According to Einstein's theory of general relativity, gravitational waves warp spacetime. In the process, they could imprint permanent changes, or "gravitational memory," on their surroundings. Researchers now think that gravitational memory could be written on photons all over the cosmic microwave background—the oldest radiation in the universe. Though gravitational memory is thought to be too subtle to be detected even by our most sensitive equipment, upcoming instruments might finally be able to pick up on a signal. If two black holes crashed into each other billions of years ago, even though we weren't around to observe, could we find out? Well, as it turns out, maybe we could. Einstein's theory of general relativity describe how the gravity of massive objects and extreme phenomena—such as black hole mergers and core-collapse supernovae (which sometimes end up as black holes)—causes ripples that permanently warp spacetime and traverse the void at the speed of light. These ripples are known as gravitational waves , and the effects of these waves thought to be permanently imprinted on their surroundings as "gravitational memory." Related Story What Exactly are Gravitational Waves? Evidence of gravitational memory, however, continues to elude telescopes. Even the Laser…