On Saturday, April 28, 2018 at 8:43:43 PM UTC-5, [email protected] wrote: > > > > On Sunday, April 29, 2018 at 1:16:37 AM UTC, Lawrence Crowell wrote: >> >> On Saturday, April 28, 2018 at 6:04:31 PM UTC-5, Bruce wrote: >>> >>> From: <[email protected]> >>> >>> 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 >>> >>> >>> I vaguely remember that from several years ago. As I recall, it was in >>> response to a claim by Vic that time reversibility of the equations meant >>> that if you measured the x-spin of a silver atom, the you could reverse the >>> result, say spin-up, and recover the initial state. That is certainly >>> impossible, since that does not take into account the phases associated >>> with the alternative result -- MWI is reversible only if you reverse all >>> the worlds. >>> >>> Besides, decoherence means that measurement resulting in classical >>> pointer-state outcomes are not reversible, even in principle, because of >>> the loss of IR photons which are never recoverable. Time reversal >>> invariance of the equations does not necessarily mean that you can actually >>> reverse things in practice. >>> >>> Bruce >>> >> >> In order to reverse a quantum system you must have the entire wave >> function. After a measurement the states are in decoherent sets, and you >> the observer "pull the marble out of the bag" and get your result. You >> would have to have access to the entire decoherent set and the prior >> superposition or entanglement phases of these states. Without that you >> can't back out squat. In fact if you have computed knowledge of the >> decoherent sets of states you still can't do anything without knowing their >> pre-measurement phases. This is the sort of thing soft measurements allow >> you to do, at least up to a point. The Schrodinger equation with time >> reversal invariance, with Wigner's requirement of complex conjugation of >> the energy operator >> >> iħ∂/∂t → i^*ħ∂/∂(-t) = iħ∂/∂t, >> >> which gives time reversal invariance. Entanglement phases evolve through >> systems accordingly, but if the reservoir of states is extremely large the >> Poincare recurrence time may be longer than the duration of the universe. >> In effect if this phase is lost the practical situation is there is a >> collapse or loss of quantum information in decoherence sets. >> >> LC >> > > > > *Aren't you describing what I've referred to as "statistical > irreversibiity", or the PRACTICAL inability to reverse a measurement, in > contrast to "irreversible in principle", by which I mean the absolute > impossibility of reversal? AGConcerning the pre-measurement phases of the > states comprising the superposition, aren't they irrelevant for calculating > probabilities? If so, why are they needed to reverse any measurement? That > is, if you can only recover the original wf up to phase angles and get the > same probabilities, why are the phases important for reversal of > measurements? AG * >
It is more than statistical. Nonquantum physics has probabilities, while quantum physics is about amplitudes that have a modulus square that is a probability. You must have control over not just probabilities, but amplitudes and by the same measure phases. LC -- 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.
