From: <agrayson2...@gmail.com <mailto:agrayson2...@gmail.com>>
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.
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