C. State-of-the-art of LIGO
Why twin detectors
The LIGO “observatory” is made up of two identical and widely separated interferometers located in two uninhabited places: LIGO Hanford in shrub-steppe region of southeastern Washington State ; and LIGO Livingston, 3002 kilometers away in a forest east of Baton Rouge,Louisiana.
" LIGO was designed with two detectors so far apart for good reason. LIGO’s detectors are so sensitive that they can 'feel' the tiniest vibrations on the Earth from sources very nearby to sources hundreds or thousands of miles away. Things like earthquakes, acoustic noise (e.g., trucks driving on nearby roads, farmers plowing fields, things that people can hear and feel), and even internal laser fluctuations can cause disturbances that can mask or mimic a gravitational wave signal in each interferometer. If the instruments were located close together, they would detect the same vibrations at the same times--both from Earth-sources and from gravitaitonal waves and it would be nearly impossible to distinguish a vibration from a gravitational wave from the local noise."\cite{7wxnuuc}
Operation
When a gravitational wave passes through the interferometer, the spacetime in the local area is altered. Depending on the source of the wave and its polarization, this results in an effective change in length of one or both of the cavities. The effective length change between the beams will cause the light currently in the cavity to become very slightly out of phase (antiphase) with the incoming light. The cavity will therefore periodically get very slightly out of coherence and the beams, which are tuned to destructively interfere at the detector, will have a very slight periodically varying detuning. This results in a measurable signal.\cite{thorne2016}
The enhanced and advanced LIGO project
Initial LIGO started to detect gravatational waves in 2002
During 2010 to 2015, LIGO since its first edition. Its sensitivity is four times of the time when it was built.