2008 Nobel Prize in Physics

The 2008 Nobel Prize in Physics goes to two different achieve- ments.  Both relate to symmetry breaking, but in very different ways.  All three recipients, Yoichiro Nambu, Makoto Kobayashi and Toshihide Maskawa, certainly deserved the prize, but Nambu should have gotten the prize years ago, and they should given the prize to Nicola Cabibbo as well—after all it is called the Cabibbo-Kobayashi-Maskawa mechanism!

In brief, this is what they did.

Nambu explained how protons and neutrons could get mass in the same way that superconductivity happens.  If that doesn't sound ground-breaking, I don't know what does!  He showed that a symmetry in something called a quantum field theory can be "spontaneously broken".   

Think of looking down at a pencil from above.  What is the most symmetric way of placing the pencil?  Why on its point!  That way, if you rotate your view, it looks the same.  But of course that is not a stable situation.  The pencil immediately falls over.  Which way?  Well, it goes spontaneously in a random direction.  And after it falls, the situation isn't invariant under rotations—the symmetry is broken.  That is in essence what Nambu showed worked in a quantum field theory—that the most stable state need not be the most symmetric one.

Now on to a completely different topic.  There are three families of quarks: up-down, charm-strange, and top-bottom (yes, these are silly names).  Back when we knew about only two of them, Nicola Cabibbo realized that they could mix together—that if you started a process with a strange quark, there was a chance you could end it with a down quark.  Such mixing is controlled by just one parameter, the Cabibbo angle.  (It's an angle of rotation in quark mixing space.)

A puzzle at the time was how to explain CP violation—the observation that the symmetries of charge conjugation (C, switching + and -) and parity (P, switching left and right) were not actually good symmetries of nature.   In other words, if you took some processes and switched both + and - charges and left and right, you didn't get the same result (trust me, that's odd).

Kobayashi and Maskawa realized in 1973 that if there were a third family of quarks (there is, but no one knew it then), one would get not only other mixing angles, but something called a phase.  This phase was just what one needed to explain CP violation.

In brief, they came up with a mechanism which explained the existing observation of CP violation by proposing that there was a third family of quarks—and lo and behold a third family of quarks was found a few years later!  Come to think of it, Cabibbo, Kobayashi, and Maskawa should have won years ago.