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How Do the LIGO Detectors Work? How are Gravitational Waves Detected?

What you see above are the instruments that created history on 14 September 2015. In a Galaxy far far away, Billion years ago 2 Black-Holes were locked into a dance till they united as one and all we hear are ripples it created in Space-time.
To hear these ripples all that was needed were the most sensitive and precise instrument on Earth that was conceptualized nearly 5 decade ago by Rainer Weiss and Kip Thorne. After constant perseverance they finally got approval for funding in 1994 by the NSF (National Science Foundation),Construction neared completion in 1997 and the detectors went online in 2002. From 2002–10 the detectors failed to register any Gravitational waves! The detectors were then upgraded from 2010–15 and new and more sensitive detectors called the Advanced-LIGO came into existence that detected the first Gravitational wave GW150814. What looked like an instant shot to fame was actually years of perseverance and confidence of both Scientists and Government in the project.

Interferometery and the Laser Interferometer Gravitational Wave observatory

  
Interference is the phenomenon observed when two waves interact with each other. When the waves interact if they are in their crest phase the resultant wave have has a crest that is summation of the crests of interacting waves and similarly if the troughs interact with crest the wave is nullified. Look at the graphic below.

When two waves are of same kind (have similar frequency,phase and amplitude) travel the same distance through same medium they have the same state (phase). I.e. if 2 similar waves travel a distance of 4 Km, then after 4 Km if one of the waves is in trough state so is the other.We exploit this property of wave along with interference in an interferometer.

Interference Patterns

  
When two beams of light waves interact they seldom create a pattern due the phenomenon of interfernce.
Look at the image above it is a pattern created by interference called Newton’s Ring it is similar to the pattern obtained in an interferometer where dark bands are the region where destructive(trough + crest) interference takes place and the Light bands(trough + trough or crest+ crest) are the region where constructive interference takes place. This pattern remains stable if the interferring waves travel a constant distance in the same medium.
The Interferometer
 

LIGO is esentially an interferometer so understanding the setup of an interferometer can suffice to give you how the LIGO works.
The Laser is our source of light waves that is directed towards the Beam-Splitter. Beam-splitter(a half-silvered mirrior) divides the beam of Laser into two similar beams. These beams travel equal lengths towards the mirriors in an L-shaped configuration where they are reflected and again reach the center to create an interference pattern similar to the diagram above. This pattern is stable till the length of both the arms are same. People at LIGO study this pattern to determine wether a Gravitational Wave has passed.

How Gravitational waves affect LIGO
  
When Gravitational waves passes through any mass it makes it contract in one direction and expand in other. This is due to the Changing intensity of Gravitational field near the object. Even our LIGO detectors are not spared from this phenomenon.


Thus when a Gravitational wave passes from an interferometer one of the arms of the interferometer are elongated and the other is contracted. This also changes the distance the light has to travel in both the arms. Now that the light waves are travelling different amount of distances due to Gravitational Waves there is a change in the interference pattern at the center of the LIGO detector. Voila! this is what indicates that a Gravitational Waves has passed and by studying the distortion it created scinentists are able to estimate the source, eneregy associated and other parameters associated with the Wave.
It is wonderful to think that a phenomnon involving one kind of wave was used to detect another.
One might also think that an earthquake can also cause the arms to change their orientation mimicking a Gravitatonal Wave. Well they are not wrong but their concerns are adeqately addressed.

Watch out for the next article on :-
How sure are we that we have detected a Gravitational Wave? How sensitive is LIGO?



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