A long-awaited, 16-year study of an orbiting pair of spinning neutron stars has yielded the most wide-ranging tests yet of Albert Einstein’s theory of gravity. Unsurprisingly, the general theory of relativity, as it’s called, passed all the tests—a sign of its soundness, but also a null result that leaves physicists looking for clues to develop a unifying theory of everything.
An international team used the unusual astronomical system to put general relativity to seven tests, including the most precise measurements yet of the energy carried away by ripples in spacetime called gravitational waves—1000 times better than ground-based gravitational wave detectors—and how those waves reduced the stars’ masses and caused their orbits to shrink. Several effects had never been measured before, including how photons from one of the stars slow down and bend as they pass through the intense gravitational field of the other.
“This is a really impressive suite of tests for general relativity—which of course passes,” says Scott Ransom of the U.S. National Radio Astronomy Observatory, who was not involved in the study.
Since its discovery in 2003, the stellar duo has become astronomers’ best laboratory for testing general relativity. Initially, “We didn’t realize what sort of a gem it was,” says Michael Kramer of the Max Planck Institute for Radio Astronomy, who led the study. The stars, collapsed stellar remnants made of tightly packed neutrons, are called pulsars because as they spin, they emit radio beams that sweep past Earth like a lighthouse. With masses greater than the Sun squeezed into balls the size of a city, the stars have intense gravitational fields. The system, named PSR J0737-3039A/B, is the only known case of two pulsars orbiting each other. Pulsars pulse with such metronomic regularity that the binary pair is like having two atomic clocks in orbit around each other. By looking for tiny variations in the pulses’ timing, researchers can put gravity to extreme tests.
To measure the pulses, Kramer and his colleagues used six large radio telescopes around the globe plus the Very Long Baseline Array, a set of 10 dishes spanning the United States. The fact that the team just kept on observing the pulsars month after month, for years, without publishing any results became a running joke among colleagues. It’s been “a long time coming!” says Cherry Ng of the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics, who commends the team for its mammoth effort. “Combining data from multiple telescopes is not an easy task.”
When the 56-page paper landed this week in Physical Review X, researchers felt it was worth the wait. “It’s an incredible paper and is incredibly thorough,” Ransom says. In total, the team says, general relativity agrees with the observations to a level of at least 99.99%.
The measurements were so precise that the team had to delve further into the theory than previous studies. In past tests, researchers could account for observations by using traditional Newtonian orbits, plus the simplest relativistic corrections, such as the way gravity slows time, shifts light toward longer wavelengths, and ratchets the axis of elliptical orbits forward slightly on each revolution, an effect called precession. But the double pulsar observations seemed to deviate from those basic models. So theorists had to factor in other, more esoteric predictions of Einstein’s theory, such as the way that a dense, spinning mass twists the spacetime around it, and the way gravity can distort the shape of an orbit. The latter is “a subtle, subtle effect that’s never been measured before,” says Ransom, who was impressed by just how many of these “higher order” effects they had to invoke.
Although confirming general relativity is satisfying, researchers want to find a chink in its armor one day. General relativity cannot be a complete description of the way the universe works because, so far as anyone knows, it cannot be reconciled with that other pillar of modern physics, quantum mechanics. An unexplained gravitational aberration could potentially offer theorists a path to a more complete theory. But the double pulsar results only make it harder to conjure up “other theories consistent with the data,” says team member Thibault Damour of the Institute for Advanced Scientific Studies near Paris.
In addition to the relativity tests, the double pulsar may offer a window into the interiors of pulsars. To understand the behavior of the densely packed neutrons, researchers need to know the pulsar density. The mass can be determined easily from its orbit, but size is harder to gauge. If observers can tease out a subtle coupling between one of the pulsars’ spin and its orbit, that will provide a way to calculate its radius. Ransom says he had hoped the double pulsar researchers might be able to pin down this number—“a holy grail,” he says—but they were only able to set size limits. “It proved much trickier to pick out.”
More patience will be needed. Ransom points out that in pulsar studies, the longer you observe, the better accuracy you achieve. In 10 or 20 years, he says, the team may be able to bear down even further. “It gets much better with time.”