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? 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/CAFxXSLTd%2BLYu9fPrq659VThQMC1V5vj_Z%3D-RrR5FNVJ03mzHhQ%40mail.gmail.com.
