On Saturday, April 28, 2018 at 6:04:31 PM UTC-5, Bruce wrote: > > From: <agrays...@gmail.com <javascript:>> > > On Saturday, April 28, 2018 at 9:33:58 PM UTC, Brent wrote: >> >> >> >> On 4/28/2018 9:39 AM, agrays...@gmail.com 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 -- 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
