Yoichiro Nambu (1/2), Makoto Kobayashi (1/4), Toshihide Maskawa (1/4).Yoichiro Nambu joins a long sequence of string theorists who have won the prestigious award: the average string theorist's chance to win the award exceeds 0.1%. For co-founders of heterotic strings, it jumps to 25% and it is over 33% for fathers of string theory, as we will see. ;-)
David Gross is one of his colleagues in this elite group. And I am not even mentioning many Nobel-prize-winning strong supporters of string theory such as Gell-Mann, Weinberg, or Smoot.
Nambu: string theory, color, and broken symmetry
Nambu, a Japanese-born American, is often described as one of the "fathers of string theory": the other two are Susskind and Nielsen. (Veneziano had the right formula but could see no strings.) Together with Goto, Nambu understood that the action of the string is proportional to the proper area of the worldsheet, analogously with the proper time of a particle's worldline.
I find it stunning that not a single media outlet or a blog besides TRF mentions who Nambu actually is. It's like if the word "relativity" were not mentioned in 1921 when Einstein picked his prize for the photoelectric effect. Well, newspapers and blogs are mostly piles of sh*t (except for Scientific American that happens to reprint a nice detailed 1995 story about the seer Nambu: and Nambu was the kind of seer whom sub-par craftsmen like Lee Smolin could not even see as a seer: a real one).
But because of Alfred Nobel's limited respect for pure theorists (recall his wife's lover but don't forget that Nobel never married), the Nobel prize is formally given to Nambu for a comparably famous discovery that is less often associated with his name, namely for his explanation of the importance of spontaneous symmetry breaking in the subatomic world. See e.g. the Nambu's and Jona Lasinio's paper (which has 2,500 citations). Well, Jona Lasinio is also alive but he is not a co-father of string theory.
I am subtly kidding, of course. There are also other reasons. What are they?
Nambu is, equally importantly, a co-author of the Nambu-Goldstone bosons of which the paper mentioned above is a particular example. For every spontaneously broken continuous [approximate] symmetry, you find one species of a [nearly] massless scalar. This massless scalar is analogous to the "phase of the collective wave function" known from superconductivity. One can prove that this is what happens in field theory. The Nambu-Goldstone bosons can also be "eaten up" by gauge bosons in spontaneously broken gauge theories, to bring the third (longitudinal) polarization of the massive gauge bosons: that's the Higgs mechanism but the 2008 prize is not going here.
In the context of QCD, the relevant nearly massless scalars are the mesons (such as pions) that arise from the chiral symmetry breaking.
Besides strings and spontaneous symmetry breaking, Nambu is also the forefather of "color" as a new kind of charge in strong interactions whose dynamics dominate QCD. His stringy work was mostly focusing on the interpretation of the theory in the context of strong interactions, see e.g. this paper, which is why his relativistic picture of the string (or a fluxtube based on the Nielsen-Olsen vortex) is also our standard qualitative explanation of confinement in QCD.
Recently, it was his namesake who was working on cosmology (hat tip: anonymous). ;-) Needless to say, he clearly deserves the award.
The CKM matrix
Kobayashi and Maskawa, who are both Japanese, receive the Nobel prize for the CKM matrix governing the quark masses, especially for the realization that a broken CP-symmetry in the quark sector requires at least three generations (but they're enough). By the way, their paper is the third most cited particle physics paper as of today.
Recall that "C" in "CKM" stands for "Cabibbo", after Nicola Cabibbo who wrote down the analogous matrix for two generations that is determined by a single angle, the Cabibbo angle. The general unitary matrix in U(3) - the case of three generations - that maps lower-quark mass eigenstates to the isospin partners of the upper-quark mass eigenstates has 9 parameters. However, 6-1 = 5 of them are phases that can be absorbed to the normalization of the six eigenvectors (one overall change of all six phases doesn't change the CKM matrix).
You're still left with 9-5 = 4 nontrivial parameters of the CKM matrix which is more than 3 parameters of an SO(3) matrix: in general, the CKM matrix must be allowed to be complex and the additional phase not included in O(3) is breaking the CP-symmetry because the mass terms in the Lagrangian are "inherently" complex while the CP-symmetry is linked to complex conjugation.
Yes, if you wrote the three previous paragraphs before they did, with a few obvious formulae around, you could probably be half a million bucks richer today. ;-) But it was hard to use the SU(3) matrices in this way because, as the Nobel committee correctly mentions, the relationships between maths and physics were lousy in the 1970s. You know, what's revolutionary here is not the mathematical exercise itself but the correct sequence of physical arguments that use these non-quite-trivial yet not-quite-hard mathematical insights to explain an aspect of the Universe.
The breaking of the CP-symmetry by the phase in the CKM matrix is the only experimentally confirmed breaking of the CP-symmetry which is a potential paradox because we know another possible source of the breaking - the QCD theta-angle (the coefficient of the trace of F wedge F in QCD). The question why the latter is small is referred to as the strong CP-problem.
Helpfully enough, the CP-violation (by the CKM matrix) is advertised by nobelprize.org as the source of matter-antimatter symmetry of the early Universe (well, there should be some stronger additional source because the CKM matrix doesn't seem enough - but it is the only known/established proof of the concept as of today). We wouldn't be here without that breaking. That's clearly one of the reasons why the Japanese contributions are more important than Cabibbo's original 2x2 matrix according to the committee. Obviously, Cabibbo's work was important (to lead people to study different eigenvectors etc. in the flavor space) but the committee has to decide in some way and none of them can be perfect for everyone.
It's a good pick! Needless to say, all three names were debated as possible candidates in the particle physics circles. I think that Lars Brink, a string theorist in the committee, may be given credit for the good choice.
Incidentally, Kobayashi said he was surprised but Maskawa said that he predicted that he would win this year because he found a pattern. ;-) However, he's not too happy about that - too much noise. However, it's great that Nambu won. (Hat tip: Willie Soon!)