Question: How an scientists discover gravitational waves and what evidence is needed?

Steve Marsden answered on 9 Mar 2016:

Great question! A little background on gravitational waves. They are emitted from huge celestial events, like two black holes colliding. The waves then travel outwards at the speed of light. As they pass over a region, they cause the space to contract and expand in a peculiar manner. By measuring the distance in two directions and comparing it, you can measure how much the space is being warped by the waves.

The two distances measured were both about 4 km long, and measured using laser beams. The change in distance caused when the gravitational wave passed over was less than the size of an atom!

There were two of these experiments set up on opposite sides of north america. This was done in order to eliminate any local sources for the change in distance. (For example a lorry driving past could potentially cause enough vibration in the ground). The same warping of space was seen in both experiments, meaning that it had to be due to a gravitational wave passing through them both.