Thursday, August 13, 2009

A moment's confusion should clear this up...

Dear Brilliant Physicists and Mathematicians,

So, there's this famous thing called the "EPR paradox", which now has oodles of experimental statistics to back it up. Correct me if I'm wrong, but the notion is some decay process emits two electrons with opposite spin in opposite dirrections (OR photons with the same polarization or whatever); so if experimenters at a space-like separation further along decide to filter said electrons by spin in PARALLEL (electron-beam transverse) magnetic fields, they'll get perfectly correlated measurements overall; and if instead they filter the electrons in PERPENDICULAR magnetic fields, they'll get perfectly UNcorrelated measurements. The thing that bugs me is that in the perpendicular case, the total spin angular momentum of the filtered electrons can't possibly be zero AFTER, so there must be some change-of-state in the total aparatus in this case. On the other hand, in the parallel case a similar change-of-state isn't necessary.

Obviously, we still expect that this doesn't permit tachyonic communication between the two experimenters; has anyone worked out the details of why we don't get a side-channel attack against the speed-limit this way?

A bemused theoretician who ought to be grading assignments


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