When the laser beams collide with the mirror and return to the base, the light wave aligns in such a way that they cancel out each other. Because of this, nothing is detected on the rear photodetector. That means normally the photodetectors present in the LIGO do not detect anything.
But when Gravitational Waves arrives on earth, it disturbs the space. Due to which there is a continuous difference happening in the length of both perpendicular tubes for some time.
Sometimes the first tube becomes slightly longer and the second shorter, sometimes the second tube becomes slightly longer and the first slightly shorter. This goes on until the gravitational wave passes through the earth and transcends it.
The instrument of LIGO is so powerful that if there is any difference between the one thousand parts of a proton in both perpendicular tubes, then they also detect it.
Due to changes in the length of both perpendicular tubes, the alignments of light waves are no longer the same, for which they cannot cancel out each other. As a result, some light reaches in the photodetector and they are detected.
The intensity of this light depends on the difference in the length of the perpendicular tubes. Measuring this intensity means measuring the Gravitational Waves.
In this way, scientists measure gravitational waves. The gravitational wave that the LIGO has the ability to detect originates from events where large amounts of energy are released.
For example - the collision between two black holes, the explosion of stars, and even the birth of the universe itself. That is, by studying the gravitational waves, we can know a lot about the universe.
Till now we have been using only Visible lights, X-rays, Radio waves, Microwaves, etc to uncover the mystery of this universe. However, for some time, we have also started using sub-atomic particles such as neutrinos to solve the mystery of this universe.
In such a situation, gravitational waves can prove to be a very unique discovery. Because gravitational waves are not electromagnetic radiation, a completely different phenomenon that not only helps us to understand gravity better but also tells us about the behavior of the black hole and other objects in this universe that is still mysterious to us.