If radio waves provide radar and sound provides sonar, then what do gravitational waves provide?

The solution may be “GRADAR” — gravitational wave “radar” — a hypothetical future device that might scan the invisible cosmos using gravitational wave reflections, according to an article approved for publication in Physical Review Letters. By searching for these signals, scientists may discover dark matter or faint, unusual stars and get insight into their interiors.

Gravity waves are vibrations in the fabric of space and time that were discovered for the first time in 2015. Astronomers use gravity waves on a regular basis in order to observe catastrophic events that are difficult to observe using light alone, such as the merger of two black holes (SN: 2/11/2016).

However, physicists have long known that gravitational waves had a seemingly worthless property: the ability to reverse path. According to Einstein’s theory of gravity, matter warps spacetime, and any wave travelling through these distortions will change path. Because of this, when anything emits gravitational waves, part of the signal travels immediately to Earth, and some of it can arrive later — much like an echo — after traveling farther and following longer paths that curve around stars and other huge objects.

Scientists have traditionally believed that these “gravitational glints” should be too faint to detect. However, physicists at Case Western Reserve University in Cleveland, Ohio, achieved a breakthrough when they used Einstein’s theory to calculate the intensity of the signal as waves propagate through the gravitational field within a star.

“The surprising thing is that you seem to get a considerably greater effect than you could have anticipated,” adds Copi. “We’re still trying to figure out where it comes from and if that’s really plausible, since it sounds too good to be true.”

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The research suggests that if gravitational glints are so powerful, astronomers may be able to utilize them to map the interiors of stars. Even large things in space that would ordinarily be hard to detect, such as globs of dark matter or solitary neutron stars on the far side of the visible universe, may be sought out by researchers.

Maya Fishbach, an astronomer at North-western University in Evanston, Illinois, who was not involved in the work, adds, “That would be a really fascinating investigation.”

However, there are still reasons to exercise caution. If this phenomenon holds up to more inspection, says Fishbach, scientists will have to better understand it before they can exploit it, which will likely be challenging.

“It’s a pretty difficult math,” admits Copi. However, comparable obstacles have been surmounted in the past. Fishbach states that the whole history of gravitational wave detection has followed this pattern. She says it was difficult to complete all the arithmetic necessary to comprehend their measures, but today the field is thriving (SN: 1/21/21). Now is the moment to use gravitational waves in an innovative manner.

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