On Sunday, April 29, 2018 at 7:17:48 AM UTC, [email protected] wrote: > > > > On Saturday, April 28, 2018 at 11:33:58 PM UTC, Bruce wrote: >> >> From: <[email protected] >> >> 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 >
Further, since I am in the Weinberg camp of finding the MWI "repellant", I conclude that irreducible randomness at the quantum level implies the arrow of time, insofar as quantum processes are strictly, in principle, irreversible. 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.
