Last week I was in CERN for various meetings. Rather unexpectedly, these included one with Roger Waters in which I totally failed to say “Welcome to the Machine” at the right moment.
The main business was CERN’s Scientific Policy Committee, followed by Council, and the first meeting of the “European Strategy Group” for particle physics which I mentioned here. As I describe in that article, much attention focusses on whether there is a case for a new big collider, and if so which one?
The LHC has transformed our view of particle physics, partly by discovering the Higgs boson and measuring its properties – especially its mass – and partly by what it has not observed. There were predictions that other new particles would show up at the same time as the Higgs, especially those predicted in an extension of the Standard Model of particle physics called “supersymmetry”. These expectations were not met, and that has led to a few different reactions amongst theorists.
Some appear to have confused the failure of theoretical predictions with a failure of the experiment. The most prominent example is probably Sabine Hossenfelder, whose initial nuanced critique of some aspects of particle theory seems to have degenerated into disillusionment with the whole idea of collider experiments. In this way of thinking, exploring and measuring new territory in physics is not enough without guarantees (as opposed to possibilities) of “finding new symmetries or solving the riddle of dark matter” (but see note added below) . We’d all like those things, but there are, as ever in real-life experiments, no such guarantees.
Others recognise that the LHC has changed the game, and are hungry for more data to stimulate new and hopefully better ideas. An example would be Nima-Arkani Hamed who, interviewed in CERN Courier, says:
Nobody who is making the case for future colliders is invoking, as a driving motivation, supersymmetry, extra dimensions or any of the other ideas that have been developed over the past 40 years for physics beyond the Standard Model.
This is because those ideas have been dealt blow after blow by LHC data. While in some cases impossible to technically rule out altogether, many of them are, as he says “either dead or on life support”. Even the underlying concepts behind the theories, such as the idea of naturalness, are under pressure. The fact that data can have this impact is a credit to good theory, by the way. A theory which endures whatever our experiments say is … suspect?
Which brings me to the third, and most bizarre reaction. A minority of theorists simply carry on as if nothing has happened. For them, supersymmetric particles are just around the next corner, as they were in 1990 (before LEP, HERA and the Tevatron Run II), in 2015 (before the LHC made its latest jump in energy) and for essentially every incremental step forward in beam energy or intensity along the way. The leader of this band is undoubtedly Gordon Kane, who has recently done it again in Physics Today.
I don’t really know what the motivation for this behaviour is, or how self-aware or sincere is the serial prediction/adjustment cycle. It even extends to silently revising books, as amusingly documented by Peter Woit here. Woit also dissects Kane’s most recent effort here, and I’ll leave it at that¹.
It is not hard to see why behaviour like Kane’s causes anger and scepticism, especially if you overestimate its importance and influence, or indeed the importance of theoretical instructions in general.
For what it’s worth, ever-imminent supersymmetry has played very little part in my own motivation. Supersymmetry is an interesting idea deserving of respect, and the various versions of it have provided some handy case studies – a testing ground for experimental design and analysis. I’ve written papers on supersymmetry, including part of my doctoral thesis. But it was never a prime motivation. My attitude to it pre-LHC is described here, in case you worry I may be letting myself off lightly in the glow of hindsight.
The prime motivation for me, and many experimentalists (probably most theorists too), was always to extend the frontier of our knowledge of how nature behaves. Extending to higher energies also means looking at higher resolution, at smaller distance scales, deeper into the heart of matter and the forces that bind it. To me at least, this seems an especially interesting direction of travel.
Perhaps we experimentalists have deferred too much to theoretical arguments in the past. (And it must be said, the Higgs one was a very good one.) It is always nice to hear about things to look out for on your travels. But if we want to continue our exploration with significant new projects we need to make it clearer than we have before that the experiments don’t exist to chase down pet theories, interesting though those may be. Nor do they exist for the technology, training and other ancillary benefits they provide, although those are necessary to make them sustainable and economically beneficial. They exist to explore. The discussion needs to centre on the value of the knowledge they uncover.
¹Except to say that I find it strange that Kane implies in his article that the first electron-positron and proton-antiproton colliders were at SLAC (California) and Fermilab (Illinois) respectively, when in fact they were, as far as I can tell, in Frascati (Italy), and CERN, Geneva. The CERN one was quite famous and won a prize.
My interpretation of Hossenfelder’s writing. She says she has never demanded guarantees, nor called the LHC a failure. I’m happy to assume she hasn’t put those statements down in so many words. However, on the first point, in my view the cumulative effect of her writing raises the bar of required expectation for a new collider to the level of “guarantee” for all practical purposes. On the second: at one point for example she writes of almost apologising to her bus driver for the fact that the LHC “… didn’t” [and] “…probably won’t” find anything new, which certainly paints a picture of perceived failure in my mind. Maybe she didn’t mean any of that and it was just unfortunate phrasing. However the “way of thinking” that I refer to is not uncommon, even amongst experimentalists.
I now wait with interest to see whether Gordon Kane also disagrees with my understanding of his words.