# The Naked Scientists Forum

### Author Topic: Do vectors militate against time reversal?  (Read 842 times)

#### Bill S

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##### Do vectors militate against time reversal?
« on: 23/06/2016 23:07:25 »
This thought experiment involves a friction-free snooker table, in a vacuum. The cue-ball is alone on the table, placed centrally on the balk cushion.

At t=0 the cue strikes it, sending it straight up the table at constant velocity, v, with constant momentum, p.

At t=1 it crosses the brown spot.
At t=2 it crosses the blue spot.
At t=3 it arrives at the pink spot; at which point, time is reversed.

It crosses the blue and brown spots at t=2 and t=1, respectively.

What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?

#### Colin2B

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##### Re: Do vectors militate against time reversal?
« Reply #1 on: 23/06/2016 23:11:42 »
..... the ball is travelling towards the balk cushion, but the vectors point away from it?
Why do you think they are pointing away from it?

#### Bill S

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##### Re: Do vectors militate against time reversal?
« Reply #2 on: 24/06/2016 01:15:30 »
Would the vectors automatically change when time reversed?

#### jeffreyH

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• The graviton sucks
##### Re: Do vectors militate against time reversal?
« Reply #3 on: 24/06/2016 01:34:44 »
That is not as straightforward as it appears. Since time is reversed. You could say that yes the vector reverses and indicates an attractive force similar to gravity. I am not sure what the alternative would be.

#### PmbPhy

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##### Re: Do vectors militate against time reversal?
« Reply #4 on: 24/06/2016 15:17:26 »
Quote from: Bill S
This thought experiment involves a friction-free snooker table, in a vacuum. The cue-ball is alone on the table, placed centrally on the balk cushion.

At t=0 the cue strikes it, sending it straight up the table at constant velocity, v, with constant momentum, p.

At t=1 it crosses the brown spot.
At t=2 it crosses the blue spot.
At t=3 it arrives at the pink spot; at which point, time is reversed.

It crosses the blue and brown spots at t=2 and t=1, respectively.

What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?
This doesn't make any sense to me. Time isn't reversing here Bill. Just because a ball is retracing its trajectory there is nothing to imply that time reverses.

#### evan_au

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##### Re: Do vectors militate against time reversal?
« Reply #5 on: 25/06/2016 12:51:44 »
Quote from: PmbPhy
Just because a ball is retracing its trajectory there is nothing to imply that time reverses.
If it is like any game of billiards I have seen, the nice, orderly triangle of balls that exist at the start of the game turn into random locations around the table (increase in entropy).

Balls moving from random locations into an orderly triangle suggests that entropy is decreasing, which is a strong indicator that time has reversed.

#### Bill S

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##### Re: Do vectors militate against time reversal?
« Reply #6 on: 25/06/2016 15:31:59 »
I had a suspiction, when I posted the OP, that it might lead to some misunderstandings, but I left the wording like that for a particular reason.

Quote from: Pete
This doesn't make any sense to me. Time isn't reversing here Bill. Just because a ball is retracing its trajectory there is nothing to imply that time reverses.

I was not suggesting that time reversed because the cue ball started going the other way. Rather, I was saying that if time were (somehow) reversed at this point, the ball would be observed retracing its path.  Does that make more sense?

The next part of the thought experiment was to consider what would happen to the vectors, v and p, after time had been reversed.

I should make it clear that I am not advocating time reversal as a physical reality, but the "possibility" is often encountered in serious scientific writing/discussion.

Quote from: Evan
If it is like any game of billiards I have seen, the nice, orderly triangle of balls that exist at the start...

That's snooker, Evan; billiards has only three balls, so any triangle would be very small, and quite rare,

#### evan_au

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##### Re: Do vectors militate against time reversal?
« Reply #7 on: 26/06/2016 12:47:09 »
Quote from: Bill S
That's snooker, Evan
Thanks, Bill....

I am showing my ignorance of all games where people chase little rounded things around fields both big and small...

...unless you consider that the LHC also fits that description?

#### The Naked Scientists Forum

##### Re: Do vectors militate against time reversal?
« Reply #7 on: 26/06/2016 12:47:09 »