On Fri, Oct 18, 2019 at 6:20 PM 'Brent Meeker' via Everything List < [email protected]> wrote:
> 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]> 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]> 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. > OK. If you recombine the beams before decoherence, you effectively erase the first erasing measurement. In which case the interference pattern disappears, and the second up-down measurement doesn't do anything except but show the diffraction patterns from each slit separately. (up-down distinguishes the L-R slits -- you know which-way!) Bruce -- 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/CAFxXSLQxpAuPtJWFgd7gNL%3D_JQHvDR_ZJxaDaxGgzY9c4h%2Bcxg%40mail.gmail.com.
