Yoichiro Nambu

yoichiro_nambu It was very sad to hear yesterday that Yoichiro Nambu has died. He was aged 94, so it was after a very long and full life.

Nambu was one of the greatest theoretical physicists of the 20th century, although not one with a high public profile. Among his contributions:

  • Being the first to really understand spontaneous symmetry breaking in quantum field theory, work for which he won a (very belated) Nobel Prize in 2008. We now understand the pion as a (pseudo-) “Nambu-Goldstone boson.”
  • Suggesting that quarks might come in three colors, and those colors might be charges for an SU(3) gauge symmetry, giving rise to force-carrying particles called gluons.
  • Proposing the first relativistic string theory, based on what is now called the Nambu-Goto action.

So — not too shabby.

But despite his outsized accomplishments, Nambu was quiet, proper, it’s even fair to say “shy.” He was one of those physicists who talked very little, and was often difficult to understand when he does talk, but if you put in the effort to follow him you would invariably be rewarded. One of his colleagues at the University of Chicago, Bruce Winstein, was charmed by the fact that Nambu was an experimentalist at heart; at home, apparently, he kept a little lab, where he would tinker with electronics to take a break from solving equations.

Any young person in science might want to read this profile of Nambu by his former student Madhusree Mukerjee. In it, Nambu tells of when he first came to the US from Japan, to be a postdoctoral researcher at the Institute for Advanced Study in Princeton. “Everyone seemed smarter than I,” Nambu recalls. “I could not accomplish what I wanted to and had a nervous breakdown.”

If Yoichiro Nambu can have a nervous breakdown because he didn’t feel smart enough, what hope is there for the rest of us?

Here are a few paragraphs I wrote about Nambu and spontaneous symmetry breaking in The Particle at the End of the Universe.

A puzzle remained: how do we reconcile the idea that photons have mass inside a superconductor with the conviction that the underlying symmetry of electromagnetism forces the photon to be massless?

This problem was tackled by a number of people, including American physicist Philip Anderson, Soviet physicist Nikolai Bogoliubov, and Japanese-American physicist Yoichiro Nambu. The key turned out to be that the symmetry was indeed there, but that it was hidden by a field with that took on a nonzero value in the superconductor. According to the jargon that accompanies this phenomenon, we say the symmetry is “spontaneously broken”: the symmetry is there in the underlying equations, but the particular solution to those equations in which we are interested doesn’t look very symmetrical.

Yoichiro Nambu, despite the fact that he won the Nobel Prize in 2008 and has garnered numerous other honors over the years, remains relatively unknown outside physics. That’s a shame, as his contributions are comparable to those of better-known colleagues. Not only was he one of the first to understand spontaneous symmetry breaking in particle physics, but he was also the first to propose that quarks carry color, to suggest the existence of gluons, and to point out that certain particle properties could be explained by imagining that the particles were really tiny strings, thus launching string theory. Theoretical physicists admire Nambu’s accomplishments, but his inclination is to avoid the limelight.

For several years in the early 2000’s I was a faculty member at the University of Chicago, with an office across the hall from Nambu’s. We didn’t interact much, but when we did he was unfailingly gracious and polite. My major encounter with him was one time when he knocked on my door, hoping that I could help him with the email system on the theory group computers, which tended to take time off at unpredictable intervals. I wasn’t much help, but he took it philosophically. Peter Freund, another theorist at Chicago, describes Nambu as a “magician”: “He suddenly pulls a whole array of rabbits out of his hat and, before you know it, the rabbits reassemble in an entirely novel formation and by God, they balance the impossible on their fluffy cottontails.” His highly developed sense of etiquette, however, failed him when he was briefly appointed as department chair: reluctant to explicitly say “no” to any question, he would indicate disapproval by pausing before saying “yes.” This led to a certain amount of consternation among his colleagues, once they realized that their requests hadn’t actually been granted.

After the BCS theory of superconductivity was proposed, Nambu began to study the phenomenon from the perspective of a particle physicist. He put his finger on the key role played by spontaneous symmetry breaking, and began to wonder about its wider applicability. One of Nambu’s breakthroughs was to show (partly in collaboration with Italian physicist Giovanni Jona-Lasinio) how spontaneous symmetry breaking could happen even if you weren’t inside a superconductor. It could happen in empty space, in the presence of a field with a nonzero value — a clear precursor to the Higgs field. Interestingly, this theory also showed how a fermion field could start out massless, but gain mass through the process of symmetry breaking.

