What you're leaving out is that there are boundary conditions that are
impossible to realize, not just because they are too complex, like a
high entropy state, but because they require infinite specifications.
Brent
On 8/6/2022 6:25 AM, Jesse Mazer wrote:
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
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