Bye, bye, little Susy

A WISE PROVERB suggests not putting all your eggs in one basket. Over recent decades, however, physicists have failed to follow that wisdom. The 20th century—and, indeed, the 19th before it—were periods of triumph for them. They transformed understanding of the material universe and thus people’s ability to manipulate the world around them. Modernity could not exist without the knowledge won by physicists over those two centuries.

In exchange, the world has given them expensive toys to play with. The most recent of these, the Large Hadron Collider (LHC), which occupies a 27km-circumference tunnel near Geneva and cost $6bn, opened for business in 2008. It quickly found a long-predicted elementary particle, the Higgs boson, that was a hangover from calculations done in the 1960s. It then embarked on its real purpose, to search for a phenomenon called Supersymmetry.

This theory, devised in the 1970s and known as Susy for short, is the all-containing basket into which particle physics’s eggs have until recently been placed. Of itself, it would eliminate many arbitrary mathematical assumptions needed for the proper working of what is known as the Standard Model of particle physics. But it is also the vanguard of a deeper hypothesis, string theory, which is intended to synthesise the Standard Model with Einstein’s general theory of relativity. Einstein’s theory explains gravity. The Standard Model explains the other three fundamental forces—electromagnetism and the weak and strong nuclear forces—and their associated particles. Both describe their particular provinces of reality well. But they do not connect together. String theory would connect them, and thus provide a so-called “theory of everything”.

String-driven things

String theory proposes that the universe is composed of minuscule objects which vibrate in the manner of the strings of a musical instrument. Like such strings, they have resonant frequencies and harmonics. These various vibrational modes, string theorists contend, correspond to various fundamental particles. Such particles include all of those already observed as part of the Standard Model, the further particles predicted by Susy, which posits that the Standard Model’s mathematical fragility will go away if each of that model’s particles has a heavier “supersymmetric” part­ner particle, or “sparticle”, and also particles called gravitons, which are needed to tie the force of gravity into any unified theory, but are not predicted by relativity.

But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase.