Fermilab’s Tevatron, the largest particle accelerator in the United States, was shut down on September 30 after a celebrated career of 28 years that has provided us with some of the greatest discoveries in particle physics. This leaves the European lab CERN (see photo on left) to lead the way into future discoveries with its Large Hadron Collider.This landmark in experimental physics is an opportunity to reexamine the theoretical model physicists have constructed and relied on in their search to understand the workings of the universe: the standard model of particle physics. The standard model is a comprehensive theory about nature’s elementary particles and the forces that control their behavior, and it has been constructed over a half-century of intensive work by many theoretical physicists as well as experimentalists. The model has worked amazingly well, harmoniously combining theory and experiments and producing extremely accurate predictions about the behavior of particles and forces. But could the model now be beginning to show some cracks?
It all started on a wintry evening in 1928. While staring at the flames in the fireplace at St. John’s College, Cambridge, Paul Dirac made one of the most important discoveries in the history of science when he saw how to combine the Schrödinger equation of quantum mechanics with Einstein’s special (but not general) theory of relativity. This achievement launched relativistic quantum field theory—which forms the theoretical basis for the standard model—and produced two immediate consequences: an explanation of the spin of the electron, and Dirac’s stunning prediction of the existence of antimatter (confirmed a few years later with the discovery of the positron).
In the late 1940s, Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, all working independently, presented the first quantum field theory, called quantum electrodynamics, which explained the electromagnetic interactions of electrons and photons. It forms the first part of the standard model by handling interactions that are controlled by the electromagnetic field. The theory’s success inspired other theoretical physicists to construct similar quantum field theories for addressing the actions of the weak and strong nuclear forces—thus together accounting for everything in particle physics except for the action of gravity, the subject of Einstein’s general theory of relativity. By the 1970s, the result, the standard model, was ready: a quantum field theory of all elementary particles—leptons and quarks and their interactions through the actions of particles (such as the photon) called bosons.
By now you’ve probably heard the widely reported news about the possible discovery of neutrinos that allegedly travel faster than light. The OPERA (Oscillation Project with Emulsion tRacking Apparatus) collaboration of almost 200 scientists working at the Gran Sasso underground laboratory in central Italy has discovered a phenomenon the physicists could simply not explain. For over three years, the scientists have been collecting data on the flight of neutrinos—those mysterious, nearly massless particles that can travel through anything at immense speed—originating in the SPS accelerator at CERN, near Geneva, and traveling underground all the way to Gran Sasso, 731 kilometers (about 450 miles) away. The experiment showed that the 16,000 neutrinos measured at Gran Sasso had traveled there through Earth’s crust at faster than light speed.
Facing a crowded lecture hall at CERN last Friday, Dario Autiero of the OPERA group explained how the researchers went to great lengths to remove any sources of error in their measurements: they measured distances using an extremely high-precision GPS called PolarX, measured time at the two locations to an accuracy of one nanosecond using cesium clocks, and accounted for the tides, Earth’s rotation, variations between day and night and spring and fall, etc. The statistical significance of the finding was six-sigma—meaning that the probability that the experimental result was a random fluke was only one in a billion. For a full hour after the presentation, Dr. Autiero was grilled by a roomful of physicists, and seemed to be able to account for all of the many potential errors brought up by the audience.
But physicists remain very skeptical. They want to see a confirmation of the findings from another experiment in a separate laboratory before they accept such a bizarre finding. After all, this result, if true, would appear to run against the spirit of Einstein’s special theory of relativity. When I showed the Gran Sasso paper to Nobel Laureate Steven Weinberg, he told me: “It looks pretty impressive, but I still think that this will go away.” The sentiment was echoed by almost every physicist I have spoken with since. The results seem mind-boggling. After all, nothing can go faster than light, right?