On Sat, Aug 6, 2022 at 12:10 PM Jesse Mazer <laserma...@gmail.com> 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

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