Just over a half-century ago, the physicist Steven Weinberg published a seminal paper titled “A Model of Leptons in the journal Physical Review Letters.” It was just three pages long, but its contents were revolutionary: in the paper, Weinberg outlined the core of the theory now known as the Standard Model, which governs elementary particles. Hailed by many since then as the most successful theory ever conceived, the Standard Model describes the universe with a comprehensiveness that is hard to understate. All the particles it predicted to exist have been found, including most recently the Higgs boson. The masses of those particles lie within 1 percent of the theoretical value anticipated by the model. And three of the four fundamental forces of nature—electromagnetism, plus the weak and the strong nuclear force—have all been shown to be manifestations of one underlying force as part of the Standard Model.
Weinberg received the Nobel Prize in Physics in 1979 for his work, but no fewer than 54 other physicists have also won the prize for their research into aspects of the Standard Model. To celebrate, eight of these Nobel laureates, along with dozens of other preeminent scientists, gathered for a special symposium over the weekend of June 1–4, 2018 at Case Western Reserve University in Cleveland. Titled “The Standard Model at 50,” the meeting was in part a celebration, in part a time to reflect and in part a time to ponder the future. If you wanted to take stock of the Standard Model’s place in physics, and meet some physics icons of the past century, this was undoubtedly the place to do it.
The first thing to emphasize is that the Standard Model is well worth celebrating. As noted by Gerard t’Hooft (Nobel laureate, 1999), no one knew in the 1960s, when he did his own seminal work in electroweak theory, that there would be something as comprehensive as the model turned out to be. But there is, and it explains all matter on all scales, from the tiniest Planck length (6.3631×10−34 inch) to the scale of the universe. “It’s gorgeous!” said David Gross (Nobel laureate, 2004, for his work on the strong force that binds atomic nuclei,) beaming like a proud father at the written equation that encapsulates the model. So precise are its predictions that physicists who rely on it at the Large Hadron Collider (LHC) near Geneva, Switzerland, have to be alert to incredibly mundane effects like trains passing by miles away, because they set up electrical fields in the rails that can affect measurements at the giant accelerator’s detectors. You don’t worry about things like that unless the predictions are incredibly spot on.
And yet, despite its robust predictions, the consensus was that today’s Standard Model is not the final one. For all its success, the Standard Model does not answer the question of what the dark matter and dark energy are that make up the majority of matter in our universe. It does not explain why neutrinos have mass. It does not explain how the fourth fundamental force, gravity, can be reconciled with the other three. And it does not explain why all the matter in our universe is here in the first place—the question of why there’s something rather than nothing.
“The bottleneck in particle physics is experimental, not theoretical,” explained Gross. The accelerators required to test the Standard Model are incredibly expensive—the LHC cost about $9 billion to build, and costs $1 billion a year to run—and finding discrepancies in experiments that could lead to a new, even more powerful theory could require even more costly experiments. Without that sort of data, however, “it’s easy to get lost in the fog,” observed t’Hooft.
Could the answers to those questions lie in extra dimensions, or string theory, or some other theory that hasn’t even been conceived yet? It’s possible, but without experimental proof, it’s easy to get carried away. “Remember,” said George Smoot, who shared the 2006 Nobel Prize for his work in characterizing the cosmic microwave background (CMB) radiation left over from the big bang, “the steady state theory for the universe [the theory ruled out by the first detection of the CMB in 1964] is extremely beautiful, but it’s also extremely wrong.”
Finally, on a weekend that was as star-studded as it gets in the physics world, it’s important to remember that all these great advances were done by actual people, with all the good and bad qualities that this entails. Someone with a resemblance to Winston Churchill showed up a few minutes late for Sunday’s talks and grabbed the seat next to me, and I was startled to notice it was Steven Weinberg (who proceeded to share his wry observations during the lecture). I found myself waiting at the bar at the reception beside George Smoot, and we proceeded to swap local craft beer tips. Helen Quinn, a brilliant Australian-American who should be on anyone’s Nobel short list for her work in particle physics, went out of her way to offer career advice to every student she came across. t'Hooft reminisced about hitchhiking through rural France during graduate school, on his way to a summer school.
And one Nobel laureate, who shall go unnamed, proceeded to frame our introduction by stating I was clearly invited because I was pretty, and that I looked old enough to finish my PhD already. (The Nobel Prize in Physics is still such an old boys club that only two women have ever won the prize out of 207 recipients. The last was in 1962—a greater gap than in any other field, and not for a lack of good scientists.)
But no matter—you can’t let a social imbecile ruin your time at a celebration, and the 50th birthday party for the Standard Model was one not to miss. It made me wonder what sort of party will be organized for its 100th. Will the speakers lecture on the solution to dark energy, or laugh at our current ideas, or still be puzzling over the answer? Will gravity be brought into the Standard Model family, or remain an outcast? It will be interesting to see if even “the most successful theory ever conceived” will stand firm over the coming years, or whether physicists will find themselves modifying it in the future.