Much as I love living in Lincoln, Nebraska, having a long residence at CERN has some advantages. For instance, we do get much better traffic of seminar and colloquium speakers here. (I know, you were thinking about chocolate.) Today’s colloquium in particular really got me thinking about how we do, or don’t, understand particle physics today.
The speaker was George Zweig of MIT. Zweig has been to CERN before — almost fifty years ago, when he was a postdoctoral fellow. (This was his first return visit since then.) He had just gotten his PhD at Caltech under Richard Feynman, and was busy trying to understand the “zoo” of hadronic particles that were being discovered in the 1960′s. (Side note: Zweig pointed out today that at the time there were 26 known hadronic particles…19 of which are no longer believed to exist.) Zweig developed a theory that explained the observations of the time by positing a set of hadronic constituents that he called “aces”. (He thought there might be four of them, hence the name.) Some particles were made of two aces (and thus called “deuces”) and others were made of three (and called “trays”). This theory successfully explained why some expected particle decays didn’t actually happen in nature, and gave an explanation for differences in masses between various sets of particles.
Now, reading this far along, you might think that this sounds like the theory of quarks. Yes and no — it was Murray Gell-Mann who first proposed quarks, and had similar successful predictions in his model. But there was a critical difference between the two theories. Zweig’s aces were meant to be true physical particles — concrete quarks, as he referred to them. Gell-Mann’s quarks, by contrast, were merely mathematical constructs whose physical reality was not required for the success of the theory. At the time, Gell-Mann’s thinking held sway; I’m no expert on the history of this period of history in theoretical particle physics. But my understanding was that the Gell-Mann approach was more in line with the theory fashions of the day, and besides, if you could have a successful theory that didn’t have to introduce some new particles that were themselves sketchy (their electric charges had to be fractions of the electron charge, and they apparently couldn’t be observed anyway), why would you?
Of course, we now know that Zweig’s interpretation is more correct; this was even becoming apparent a few short years later, when deep-inelastic scattering experiments at SLAC in the late 1960′s discovered that nucleons had smaller constituents, but at that time it was controversial to actually associate those with the quarks (or aces). For whatever reason, Zweig left the field of particle physics and went on to a successful career as a faculty member at MIT, doing work in neurobiology that involved understanding the mechanisms of hearing.
I find it a fascinating tale of how science actually gets done. How might it apply to our science today? A theory like the standard model of particle physics has been so well tested by experiment that it is taken to be true without controversy. But theories of physics beyond the standard model, the sort of theories that we’re now trying to test at the LHC, are much less constrained. And, to be sure, some are more popular than others, because they are believed to have some certain inherent beauty to them, or because they fit well with patterns that we think we observe. I’m no theorist, but I’m sure that some theories are currently more fashionable than others. But in the absence of experimental data, we can’t know that they are right. Perhaps there are some voices that are not being heard as well as they need to be. Fifty years from now, will we identify another George Zweig?