Particularly energetic events like certain stellar explosions can accelerate mass and energy in ways that create ripples in the fabric of space-time. In a sense these events are like giant ’cosmic storms’. Every surfer knows that when she’s enjoying a good swell, she’s actually surfing waves produced by a very energetic event that occurred somewhere far away in the ocean. A smart surfer knows that looking at the direction and other properties of the waves she’s on, she can in principle reconstruct important facts, like how intense the storm was and where it occurred. This is what astronomers hope to do by detecting the passage of gravitational waves on Earth. The big difference? The ’cosmic storms’ that produce gravitational waves are usually so far, that by the time the gravitational waves reach the Earth, they are extremely tiny. How tiny? The displacement produced by the gravitational waves expected to be observed with the LIGO detectors are about \(10^{21}\) times smaller than the displacement a surfer usually experiences when riding a wave. The LIGO detectors, with their 4km-long arms, can detect ripples in space-time that change the distance between two objects by \(10^{-18}\)m, which is about 1000 smaller than the size of a proton. An incredibly tricky measurement, and quite a flat day for a surfer.