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 <johnkcl...@gmail.com> wrote: > On Fri, Aug 5, 2022 at 6:47 PM Jesse Mazer <laserma...@gmail.com> 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 everything-list+unsubscr...@googlegroups.com. > 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 everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAPCWU3K%2BvXBVVc1p9cXr6yAh1Q8j9DxU79DcyvzUTknhXKs%2Byw%40mail.gmail.com.
