# Reverse Universe

1. Nov 16, 2009

### Vincit

Among the physical laws it is a general characteristic that there is reversibility in time; that is, should the whole universe trace back the various positions that bodies in it have passed through in a given interval of time, but in the reverse order to that in which these positions actually occurred, then the universe, in this imaginary case, would still obey the same laws.

To test this supposed reversibility, we may imagine what we call 'Reverse Universe'. So, for example, if we had an imaginary universe, that universe would experience the same position of all bodies at various moments of time the same as the real universe; in reverse order.

But what would happen to a problem such as acceleration? I know that acceleration is the change of velocity divided by the interval of time required to produce this difference. So, if velocity A changed to velocity B, the equation(vectorially) would be B-A/T. So, in the corresponding motion of the reverse universe, wouldn't the velocity change from -B to -A, so that the acceleration is [(-A)-(-B)]/T. This is equal to B-A/T. Does that show that the magnitude of the acceleration will be the same in the reverse universe and the existing universe, but just reverse in order?

:/ I am being sidetracked by slight nuisances; help would be appreciated.

2. Nov 17, 2009

### pseudophonist

Most laws are like that except the second law of thermodynamics, part of which states that for spontaneous (read: possible) events will cause a net increase in entropy in the universe.

The maths from your acceleration scenario isn't too surprising. If we take some numbers, say A = 2m/s, B = 10 m/s and T = 4s, we get an accelleration of 2m/s^2. Now going from -10m/s to -2m/s is still an increase in speed of 8m/s in 4s, giving the same accelleration.
One is an increase in speed in the positive direction and the other is a decrease in speed in the negative direction, which is the same accelleration.

3. Nov 17, 2009

### schangtze

I have a question, why the reverse U's velocity changes form -A to -B while the real U changes from B to B? Shouldn't it be from -B to -A ?