Whether the theory of supergravity, an attempt to unify all the forces of nature, is a true description of the world still hangs in the balance more than 40 years after it was proposed. Nonetheless it has now nabbed its founders one of the most lucrative awards in science: a shared US$3-million Special Breakthrough Prize in fundamental physics.
Supergravity1 was devised in 1976 by particle physicists Sergio Ferrara of CERN, Europe’s particle-physics laboratory near Geneva, Switzerland; Daniel Freedman of the Massachusetts Institute of Technology in Cambridge; and Peter van Nieuwenhuizen of Stony Brook University in New York. The selection committee that awarded the prize chose to honour the theory, in part, for its impact on the understanding of ordinary gravity. Supergravity also underpins one of physicists’ favourite candidate ‘theories of everything’, string theory. The latter asserts that elementary particles are made of tiny threads of energy, but it remains unproven.
“Supergravity has been transcendently important in the development of physics for the past 40 years and in our exploration of what might lie beyond what we know about nature,” says string theorist Andrew Strominger at Harvard University in Cambridge, Massachusetts, who sat on the prize’s selection committee.
Russian entrepreneur Yuri Milner launched the Breakthrough Prizes in 2012, and funders now include Google co-founder Sergey Brin and Facebook’s Mark Zuckerberg. Awards are given out towards the end of each year, across a range of fields in science and mathematics. But the selection committee — picked from the pool of previous Breakthrough prizewinners — can make special awards to recognize exceptional work. In 2013, for example, Stephen Hawking won for his theory — also still untested experimentally — that black holes give off radiation.
By the early 1970s, physicists had constructed the standard model of particle physics, in which three of the four fundamental forces of nature are associated with their own particle: the electromagnetic force is carried by the particle of light, the photon; the strong force that binds atomic nuclei is mediated by the ‘gluon’; and the weak force that governs radioactive decay is associated with ‘W’ and ‘Z’ particles. All these particles have been observed experimentally. But the fourth fundamental force, gravity, resisted efforts to include it in the model. Supergravity was an early attempt to do so, combining particle physics with Einstein’s theory of gravity, general relativity.
Ferrara, Freedman and van Nieuwenhuizen drew inspiration from supersymmetry, an extension of the standard model first proposed in 1973. Supersymmetry asserts that each known particle has a heavier, and as yet undiscovered, twin. Models that try to bring the final fundamental force, gravity, into the mix assign it a hypothetical ‘graviton’ particle. The team proposed a super-twin for the graviton called the gravitino. Van Nieuwenhuizen remembers the night he watched their computer program crunch through the supergravity calculations, fearful it would prematurely grind to a halt, indicating that the theory was wrong. “I sat there with mounting tension,” he says. But when the program reached its conclusion successfully, he was convinced supergravity was real.
Forty years later, van Nieuwenhuizen was left speechless by the news of the award. “It was a total surprise,” he says. “I’d given up hope it would happen.”
David Tong, a string theorist at the University of Cambridge, UK, says that the innovation behind supergravity was “astonishing”, given that at the time particle physicists and gravity researchers rarely interacted. “Here, the team was applying particle-physics techniques to gravity and then testing them computationally, when nobody was using computers to do this sort of thing,” says Tong.
Today, supergravity is a cornerstone of string theory, which is a popular candidate for the ultimate description of reality. But for decades, particle accelerators, including CERN’s Large Hadron Collider (LHC), have failed to spot any signs of supersymmetry particles or the gravitino, or any evidence for string theory — although this does not rule it out completely. “These ideas may just not be testable in our lifetime,” says Tong.
A lack of evidence should also not detract from supergravity’s achievements, argues Strominger, because the theory has already been used to solve mysteries about gravity. For instance, general relativity apparently allows particles to have negative masses and energies, in theory. “If that was true, some things wouldn’t fall to Earth when dropped, but fall into space,” says Strominger. That does not happen, but no one could explain why not. Turning supergravity’s mathematical machinery to general relativity, however, enabled physicists to prove that particles cannot have negative masses and energies. “Those results will hold whether or not supergravity actually exists in nature,” says Strominger.
But Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany, warns that the LHC’s failure to find supersymmetry particles deals a near fatal blow to supergravity’s chances of being true. She says that the winners have “done great mathematical work that deserves recognition”, adding, “but perhaps the award should be for pure mathematics, because this is not physics.”