Can't your argument be extended to the question of whether time is irreversible FAPP, or IRREVERSIBLE IN PRINCIPLE. For example, consider a gas at some temperature in an enclosure which is cooling. We might conclude the time is irreversible FAPP, but quantum theory does not give any information about the direction of the emitted thermal photons. So I conclude, based on present theory, that time is strictly irreversible, that is, IRREVERSIBLE IN PRINCIPLE. AG
On Friday, August 5, 2022 at 10:35:18 PM UTC-6 Bruce wrote: > On Sat, Aug 6, 2022 at 12:10 PM Jesse Mazer <[email protected]> wrote: > >> 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. >> > > I think I was drawing a distinction between time reversible laws and > processes as things that happen to particular "bits of matter". The laws > might be time reversal invariant, but particular processes might not be > reversible. It makes little sense to restrict one's attention to > reversible laws when one is asked whether a particular process can be > reversed or not. There are clearly processes that cannot be reversed, in > principle and not just FAPP. The emission of photons into an expanding > universe is just one example, even though the emission process might be > governed by reversible laws. The emitted photon cannot be caught and > returned. That is all that is meant by saying that it is not reversible. > This is relevant to the question as to whether a quantum measurement is > reversible or not. Quantum evolution is unitary, but generally the process > of measurement is not reversible, even in principle. Take the spin > measurement of a spin-half particle. Given an "up" result for instance, one > cannot reverse this to determine the spin state of the particle prior to > the measurement. Many worlds do not help here, because one has no access to > other worlds. > > 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/07e5261b-df34-488d-8ffc-6af4c50d3452n%40googlegroups.com.
