The LHC has broken physics, according to this philosopher

The Large Hadron Collider (LHC) has not found any new, fundamental particles besides the Higgs-boson. This is a slap in the face of thousands of particle physicists who were confident the mega-collider should see more: Other new particles, additional dimensions of space, tiny black holes, dark matter, new symmetries, or something else entirely. None of these fancy things have shown up.

The reason that particle physicists were confident the LHC should see more than the Higgs was that their best current theory – the Standard Model of particle physics – is not “natural.”

The argument roughly goes like this: The Higgs-boson is different from all the other particles in the standard model. The other particles have either spin 1 or spin ½, but the Higgs has spin 0. Because of this, quantum fluctuations make a much larger contribution to the mass of the Higgs. This large contribution is ugly, therefore particle physicists think that once collision energies are high enough to create the Higgs, something else must appear along with the Higgs so that the theory is beautiful again.

However, that the Standard Model is not technically natural has no practical relevance because these supposedly troublesome quantum contributions are not observable. The lack of naturalness is therefore an entirely philosophical conundrum, and the whole debate about it stems from a fundamental confusion about what requires explanation in science to begin with. If you can’t observe it, there’s nothing to explain. If it’s ugly, that’s too bad. Why are we even talking about this?

It has remained a mystery to me why anyone buys naturalness arguments. You may say, well, that’s just Dr Hossenfelder’s opinion and other people have other opinions. But it is no longer just an opinion: The LHC predictions based on naturalness arguments did, as a matter of fact, not work. So you might think that particle physicists would finally stop using them.

But particle physicists have been mostly quiet about the evident failure of their predictions. Except for a 2017 essay by Gian-Francesco Guidice, head of the CERN theory group and one of the strongest advocates of naturalness-arguments, no one wants to admit something went badly wrong. Maybe they just hope no one will notice their blunder, never mind that it’s all over the published literature. Some are busy inventing new types of naturalness that would predict new particles would show up only at the next larger collider.

This may be an unfortunate situation for particle physicists, but it’s a wonderful situation for philosophers who love nothing quite as much as going on about someone else’s problem. A few weeks ago, we discussed William Porter’s analysis of the naturalness crisis in particle physics. Today I want to draw your attention to a preprint by the philosopher David Wallace, titled “Naturalness and Emergence.”

About the failure of particle physicists’ predictions, Wallace writes:

“I argue that any such naturalness failure threatens to undermine the entire structure of our understanding of inter-theoretic reduction, and so risks a much larger crisis in physics than is sometimes suggested.”

With “inter-theoretic reduction” he refers to the derivation of a low-energy theory from a high-energy theory, which is the root of reductionism. In the text he further writes:

”If Naturalness arguments just brutely fail in particle physics and cosmology, then there can be no reliable methodological argument for assuming them elsewhere (say, in statistical mechanics).”

Sadly enough, Wallace makes the very mistake that I have pointed out repeatedly, that I frequently hear particle physicists make, and that also Porter Williams spells out in his recent paper. Wallace conflates the dynamical degrees of freedom of a theory (say, the momenta of particles) with the parameters of a theory (eg, the strength of the interactions). For the former you can collect statistical data and meaningfully talk about probabilities. For the latter you cannot: We have only one universe with one particular set of constants. Speaking about these constants’ probabilities is scientifically meaningless.

In his paper, Wallace argues that in statistical mechanics one needs to assume an initial probability distribution that is “simple” in a way that he admits is not well-defined. It is correct, of course, that in practice you start with such an assumption to make a calculation that results in a prediction. However, this being science, you can justify this assumption by the mere fact that the predictions work.

The situation is entirely different, however, for distributions over parameters. Not only are these distributions not observable by construction, worse, starting with the supposedly simple assumptions for this distribution demonstrably works badly. That it works badly is, after all, the whole reason we are having this discussion to begin with.

Ironically enough, Wallace, in his paper, complains that I am conflating different types of naturalness in my paper, making me think he cannot have read what I wrote too carefully. The exact opposite is the case: I am not commenting on the naturalness of statistical distributions of degrees of freedom because that’s just a different story. As I say explicitly, naturalness arguments are well-defined if you can sample the probability distribution. If, on the other hand, you have no way to ever determine a distribution, they are ill-defined.

While I am at it, a note for the folks with their Bayesian arguments. Look, if you think that a supersymmetric extension of the standard model is more predictive than the standard model itself, then maybe try to figure out just what it predicts. Besides things we have not seen, I mean. And keep in mind that if you assume that the mass of the Higgs-boson is in the energy range of the LHC, you cannot count that as a prediction. Good luck.

Back to Wallace. His argument is logically equivalent to crossing the Atlantic in a 747 while claiming that heavier-than-air machines can’t fly. The standard model has so far successfully explained every single piece of data that’s come out of the LHC. If you are using a criterion for theory-evaluation according to which that’s not a good theory, the problem is with you, not with the theory.