There’s nothing magical about reversing particle speeds. For entropy to decrease to the original value you would have to know and be able to change the speeds with perfect precision, which is of course meaningless in physics. If you get it even the tiniest bit off you might expect _some_ entropy decrease for a while but inevitably the system will go “off track” (in classical chaos the time it’s going to take is only logarithmic in your precision) and onto a different increasing-entropy trajectory.
Jaynes’ 1957 paper has a nice formal explanation of entropy vs. velocity reversal.
There’s nothing magical about reversing particle speeds. For entropy to decrease to the original value you would have to know and be able to change the speeds with perfect precision, which is of course meaningless in physics. If you get it even the tiniest bit off you might expect _some_ entropy decrease for a while but inevitably the system will go “off track” (in classical chaos the time it’s going to take is only logarithmic in your precision) and onto a different increasing-entropy trajectory.
Jaynes’ 1957 paper has a nice formal explanation of entropy vs. velocity reversal.
(the paper: https://journals.aps.org/pr/abstract/10.1103/PhysRev.106.620)