When it comes to our ability to comprehend the nature of space, time, and the material universe, to understand the fabric of physical reality, the “quantum theory of gravity” represents one the most significant scientific theories of our age. Today, we describe the expansion of the universe using Albert Einstein’s general theory of relativity, which explains how gravity works. But to deal with the physics of very small things we reach for a completely different theory, called quantum mechanics.
Now both of these venerable theories work extraordinarily well, but they are also fundamentally incompatible. Despite many efforts over many years, there is no consensus among physicists about how they should be brought together.
You might therefore be surprised to learn of the award of a $3m Breakthrough Prize for Fundamental Physics to the pioneers of “supergravity”—a particular quantum theory of gravity—which they published over 40 years ago. Even more curiously, supergravity is based on something called “supersymmetry” for which, despite many efforts over many years, physicists have failed to find any evidence. In the media splash following the announcement in early August, some commentators lauded the decision, declaring the prize “well deserved.” But for others this is just another symptom of the malaise confounding foundational theoretical physics. One commentator tweeted that this is a “Breakthrough Prize for fiction.”
Physics is the hardest of the “hard sciences” and to count as science, theories can’t be considered in the absence of some kind of supporting evidence, or at least the promise of evidence. This is why Peter Higgs and François Englert had to wait 49 years before their efforts—first published in 1964—were recognised with a Nobel Prize, following the discovery of the Higgs boson at CERN’s Large Hadron Collider. No matter how talented you are, you don’t win the Nobel just for having a few good ideas. The prize is not awarded for speculative theories that “might be true.”
This really ought to be the end of the matter, but we find ourselves living through a rather extraordinary period in the history of foundational physics. Ideas are not all that hard to come by, but gathering empirical evidence often requires expensive particle colliders or satellite-borne instruments, and can take decades. We are ideas-rich, but data-poor, and speculation is cheap.
Speculative theorising led in the early 1970s to the idea of supersymmetry. There are some aspects of particle physics that theorists can’t really explain, such as why the Higgs boson has the mass it does. Struck by the collection of marvellous symmetries that have been revealed in nature, theorists speculated that there must exist a supersymmetry, connecting matter particles (called fermions) with other kinds of particles (called bosons).
If we assume that the spacetime of general relativity is “supersymmetric” then we arrive at supergravity. Amongst the earliest examples were versions developed in 1976 by Sergio Ferrara, Daniel Freedman, and Peter van Nieuwenhuizen (the recipients of the $3m prize) and, independently, by Stanley Deser and Bruno Zumino. These theorists discovered that some of the difficulties associated with a quantum theory of gravity were somewhat (but not totally) eased if supersymmetry was assumed.
Excitement built up around an extended version of supergravity based on eight different kinds of supersymmetry. This theory accommodates not only the “graviton,” the hypothetical carrier of the gravitational force, but eight supersymmetric partners called “gravitinos” and 154 other particles. For a short time, this version of supergravity was believed to be the “real deal.” In 1980 Stephen Hawking asked if the end was in sight for theoretical physics. At the time the jury was still out, but Hawking argued that supergravity was “the only candidate in sight.”
Although supergravity appeared to promise much, it proved to be impossible to resolve all its problems and, by the early 1980s, interest waned. It didn’t matter that Loop Quantum Gravity, an alternative approach that doesn’t require supersymmetry, was established in the 1990s. It was already too late. Supersymmetry was by now an essential component of “superstring theory,” based on the idea that all elementary particles are vibrational patterns in “strings” of force. This became the dominant paradigm among particle theorists, who continue to regard it as the “only game in town.” Despite its problems, 11-dimensional supergravity took its place alongside five (yes, five) different variants of superstring theory in “M-theory,” conjectured in 1995 to be an overarching structure, which to date nobody has been able to write down.
But, as we’ve seen, supersymmetry, on which all these theories are based, implies a host of supersymmetric particles, partners of the particles we already know. The electron is partnered by the “selectron.” Every quark is partnered by a corresponding squark. The theorists have built a veritable industry out of predicting these particles’ existence. There has typically been a flurry of predictions just as a new particle collider is about to be commissioned. When the collider fails to find evidence for them, the predictions are simply shifted to a higher energy range. When a larger collider then fails to find evidence for them, the game is repeated, and we’re reminded of Karl Popper’s assertion: “it is a typical soothsayers’ trick to predict things so vaguely that the predictions can hardly fail: that they become irrefutable.”
The theorists’ assertions that supersymmetry is so neat that it “must be true” have rung pretty hollow, and the failure to find supersymmetric particles has caused some considerable discomfort. Although the case for building the Large Hadron Collider was based principally on the search for the Higgs boson, much was made of the promise that we would find supersymmetric particles, too. The promise was over-sold, and in the conceptual design report for CERN’s next-generation Future Circular Collider you will find this sober assessment: “…any expectations for early discoveries [of new particles] at the LHC—often based on theoretical, and in some cases aesthetic, arguments—were misguided. In times like this, when theoretical guidance is called into question, experimental answers must be pursued as vigorously as possible.” Ouch.
Many theorists argue that supersymmetry is far from discredited. “Supersymmetry is still alive and kicking,” says British string theorist Michael Duff, “and supergravity was at the heart of all this progress.” It might be alive and kicking in the minds of some theorists, but without any supporting evidence supersymmetry remains a metaphysical construction. It is just an idea.
And here we come to the most curious feature of these extraordinary times. It doesn’t matter. Evidence is no longer relevant. Just as truth has been sacrificed on the altar of the post-truth politics of Trump and Brexit, so evidence is sacrificed in the interests of perpetuating the oxymoron that is post-empirical science.
The Breakthrough Prize was established in July 2012 by Russian entrepreneur Yuri Milner, a former particle theorist who had studied at Moscow State University and the Lebedev Institute. He decided to help recognise the achievements of others by using part of his fortune to establish an annual $3m Prize to rival the Nobel (currently valued at a little over $1m). The programme is now sponsored by Milner and his wife Julia, a contemporary artist, along with Google co-founder Sergey Brin, tech entrepreneur Anne Wojcicki, Chinese entrepreneurs Jack Ma and Pony Ma, and Mark Zuckerberg and his wife, philanthropist Priscilla Chan.
A new prize allows for a new set of rules for awarding it, so by-passing the Nobel committee’s insistence on evidential support. Although Breakthrough Prize award decisions are made by a team of professional physicists, from its inception this was dominated by superstring theorists, reflecting Milner’s scientific background. To be fair, the important role of experiment has been acknowledged, but prizes have been liberally distributed to physicists working on theories that have to date failed to find empirical support. It’s difficult not to perceive this as an exercise in rather lavish self-congratulation.
I have the utmost respect for these theorists’ capabilities and have no real issue with recognising their achievements. But the prize is proclaimed to be one of fundamental physics, and this simply underlines the impression that ideas are everything, and empirical evidence no longer matters.
Of course, the sponsors are free to spend their money however they like. But if the purpose of such philanthropy is to encourage recognition and support progress in physics, then surely founding an institute or funding postgraduate studentships and post-doctoral positions would be more enlightened.
Jim Baggott is a freelance science writer and author of Quantum Space, published by OUP