But reversibility as understood by physicists isn't about whether you could "create" the appropriate type of system with advanced technology or whatever, it's about the abstract question of whether the time-reversed version is a valid solution to the same dynamical laws of physics. One could imagine a god who can create multiple universes, with the only constraint that they all obey the same laws of physics--if the laws and dynamics are reversible, that implies that for any given universe, the god can also create a distinct universe that behaves like a time-reversed version of the first one.
On Fri, Aug 5, 2022 at 8:18 PM Brent Meeker <meekerbr...@gmail.com> wrote: > I'm pointing out that in some cases creating the reverse boundary > conditions is impossible in principle because they are at infinity. > > Brent > > On 8/5/2022 3:47 PM, Jesse Mazer wrote: > > 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 <meekerbr...@gmail.com> 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 <meekerbr...@gmail.com> >> 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 <agrays...@gmail.com> >>> >>> 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 everything-list+unsubscr...@googlegroups.com. >>> >> 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 everything-list+unsubscr...@googlegroups.com. >>> 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 everything-list+unsubscr...@googlegroups.com. >> 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 everything-list+unsubscr...@googlegroups.com. >> 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. > 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/CAPCWU3J74eo_pbXCLMe0L%3Dom7qR-bHeKVw6XJ3UhUJT-cT88Ng%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAPCWU3J74eo_pbXCLMe0L%3Dom7qR-bHeKVw6XJ3UhUJT-cT88Ng%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 everything-list+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/4f5861ca-6b04-dcb1-64f5-dfeaead00141%40gmail.com > <https://groups.google.com/d/msgid/everything-list/4f5861ca-6b04-dcb1-64f5-dfeaead00141%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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