A bet about a cherished theory of physics may soon pay out

IN 1994, on a warm summer’s evening in Erice, in Sicily, in the midst of a pleasantly well-lubricated dinner, two physicists made a wager on the laws of nature. The bet between Kenneth Lane and David Gross concerned supersymmetry, or “Susy” for short, a theory which stipulates that all known fundamental particles have heavier, supersymmetric counterparts called sparticles.

When the bet was laid, no sparticles had been spotted. Yet plans for a powerful particle accelerator called the Large Hadron Collider (LHC) were being drawn up. Dr Lane proposed that if the new machine found evidence for the theory, he would buy the table dinner at Girardet’s, an expensive restaurant in Switzerland considered by some the best in the world. If not, then dinner would be on Dr Gross. The terms, scribbled on a napkin, stipulated that the bet would be payable once the LHC had produced enough data to be sure of the outcome. The chosen figure, in the obscure units used by physicists, was 50 inverse femtobarns, or roughly 5 quadrillion of the high-energy collisions between particles that the LHC is designed to produce.

Two decades on, Girardet’s is no more. But the LHC is in rude health. It has, since 2010, collected about 60 inverse femtobarns of data. With no sightings of the particles that Susy predicts, Dr Lane says it is time for Dr Gross (who won the Nobel prize in 2004) to cough up—if not with dinner at Girardet’s then at another suitably ritzy venue. After receiving no response to several e-mail prompts, however, Dr Lane is growing impatient. “David appears to be welshing on our bet,” he says.

One indication of the strength of feeling surrounding Susy is that the Erice bet is not unique. Another, wagering a bottle of cognac on the discovery of a sparticle by June 2016, was settled, in the sceptic’s favour, over the summer.

Collision course

Susy has many fans. That is because, if it is true, it could help solve many physical puzzles. Dark matter, for instance, is a mysterious substance known to make up about 27% of the total amount of stuff, both matter and energy, in the universe. The particles predicted by Susy are one plausible dark-matter candidate. A “grand unified theory”, for which physicists have been hunting for decades, would explain how fundamental forces such as gravity and electromagnetism merge into a single force at the very high temperatures thought to have existed shortly after the Big Bang. Susy can help build such theories.

It could also make sense of a peculiarity of the Higgs boson, a long-sought particle discovered by the LHC in 2012. The Higgs interacts with many other particles. Summed together, these give it its mass. But trying to predict that mass by calculating it yields a number about 10 quadrillion times larger than its actual value. Fixing the maths requires a large and ugly fudge. Susy’s hypothesised sparticles cancel out the contributions from their “real” partners, meaning no fudge is needed.

Strictly speaking, Susy can never be formally disproved. It can always be tweaked so that sparticles appear only at energies that are just out of reach of the best existing colliders. Yet the more such tweaks are applied, the more they erode the elegance for which the theory is admired.

In light of the LHC’s failure to find evidence for Susy, more physicists are arguing that the field’s obsession with the theory is a waste of time and effort. Scientists at the LHC filter the data they record by looking first for particles predicted by favoured theories, including Susy. Less popular ideas get a smaller share of the resources. That could delay other discoveries. Dr Lane, for instance, thinks so-called composite-Higgs models, which assume the Higgs is made up of even smaller constituent particles, should get more attention.

Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies, is one of many who think it is time for theorists to focus on other problems—how gravity behaves at the very small scales of quantum mechanics, for instance. If the LHC finds no trace of sparticles in this year’s data, she believes the last thing the field needs is another round of Susy model adjustments. “That’s not science,” she says. “That’s pathetic.”

Dr Gross is not ready to concede quite yet. The data are in, but their analysis is not complete. “It looks like I will lose this bet by the end of the year,” he says, “but we should await the word from the experimenters themselves.” (Dr Lane says the original terms have been met and Dr Gross should throw in the towel.)

In the longer term, there is more at stake than a fancy dinner and a firmer understanding of the nature of reality. Colliders are expensive—the LHC cost $5bn to build. It has many years still to run, and plenty of time to discover something new. But its apparent failure to find convincing evidence for Susy has some worried that, if the LHC fails to turn up much new physics of any sort, plans for yet bigger colliders will be harder to justify.

Others are more sanguine: the history of science is, after all, littered with much-loved but wrong theories, from the idea that Earth is the centre of creation to the “luminiferous ether” that was thought, in the 19th century, to suffuse the universe. If Susy comes to nothing, Dr Gross hopes that will inspire new ideas from young theorists. “That”, he adds, “is a category that does not contain either Lane or me.”