Gravitational waves is the last major prediction of general relativity to be directly observed. We have indirect evidence of gravitational waves through phenomena such as binary pulsars, but so far attempts at direct observations have yielded nothing. Currently the main effort to detect these waves focuses on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which uses laser interferometry to measure tiny shifts in the position of masses. The LIGO uses laser interferometry along a path 4 kilometers long, but even then, the expected distortion would be about a billionth of a nanometer. This is about the same level as the background noise of LIGO itself, so finding a gravitational signal in the noise is difficult at best.
One of the major problems with LIGO is that it is ground based. Any vibration of the ground, such as a truck driving on a road miles away, can cause noise in the signal. A better alternative would be to put the LIGO project in space. This is the idea behind the Evolved Laser Interferometer Space Antenna (eLISA), which would precisely measure the position of masses orbiting in space. While the project isn’t scheduled to launch until at least 2034, tomorrow the first test project will launch. Known as LISA Pathfinder, the spacecraft will put two masses in free fall about 38 centimeters apart.
You might think the spacecraft itself will be in free fall, so what’s the big deal. In this case the two masses need to be completely untouched by the spacecraft. While the masses are in free fall, the spacecraft will adjust its position to stay around them. A laser interferometer housed in the spacecraft will measure the relative positions of the two masses to within a hundredth of a nanometer.
Making such precision measurements in space is a big challenge, and LISA Pathfinder is an important step toward (hopefully) measuring gravitational waves.