Theoretical physicists have found a new way to test Albert Einstein’s theory of gravity or general relativity and — just maybe — to explore the distant universe for small, elusive objects. Gravitational Wavesripples in space created when massive objects such as black holes whirl and collide —should bounce off other massive objects to produce echoes of the signals coming directly to Earth, the theorists predict. Such “gravitational glare” could serve as a kind of radar to detect white dwarfs, neutron stars and other stellar bodies that are hard to see outside our galaxy.
If general relativity is correct, the echo must exist at some level, said Craig Copi, a theoretical physicist at Case Western Reserve University and lead author of the paper. Still, he cautions, “that’s no guarantee it’s detectable.”
According to general relativity, massive objects such as stars and planets distort spacetime to create the effect we call gravity. When two massive objects such as a pair of black holes swirl together, the collision should radiate gravitational waves in all directions.
Since 2015, scientists detect those incredibly weak waves, using huge L-shaped optical instruments called interferometers, such as the two from the Laser Interferometric Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state, and the Virgo detector near Pisa, Italy. Together, the detectors detected dozens of fleeting gravitational wave signalsmost of which come from the merger of two black holes.
But sometimes such a signal would have to be accompanied by a hefty echo that comes a fraction of a second later, predict Copi and Glenn Starkman, a theorist at Case Western. They consider a compact object such as a white dwarf or a neutron star that is close, but not directly, in the line of sight of the merging black holes. Using general relativity, they calculate that the gravitational waves scattering off the object can reproducing the signal coming directly from the sourcereport them this week Physical Assessment Letters†
The physics is subtle. The waves scatter not from the object’s material – through which they pass – but from the object’s gravitational field. Theorists had previously calculated that scattering from an infinitesimal point-like object such as a black hole would produce only a very faint scattering. This is probably due to the specific mathematical nature of a point source’s field, whose strength varies inversely with the square of the distance from the point.
Instead of a point, Copi and Starkman analyzed the scattering of a dense spherical object that looked more like a bowling ball. They had expected it to also produce an echo that was too small to be detected. “The shocking thing we found is that it isn’t,” says Copi. The key to the effect is that within the sphere, the gravitational field is changed relative to the point source shape, he explains.
Other types of echoes are possible. Some physicists have calculated that if the general theory of relativity is modified in certain ways by quantum mechanics, the end of the signal from the merger of two black holes should show a pulsating reverberation. But that effect requires new physics and produces a succession of imperfect echoes. The glint of gravity produces a single, faithful echo of the entire signal, notes Madeline Wade, a gravitational wave physicist at Kenyon College. “I’ve never heard of a prediction like this where… [the echo] is a kind of copy and paste of the signal with some delay.”
There’s another standard way to produce multiple signals, says Neil Cornish, a gravitational wave astronomer at Montana State University. If a dense object sits exactly along the line of sight to a source of the gravitational waves, it can act as a lens to produce multiple “images” of the event. But, he says, the chances of seeing such a lens event should be much smaller.
Assuming nominal populations of neutron stars, white dwarfs and other compact objects, an echo of one-third of the original signal should accompany about one in every 225 gravitational wave events, Copi and Starkman estimate. So one or two big echoes can hide in the 90 events LIGO and Virgo have already seen, says Leslie Wade, a LIGO member and gravitational wave physicist at Kenyon. So the Wades are getting ready to go fishing for them. “The win is big, while the cost of looking for these things would be small,” says Leslie Wade, “So let’s go for it.”
Cornish, also a LIGO member, notes that the ever-improving detectors should detect thousands of events over the next decade. Seeing just one or two glints would serve as a sort of “gradar” to give scientists a rough estimate of the number of compact objects like neutron stars and white dwarfs far beyond our galaxy, he says. “It’s kind of like the blind man feels the elephant,” Cornish says. “You don’t become like a super-sharp probe here, but it would still be information that we wouldn’t have otherwise.”