As brilliant as it was, Nambu’s suggestion of spontaneous symmetry breaking came with a price. While his models gave masses to fermions, they also predicted a new massless boson particle — exactly what particle physicists were trying to avoid, since they didn’t see any such particles created by the nuclear forces. Soon thereafter, Scottish physicist Jeffrey Goldstone argued that this wasn’t just an annoyance: this kind of symmetry breaking necessarily gave rise to massless particles, now called “Nambu-Goldstone bosons.” Pakistani physicist Abdus Salam and American physicist Steven Weinberg then collaborated with Goldstone in promoting this argument to what seemed like an air-tight proof, now called “Goldstone’s theorem.”

One question that must be addressed by any theory of broken symmetry is, what is the field that breaks the symmetry? In a superconductor the role is played by the Cooper pairs, composite states of electrons. In the Nambu/Jona-Lasinio model, a similar effect happens with composite nucleons. Starting with Goldstone’s 1961 paper, however, physicists become comfortable with the idea of simply positing a set of new fundamental boson fields whose job it was to break symmetries by taking on a nonzero value in empty space. The kind of fields required are known as a “scalar” fields, which is a way of saying they have no intrinsic spin. The gauge fields that carry forces, although they are also bosons, have spin equal to one.

If the symmetry weren’t broken, all the fields in Goldstone’s model would behave in exactly the same way, as massive scalar bosons, due to the requirements of the symmetry. When the symmetry is broken, the fields differentiate themselves. In the case of a global symmetry (a single transformation all throughout space), which is what Goldstone considered, one field remains massive, while the others become massless Nambu-Goldstone bosons — that’s Goldstone’s theorem.

This entry was posted in Science. Bookmark the permalink.

11 Responses to Yoichiro Nambu

  1. Amarashiki says:

    This book is a “must read” for any respetable theoretical physicist in his/her free time:


    I would like to add his idea of “Nambu Mechanics”…Something that we have not yet understood…

  2. Joan Winstein says:

    Even without the shout-out to Bruce, this was a lovely post, Sean. He was a devoted husband too, skipping the Nobel ceremony and staying at his wife’s side, because his wife was too ill to travel at that time. How many would do that?

  3. Pingback: Sean Carroll: Yoichiro Nambu | Por amor a la ciencia

  4. Bob Zannelli says:

    Sad, but he had a life of great accomplishments. That’s all there can be in any life , most of us don’t even get close.

  5. J. Ricardo says:

    Sad, indeed. Nambu was my academic grand-grandparent, since he advised the advisor (Roland Koberle) of my advisor (Francisco C. Alcaraz). Among his many contributions we shouldn’t forget the Nambu-Jona-Lasinio model, which provided us with several physical insights.

  6. John Barrett says:

    It’s a shame really. It seemed like he would have been a lot more interesting to talk to than most other physicist, from what I have heard about him.

    Looking into the Nambu-Goto action it immediately mentions Cosmic strings; there is something I haven’t heard of in a while. It makes me wonder if he supported the existence of Cosmic strings or if they are necessary due to the Nambu-Goto action. Or is it just a tool that proprietors of Cosmic string theory used to describe them? It seems rather odd, being that Cosmic strings would be so different than strings in string theory…

  7. Sean Carroll says:

    Cosmic strings and fundamental (“string theory”) strings aren’t that different, actually. They’re both kinds of relativistic strings. Indeed, in some models they are dual to each other. http://arxiv.org/abs/0911.1345

  8. John Barrett says:

    That is an interesting paper, and it answered a lot of my questions. I am surprised to see that work is still being done with cosmic strings. Although, I can’t help but get the feeling that they are going about it all wrong when reading about it. It would seem like, if they approached the problem like a particle physicist, they would try to avoid the presence of cosmic super strings in GUT’s or M-Theory, since they have not been able to be detected. That is what is observed, so that is how they should try to find their accurate depiction of reality.

    Direct observational evidence would be more appealing, but if they could derive the presence of a cosmic super string in a GUT, then it could help them pick out the right version of super string theory that describes our universe. I guess they never proved that the evidence from the south pole was B-mode polarization, like they hoped for finding in the paper. Then that might take them down the wrong path as well just assuming cosmic super strings don’t exist, if there was just no such thing as gravitational waves…

    Who knows, really? One day cosmic super string theory could become the super physicist guide book on what GUT or version of string theory not to use. For all we know, we could be living inside of a cosmic string or the cosmic super string dissipated to turn into the super strings we all know and love…

  9. Shaun M. says:

    Beautiful post Sean.

  10. phumurn says:

    The development of science has not been fully disclosed ,I mean some of the founding fathers were side lined Yoichiro Nambu may his legacy live on .