Unusual three-star system promises new test of gravity

Measurements of pulsar and white dwarfs could bolster or dethrone general relativity

TRIPLE THREAT  A pulsar (left) is orbited by two white dwarfs — one close in, one farther away — in this illustration of the system PSR J0337+1715. The curved blue lines show the pulsar’s magnetic field and the blue cones show the radiation the star sends into space. Astronomers measuring this radiation will try to test general relativity, the leading theory of gravity, to unprecedented precision.

Bill Saxton, NRAO/AUI and NSF

A unique threesome of stars locked in tight, circular orbits could help astronomers test the leading theory of gravity to unprecedented precision. The discovery of the celestial trio is reported January 5 in Nature.

“We should be grateful to the universe for making such things,” says Paulo Freire, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany, who applauds the finding. “Part of me wishes I were involved.”

Our galaxy is full of stellar couples and trios. But the formations and motions of the stars in PSR J0337+1715 make the system unique among those found by astronomers. The triad consists of an extremely dense, fast-rotating stellar corpse called a pulsar and two less massive dying stars known as white dwarfs. Pulsars form when stars at least 1.4 times larger than our sun blow up in supernovas; these powerful explosions usually knock nearby stars out of their orbits. In PSR J0337+1715, however, all three stars remain closer to each other than the sun is to Earth and travel in nearly circular orbits, with one white dwarf much closer to the pulsar and one farther away.

Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va., who discovered PSR J0337+1715 in telescope data, says he and his colleagues don’t know how the stars could have come together in this way. But the researchers are excited because the system’s arrangement will allow them to probe gravity. As the pulsar spins 366 times per second, its magnetic field sweeps through space like a lighthouse beam. By measuring minute variations in the beam’s timing, Ransom and his colleagues can precisely track the motions of all three stars in the system.

The team hopes to see if the stars are pulled toward each other as general relativity, the leading theory of gravity, predicts they should be. When a hyperdense object like a pulsar forms, part of its mass converts to energy that binds the object together. General relativity’s strong equivalence principle states that gravity should have the same effect on this binding energy as it would on an equivalent amount of mass. That means the pulsar and the inner white dwarf in PSR J0337+1715 would fall toward the outer white dwarf at the same rate. In nearly all competing theories of gravity, binding energy interacts with gravity differently than with mass, which would cause the pulsar and inner white dwarf to fall at slightly different rates.

Physicists believe general relativity must eventually yield to a theory compatible with quantum mechanics, which describes nature at its smallest scales. All such theories that have been proposed would not contain the strong equivalence principle, says Ransom. Even though his team should soon be able to measure gravity a hundred times more precisely than ever before, Ransom hesitates to bet his measurement will be the one to overthrow general relativity, which has reigned since shortly after Albert Einstein published it nearly a century ago. “Everything that we’ve ever looked at has shown that it works beautifully,” he says.

Ransom’s group has an opportunity to do a beautiful test of general relativity, University of Florida physicist Clifford Will says. “If they could pull it off in the next year or two, it would be a great 100th birthday present for Einstein’s theory.”

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