The Fermilab Muon Measurement May or May Not Point to New Physics, But ...

On April 7, particle physicists all over the world were excited and energized by the announcement of a measurement of the behavior of muons—the heavier, unstable subatomic cousins of electrons—that differed significantly from the expected value.

A century from now, looking back on this moment, will historians understand this excitement? They certainly won’t see a major turning point in the history of science. No puzzle was solved, no new particle or field was discovered, no paradigm shifted in our picture of nature. What happened on April 7 was just an announcement that the muon’s wobble—its value is called g-2—had been measured a little more precisely than before, and that the international high-energy physics community was therefore a little more confident that other particles and fields are out there yet to be discovered.

Nevertheless, historians of science will see this as a special moment, not because of the measurement but because of the measuring. The first results of the experiment at Fermilab was the outcome of a remarkable and perhaps even unprecedented set of interactions between an extraordinarily diverse set of scientific cultures that, over 60 years, evolved independently yet required each other.

Early theoretical calculations of g-2 according to quantum electrodynamics received a jolt in 1966 when Cornell theorist Toichiro Kinoshita realized that his previous studies had well-prepared him to work out its value. His first calculations were by hand, but soon his calculations became too unwieldy to be performed that way and he became dependent on computers and special software. To make the prediction ever more precise, he had to incorporate work by different groups of theorists who specialized in the vast and diverse panoply of interacting particles and forces that subtly influence the g-2 value. ((Kinoshita is retired, and today the theoretical value is worked on by more than 100 physicists.) The result was a specific prediction, relying on the contributions of many theorists, with a minuscule error bar that made a clear experimental target.