Now that you know how an interferometer works it’s obvious your next question would be to know how sensitive are they ? Here I take the second question first and then move on to answer how sure are we that we have detected a Gravitational Wave. In case you are here for the first time or have no clue about interferometers or Gravitational wave astronomy you might want to read my previous article first. 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… medium.com A bit of History The original Michelson Morley experiment. You can see the mirrors and beam-splitters mounted a the top. The box in the bottom is filled with a pool of Mercury. [Image Courtesy:By Case Western Reserve University — http://www.cellularuniverse.org/AA2MM_Aether.htm , Public Domain, https://commons.wikimedia.org/w/index.php?curid=48697725] Once Maxwell