Gravitational waves—ripples in the geometry of space-time—are analogous to electromagnetic waves. The challenge in trying to observe these waves directly is that they are extremely weak. To make waves large enough to be (i) , the most (ii) events in the universe are required: supernova explosions, the formation of black holes, or the collision of stars. Even so, the effects are (iii) . The geometry changes so little that a distance of several kilometers changes by less than the diameter of a proton.