Name: Sean Carroll: Are we living in a clockwork universe?
Uploaded: 2025-10-16T18:05:16.000Z
Duration: 9 min 29 s
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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

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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 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 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.

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 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 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 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 then 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.

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.

Newton was building on the shoulders of giants.

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.

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

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.

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 in 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.

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

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.

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 Simone Laplace, over 100 years after Newton.

But the structure of classical mechanics implies that if you knew the position and velocity not just of one particle, but

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 is quite imaginative about these things, put it in terms of a metaphor.

Later, commentators dubbed it Laplace's demon.

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 want to think, well, wait a minute, I'm a person, I'm a human being.

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.