Question: What do you do with the lasers?

David Robertson answered on 15 Jun 2016:

We use lasers as very good rulers to measure Gravitational Waves.

Gravitational waves are wobbles in space, or more fancily: “ripples in the curvature of spacetime”. They squash and stretch space and everything in it, so you are being squashed and stretched all the time, but by very, very small amounts.

Detecting them is easy – all you have to do is to measure the squashing and stretching of space. You need some very still points, and a very good ruler to measure between them. The only difficult thing is doing all of this well enough. We use a laser to measure the movements of very still mirrors. The laser acts as a very good ruler. You take a single laser beam and split it, send it down two arms to two far away mirror and reflect them back. The returned beams are recombined. Any change in the arms causes a change in the pattern of the recombined beams. This is known as a Michelson Interferometer
https://en.wikipedia.org/wiki/Michelson_interferometer
and is the principal used in eLISA
http://www.elisascience.org
and in LIGO
http://www.ligo.caltech.edu