Interferometer

An interferometer splits a single beam of light into two paths, sends them along separate arms, and recombines them. Because light is a wave, the recombined beam shows constructive or destructive interference depending on the difference in path length between the two arms — measured in fractions of a wavelength. A change in either arm's length by a fraction of the laser wavelength shifts the interference pattern, so the device converts a minuscule length change into a measurable change in brightness at the output.

The configuration used in gravitational-wave detection is the Michelson interferometer: a beam splitter divides the laser into two perpendicular arms, each terminated by a mirror, and the returning beams recombine at a 'dark port' photodetector tuned to see blackness when the arms are balanced. A passing gravitational wave stretches one arm and squeezes the other, unbalancing the arms and lighting the dark port. LIGO's 4-kilometer arms, Fabry–Pérot cavities that fold the light hundreds of times, and exquisite isolation from seismic and thermal noise let it resolve a differential arm change of about 10⁻¹⁸ meters — roughly a thousandth of a proton's diameter.

The same instrument has a long pedigree in physics: the 1887 Michelson–Morley experiment used a tabletop Michelson interferometer to search for the luminiferous aether and found none, a null result that helped clear the ground for special relativity. The gravitational-wave detectors are the same idea scaled up by roughly a billion in sensitivity.

Albert A. Michelson built the first such device in the 1880s; his and Edward Morley's 1887 aether experiment is the canonical example. Rainer Weiss adapted the Michelson layout for gravitational-wave detection in his 1972 MIT progress report, leading eventually to LIGO and the 2015 detection of GW150914.