Gravity is a strange thing. In our daily life we consider it a force. It draws us to Earth and keeps planets in orbits around their stars. But gravity is not a force. It is a distortion of space and time that bends the trajectory of objects. Throw a ball into space and it moves in a straight line according to Newton's first law of motion. Throw the same ball near the earth's surface and it will follow a parabolic trajectory caused by the distortion of space-time around the earth.
Since matter bends space, it can also deflect the path of light. We call this effect gravitational lenses. It usually happens when light from a distant galaxy or quasar is deflected by the mass of a closer galaxy on its way to us. Gravitational lenses can focus the light and make a distant galaxy appear brighter. This has helped us observe some of the most distant galaxies.
Animated representation of a gravitational wave. Photo credit: ESA – C. Carreau
The warping of space by matter can also create gravitational waves when matter moves through space like waves in a pond. Most of these gravitational waves are too small to see, but we can see strong waves caused by black hole fusions. By observing the waves of several fusions, we have confirmed that gravitational waves travel at the speed of light, as predicted by general relativity. And that means that gravity can deflect its own waves too.
Warped space changes the distance that light has to travel. Photo credit: APS / Alan Stonebraker
Gravitational waves and light both move at the same constant speed in a vacuum. Your paths can be distracted because this speed is finite. It takes time for the waves to travel a certain distance. When a galaxy or black hole warps its surrounding space, the part of a wave near the object has to move a little further than the rest of the wave, so it takes a little longer to reach us. As a result, the wave is deflected towards the mass. So if the merging of black holes sends out a burst of gravitational waves, those waves could be captured by nearby galaxies.
The chirping of a gravitational fusion is clear. Photo credit: LIGO / Caltech / MIT / University of Chicago (Ben Farr)
At least that's the theory. Recently, a team studied this effect and how it would change the appearance of gravitational wave events. They found a couple of interesting things. The first is that lens gravity waves make the source appear closer than it actually is, much like lens light can appear brighter. This could have a significant impact on the observed gravitational fusions as astronomers use their distances to measure the size and extent of the universe. However, the team also found that lens fusion events would have a significantly different overall shape, so we can, in principle, tell the difference between lensed gravitational waves and lensless. When they looked at the black hole fusions that LIGO and VIRGO had discovered so far, they found that none of them were burdened by gravitational lenses.
It will be a while before we can really study gravitational waves with lenses. While the effects are clear, they will require more sensitive gravitational wave observatories to see them in detail. However, this work shows how gravitational wave astronomy will provide astronomers with a wealth of information in time.
Reference: Buscicchio, Riccardo, et al. "Restricting the lensing of binary black holes from their stochastic background." Physical Review Letters 125.14 (2020): 141102.