There is something about the time reversal principle that I dont completely understand.

Consider an isolated gas of molecules that has undergone expansion in volume. Now, we know that Newtonian dynamics obeys time reversal symmetry, ie if I reverse the velocities of all particles in the system at some instant, the evolution from then on would be backward. In other words the system would retrace its path. Atleast , this definitely holds when there are no velocity dependent interactions, which we can assume here for the sake of simplicity.
However, we also know for sure (from both second law of thermodynamics and common experience) that isolated gas molecules would never contract (statistically speaking) and decrease in overall volume.

How then does one account for this apparent contradiction?

Posted By :Venkateshan