bilbo.niket@LOCUS.UCLA.EDU ("Niket K. Patwardhan") (12/04/85)
>(9) " Aspect's Paris experiment shows the reality of nonlocal photon spin- > spin quantum correlations over faster than light space-like > separations between the detections of photons emitted in a > double quantum jump. " > > Not a fair presentation. The experiment shows, that if one of two > correlated photons is perturbed, the other changes state in a manner > that implies awareness of the perturbation. To an external observer, > the photons may be some distance apart. But to an observer on one of > the photons, they are still in contact (Lorentz equation), and so it > is not anomalous that one should affect the other. > The original message to which this was a reply to, was pure gobbledygook to me. (I thought it was a joke!) But.... since enough people seem to have taken it seriously.... does the above mean that INFORMATION can be transmitted instantaneously? I couldn't care less what the observers on the photons would think, the external observer would see an instantaneous transmission of information. I like the explanation though... it has a nice satisfying feeling to it.
FIRTH@TL-20B.ARPA (12/05/85)
Well, I'm also not sure how much of the original post was a joke, but decided to say something about the parts that seem to make sense. The simplest experiment that illustrates the "instantaneous action" problem is the Wu experiment: Take a source of photon pairs, eg an electron/positron annihilation. This emits two photons in opposite directions (conservation of momentum), and orthogonally polarised. Now put a polarising filter in the path of one of the streams of photons. Of course, half get through. Do the same for the other stream - again, half get through. Now try this: what fraction of PAIRS get through BOTH filters? (You detect that a pair gets through because your two detectors ping at the same time). Well, if the photons were randomly polarised, the probabilities multipl;y, and the answer would be 1/4. Since they are orthogonally polarised (correlated), the calculation is a bit harder, and comes to filters parallel: 1/8 filters perpendicular: 3/8 all by good classical probability theory. Now try the experiment. The result is filters parallel: 0 filters perpendicular: 1/2 One explanation is this: the photons are not only initially correlated bu remain correlated. If one is perturbed by a filter (eg has its direction of polarisation rotated) then the other changes ITS direction of polarisation to remain orthogonal. The photons are coupled. Now put the filters, and detectors, VERY far apart (eg one in Pittsburgh and the other in the Imperial library on Trantor). We have no reason to suppose we won't see the same result: the photons are still correlated. So, in the old, particle way of thinking, photon A (in Pittsburgh) has just got through the filter and says "Hey, sister on Trantor, you better rotate left pi/8 to stay orthogonal!". The assumption that the photons become uncorrelated when they are some distance apart is the assumption of "locality" (incidentally, one of the key assumptions in the Einstein-Podolsky-Rosen thought experiment), and experiments such as the above lead one to suspect that this assumption is not always true. Well, you could say that something - even if only a perturbation in a wave function - "travels faster than light", though I find that model unhelpful. But, as far as I know, nobody has been able to imagine a device that would use this effect to send specific SIGNALS faster than light. Robert Firth PS: should you care, I happen to believe it IS possible to move information faster than light, we just haven't found out how, because our model of reality is defective. -------
space@ucbvax.UUCP (12/06/85)
You should take a look at the book "In Search of Schrodinger's Cat" which describes a method of putting liquid helium in a state where a whole drop acts like a single quantum particle, responding to stimulus instantly over its whole surface. It was suggested this technology be used for computer buses of infinite speed!