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…