On Saturday, April 28, 2018 at 11:59:27 PM UTC, Brent wrote: > > > > On 4/28/2018 4:28 PM, [email protected] <javascript:> 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 -- 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.
