you knew exactly everything that was going on in the universe, you could predict the future. Now, classical mechanics isn’t quite right. Eventually, we’re gonna talk about quantum mechanics, so that’s another thing you have to keep in mind. But to the approximation that classical mechanics is good, you are determined in what is going to happen. But guess what? You don’t know all the positions of all the atoms and all the molecules that make up you. Indeed, you literally cannot know them because the memory storage capacity to know all that would be at least as big as your brain, if not bigger. And if you made your brain bigger, now you just have more molecules to keep track of. It is impossible to actually have a real Laplace’s demon in the universe. It’s just a thought experiment to make vivid the implications of determinism. So philosophers have settled on what they decided to call compatibilism in the sense that on the one hand, the deep down microscopic laws of physics are perfectly deterministic, or they’re not if you’re in quantum mechanics, but they’re pretty deterministic anyway. But since you don’t know it, you should be asking yourself, what is the best I can do? What is the best way that I can try to understand human beings given the vastly incomplete information I have? I know about, you know, my friends’ personality, and you know their predilections and their traits, but I don’t know every neuron in their brain. And under those circumstances, you will model, you will think about a fellow human being or about yourself as an agent capable of making choices. Everyone does that, and that’s the right thing to do because you are not Laplace’s demon.