MJackson.Wbst@Xerox.ARPA (07/15/85)
I assert that it is *not* possible, except perhaps in ways which admit of (speed-of-light-or-less) propagation of the "effect" from A to B. You may take that as a challenge, but be warned that this is a pretty well-established physical principle. Note, incidentally, that the thought experiment we have been describing doesn't need to involve QM at all! The situation does not, in and of itself, rule out the possibility that the particles pairs emitted at E are "prepared" in states for which the results of all possible directions of spin measurement are determined in advance, with the probability matrix given. Of course, other experiments show that this is not the case for particle spin. Historically, Einstein, Podolsky, and Rosen [Phys. Rev. 47, 777 (1935)] first formulated this "paradox" in terms of two diverging, correlated particles in which a measurement at A of position or momentum established the position or momentum of the particle at B. What this leaves one with is either (a) the position and momentum of B *really* were determined at emission, and the uncertainty principle *only* represents a limit on our knowledge, or (b) some kind of "spooky actions at a distance" by which the measurement at A affects "reality" at B. For EPR the latter was sufficiently repugnant that they considered the argument a proof of the former, but this of course is a matter of taste in the absence of some experimental distinction. The possibility of such a test was first proposed by John Bell [Physics 1, 195 (1964)], flexing off a suggestion by David Bohm. He constructed a thought experiment in which if the states are unambiguous (even though unobservable, and however distributed) at emission there would be clear limits (hence the term "Bell's inequality") on the statistical distributions of spins recorded, which limits are predicted by very simple QM to be violated. Experiment (the major recent work is by Alain Aspect and co-workers at Orsay) supports QM; "spooky actions at a distance" cannot be avoided. In the next couple of days I hope to find time to relate the very simple thought experiment used by David Mermin [Physics Today, April 1985, p. 38] to make this concrete. There is no way I could do justice to the entire article, which is excellent, so let me again urge the interested to check it out. The subject line in the header of this message is the title of the article, and the quote below is taken from it. Mark "Anybody who's not bothered by Bell's theorem has to have rocks in his head."