33.8 Interferometers
Scientists and engineers have devised many ingenious methods for using interference to control the flow of light and to make very precise measurements with light waves. A device that makes practical use of interference is called an interferometer.
Interference requires two waves of exactly the same wavelength. One way of guaranteeing that two waves have exactly equal wavelengths is to divide one wave into two parts of smaller amplitude. Later, at a different point in space, the two parts are recombined. Interferometers are based on the division and recombination of a single wave.
LIGO, the Laser Interferometer Gravitational-Wave Observatory, detects cosmic gravitational waves with a Michelson interferometer that has 4-km-long arms. A passing gravitational wave changes the arm length by roughly 1/1000 the diameter of a proton, but modern interferometric techniques can detect this.