[jethkl]

Sharing a simple thought experiment that was shared with me years ago that explains (to me at least) why this is an interesting question. Imagine a billiard ball with nonzero velocity bouncing around an enclosed box. When the ball encounters a side of the box, it bounces off elastically. A replay of this ball's path over time is equally plausible if the replay were run forward or reversed. The preceding is also true if one imagines 2 or 3 balls, with the only difference being that the balls may also bounce off each other elastically. Even in this scenario, reversibility of playback holds no matter the configuration of the balls: they could all start clustered or be scattered and the replay would be plausible when played in either time direction. But this is no longer true when the box (now much larger) contains millions of billiard balls. 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. This is because 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. To summarize, in the "few scenario" we can plausibly reverse time but in the "many scenario", we cannot. The only difference between scenarios is the number of balls in the box, which suggests that time is an emergent property. layer8's answer elsewhere in this thread says the same, but more succinctly.

[aurareturn]

  But this is no longer true when the box (now much larger) contains millions of billiard balls. 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.

I don't understand how increasing the number of balls means you can't reverse the playback.

[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.