Gravitational wave ‘radar’ could help map the invisible universe

If radio waves give you radar and sound provides you sonar, what do gravitational waves give you? It seems like the setting for a comedy.

According to researchers in an article published to Physical Review Letters, the solution may be "GRADAR," or gravitational wave "radar," a hypothetical future device that might utilize gravitational wave reflections to map the invisible cosmos. Scientists may be able to locate dark matter or faint, exotic stars by searching for these signals and learning about the inner workings of these objects.

Gravitational waves, which were originally discovered in 2015 and are moving ripples in the very fabric of space and time, are frequently used by astronomers to see catastrophic events that are challenging to analyze with light alone, such as the merger of two black holes (SN: 2/11/2016).

However, physicists are also aware of gravitational waves' apparently pointless ability to shift direction. According to Einstein's theory of gravity, matter warps spacetime, and any wave travelling through these distortions will change direction. As a result, when anything releases gravitational waves, some of the signal may arrive directly at Earth while others may reach later — like an echo — after traveling along longer pathways that curve around a star or other large, heavy object.

These subsequent signals, known as "gravitational glints," had long been assumed to be too faint to be seen by scientists. But Cleveland, Ohio-based physicists Craig Copi and Glenn Starkman made a quantum leap: Using Einstein's theory as a foundation, they computed the strength of the signal that would result from waves scattering across the gravitational field within a star itself.

“The shocking thing is that you seem to get a much larger result than you would have expected,” according to Copi. “It’s something we’re still trying to understand, where that comes from — whether it’s believable, even, because it just seems too good to be true.” 

According to the study, astronomers might be able to map the interiors of stars using gravitational glints if they can be that powerful. Even large things in space that would be difficult to find normally, such as clumps of dark matter or lone neutron stars on the far side of the observable universe, may be searched for by researchers.

“That would be a very exciting probe,” explains Maya Fishbach, an astrophysics professor at Northwestern University in Evanston, Illinois, who was not involved with the study.

Still, there are reasons to exercise caution. If this phenomena can withstand closer examination, Fishbach adds, scientists would first need to better understand it before they could put it to use. This will likely be challenging.

“It’s a very hard calculation,” adds Copi. 

But comparable difficulties have been overcame in the past. “The whole story of gravitational wave detection has been like that,” according to Fishbach. She claims that while understanding their measures required a lot of arithmetic, the discipline is currently advancing rapidly(SN: 1/21/21). “This is the time to really be creative with gravitational waves.”

C. Copi and G.D. Starkman. Gravitational glint: detectable gravitational wave tails from stars and compact objects. Physical Review Letters. In press, 2022.