General relativity postulates that massive objects curve space and time. Gravitational lensing occurs when these objects distort or bend light curves. Similarly to light, gravitational waves can be gravitationally lensed.
However, unlike in lensing of light where one detects particles, in gravitational-wave lensing we detect waves. Consequently, methods to detect and utilize gravitational-wave lensing are entirely different from that of light and are being actively developed.
Specifically, when a pair of gravitational waves travel through a galaxy, the gravity of the galaxy curves the trajectories of the waves, focusing them toward us. As we observe the waves, they will have been amplified due to the focusing by lensing and will arrive at different times due to having traveled different trajectories at the same speed. However, unlike in light lensing, where one has to classify incoming photons as lensed by their angular direction (as no signal templates exist), we can classify gravitational waves as lensed by statistically distinguishing them as identical events using gravitational-wave templates and Bayesian analysis. Moreover, if there are substructures on the lensing path, the galaxy will amplify their lensing effects, resulting in wave diffraction and “beating patterns”. The patterns occur at characteristic lensing time-scale and could potentially be resolved to infer the properties of the structures in LIGO/Virgo/LISA if the events occur. Such wave diffraction does not take place in light lensing.
In our recent work, we investigate if there are signatures of gravitational-wave lensing within the LIGO/Virgo ground-based gravitational-wave detectors. In particular, we look for three effects: 1) evidence of lensing magnification in the individual signals due to galaxy lenses, 2) evidence of multiple images due to strong lensing by galaxies, 3) evidence of wave optics effects due to point-mass lenses.
We find no compelling evidence of any of these signatures in the observed gravitational wave signals in our work. However, as the sensitivities of gravitational wave detectors improve in the future, detecting lensed events is estimated to become likely (see 1,2,3).
Link to our work published in the Astrophysical Journal Letters