Michelson-Morley experiment

Michelson-Morley interferometer

The Michelson interferometer consists of a diagonal mirror (1) which divides the beam from the lamp in half; one half is transmitted to the mirror (C) and the other reflected to the mirror (D). A glass plate (2) is inserted to equalize the path lengths. The beams then return and recombine towards the left where fringes form. An observer sees both the image of C and D. If the planes of these two are parallel, circular fringes appear. Twyman (1918) modified Michelson's interferometer for looking for flaws in glass prisms and lenses (B). The prism to be tested (3) is placed in the light path. If it has imperfections the resulting image will be distorted.

The Michelson-Morley experiment was an important experiment whose results, by showing that the ether does not exist, substantially contributed to Einstein's formulation of relativity theory.


Its genesis was the development by Albert Abraham Michelson (1852–1931) of an interferometer (1881) whereby a beam of light could be split into two parts, sent at right angles to each other, and then brought together again. Because of the Earth's motion through space, the "drag" of the stationary ether should produce interference effects when the beams are brought together: his early experiments brought no such effects. With Edward Morley he improved the sensitivity of his equipment, and by 1887 was able to show that there was no "drag," and therefore no ether.


Michelson, awarded the 1908 Nobel Prize in Physics, was the first US Nobel prizewinner.