On Saturday, April 28, 2018 at 11:33:58 PM UTC, Bruce wrote: > > From: <[email protected] <javascript:> > > 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? > > > The short answer is, No. Reversible means unitary evolution. Schrödinger > evolution is unitary only with MWI. So reversible implies MWI. And since we > don't have access to other MWI worlds, reversiblity is impossible for us > "*in principle*. > > Bruce >
I'm in Cali, Colombia, on a Saturday night, music late at night and I can't sleep. Then it hit me. In a one world analysis using standard QM, there is no process for going from the SWE to a definite measurement; that is, there is no process for the transition of the system being measured to the eigenfunction for which the measured value is an eigenvalue. So although the SWE is time symmetric, the measuring process is NOT. Standard QM does not tell us how we transition from the SWE to a particular measurement and eigenstate. So, if it doesn't tell us how we get TO the measured value and the final eigenstate, it surely can't tell us how to go in the opposite direction, back to the original state, which would be the reversed process. IOW, after a measurement occurs, there is no way to recover the original wf. This means that standard one world QM is irreversible "in principle". Playing the movie backward is totally misleading. AG > > 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 > > -- 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.
