But when physicists say that a given system's dynamics are "reversible" doesn't this generally involve an appeal to different initial boundary conditions? (The end conditions with all the velocities reversed and treated as a new system's initial conditions, for example.) Are you using reversible/irreversible in a more colloquial sense?
On Fri, Aug 5, 2022 at 5:57 PM Brent Meeker <[email protected]> wrote: > That's why I wrote, "The arrow of time comes from the boundary condition." > > Brent > > On 8/5/2022 2:54 PM, Jesse Mazer 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? > > On Thu, Aug 4, 2022 at 5:10 PM Brent Meeker <[email protected]> wrote: > >> If a photon is emitted into an infinite universe it is irreversible in >> principle, not just FAPP. But it doesn't mean the physical theory is >> irreversible. The arrow of time comes from the boundary condition. >> >> Brent >> >> On 8/4/2022 8:47 AM, smitra wrote: >> > On 04-08-2022 17:41, Alan Grayson wrote: >> >> I recall Bruce giving an example of an irreversible process, but I >> >> can't recall the details. AG >> >> >> > >> > Probably a FAPP irreversible process. >> > >> > Saibal >> > >> > >> >> On Thursday, August 4, 2022 at 6:39:04 AM UTC-6 Jason wrote: >> >> >> >>> On Thu, Aug 4, 2022, 5:23 AM Alan Grayson <[email protected]> >> >>> wrote: >> >>>> I meant to write that information conservation depends on >> >>> reversibility! How solid is that assumption? AG >> >>> >> >>> I think it is pretty good. >> >>> >> >>> I think reversibility is part of it. Certainly in a reversable >> >>> Newtonian kind of physics (no GR and no QM, full determinism), >> >>> reversability would imply an inability to destroy information. >> >>> >> >>> In reversible computers, information can't be deleted, only shuffled >> >>> around, so in this simplistic model, reversibility (in a Turing >> >>> machine) implies conservation of information. >> >>> >> >>> In GR, matter falling into black holes was originally thought to be >> >>> an irreversible process. This led to the "black hole war". >> >>> https://en.wikipedia.org/wiki/The_Black_Hole_War which was >> >>> eventually settled by concluding information isn't destroyed in a >> >>> black hole, therefore the pattern of black hole radiation must >> >>> somehow indicate or encode what has fallen in to it. >> >>> >> >>> In QM, wave function collapse was thought to be an example of an >> >>> irreversible process. Yet from the global view of all the branches >> >>> and many world's it is not. >> >>> >> >>> But moreover, despite the apparent irreversibility if collapse from >> >>> the confines of any one branch, the information available within any >> >>> single branch still seems to be conserved (just as matter and energy >> >>> are). This lead to a kind of: energy-matter-information equivalence. >> >>> >> >> >> https://en.wikipedia.org/wiki/Holographic_principle#Energy,_matter,_and_information_equivalence >> >> >> >>> >> >>> This question, I think, probes at the very deepest levels of >> >>> physics. I have some more thoughts on this written here: >> >>> >> >>> >> >> >> https://alwaysasking.com/why-does-anything-exist/#Information_as_Fundamental >> >> >> >>> >> >>> Jason >> >> >> >> -- >> >> 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/1c5ab1b8-fef6-4a5c-bd88-fb7b24d0e4b8n%40googlegroups.com >> >> >> >> [1]. >> >> >> >> >> >> Links: >> >> ------ >> >> [1] >> >> >> https://groups.google.com/d/msgid/everything-list/1c5ab1b8-fef6-4a5c-bd88-fb7b24d0e4b8n%40googlegroups.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/2f8cd146-3c6d-a542-7618-75b2d05c0073%40gmail.com >> . >> > -- > 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/CAPCWU3%2BWyDWDQDNPzoXUM4gqFv0HjWQ2OrnQdPGQmgmJKqWtOw%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAPCWU3%2BWyDWDQDNPzoXUM4gqFv0HjWQ2OrnQdPGQmgmJKqWtOw%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/43ba70f3-6535-58f9-ca9c-9b11cb6a7f38%40gmail.com > <https://groups.google.com/d/msgid/everything-list/43ba70f3-6535-58f9-ca9c-9b11cb6a7f38%40gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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