On 4/28/2018 5:24 PM, [email protected] wrote:
On Saturday, April 28, 2018 at 11:59:27 PM UTC, Brent wrote:
On 4/28/2018 4:28 PM, [email protected] <javascript:> wrote:
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? 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
No. Suppose you have filmed (is "videoed" a word?) a stream of
electrons, all prepared as |up> entering and SG oriented
left/right. So the film shows a stream electrons exiting in two
streams, one with the electrons oriented |left> and one with them
oriented |right>. Now you play it backwards and you see the two
streams of electrons, one with the electrons oriented |left> and
one with them oriented |right>, entering the SG. They come out as
a stream of |up> electrons in the reversed movie. But
nomologically that is impossible (has infinitesimal probability);
in an actual experiment they would come out with their |left> or
|right> orientation intact.
Brent
In my effort to clarify this subject, I keep saying that if something
can happen, even with infinitesimal probability, I will say it is
"statistically irreversible" -- meaning it CAN in PRINCIPLE be
reversed. This I distinguish from irreversible in principle, meaning
the process can never be reversed. So, given a film which contains
each and every interaction of any process, and the fact that the
equations of physics are time reversible, I conclude that every
physical process, without exception, is either easily reversible or
worst case statistically irreversible (meaning reversibility is
POSSIBLE, even if hugely unlikely). I am probably wrong. LOL. AG
The problem is that your film would have to record both branches of the
wave-function, i.e. both "worlds" for each electron so that in the
reversal the phase information would be available. This would allow the
reversal to the original state of the wave function. But having the
original wave function doesn't mean you can measure it and get the same
results as if you had measured it originally. The wave function still
only encodes probabilities insofar as your measurements and perceptions
are concerned. So it would be like in some SciFi stories, when you go
back in time it's to a different "branch" of the MWI.
Brent
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