Stephen Wolfram

and you run that movie in reverse, you can't tell which way was the forward direction of time and which way was the backward direction of time.

You're just looking at individual billiard balls.

By the time you've got a whole collection of them, a million of them or something, then it turns out to be the case, and this is the mystery of the second law, that

the orderly thing, you start with the orderly thing and it becomes disordered, and that's the forward direction in time.

And the other way around of it starts disordered and becomes ordered, you just don't see that in the world.

Now, in principle, if you traced the detailed motions of all those molecules backwards,

you would be able to, it will, the reverse of time makes, as you go forwards in time, order goes to disorder.

As you go backwards in time, order goes to disorder.

Perfectly so, yes.

Right.

So the mystery is, why is it the case

that one version of the mystery is, why is it the case that you never see something which happens to be just the kind of disorder that you would need to somehow evolve to order?

Why does that not happen?

Why do you always just see order goes to disorder, not the other way around?

So the thing that I kind of realized, I started realizing in the 1980s, it's kind of like, it's a bit like cryptography.

It's kind of like you start off from this key that's pretty simple, and then you kind of run it, and you can get this complicated random mess.

And the thing that...

Well, I sort of started realizing back then was that the second law is kind of a story of computational irreducibility.

It's a story of what we can describe easily at the beginning, we can only describe with a lot of computational effort at the end.

Okay, so now we come many, many years later, and having done this big project to understand fundamental physics, I realized that a key aspect of that is understanding what observers are like.