On Sunday, April 29, 2018 at 12:40:51 AM UTC, Brent wrote: > > > > On 4/28/2018 5:24 PM, [email protected] <javascript:> wrote: > > > > On Saturday, April 28, 2018 at 11:59:27 PM UTC, Brent wrote: >> >> >> >> On 4/28/2018 4:28 PM, [email protected] wrote: >> >> >> >> On Saturday, April 28, 2018 at 11:17:54 PM UTC, Bruce wrote: >>> >>> From: <[email protected] >>> >>> >>> On Saturday, April 28, 2018 at 10:55:13 PM UTC, [email protected] >>> wrote: >>>> >>>> >>>> >>>> On Saturday, April 28, 2018 at 9:33:58 PM UTC, Brent wrote: >>>>> >>>>> >>>>> >>>>> On 4/28/2018 9:39 AM, [email protected] wrote: >>>>> > Is it a settled issue whether measurements in QM are strictly >>>>> > irreversible, >>>>> >>>>> There are interactions that, if you did not arrange that they be >>>>> erased, >>>>> would constitute measurements. Whether you say they were measurements >>>>> and then got erased or they are not measurments because they didn't >>>>> produce an irreversible record is a phlosophical or semantic question. >>>>> >>>>> > that is irreversible in principle, or just statistically >>>>> irreversible, >>>>> > that is, reversible but with infinitesimal probability? TIA, >>>>> >>>>> The equations are all reversible so you might say they are reversible >>>>> with infinitesimal probability...but in most cases that reversal would >>>>> mean catching and reversing photons that are already on their way >>>>> outbound beyond the orbit of the Moon. >>>>> >>>>> Brent >>>>> >>>> >>>> Are there any measurements that can't be reversed regardless of the >>>> fact that the equations of physics are time reversible? I could swear, >>>> and I DO, that Bruce demonstrated such a case for spin 1/2 particles >>>> measured by SG device. AG >>>> >>> >>> You can always take a movie of the measurement and play it backward. >>> Does this say anything about reversal in principle; that every >>> measurement >>> is in principle reversible? AG >>> >>> >>> That was the trap Vic fell into. Playing the movie backwards is not >>> generally equivalent to time reversal. It is in classical physics, but in >>> the quantum case, the movie is taken in only one world after the decoherent >>> splitting of the MWI , so playing it backwards does not reverse the other >>> worlds. >>> >>> Bruce >>> >> >> Can't we analyze this problem without bringing the MWI? If we play the >> movie backward, and the movie is good enough to include all IR photons >> involved in the process, won't the movie played backward indicate the every >> measurement, indeed every physical process, is in PRINCIPLE reversible? AG >> >> >> No. Suppose you have filmed (is "videoed" a word?) a stream of >> electrons, all prepared as |up> entering and SG oriented left/right. So >> the film shows a stream electrons exiting in two streams, one with the >> electrons oriented |left> and one with them oriented |right>. Now you play >> it backwards and you see the two streams of electrons, one with the >> electrons oriented |left> and one with them oriented |right>, entering the >> SG. They come out as a stream of |up> electrons in the reversed movie. >> But nomologically that is impossible (has infinitesimal probability); in an >> actual experiment they would come out with their |left> or |right> >> orientation intact. >> >> Brent >> > > In my effort to clarify this subject, I keep saying that if something can > happen, even with infinitesimal probability, I will say it is > "statistically irreversible" -- meaning it CAN in PRINCIPLE be reversed. > This I distinguish from irreversible in principle, meaning the process can > never be reversed. So, given a film which contains each and every > interaction of any process, and the fact that the equations of physics are > time reversible, I conclude that every physical process, without exception, > is either easily reversible or worst case statistically irreversible > (meaning reversibility is POSSIBLE, even if hugely unlikely). I am probably > wrong. LOL. AG > > > The problem is that your film would have to record both branches of the > wave-function, i.e. both "worlds" for each electron so that in the reversal > the phase information would be available. This would allow the reversal to > the original state of the wave function. But having the original wave > function doesn't mean you can measure it and get the same results as if you > had measured it originally. The wave function still only encodes > probabilities insofar as your measurements and perceptions are concerned. > So it would be like in some SciFi stories, when you go back in time it's to > a different "branch" of the MWI. > > Brent >
Why are the phase relations of the waves comprising the original wf, of what is presumably a coherent wave structure, lost when the measurement occurs? TIA, AG more options, visit https://groups.google.com/d/optout. > > -- 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 post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
