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.
