Sean Carroll: Can we ever escape the logic of a clockwork universe?

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 want to 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 am 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

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 because 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 going to 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 want to 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 going to 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 will 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 going to 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 going to 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 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. 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 moment 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

was 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 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 want to 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 going to 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. 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 my friend's personality and 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