Explaining gravitational waves

Paul McNamara, ESA LISA Pathfinder Project Scientist, explains gravitational waves.
ESA’s LISA Pathfinder lifted off 3 December on a Vega rocket from Europe’s spaceport in Kourou, French Guiana, on its way to demonstrate technology for observing gravitational waves from space.
Gravitational waves are ripples in the fabric of spacetime, predicted a century ago by Albert Einstein’s General Theory of Relativity, published on 2 December 1915.
LISA Pathfinder will test the extraordinary technology needed to observe gravitational waves from space. At its core is a pair of identical 46 mm gold–platinum cubes separated by 38 cm, which will be isolated from all external and internal forces acting on them except one: gravity.