This increases the photon count and the sensitivity of the detector to gravitational waves by producing larger phase shifts for a given displacement of the test masses. The detection process depends on interference at the photodetector. If no gravitational wave is present and there are no local anomalies affecting the beams, then the light from each leg will join back at the corner of the L destructively, meaning no light will be seen at the photodetector, the so-called dark port. If a gravitational wave is present, it will stretch the distance between mirrors on one leg of the L and shorten the distance between mirrors on the other leg. This path difference will come together constructively at the corner of the L and exhibit itself as incoming light at the photodetector.
Gravitational waves are extremely weak.
Each leg will be encased in vacuum. The initial detector will have a pressure of 10- 6 torricellis. There are still about 1010 particles per cubic centimeter in the residual gas. The trick in making the vacuum system is not that we are breaking new ground in vacuum technology but rather that we are trying, because of the enormous size of the vacuum system, to do it economically. The engineers need to reduce the natural outgassing rates of the metal and to economically clean the inside of the beam tubes, mainly to keep them free of hydrocarbons, which can contaminate mirror coatings and which also have a larger scattering probability than smaller molecules.