On Friday, August 5, 2022 at 6:14:42 PM UTC-5 Bruce wrote: > On Sat, Aug 6, 2022 at 7:54 AM Jesse Mazer <laser...@gmail.com> 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 >

It is the case of a billiard ball impacting another vs a set of racked billiard balls. If I were to take a video of a billiard ball impacting another, framed this in the center of mass of the balls and mask any perception of the rotation of the balls, the video run backwards and forwards would by very similar. There would be nothing to distinguish the forwards and backwards video. It is perfectly time reverse invariant. Now consider the racked balls impacted by the cue-ball. It is pretty easy to see which is forwards in time, as we do not expect balls to rush inwards and align themselves in an ordered set and eject another. However, if the table were "perfect," it had frictionless surface and the balls reflected off the sides perfectly, if we wait long enough it will return to its original state. This is Poincare recurrence. You have to wait a lot longer than the duration of the universe so far. The same happens with quantum mechanics. There is a Poincare recurrence, given by the exponential of the Euclideanized action. However, there is an additional phase, which defines the quantum complexity and the recurrence time on that is the exponential of the Poincare recurrence time. Quantum complexity is interesting, and I think it involves the Hilbert-Polya conjecture concerning the Riemann zeta function and the zeros being mapped to the eigenvalues of a Hermitian operator. In this case the recurrence is a vast period of time, as long as the stability of the de Sitter manifold of the cosmos. In effect we have limitations on what we can observe and account for, but ultimately the universe may have an accounting of quantum information, at least for unitary systems and quantum gravitation with Petrov types that have Killing vectors. When it comes to the universe at large, that may be a different matter. Such ideas may turn out to be false. LC -- 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 everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/f2cf6a7e-a90f-4f22-9f88-84bf08417581n%40googlegroups.com.