This comes from Greene's Fabric of the Cosmos: Wheeler imagines a cosmic version of the delayed choice experiment in which the light source is not a laboratory laser but instead, a powerful quasar in deep space. The beam splitter is not a laboratory variety, either, but is an intervening galaxy whose gravitational pull can act like a lens that focuses passing photons and directs them toward earth. if enough photons from the quasar are collected, they should fill out an interference pattern on a photographic plate. But if we were to put another photon detector right near the end of one route, it would provide which-path information for the photons, thereby destroying the interference pattern. the photons could have been traveling for many billions of years. I'm confused about the words in bold. I don't see how a gravitational pull can act like a lens that focuses photons.