"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. 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/CAFxXSLQw_%2B17D1CuCDz1%3Dn35uk2vWUhxMZmCf7hyV8ucbtDH%2Bw%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAFxXSLQw_%2B17D1CuCDz1%3Dn35uk2vWUhxMZmCf7hyV8ucbtDH%2Bw%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/CAPCWU3KaEhcgb%2BCRCBpkQ%3De_gs90_g4b_LJ72wbKz%3D7Q_CYbzA%40mail.gmail.com.
