Michelson moved his light experiment out of Berlin because horses shook the ground
In 1881 Albert Michelson built an instrument to detect Earth's motion through space by comparing two beams of light. Central Berlin's horse traffic caused too much vibration, so he ran it in the basement of the Potsdam Observatory. He found nothing, a null result that helped pave the way for relativity.
Interferometers exploit the way waves add up. Split one beam of light into two, send them along different paths and bring them back together. If the crests line up, the light brightens; if crest meets trough, it cancels; anything between gives intermediate brightness. The resulting pattern of light and dark bands, called fringes, reveals tiny differences in how far each beam travelled or how much it was slowed by the material in its way. Such instruments are the most precise length-measuring devices in existence, able to gauge the shape of optical parts to within nanometres.
The idea took time to be accepted. Thomas Young described the interference of light to the Royal Society in 1803, after trying the double-opening experiment with ripples in a water tank, but most scientists rejected it because Isaac Newton's particle theory of light still dominated. In France, Augustin Fresnel, unaware of Young, developed a wave theory, and between 1816 and 1818 François Arago built the first interferometer at the Paris Observatory to measure how damp air bends light differently from dry air, a problem for pinpointing star positions.
Young's experiment has since been repeated with single photons, with electrons, and even with buckyballs, carbon molecules big enough to see under an electron microscope. Humphrey Lloyd used a mirror at a grazing angle in 1834 to show that light reflecting off a front surface flips its phase, because the band nearest the mirror came out dark instead of bright.
The same mixing principle appears in radio. The superheterodyne receiver, invented around 1917 and 1918 by Edwin Armstrong and Lucien Lévy, combines an incoming signal with a local oscillator to shift it to a fixed lower frequency that is easier to amplify. Astronomers link separate dishes so that together they resolve detail like a single telescope as wide as the gap between them.
Source: Interferometry
