-ecriture d'adulte
https://forumserver.twoplustwo.com/47...oblem-1652550/
Is there something unexplained/mysterious about the arrow of time?
The AoT is, roughly, the observational fact that there is a massive asymmetry between past states and future states despite the fact that the microscopic laws governing this evolution are completely time symmetric. Unlike other issues in modern physics, one of the main points of contention seems to be whether this is a problem at all! From my passive information gathering, it seems that cosmologists and philosophers are more likely to think there is a real problem, while particle/string theorists don’t see one. I largely sympathize with the latter, fwiw.
Just for background, the obvious one word answer to why the future is different from the past, why do eggs break but not unbreak etc is entropy. There are something like ~10^23 more ways for an egg to be broken than for it to be unbroken. So an unbroken egg can change to a broken one, while the opposite requires incredible fine tunings/correlations; so much so that the universe is not old enough for an event that rare to have occurred yet.
This was largely understood in the 1800s. There was also an obvious objection (also raised in the 1800s) that this doesn’t really explain anything. Namely, the argument works exactly the same backwards in time (let t---->-t). Given we see some low entropy state now (an unbroken egg) it should have been broken in the past simply because there are again ~10^23 more ways for an egg to be broken than not.
The response is usually to just put in time asymmetry by hand, as a boundary condition. The past is given a “fixed” lower entropy condition while the future is “unfixed” and therefore simply evolves into more likely states. Cosmologists and philosophers claim that this, again, doesn’t really solve anything because we’re left with the problem of why the big bang itself started, or very soon after found itself in an incredibly low entropy state.
Hawking, at one point, honestly held the belief that if the universe was not expanding but contracting, eggs would be observed to unbreak rather than break!!!!! He’s since been convinced this is wrong. OP is long enough so I’ll stop here, ignoring most of the cosmological issues, but I'll add that I'm still not sure there is a problem to begin with. Any thoughts?