 Michelson interferometers look like the best chance to detect these waves. You'll recall that we covered the interferometers in the first chapter of this video book. The arms on that one were 11 meters long, and its sensitivity was nowhere near what is needed for gravitational waves. Today, we have LIGO, the Laser Interferometer Gravitational Wave Observatory that has built two identical interferometers 3,000 kilometers apart with one near Hartford, Washington, and the other near Livingston, Louisiana. Here are the L-shaped LIGO instrument components. It has a powerful near-infrared laser with an output after amplification that reaches 200 watts of 1064 nanometer light. The beam splitter and mirrors that act as test masses are 40 kilogram objects suspended via fused silica glass fibers to minimize noise due to vibrations. Additional internal and external active vibration minimization technologies eliminate the effects of everything from nearby traffic to lunar tidal forces. The four kilometer arms are 10,000 cubic centimeters of ultra-high vacuum equal to one trillionth of an atmosphere. In addition, each arm contains reflective mirrors that route the light back and forth inside the arms 280 times before it hits the exits for recombination. The photodetector is a state-of-the-art indium gallium arsenide photodiode array with a high quantum efficiency designed to detect extremely small amounts of light at a wavelength of 1064 nanometers. The laser light is split and sent to the two mirrors. On return, they are recombined and sent to the photodetector. The beams returning from the two arms are kept out of phase so that when the arms are both in sync as when there is no gravitational wave passing through their light waves subtract and no light arrives at the photodetector. When a gravitational wave passes through the interferometer the distance along the arms of the interferometer are shortened and lengthened causing the beams to become slightly out of sync. Hence, some light arrives at the photodetector indicating a signal. Given LIGO's extra 280 passes through the tube a gravitational wave strain amplitude of 10 to the minus 21 would displace the mirrors by 10 to the minus 18th meters. That's one thousandth the diameter of a proton. On our sensitivity graph we see where LIGO's characteristics fit. This is a range where powerful binary system mergers within the Virgo supercluster should be detectable.