Gravitational Waves

Gravitational waves have been observed. for the first time. LIGO, the Laser Interferometer Gravitational-Wave Observer, detected gravitational waves created when two massive black holes merged over 1.3 billion years ago. Einstein predicted that they could exist, but up until now, we did not have the technology to detect them.


What exactly are Gravitational Waves? Think of them as ripples in space-time. When you drop a pebble in a still body of water, circular waves move out from where the pebble hits. A similar thing happens when an object with large masses on either end rotate. The masses create disruptions in space-time and as the masses rotate, those distortions propagate radially from the center of mass. When the two black holes were colliding, they were rotating about their common center of mass, and the black holes themselves created the ripples. However, the size of these ripples are extremely small. So how did LIGO detect these waves?


When a gravitation wave passes through a point in space, it can distort the point perpendicularly to the direction of travel. In one direction, space is stretched and in the perpendicular direction, space is squeezed. When the next wave passes, the stretching and squeezing are reversed.

LIGO is an interferometer which is a system of telescopes that uses two or more telescopes to act effectively as a single telescope. What LIGO did to find gravitational waves was when one passed by, one arm of the interferometer was longer than the perpendicular arm. However, this difference is so small (1/10,000th the size of a proton), the precise measurements had to be made. That is why it took almost 100 years to detect gravitational waves.