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.

Thursday, 9 October 2008

Keep your eyes on the Prize

I guess you-all have noticed that the 2008 Nobel Prize for Physics has been announced, and once again it's gone to a bunch of guys for work in particle physics: Yoichiro Nambu, Makoto Kobayashi, and Toshihide Maskawa. No complaints from me.

The results of my own experiment, Belle, have been instrumental in vindicating this work, particularly that of Kobayashi and Maskawa. We mention them daily, and not just because the experiment is based in Japan; the situation is the same on the other side of the Pacific, at the friendly-rival experiment BaBar at Stanford.

These gentlemen's work concerns spontaneously broken symmetries: the universally used example is balancing a pencil on its point. This situation is symmetric, but unstable. The stable configuration --- the configuration with lowest energy --- has the pencil dropping flat onto the table, picking out one direction at random over all the other possible directions it could equally well have chosen. The phenomenon is quite common in physics: the achievement of Nambu was to apply this to particle physics, in particular the strong interaction that binds the quarks, the atomic nucleus, and so on; the achievement of Kobayashi and Maskawa was to further explain the breaking of the "CP" symmetry, and in the process predict the existence of six types of quark. At the time they wrote their paper, three types were known, and the other three were found, one after the other, over the next twenty-two years. We set great store by this sort of bold predictive power. The final, spectacular confirmation of Kobayashi and Maskawa's work was the observation of CP violation in the B-meson system by Belle and BaBar in 2000-2002, with precisely the value expected on their model. At which point the entire field cried "Respect!", looked at their watches, and started counting down to the inevitable award of the Nobel. The real award, of course, had already been given in full.

[The Italian physicist Nicola Cabibbo has been left out of the party by the Swedish Academy. It was Cabibbo who first established the idea of "mixing" between types of quarks (in his case, "down" and "strange") that was then extended by Kobayashi and Maskawa. Including him would have spoiled the symmetry-breaking focus, but still: his friends have grounds to be disappointed on his behalf.]

There are various discussions, aimed at both the public and the press, at the Nobel Prize site; there's also a longer and rather Nobel-Prize-obsessed technical account by the Academy [warning! physics background required!] on the physics involved. As usual, there is also an accessible report in the New York Times science pages.

Why should you care? Well, CP violation is one of Sakharov's three necessary elements to explain how the universe can have lots more matter than antimatter: why the place is full of stuff, rather than the stuff (the matter and antimatter) having all just annihilated away to leave radiation alone. Why, in other words, it's possible for you to be here.