Watsisname wrote:Mouthwash, Doc and I have been talking about this a bit and we both conclude that a universe with a reversed second law would simply be equivalent to reversing time.

A key example is that of a black hole, which is the maximal entropy state for any system. For entropy to decrease, the black hole must be replaced by a white hole, which is a time reversal.

The second law says that a box filled with air quickly achieves an equilibrium where the air is uniformly distributed. If you have the air all on one side blocked by a barrier, opening the barrier quickly results in the air filling the box. It does this because there is a vastly greater number of ways that particles can be arranged uniformly through the whole box, rather than all bunched up on one side, so the system naturally evolves to the higher entropy state.

In a universe where entropy of a closed system decreases, then this implies a box filled with air will naturally evolve to having the air bunched up into highly structured states. This would appear very strange, as if complex things arise for no reason. This rise of complexitydoesoccur in our universe with time going forwards, but it is allowed by an increase in entropy elsewhere. For instance, the complexity of life on Earth is due to the hot Sun providing energy in the form of low-entropy sunlight, and a cold dark sky acting as a sink for the high-entropy energy to flow out. Life is like an eddy in a stream, where some water can locally move upstream while the bulk of the flow moves downward.

Reversing the increase of entropy for the Earth system would be like having infrared energy coming in from the surrounding space, driving processes on Earth, and then being shot out as low-entropy sunlight directly into the Sun, which then flows to the core to drive fusion processes in reverse. Very strange.

Could a universe exist where the entropy stays constant globally? Yes. It would have to be completely lacking in evolution of structure. No stars, no galaxies, no life. All "irreversible" processes increase entropy by turning useful energy into waste heat. To avoid that requires a universe with only purely reversible processes. The expansion of the universe itself is isentropic (does not change the entropy), but not when you include matter which can collapse under gravity and do things. Gravitational collapse of matter into structures increases entropy.

So in a nutshell: a universe where entropy decreases requires reversing time, and a universe with no entropy change would be very boring.

Cool. However, since quantum effects seem to violate basic laws of the universe (e.g. causing matter to escape black holes, instantaneous connections), it's also conceivable for certain substances or interactions to give out more energy than they take in. Instead of positing a universe where entropy goes in reverse, how about a universe filling up with this magically produced energy?