According to general relativity, massive objects warp space and time around them, like a bowling ball dropped onto a sheet of rubber, causing passing objects, and even light, to travel along curved paths. When two extremely dense objects, such as neutron stars (stars so dense the protons and electrons in their atoms collapse to form neutrons) or black holes, orbit each other in binary pairs, their interactions should create ripples in the fabric of space-time called gravitational waves. The most powerful waves would arise when two neutron stars or black holes merge together.
Those waves should be detectable by experiments called Advanced LIGO (Laser Interferometer Gravitational Wave Observatory) and Advanced Virgo, set to come online in 2017. They will each use giant L-shape detectors in Louisiana, Washington and Italy to search for minute changes in the lengths of the detectors' arms caused when gravity waves pass through. At the corner of the "L," a laser is split into two beams that travel back and forth down the length of the two arms (each between 1.2 miles, or 2 kilometers, and 2.5 miles, or 4 kilometers, long), bouncing off mirrors at each end. If a gravitational wave passed through, it would stretch and compress these lengths, depending on its orientation, creating a small but detectable difference in the length of the two arms.