Symmetry

The Nobel Prize in Physics this year went to three scientists -- two Japanese nationals, the other a US citizen who, as it happens, was born in Japan.

The Japanese-American, Yoichiro Nambu, of the University of Chicago, is the senior of the three, and his share of the award is for work done earlier than that of his co-honorees. He receives half of the prize money, whereas Makoto Kobayashi and Toshihide Maskawa share the other half.

The idea behind the work of all three is the intriguing one of "spontaneous broken symmetry" on the sub-atomic level.

We tend to think of symmetry as the "default mode" of nature, so that anything asymmetrical requires explanation. Imagine a teetering boulder poised on the peak of a mountain, in such a way that it seems it COULD easily roll down two or more possible sloped paths. The situation is symmetrical, and this symmetry is broken when the boulder in fact does start to roll -- because of course it will only roll down one of them. We'd want to know: why? Was there a sudden gust of wind in one direction rather than the other?

Maybe in some cases there isn't any explanation. The boulder simply did roll one way rather than another, and at bottom there's no better answer than "just because." In other words, there may be breaks in symmetry that are "spontaneous." Indeed, the world may owe its existence to spontaneous broken symmatry. After all, imagine a Big Bang that produces as much anti-matter as matter. Shouldn't the two have annihilated each other and left the world with ... nothing? There must have been a break in the symmetry then. An excess of matter over anti-matter, in order to get the world underway.

In the world of sub-atomic particles, there are three sorts of symmetry: mirror (or positional) symmetry, such as that of the boulder, charge, and time: called P, C and T respectively.

In the 1950s, studies of the decay of the atomic nucleus in the element cobalt 60 revealed that it didn't follow P symmetry. Particles leaving that nucleus preferred one direction over another.

Nambu entered the picture in 1960, while he was working on superconductivity, i.e. the resistence-free flow of electrical currents. Spontaneous symmetry violations showed up as he worked on the theoretical description of superconductivity. Later in the 1960s he translated these violations into the world of subatomic particles. The Nobel committee says that "his mathematical tools now permeate all theories concerning the Standard Model." Hence his award this year, nearly a half-century later.

Meanwhile, a consensus view was developing that the really important and unbreakable symmetry at the subatomic level is CP, i.e. the combination of charge with position. Symmetry can be broken in either C or P, but surely not in both at the same time, the reasoning went.

Experimenters soon shattered that consolation. CP symmetry fails, too. Makoto Kobayashi and Toshihide Maskawa, working together in the early 1970s, developed a theoretical explanation for why even the breaking of the CP symmetries as observed need not shatter the "standard model." [I won't even try to explain what the "standard model" means in this context. It's a model, it's standard, and they saved it. That's all we need to grasp.]

The Kobayashi/Maskawa theory predicted the existence of three hypothetical families of quarks. Lo! and behold, the experimenters have recently (2001) confirmed their predictions, finding those quarks as described.

Fascinating stuff. I'm not at all confident that I understand the material I've just bee summarizing, but I'm quite certain these three gentlemen deserve their recognition. Bonzai!