The universe as an equation
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’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 when you hear hose 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.
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’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, 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.
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