On 10/17/2019 8:38 PM, Bruce Kellett wrote:
On Fri, Oct 18, 2019 at 2:08 PM 'Brent Meeker' via Everything List
<[email protected]
<mailto:[email protected]>> wrote:
On 10/17/2019 4:34 PM, Bruce Kellett wrote:
On Fri, Oct 18, 2019 at 10:05 AM 'Brent Meeker' via Everything
List <[email protected]
<mailto:[email protected]>> wrote:
But I wonder what happens in Carroll's experiment if, after
measuring in the left/right basis and noting that two
different interference patterns can then be discerned by
considering either those due to left spin recording particles
or considering right spin particles, one measures the
recording particles again in the up/down basis. The overall
pattern is the same, it's just that you've relabeled spots
on the screen according to whether the second measurement of
recording particles assigned them to UP or to DOWN. Now you
can consider the subset labeled UP (or DOWN). This should be
a superposition of ensembles randomly selected from the left
and right ensembles and in that case would not show an
interference pattern...but the information has certainly been
erased (twice)?
If I understand you, what you are suggesting is that either the
left polarized, or right polarized, are measured again in the
up-down direction. I think that if you do this second
measurement, you will simply reduce the intensity by a factor of
two.
No. You just partitioning the spots on the screen in a different
way, so there are the same number of spots. After the first
measurement of the recording particles spins, in the left/right
basis, you labeled the spots on screen according to left or
right. And when you looked only at the left labeled spots they
showed an interference pattern. And necessarily the right labeled
spots were the complement relative to the no-interference pattern.
So there are two implicit complementary interference patterns
hidden in the no-interference pattern. But on the second
measurement of the recording particles in the up/down basis each
one should be up or down with probability 1/2. So all those
measuring UP is just a random selection of the overall ensemble,
the ensemble that showed no interference. So yes it's intensity
is reduce (only half the spots end up labeled UP) but it's a
no-interference pattern.
The welcher weg information was permanently erased by the first
left-right measurement.
Right. So why doesn't the interference pattern persist after the
second measurement of the recording particles? I suppose the
answer is that it does, we just don't have the information
necessary to pick it out anymore. Still it seems curious that we
can erase the which-way once and, by looking at the results, find
the interference pattern. But if we erase twice we can't find it.
Are you suggesting that we lose the original left-right labels? I
thought that if you select 'left', then re-measure just those photons
in the up-down basis, you still get the 'left' interference pattern,
with the spots now randomly labelled 'up' or 'down'. If you put both
the 'left' and 'right' photons through the second measurement, and
lose the original labels, then the interference pattern may vanish,
and you get randomly scattered 'up' and 'down' spots. But that is
because you forgot the original separation -- it is still there, you
just labelled things differently.
Or am I still missing your point?
I was thinking of the second, in which you do the two measurements in
succession, like one SG after another so that you never looked at or
recorded the first measurement result. But that would mean you'd have
to put the beams back together between the measurements. In that case
the second measurement would be just like the first hadn't happened, and
you'd be able to discern an interference pattern.
Brent
--
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 [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/everything-list/97b07469-891d-d4a3-492d-ba18e70000f3%40verizon.net.
