A Special Breakthrough Prize in Fundamental Physics, worth $3 million, has been awarded to three researchers who devised a theory in the 1970s called supergravity, which attempts to unify all of the four fundamental forces of nature. Daniel Freedman, now at the Massachusetts Institute of Technology, Sergio Ferrara, now at the University of California, Los Angeles, and Peter van Nieuwenhuizen of Stony Brook University collaborated on this approach to resolving the apparent conflicts between the two most fundamental theories of physics: quantum mechanics, which describes the microscopic world of atoms and particles, and general relativity, which describes the force of gravity and its influence on cosmic scales.
Michael Duff of Imperial College London, who has worked on quantum gravity since the 1970s, welcomes the award and calls the three recipients “worthy winners.” Four decades after supergravity was devised, there is still no empirical evidence that the idea is correct. But the Breakthrough Prize in Fundamental Physics has a well-established record of rewarding ideas that still lack experimental verification, in marked distinction to the Nobel Prize’s requirement that concepts be considered confirmed by observation.
Spinning Up Supergravity
Supergravity was born from the quest to find simplicity and unity among the particles and forces of nature. All known particles are encompassed within the theoretical framework known as the Standard Model of particle physics, which was completed in 2012 with the discovery of the Higgs boson at the Large Hadron Collider (LHC) at CERN, the European particle-physics center near Geneva. Within a formulation of quantum mechanics called quantum field theory, three of the Standard Model’s fundamental forces—electromagnetism and the so-called strong and weak forces, which act inside atomic nuclei—are represented by the exchange of particles called bosons between other, interacting particles called fermions. All particles possess a quantum mechanical property called spin, which, for bosons, has an integer value (0, 1, 2, and so on). Bosons include photons, the particles of light and the force carriers of electromagnetism, and gluons, the particles that convey the strong force. Fermions include electrons and the quarks that are the constituents of protons and neutrons in atomic nuclei. Fermions have a half-integer spin: 1/2, 3/2, and so on.
But the Standard Model does not embrace the fourth fundamental force: gravity. Even so, it has long been agreed that gravity should have a corresponding boson called the graviton, which would possess a spin of 2. The observation of gravitational waves in 2015 (which was also rewarded with a Breakthrough Prize, as well as a Nobel) essentially confirmed this picture, Ferrara says.
In the early 1970s several researchers independently proposed that bosons and fermions might be related to one another via a fundamental symmetry called supersymmetry. In this view, very early after the big bang that began our universe, a single type of particle split into these two families in a process of “symmetry breaking,” rather like the branching of a river network. Supersymmetry predicts that every known particle has an as yet undetected supersymmetric partner: bosons, for example, have “bosino” siblings, such as the gluino.
In 1975 Freedman realized that supersymmetry could be extended to include gravity. That inclusion would imply that the graviton has a supersymmetric partner called the gravitino, which the theory predicts to (uniquely) have a spin of 3/2. He and van Nieuwenhuizen, working at Stony Brook, began pooling their expertise to think about the problem. The theory really began to take off when, on a visit to Paris, Freedman met Ferrara, who was then working at CERN. When he returned to the U.S. later that year, Freedman says, “I thought I’d find the rest in two weeks. But it didn’t work out that way.”
In fact, it took him and van Nieuwenhuizen many months of laborious calculations, some performed using the computer facilities at Brookhaven National Laboratory. For the theory to work, they needed to show that around 2,000 terms in their complex equations each canceled to precisely zero. Van Nieuwenhuizen recalls the night when the results came down the phone line from Brookhaven—all 2,000 zeros, one at a time. “My whole life was completely changed that night,” he says. Freedman, van Nieuwenhuizen and Ferrara published their theory in 1976.
Some of these ideas were later used in the 1980s to develop superstring theory, a version of string theory—in which particles are represented as vibrating one-dimensional objects called “strings”—that incorporates supersymmetry. “Supersymmetry and supergravity were key elements in the ambitious program of using strings to make a consistent quantum theory of gravity,” says particle physicist John Ellis of CERN.
Many researchers agree with Ellis that string theory is now the best hope for a theory of quantum gravity—a reason, no doubt, for the Breakthrough Prize’s previous awards for string theory work. But despite intensive development of the idea, string theory has been unable to furnish any predictions amenable to experimental tests with the current generation of particle colliders—the energies needed are vastly too great. This situation has sparked heated debate about whether string theory can be considered “real science” at all.
