Gravitational wave radar could probe deep space for tiny stellar objects

Theoretical physicists have hit on a new way to test Albert Einstein’s theory of gravity, or general relativity, and—just maybe—probe the distant universe for tiny, hard to detect objects. Gravitational waves—ripples in space set off when massive objects such as black holes whirl together and collide—should bounce off other massive objects to produce echoes of the signals coming directly to Earth, the theorists predict. Such “gravitational glints” might serve as a kind of radar to detect white dwarfs, neutron stars, and other stellar corpses that are difficult to see beyond our galaxy.

If general relativity is correct, the echo has to exist at some level, says Craig Copi, a theoretical physicist at Case Western Reserve University and lead author on the paper. Still, he cautions, “that does not guarantee that it’s observable.”

According to general relativity, massive objects such as stars and planets warp 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 have been able to detect those incredibly faint waves, using enormous L-shaped optical instruments called interferometers, such as the two of the Laser Interferometric Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state, and the Virgo detector near Pisa, Italy. Together, the detectors have observed dozens of fleeting gravitational wave signals, most coming from the merger of two black holes.

But sometimes, such a signal ought to be accompanied by a sizable 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 lies close to, but not directly on the line of sight to the merging black holes. Using general relativity, they calculate that the gravitational waves scattering off the object can reproduce the signal coming straight from the source, they report this week in Physical Review Letters.

The physics is subtle. The waves scatter not off the material of the object—which they go right through—but from the object’s gravitational field. Theorists had previously calculated that scattering from an infinitesimally small pointlike object such as a black hole should only produce a very feeble scattering. That’s probably because of the specific mathematical nature of the field of a point source, whose strength famously varies inversely with the square of the distance to the point.

Instead of a point, Copi and Starkman analyzed the scattering from a dense spherical object more like a bowling ball. They had expected it to also produce an echo too small to be detected. “The shocking thing we found is that it’s not,” Copi says. The key to the effect is that within the sphere, the gravitational field is modified from the point-source form, he explains.

Other kinds of echoes might be possible. Some physicists have calculated that if general relativity is modified in certain ways by quantum mechanics, then the tail end of the signal from the merger of two black holes should exhibit a pulsing reverberation. But that effect requires new physics and produces a sequence of imperfect echoes. The gravitational glint produces a single, faithful echo of the entire signal, notes Madeline Wade, a gravitational wave physicist at Kenyon College. “I have never heard of a prediction like this, where [the echo] is kind of a copy and paste of the signal at some time delay.”

There is one other 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, then it can act like a lens to produce multiple “images” of the event. But, he says, the chances of seeing such a lensing event should be far smaller.

Assuming nominal populations of neutron stars, white dwarfs, and other compact objects, an echo one-third the size of the original signal should accompany roughly one in every 225 gravitational wave events, Copi and Starkman estimate. So, one or two large echoes could be hiding in the 90 events LIGO and Virgo have already spotted, says Leslie Wade, a LIGO member and gravitational wave physicist at Kenyon. So, the Wades are gearing up to trawl for them. “The win is large whereas the cost of looking for these things would be small,” Leslie Wade says, “So, let’s go for it.”

Cornish, also a LIGO member, notes that the ever-improving detectors ought to spot thousands of events in the next decade. Spotting just one or two glints would serve as a kind of “gradar” to give scientists a crude estimate of how many compact objects such as neutron stars and white dwarfs exist far beyond our galaxy, he says. “It’s a bit like the blind man feeling the elephant,” Cornish says. “You’re not getting like a supersharp probe here, but it would still be some information we wouldn’t otherwise have.”