# Entropy and time reversal

1. Jul 12, 2006

### leonchik1976

How time reversal of wave function and 2nd law of thermodynamics of entropy are settle together?

2. Jul 12, 2006

### vanesch

Staff Emeritus
Several answers are possible. One possible answer is that some people will say that projection is not time-reversible, so if projection is taken seriously, this kills time-symmetry in physics.

But even if you do not consider this possibility, I'd say that the answer is the same as in classical physics, where time-reversible LAWS can give rise to non-time-reversible *coarse-grained* dynamics: namely by special initial conditions.
If you start out (in a classical, time-reversible dynamics) with a highly peculiar initial state, and you only look at low-order correlation functions (coarse-graining), then you obtain a time-irreversible dynamics of these correlation functions until they reach their equilibrium values.

Mind you, I'm not saying that this is what explains finally the entropy increase in our universe. I'm only giving the example that there is no contradiction between time-reversible dynamics, and a second law of thermodynamics which prescribes the irreversibility of low-order correlation functions: it is sufficient to take a peculiar initial state.
It might of course be that there are genuinly irreversible laws too.
But there is no *contradiction* between the second law of thermodynamics, and time-symmetrical microdynamics.

3. Jul 13, 2006

### lalbatros

It is just a question of time !

4. Jul 13, 2006

### icarus3k

From what i know of, irreversible wave function collapses are supposedly the reason for the second law of thermodynamics.

Am I right here?

----------------------------------
Daring to Fly
http://icarus3k.blogspot.com

5. Jul 14, 2006

### wavemaster

I guess you can live without believing irreversible wavefunction collapse.
Following Feynman's example, increasing entropy is a consequence of statistical behavior.

Last edited: Jul 14, 2006