Is supersymmetry a "speculative idea"?

Matt Strassler wrote a mostly sensible text

A Couple of Rare Events

on the media's reaction to the LHCb and CMS' measurement of the decay rate of the rare process \(B_{(s)}^0\to \mu^+\mu^-\). For the first time, the decay rate was measured to be nonzero and it agrees with the Standard Model within something like a 30% error margin. Matt correctly says that some media sell it as a breathtaking success of the Standard Model that nearly kills all the competitors. And he correctly points out that the media apparently think that the supersymmetry is the only competitor of the Standard Model.

Related: The Huffington Post wrote a story about the 4.5-sigma LHCb anomaly (TRF) pointed out by Descotes-Genon, Matias, Virto

I agree with much of what he says. In reality, there are other theories beyond the Standard Model; the precision with which the rare decay was measured isn't too great; some values of parameters of beyond-the-Standard-Model theories have been excluded while others remain perfectly fine, in contradiction with the main message of the media. Well, I have some understanding for the media's approach: supersymmetry is the #1 well-motivated theory for beyond-the-Standard-Model physics which is why supersymmetry is sometimes sloppily used as a shortcut for the whole set. In fact, the relative likelihood that SUSY is the first new physics that will be discovered is getting larger which means that this approximation of "Beyond the Standard Model physics" by "supersymmetry" is arguably becoming increasingly accurate.

However, there's one detail in Matt's text that I simply can't swallow. He uses the word "speculative" a whopping eight times for supersymmetry and all other ideas for beyond-the-Standard-Model physics. I think that this adjective – something that Matt has clearly adopted as a major part of his idiosyncratic language – is totally inappropriate for these theories. Why?




The adjective "speculative" is being used in many ways and the precise purpose of this word depends on the discipline. However, I believe that "speculative" is an international word so even though I am not a native speaker, I can discuss the fine nuances because the Czech word "spekulativní" is arguably used in the same way.




In physics, a "speculative idea" is one that is sort of cool, from a certain viewpoint, but it is an idea that isn't directly connected with the "standard research" that is going on right now, through the usual combination of experimental and theoretical advances. A "speculative idea" is disconnected in this way either because it is

• apparently manifestly wrong – the word "speculative" is just a codeword for "wrong" that allows the deluded researcher to be treated on par with the proper researchers although he or she is apparently not
• very unlikely – it depends on assumptions whose probability seems small and that don't seem to be sufficiently justified by any results that are more or less established
• too philosophical – the motivation for the idea or the assumptions behind it are philosophical if not ideological in character; they are not empirically scientifical in nature
• too futuristic – the idea is just not appropriate for research by the bulk of the current researchers because the links between that idea and the industry of doable experiments or calculations are very scarce and the situation may only be expected to change in a distant future.

These four descriptions are sort of related, they're not quite independent, and they don't quite exhaust what we mean by the word "speculative" but I think that they fairly capture the point.

Whatever is your favorite defining point of a "speculative idea", I believe that none of them is legitimate to be used for the beyond-the-Standard-Model theories of particle physics, especially not for supersymmetry.

Supersymmetry in particular – and at least some other BSM models to a lesser extent as well – is surely not "manifestly wrong". In fact, there are good top-down arguments suggesting that supersymmetry is, on the contrary, "manifestly right", a mathematically necessary part of any consistent theory of quantum gravity that at least remotely resembles the real world.

One shouldn't assume that supersymmetry has been established as a key part of the description of the collisions at the LHC because it hasn't been established. But its irrelevance hasn't been established, either. Even for a "theoretical agnostic" – I mean someone who doesn't give a damn about theoretical arguments at all – it is right to be neutral about the validity of supersymmetry. There clearly exist supersymmetric scenarios that are compatible with all the observed LHC phenomena. And there also exist non-supersymmetric low-energy effective theories (but not necessarily complete theories including gravity) that are compatible with all the LHC data so far.

An impartial "theoretical agnostic" should assign comparable Bayesian prior probabilities to the "supersymmetric models of Nature" and to the "non-supersymmetric models of Nature". Doing something entirely different – e.g. describing one class as the "default one" and the other class as a "speculative one" – only reveals the bias of the "theoretical agnostic". So this is my recipe of "impartiality" for someone who doesn't assign any weight to too sophisticated theoretical arguments. Matt has violated this rule.

However, I don't really believe that a good physicist should be a "theoretical agnostic". Arbitrarily advanced or abstract arguments that are relevant for finding the right answers to physical questions are things that a good physicist should be interested in and pay attention to. From this viewpoint, Matt's attitude is even more invalid or outdated, if you wish. The top-down research – the only other research aside from the bottom-up research that may introduce any asymmetry between our belief in the supersymmetric and non-supersymmetric theories – actually favors the supersymmetric class. At the scale of quantum gravity, the Planck scale, or at much lower scales, supersymmetry should simply get restored. There don't seem to be any stable, consistent theories including quantum gravity that refuse to respect supersymmetry even at the Planck scale!

So if one introduces an asymmetry and decides to call one class "default" and one class "speculative", it should be done exactly in the opposite way than Matt's way.

Matt's wording is not only wrong but also counterproductive for the research in particle physics because the word "speculative" suggests that the research of these models is something else than the usual empirical science rooted in the empirical data. But the supersymmetric model building in particular is as much standard science that hugely cares about the empirical data as you can get. Matt's adjective "speculative" makes SUSY – and all BSM physics research – sound like a speculative, philosophical, futuristic research or a research of unhinged crackpots. It's none of these things.

I am also dissatisfied with Matt's description of supersymmetry as "one of the speculative ideas to solve the hierarchy problem". The potential to solve or partially solve the hierarchy problem is just one argument among many – and not the most important one in my reading of the situation – in favor of the supersymmetry. The idea that the hierarchy problem is the only justification of SUSY or the only thing to care about and judge new ideas by seems extremely narrow-minded and primitive to me.

Supersymmetry is primarily a symmetry whose restoration seems inevitable at the Planck scale or lower in consistent theories of quantum gravity. It is also a symmetry that produces the most natural dark-matter candidates we know in literature. It is a symmetry that must be assumed to make the minimal particle content compatible with the gauge coupling unification. Yes, it is also the most natural known explanation of the hierarchy problem but also a viable method to achieve other cancellations, even a way to make the C.C. problem less severe quantitatively. SUSY seems also necessary if you think that the number 3/2 shouldn't be missing in the list of integers and half-integers between zero and two. This list is in no way complete.

So the media are conveying a heavily oversimplified and ultimately distorted picture of the reality and Matt Strassler is corrected some of it. But his story still seems excessively oversimplified and distorted to me. Sorry.