EZ-PEAS t1_iza4krp wrote on December 7, 2022 at 4:35 PM

Reply to comment by ruamru123 in Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science by AutoModerator

Entropy isn't a physical thing, it's a computed quantity that informs us about the state of a system. A decreasing-entropy universe requires some interpretation of what decreasing entropy means in the specific contexts where we talk about entropy.

For example, entropy in thermodynamics means that heat spreads out rather than concentrates, gasses diffuse instead of concentrating, etc. Maybe your interpretation of negative-entropy means that fires suck heat out of the surrounding areas. Maybe it means that gasses concentrate naturally, so if you had a closed box and you left it alone then the atmospheric gas inside would separate into layers of oxygen, nitrogen, etc. by density. Maybe it means that if you shatter a teacup the pieces slowly glue themselves back together.

Entropy is just an observational quantity that describes what we see in the real world. It describes the physical behavior we see in the real world, and the "opposite" of real behavior is not well defined. There is only one realistic, correct behavior. There are infinitely many incorrect behaviors you can pick from.

davidjara t1_izajt0c wrote on December 7, 2022 at 6:14 PM

I strongly disagree. "physical things" are precisely "observational quantities" and entropy is just as real as temperature/pressure or whatever thermodynamical quantity.

Going back to OP's question. The only physical law that differentiates past from future is entropy, all other (fundamental) physical laws are time reversal invariants. Many people believe that the passing of time is how we experience a gradient of entropy (Hawking has a beautiful argument of why the only way we can remember something is by increasing entropy in his brief history of time, I can find the exact part if you are interested). From this perspective, it is not that it just so happens that entropy increases with time, it is that we can only remember stuff with less entropy and the stuff we remember is what we call the past.

MrDownhillRacer t1_izbuq0a wrote on December 7, 2022 at 11:28 PM

I would think that it isn't too hard to imagine what the universe would be like if all the fundamental dynamical laws were the same, except entropy decreased over time instead of increased.

It would look like our universe in reverse. It would begin from heat death and end in a Big Crunch. Puddles would form into ice cubes. A wave would converge from disparate sources and carry enough energy to eject a penny out of a fountain and into your hand.

I think the difficulty is: can we meaningfully distinguish that sort of universe from our own, or are we just describing the universe we already live in from a different angle? It might be like giving a description of a pencil that starts with its point and ends with its eraser, and then giving a description of an otherwise identical pencil that starts with its eraser and ends with its point. These could just be descriptions of the same pencil.

I suppose there is another ways that you could have a universe in which entropy always decreases. It could start in a state of low entropy instead of with a Big Bang, and just get even lower in entropy from there.