Michelson Experiment

an experiment designed by A. Michelson in 1881 to measure the effect of the earth’s motion on the velocity of light. The negative result of the Michelson experiment is one of the fundamental experimental facts underlying the theory of relativity.

Late 19th-century physicists believed that light propagates in some universal medium called the ether. A number of phenomena (aberration of light, Fizeau experiment) led to the conclusion that the ether is stationary or is carried along to some extent by bodies in their motion. According to the hypothesis of a stationary ether, it is possible to observe an “ether wind” as the earth moves through the ether, and the velocity of light with respect to the earth must depend on the direction of the light beam with respect to the direction of the earth’s motion in the ether.

The Michelson experiment was conducted by means of a Michelson interferometer with arms of the same length. One arm was directed along the earth’s motion, and the other, at right angles to it. As the entire instrument is rotated by 90°, the path difference of the two beams necessarily changes sign, as a consequence of which the fringe pattern must shift. A calculation shows that this shift, expressed as a fraction of the width of an interference fringe, is given by Δ = (2l/λ) (v²/c²), where l is the length of the interferometer arm, λ the wavelength of light used (the yellow Na line), c the velocity of light in the ether, and v the earth’s orbital velocity. Since v/c is of the order of 10^-4 for the earth’s orbital velocity, the expected shift is extremely small and amounted to only 0.04 in the first Michelson experiment. Nevertheless, Michelson became convinced of the falseness of the stationary ether hypothesis solely on the basis of this experiment.

The Michelson experiment was subsequently repeated several times. In experiments that Michelson conducted with E. W. Morley between 1885 and 1887, the interferometer was mounted on a massive slab floating in mercury to ensure smoothness in the rotation of the instrument. The optical path length was increased to 11 m by means of multiple reflections from mirrors. The expected shift Δ was close to 0.4 in this case. The measurements confirmed the negative result of the Michelson experiment. The absence of a stationary ether was demonstrated once again in 1958 at Columbia University in the USA. Beams of radiation from two identical quantum generators of microwaves—masers—were directed in opposite directions, along the earth’s motion and opposite to it, and their frequencies were compared. It was established to an extremely high accuracy (approximately 10^-9 percent) that the frequencies remained identical, although an ether wind would have led to a difference between the frequencies equal to a value nearly 500 times greater than the measurement error.

The negative result of the Michelson experiment could not be understood within the framework of classical physics or reconciled with other phenomena in the electrodynamics of moving media. In the theory of relativity the constancy of the velocity of light for all inertial frames of reference is adopted as a postulate to be confirmed by a large number of experiments.