At the heart of our Galaxy lives the supermassive black hole Sagittarius A*. If an invisible companion orbits nearby, their dance can be spotted through the light of other stars. The pair's gravity works like a lens: as the black holes pass in front of a background star, its light briefly brightens and splits into two.
This method is called gravitational lensing. It helps find binary black holes where ordinary telescopes are powerless. Especially intriguing is a new approach — quasi-periodic lensing of starlight (QPLS). Here, the light brightens not once but rhythmically, with the orbital period of the pair. It's like a lighthouse that doesn't flash on its own but because of a warped mirror in the beam's path.
New telescopes like the ELT will be able to spot the second, fainter image of a star near the Milky Way's center. And surveys like LSST will find hundreds of rhythmically blinking quasars. These data will merge with signals from gravitational wave detectors like LISA and pulsar timing arrays, and then, for the first time, we'll both see and hear the same cosmic duet.
If a supermassive black hole passed exactly between us and a distant star, we wouldn't see a point but a perfect ring of light, an Einstein ring.
In the movie Interstellar, the supermassive black hole Gargantua is shown through gravitational lensing of the background: this very effect helps us search for real black hole pairs.