Are you defining "process" as a *pattern* of behavior which can be duplicated with different bits of matter, or as something that refers to some specific bits of matter, so that reversing a process would require doing it to the same bits of matter that underwent the original process? I think if a physicist talked about a "process" being reversible or not, they would be referring to the pattern-based notion. For example, take the process of a rogue planet coming close to a planetary system and getting captured by its gravitational interactions with the star and the planets in the system. With a pattern-based notion of process, that process is reversible in the sense that one could have a different star and different planets with identical masses, where the initial conditions were such that the planet got ejected from the system in a perfect time-reversed version of the behavior of the first system.
On Fri, Aug 5, 2022 at 7:44 PM Bruce Kellett <[email protected]> wrote: > On Sat, Aug 6, 2022 at 9:29 AM Jesse Mazer <[email protected]> wrote: > >> "The time invariance of the laws means that a photon coming in from outer >> space is consistent with the laws. But that cannot be the same photon." >> >> But "reversibility" as physicists define it has nothing to do with >> actually causing the same system to reverse itself, it's a more abstract >> notion that you could have a different system obeying the same dynamical >> laws whose behavior over time would be a perfectly time-reversed mirror of >> the first system's behavior. If you think it's about a single system >> evolving one way for some period of time and suddenly reversing itself so >> that its subsequent behavior looks like a reversed version of its initial >> behavior, that's just a misunderstanding of the concept. >> > > You are talking about the time-reversal invariance of the laws of physics. > That is one thing, but when people ask whether irreversible processes are > possible, then the emphasis is on the process, not the underlying laws. So > the issue is whether there are individual processes that cannot be > reversed, not whether there can exist separate processes that look like the > original process in reverse. > > This is important in the context of unitary evolution in quantum > mechanics. Unitary time evolution obeys time symmetric laws, but the > emission of a photon into an expanding universe, while consistent with > unitary evolution, is not a reversible process. > > Bruce > > > On Fri, Aug 5, 2022 at 7:14 PM Bruce Kellett <[email protected]> >> wrote: >> >>> On Sat, Aug 6, 2022 at 7:54 AM Jesse Mazer <[email protected]> wrote: >>> >>>> Why do you say it's irreversible in principle? Wouldn't the >>>> time-reverse of that just be a photon traveling towards an atom and being >>>> absorbed, which is permitted by the laws of physics given a different set >>>> of initial boundary conditions? >>>> >>> >>> The laws of physics are invariant under the time-reversal operation. >>> That does not imply that irreversible processes are impossible. Brent has >>> pointed out that sending a photon out into an expanding universe is a >>> process that is irreversible in principle. The time invariance of the laws >>> means that a photon coming in from outer space is consistent with the laws. >>> But that cannot be the same photon. The idea that you can surround >>> everything with a perfectly reflecting mirror, so that all emitted photons >>> are returned, is just a fanciful diversionary tactic -- no such >>> reflective surrounds exist. Besides, reflecting photons back is not a >>> process reversal in an expanding universe. The red shift induced by the >>> expansion means that the returning photon inevitably has lower energy than >>> the emitted photon. >>> >>> Bruce >>> >> -- > 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/CAFxXSLQEa1Xt7Q0sGMHUy7crGB4G5NJQvbdD7FEbzuxmhjD1Tg%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAFxXSLQEa1Xt7Q0sGMHUy7crGB4G5NJQvbdD7FEbzuxmhjD1Tg%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/CAPCWU3Lei1xd4rzjWSkUpLH%3DRGA%3DsSY8X0TFXGbKSJLnzg%2BWLQ%40mail.gmail.com.
