On 4/28/2018 5:57 PM, [email protected] wrote:
On Sunday, April 29, 2018 at 12:40:51 AM UTC, Brent wrote:
On 4/28/2018 5:24 PM, [email protected] <javascript:> wrote:
On Saturday, April 28, 2018 at 11:59:27 PM UTC, Brent wrote:
On 4/28/2018 4:28 PM, [email protected] 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
Why are the phase relations of the waves comprising the original wf,
They're not lost. They're what make the different branches of the
wave-function (approximately) orthogonal.
Brent
of what is presumably a coherent wave structure, lost when the
measurement occurs? TIA, AG
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