PRL Top Ten

NUMBER TEN

Unity of All Elementary-Particle Forces
H. Georgi and S. L. Glashow, Phys. Rev. Lett. 32, 438 (1974), 1946 citations

With this article, APS News begins a special feature by James Riordon on the ten most-cited papers published in Physical Review Letters since its inception in 1958. The citation data have been provided, not quite free of charge, by the Institute for Scientific Information (ISI), publishers of the Science Citation Index.

In counting down in the coming months to the top-cited paper of all time, our intention is not just a mad desire to mimic David Letterman or the DJ on the local radio station. We want to provide some insight into what makes these papers significant and how they came to be written. Our treatment will be more descriptive than scholarly: space will not permit us to include numerous references nor to branch out to discuss related work in the text.

Even though PRL is the most highly-regarded physics journal, and citations are certainly one possible measure of a paper's significance, we do not claim that the ten papers to be featured here are necessarily more significant than many other papers that have appeared in the literature over the same span of time. But these papers range over many different fields of physics, and their impact has been great. We hope our readers will enjoy finding out more about them.

The tenth paper in our list of the top ten, most-cited Physical Review Letters is the seminal work that instigated serious pursuit of Grand Unified Theories (GUTs). "It was a fun paper," laughs Howard Georgi when asked about the Unity of All Elementary- Particle Forces., "It was very early, well before we really understood all the pieces. It was just fun finding this way to put them all together."

Progress in particle physics had been relatively slow through much of the 1960's, but Georgi knew he was in the right place at the right time when he accepted a postdoctoral fellowship at Harvard in 1971. Gerard 't Hooft had just published his proof of the renormalizability of theories such as Steven Weinberg's gauge model unifying the weak and electromagnetic forces. A review of 't Hooft's arguments by Ben Lee in late 1971 inspired a host of Harvard theorists to set to work on various aspects of gauge theory. Georgi soon joined Sheldon Glashow in pondering renormalizable gauge models.

Together, Georgi and Glashow attempted to unify SU(2) and U(1), with little success, until the end of 1973. A breakthrough finally came in January 1974. Georgi explains that their key revelation essentially resulted from an act of desperationthe attempt to incorporate strong forces into their gauge models. "Shelly and I realized that the strong interactions didn't really have to be strong, and that one could imagine unifying them with the weak and electromagnetic interactions." In a single day, that revelation led Georgi to the SU(5) group. "It happened in two stages," says Georgi, "I actually found the SO(10) model before the SU(5) model, by about an hour, because it was easier to see what to look at. It was only after I found out that I had managed to take it apart into SU(5) that I realized how obvious it was."

"I had gone home and, after dinner, worked it all out in a few hours," recalls Georgi. The SU(5) model he discovered was astonishingly simple, and economically fit leptons and quarks into a single group. But Georgi's newfound theory apparently had one unavoidable drawback it predicted that the proton should decay into a positron and a neutral pion. "I finally went to bed when I found out that the proton decayed, "says Georgi, "which to me was rather depressing." After all, as far as he knew, the proton was stable. "When I came back and described this to Shelly, that's what got him excited. He was right, of course, because that was the interesting experimental consequence of all this. At that point, we worked out in more detail what the bounds on the proton decay were at the time and wrote the paper up."

In the decades since Georgi and Glashow published their revolutionary paper, experimentally determined lower limits on the proton lifetime have eliminated a host of GUT models, including the SU(5) model. But their 1974 work is frequently cited as the prototypical grand unification model, particularly in the introductory sections of the latest GUT papers.

Georgi is still at Harvard, and now holds the Mallinckrodt chair in the Physics Department. Sheldon Glashow shared the 1979 Nobel Prize with Abdus Salam and Steven Weinberg for efforts leading to electroweak unification, and is currently Metcalf Professor of Math & Science at Boston University.

Although Georgi rarely works on grand unification theory these days, he finds the tools he picked up during the golden GUT days of the seventies handy in his current work on gauge theories, and has written a graduate Lie algebra text to introduce the powerful techniques to a new generation of theorists.

Georgi believes the jury is still out when it comes to grand unification schemes. "It depends on the day," he replies when asked if he thinks we will ever find the correct grand unified theory. "Who knows? It would be nice to see proton decay, if it's really there. The trouble is that we don't have very many experimental handles on this," says Georgi. "At the moment it's a little bit ethereal. But it's absolutely gorgeous the way these things fit together. That was, for me, always the extraordinary thing."