In 1915, Einstein published his theory of general relativity. It stated that three-dimensional space is connected to a fourth dimension: time. Einstein described this concept, known as spacetime. He proposed that the presence of matter and energy causes spacetime to curve. That curvature creates gravity, the force that attracts objects to one another (see Warped Space).
General relativity predicts that in some areas of spacetime, called black holes, gravity is so strong that even light can’t escape. Einstein’s theory suggests that explosions and violent collisions in space, like one between two black holes, send gravitational waves through spacetime, much like the way a dropped pebble sends ripples across a pond. These waves “change the shape of spacetime itself, which stretches and shrinks as they move,” says Marco Cavaglià, an astrophysicist at the University of Mississippi.