Albert Einstein died on April 18, 1955 (exactly 50 years ago). His papers on (Special and General) Relativity had a huge impact on 20th century physics and philosophy. One of the predictions of this theory relates to Gravitational Waves, which are ripples in space-time, caused by motions of matter. Imagine a ball on a rubber sheet, causing the sheet to deform. As the ball moves on the sheet, the deformation of the sheet changes. This change in deformation (ripples) will be felt by say, an ant standing on the sheet some distance away! Similarly, motions of stars and planets (any mass for that matter) should create ripples in 3D space, and should be detectable by sensitive instruments.

Proof mass on one craft (Courtesy: Max-Planck Institute)The joint ESA-NASA LISA (Laser Interferometer Space Antenna) mission hopes to detect gravitational waves in space. To be launched in 2013, the LISA mission will use laser interferometers - very sensitive tools to measure tiny variations in the distance between objects – and proof masses on board three spacecraft flying in formation. The three crafts will be flown in a triangular formation, each 5 million kms away from the other two. The setup will fly some 50 million kms from the Earth, so as to not get affected by Earth's gravity.
Infrared lasers will be beamed between the spacecraft, arriving on small 2-kilogram proof masses, 4-centimetre cubes made of Gold and Platinum. As the setup flies through gravitational waves (created by the blast from the Big Bang, by Earth, Sun, the planets, and so on), the lasers will fluctuate as the triangle gets deformed (imagine a triangle moving on a curved surface). The fluctuations will be measured, and the measurements relayed back to Earth.
Since gravitational waves are extremely weak (gravity is the weakest force in nature), extreme sensitivity is required. For example, the lasers need to measure fluctuations of an order of a 10th of the radius of an atom! The setup will also be able to measure the direction from where the waves are coming, which will help it locate perhaps some dark matter and other exotic sources.