On Sunday, April 29, 2018 at 3:40:19 AM UTC, Brent wrote: > > > > On 4/28/2018 5:57 PM, [email protected] <javascript:> wrote: > > > > On Sunday, April 29, 2018 at 12:40:51 AM UTC, Brent wrote: >> >> >> >> On 4/28/2018 5:24 PM, [email protected] 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, > > > They're not lost. They're what make the different branches of the > wave-function (approximately) orthogonal. > > Brent >
I see I have to be very careful with my words. I meant to say, why are the phase relations *changed* in a measurement? As for the different branches, I suppose that's part of the MWI. I don't acknowledge the existence of different branches. AG > > 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] <javascript:>. > To post to this group, send email to [email protected] > <javascript:>. > Visit this group at https://groups.google.com/group/everything-list. > For 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.
