On 4/28/2018 6:43 PM, agrayson2...@gmail.com 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: <agrays...@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.
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? AG
Concerning the pre-measurement phases of the states comprising the
superposition, aren't they irrelevant for calculating probabilities? *
No, in general they're not irrelevant for calculating probabilities.
It's their interference that causes you to be here and not there, causes
interference fringes, causes your brain state that saw UP to be
orthogonal to your brain state that saw DOWN.
Brent
*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 *
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