So long, Susy?

In 2017 the idea of Supersymmetry will either be seen to be true, or die

Science and technology

“The great tragedy of science”, said Thomas Huxley, “is the slaying of a beautiful hypothesis by an ugly fact.” That, though, is the scientific method. If nature provides clear evidence that a hypothesis is wrong then you have to abandon it or at least modify it. It is psychologically uncomfortable, no doubt, for those with an interest in the correctness of the hypothesis in question. But at least everybody knows where they stand.

What happens, though, in the opposite case: when nature fails to contradict a hypothesis but stubbornly refuses to provide any facts that support it? Then nobody knows where he stands. This is fast becoming the case for a crucial hypothesis in physics, called Supersymmetry—or Susy, to its friends. Susy attempts to tie up many of the loose ends in physical theory by providing each of the known fundamental particles of matter and energy with a “supersymmetric” partner particle, called a sparticle. It is neat. It is elegant. But it is still unsupported by any actual facts. And 2017 looks like the year when the theory will either be confirmed or dropped.

Supersymmetry is not a new idea. In its modern form it dates back to 1971. But, until recently, no one worried too much about the failure of sparticles to turn up in particle accelerators because Susy predicts they will be heavier than their familiar partners, and thus require more energy to make. How much more energy depends on which version of Susy you subscribe to. But enough versions said “quite a lot more” for it not to matter that the machines that were around then could not produce them. That changed, though, with the recent upgrading of the Large Hadron Collider (LHC) at CERN, Europe’s particle-physics laboratory on the outskirts of Geneva. This machine reopened in 2015 and is now powerful enough for the sparticles predicted by all of the simple and straightforward versions of Susy to manifest themselves.

They have not. Even a flurry of reports in 2016 that the LHC had found a new particle, which turned out to be incorrect, did not tend to suggest the putative discovery was a sparticle. And, at a subsequent conference in Copenhagen, the absence of sparticles was formally acknowledged in a time-hallowed tradition within theoretical physics. A group of scientists who had bet that sparticles would be found by June 2016 duly handed over the prize to a group of sceptics who had bet the opposite. The stake: a bottle of cognac.

If, despite that bet, sparticles do turn up in 2017, everyone will cheer and physics will be back on track towards explaining such mysteries as dark matter (a constituent of the universe, detectable only by its gravitational effect, which may be made of sparticles) and whether the various forces of nature are, underneath it all, aspects of the same thing. If sparticles do not turn up, though, Susy will (at best) have to be put on life support.

There are modified versions of the theory that might let it linger on for a while. One, for instance, posits that only half of known particles have sparticle partners, and that these are conveniently out of the LHC’s reach at the moment. But such finagling smacks of desperation, and it contrasts noticeably with the triumphalism that surrounded theoretical physics in 2013, when the long-predicted Higgs boson was discovered. To say when, precisely, a theory died is often harder than saying when it was born. But unless sparticles decide to reveal themselves at last, the history books may well agree that 2017 was the date of Susy’s demise.

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