When I was a teen-ager, I spent many nights gazing through a telescope at an amateur observatory in Cranford, New Jersey. Saturn, to the naked eye, is a shining white dot. But through the telescope it became a yellow-white sphere, with softly glowing rings. It looked too perfect to be real. I could see the planet’s moons scattered around it—Titan, Enceladus, Mimas, Dione—and the Cassini gap in its rings.
That telescope had lenses that were ten inches wide, which were made in-house at the observatory. Put simply, converging lenses, like those in telescopes, work by bending rays of light until they come together at a single point, known as a focal point. The bigger the lens, the more light it can capture, and the fainter the objects you can see. Now, if a ten-inch lens can let you see the rings of Saturn, think about what you could observe with a lens that is hundreds of thousands of miles wide.
In a paper published in 1936, Einstein worked out the basic idea of what is now called gravitational lensing. Because gravity bends light, or any other kind of radiation, a large mass can act like a giant lens, bending approaching light and radio waves around it, and magnifying them. The sun can be thought of as a gravitational lens. So can any star, or galaxy, or even cluster of galaxies.