If supersymmetric string theory is correct, however, so is supergravity: Freedman explains that the latter is what emerges from the theory at relatively low energies, rather like how Newtonian mechanics and gravity represent the low-energy limits of Einstein’s special and general theories of relativity. Supergravity also underpins several other advances rewarded by previous Breakthrough Prizes, such as the late Stephen Hawking’s work on black hole thermodynamics—which earned a Special Breakthrough in 2013—and the so-called anti–de Sitter/conformal field theory correspondence, a link between string theory and quantum field theory proposed in 1997 by Juan Maldacena, now at the Institute for Advanced Study in Princeton, N.J. The gravitino predicted by supergravity has also been posited as a candidate for the mysterious dark matter thought to outweigh the universe’s visible matter by about a factor of five. “Supergravity was where all the action was in the late 1970s and early 1980s,” says writer and former physicist Graham Farmelo, whose 2019 book The Universe Speaks in Numbers explores string theory.
Waiting for a Breakthrough
Supersymmetry has come under fire after the LHC failed to find evidence of the new particles it demands. But Duff says that failure does not, by any means, signal problems with the basic idea. “String theory is silent about the energies at which supersymmetry would reveal itself,” he says—it could be that much higher energies will be needed than are currently accessible. “Supersymmetry is still alive and kicking, and supergravity was at the heart of all this progress,” Duff says.
Besides, some feel that the Nobel committee’s demands for empirical proof look increasingly outdated. The Breakthrough Prize’s position, Farmelo says, “will be seen as the wiser choice in the long term.” Some researchers, for example, fault the Nobel for denying a prize to Hawking, whose research on black hole thermodynamics in the 1970s is widely considered to be a correct description of nature.
Andrei Linde, who, as one of the previous recipients of a Breakthrough Prize, is now part of the committee that bestows the awards, says that their purpose is to “reward extraordinary ideas.” “If you have thousands of people influenced by a single bright idea,” he adds, then its influence deserves recognition, whether it is experimentally proved or not. As testament to that position, he says that despite the fact that supergravity is fundamentally about particle physics, “I use it, too, even though I’m a cosmologist.”
“I think it is good to have a spectrum of prizes that recognize different aspects of science,” Ellis says. “It’s my impression that Nobel Prizes sometimes go to experimentalists rather than people who proposed the underlying theory.”
The Breakthrough Prize in Fundamental Physics was founded in 2012 by investor and philanthropist Yuri Milner. In contrast to the annual Breakthrough, the “Special” prize can be given at any time “in exceptional cases.” The awards are increasingly seen as comparable to the Nobel Prizes not only in monetary value (a Nobel is worth around $1 million) but also prestige. Being conferred by a committee of world-renowned specialists, Ferrara says, makes the award “something special” and “the most important prize in my career.” For Freedman, “this one takes the cake—it is the cap of my long career.”
Van Nieuwenhuizen heard about the award from Ed Witten, a prominent string theorist and one of the inaugural 2012 Breakthrough Prize laureates who comprise the selection committee. “I was sitting at home and saw a message on my screen from Ed,” van Nieuwenhuizen says. “I was very worried he’d ask me some difficult question about supergravity to which I’d not know the answer.” But when Witten then phoned to tell him the real reason for the message, he was speechless. “I’d known we might be candidates in the past,” he says, “but I had completely given up hope of getting it.”
One potentially controversial aspect of the decision is that the supergravity picture was also formulated independently by Bruno Zumino, a pioneer of supersymmetry, and Stanley Deser, now at Brandeis University, who also published their work in 1976—initiating disputes over priority. Zumino died in 2014, but the omission of Deser from the award seems puzzling, Duff says, given that there is no restriction on the number of recipients.
That situation aside, Linde admits that it is surprising, given the importance of supergravity, that its architects were not rewarded sooner. But what are the prospects of seeing the theory put to the test? Detection of any supersymmetric particle would strongly suggest the theory is correct, Farmelo says, because there are arguments that supersymmetry is “the one and only possible way to extend the symmetry of spacetime”—the fundamental canvas of gravity as described in general relativity—“to ensure it is quantum-mechanical.” Clinching evidence, though, would come from detection of the gravitino itself. “That would be wonderful,” Freedman says. But he admits that it will be extremely hard to achieve, because the gravitino should interact so weakly with any other particles. We need to be patient, Ferrara says, pointing out that the Higgs boson was not observed until five decades after it was first predicted. For supersymmetric particles such as the gravitino, he says, “we still have some decades to go” before considering it overdue.
Van Nieuwenhuizen has hopes that a new collider with greater energies than the LHC planned in China might see supersymmetric particles. He reckons on a 50 percent chance of that happening in his lifetime. To its advocates, though, supersymmetry and its concomitant supergravity seem not just likely but virtually inescapable. “I think it’s inevitable that the spin-3/2 particle [gravitino] is realized in nature,” Freedman says. “There is no comparable theory,” Ferrara argues, “and it would be really a pity if nature has not used this one.”