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] <javascript:>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? AGThat 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. BruceCan'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.
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