In the previous post, “Running over the same old ground?”, I outlined three types of reaction to the lack of evidence (so far, at the LHC) for physics beyond the Standard Model. A conversation afterward with Prof Michael Duff reminded me there is a fourth type, which he exemplifies.

I was being grumpy about physicists (particularly Gordon Kane, in this case) who continue as if nothing happened, despite having predicted firmly that new particles would be discovered. In the context of a proper theory, if a prediction is real then its falsification surely tells us something. It changes things, so carrying on regardless with new predictions based on the same ideas seems strange.

Physicists like Mike Duff, however, can carry on as if nothing has changed because nothing of relevance to their work has.

Mathematical consistency requirements can lead theoretical physics to make powerful predictions. Examples would be Dirac predicting the existence of antimatter, or Brout, Englert, Higgs and others predicting the existence of the Higgs boson. Much of the motivation for supersymmetry is about the mathematically consistent incorporation of gravity into a quantum framework. Nothing about this requires that supersymmetry be visible at the LHC or any feasible future collider. In fact, nothing currently measurable is so far predicted.

This is a failing acknowledged by proponents of the approach. It frustrates and worries many people (see here for a discussion between Mike and science writer Jim Baggott on the topic). Regardless, it is worth distinguishing this kind of supersymmetry theorist from the kind that is under pressure at the LHC.

There is an argument that supersymmetry is present in nature. There is another argument that new physics should be visible at the LHC, or at energy scales in the region.

Having supersymmetry appear at a collider would satisfy both arguments. That makes, or made, it quite a compelling possibility. Some theorists have claimed to directly connect the two arguments, but such connections are not widely accepted. That is, the non-observation of supersymmetry at colliders does not mean it can’t still exist at some energy scale way up high.

It may be that one or both of these arguments fails. If we do enough experiments we can settle the matter on the second, but we can have no impact of the first, unless and until some solid, testable predictions emerge. Still, whatever you think about the mathematical quest for a better theory, it doesn’t make sense to simultaneously criticise the same theorists for (a) not making predictions and (b) making bad ones.