Physicists may distinguish between time-reversibility of the dynamics, also called "microscopic reversibility" at https://en.wikipedia.org/wiki/Microscopic_reversibility , vs. "macroscopic" or "thermodynamic" irreversibility, which as you say is ultimately thought to be a statistical consequence of the low-entropy conditions at around the time of the Big Bang. But from the dynamical point of view you could have a valid solution with a universe that's been eternally contracting towards a Big Crunch, with a low-entropy state near the Big Crunch, which would be the time-reverse of our universe's evolution. Huw Price's book Time's Arrow and Archimedes' Point has a good discussion of the issues surrounding microscopic reversibility vs. thermodynamic irreversibility, and the role of low-entropy boundary conditions.
On Sat, Aug 6, 2022 at 7:02 AM John Clark <[email protected]> wrote: > On Fri, Aug 5, 2022 at 6:47 PM Jesse Mazer <[email protected]> wrote: > > *> But when physicists say that a given system's dynamics are "reversible" >> doesn't this generally involve an appeal to different initial boundary >> conditions?* >> > > If at the time of the Big Bang the universe was it in an extremely low > entropy state then even if the laws of physics were 100% deterministic > and even if X and Y always produced Z and nothing except X and Y could > produce Z the second law of thermodynamics would still insist that things > are irreversible because there are an astronomical number to an > astronomical power more ways for something to have high entropy than low > entropy. > > John K Clark See what's on my new list at Extropolis > <https://groups.google.com/g/extropolis> > 2le > > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAJPayv2PdgHraMf-NMt0%3Dhs-QXw4nqC6BMv%2BEWMWO8zbsDLCFg%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAJPayv2PdgHraMf-NMt0%3Dhs-QXw4nqC6BMv%2BEWMWO8zbsDLCFg%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAPCWU3K%2BvXBVVc1p9cXr6yAh1Q8j9DxU79DcyvzUTknhXKs%2Byw%40mail.gmail.com.
