salvestrom said:
My remark about additional energy was referring to the energy 'lost' when the cue strikes the ball producing a sound. In order to physically reverse the process that energy needs to be put back into the cue. While this is mathematically a reversible process, physically I know of no process that would allow it, anymore than the embers of a fire can regain the radiated heat. I consider these things beyond unlikely.
Precisely. That is what defines the direction of time. We do not see shards of glass leap together then back onto a shelf where they nudge a person's elbow.
The pool cue and break is analogous to the universe
right now. Low entropy, clear direction of time.
The pool table after ten minutes is analogous to the universe trillions of years from now. Very high entropy, no direction of time.
I film the billiard table on a video camera for ten minutes. I play it back to you but only the last minute - and I don't tell you whether I play that minute forward or backward. Can you tell by looking at that minute which direction I played it? Nope.
(Note, by the way that it works whether or not you include friction. If you allow friction then, after 9 minutes all the balls are motionless - no direction of time. If the billiard table and balls are frictionless, then after 9 minutes they are still careening around completely randomly. Either way you have lost the arrow of time in the video I show you. The universe, being a closed system, conserves its energy, thus it is equivalent to the frictionless version of the pool table.)
The original point of comparing the universe to the billiards table was that, trillions of years in the future, there
are no low-entropy objects such as pool cues or atmospheres. You just have a uniform soup of billiard balls all with random motion. They carom off each other but, since they're all just billiard balls bouncing around, there is no further increase in entropy, no increase in disorder. Thus the arrow of time is lost.
See?