[jethkl]

you can reverse the playback, all the physics of billiards bouncing around works equally well in either time direction.

> If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction.

It is extremely unlikely that all the billiards will, simply by chance at some point in the future, collect together so they are contained within a very small volume. this shows that there is asymmetry in which direction time flows.

[zmgsabst]

If there’s only finitely many states, they will eventually — unless something prevents it.

https://en.wikipedia.org/wiki/Poincaré_recurrence_theorem

[aurareturn]

How did the balls start clustered?

I suppose we're now entering into the realm of what happened before the big bang?

[tsimionescu]

Actually no, this is a good question that hits at a tension between the second law of thermodynamics and the big bang model. The aftermath of the big bang and the inflation period are known to be times where the universe was extremely hot and dense. And yet, by the rule that entropy can't increase over time, it follows that they were the lowest entropy state of the universe: definitely lower entropy than what we have today. But how can an extremely hot plasma made up of all of the particles that today make up stars and planets and so on have been a lower entropy state than the galaxies of today?

[int_19h]

That's the thing - if the balls don't start clustered, then you "lose" the time direction again; now it doesn't matter which way you replay the trajectories, they are equally plausible in either direction. So if that's what you ultimately base the definition of time on, this implies that it is an emergent property.

[metalman]

it does not need anything other than plausible start and stop conditions to point out how time reversal is implausable, the time reversal idea only works under the implausable condition of only happening in a pefect steady state, no begining no end, just threading a piece of film in backwards, after editing out the set dressers at work without answering the fundamental question of "how did we get here anyway" its not a theory, its slight of hand edit: if it was an educational, thought experiment and labeled as such fine, sure, but its nothing more than that, :) , but could be less