LISA Pathfinder (the acronym stands for Laser Interferometer Space Antenna) is intended to demonstrate the technology needed to detect waves of gravity, rather than light, X-rays or gamma rays. Gravitational waves are rippling distortions in space given off when massive bodies are accelerated. They are predicted by Einstein’s general theory of relativity, which celebrates its centenary this year. Physicists have compelling indirect evidence that they are real. (The 1993 Nobel prize for physics was awarded for observations of a pair of superdense stars whose orbits around each other are decaying in a way that can be accounted for only if gravitational waves are carrying away some of their momentum.) But researchers have never seen a gravity wave directly.