Monday, 13 October 2008

The Green Baize Table Conspiracy

The insanely overdue writeup of my review talk, Quantum entanglement at the psi(3770) and Upsilon(4S), is now on the arXiv public preprint server, and will be included in the proceedings of the Flavor Physics and CP Violation conference. I was asked to give the review because of my involvement in a test of quantum mechanics by the Belle experiment: I wrote about this last year in the post Tangled up in (quantum) blue.

When I have spoken on these results, the most popular part of the talk has always been my explanation (included in the writeup) of an important counter-example, in the form of a conspiracy theory involving the Cigarette-Smoking Man. It seems that more physicists watch the X-Files than would generally admit to it.

The burden of the counter-example is that, if one is willing to countenance bizarre and conspiratorial alternative theories, the sort of study we did at Belle doesn't establish the quantum mechanical result it's trying to test. So we just do the best we can. However there are optical experiments that are immune to (at least this type of) conspiracy-theory explanation. This is the really remarkable thing, and why Bell inequality tests are considered such a big deal in physics. Based on such experiments, we conclude that the weirdness of quantum mechanics is a real feature of the world --- independent of whether quantum mechanics itself is ultimately correct. We might one day learn more, and go beyond the understanding we have from QM, but even so we'd be stuck with the fact that two objects "separated" in space, even many kilometres apart, can be (in a sense) inseparable, forming a single object, a single system. A whole, rather than two parts.

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