The Michelson-morley Experiment
The observation and explanation of interference fringes dates back to Robert Hooke (1635) and Isaac Newton (1642-1727), but the invention of interferometry is generally attributed to the American physicist Albert Michelson (1852-1931). The Michelson interferometer consists of two perpendicular arms with a half-silvered mirror (a beamsplitter) at the intersection (see Figure 1).
Each arm has a mirror at one end. Light from the source enters the interferometer along one arm, and is equally split at the beamsplitter. Half the light travels to mirror #1 and reflects back toward the beamsplitter. It passes through the beamsplitter and continues to the detector, which can be like a motion picture screen. The other half of the light first travels straight through the beamsplitter, reflects off mirror #2, and then returns to the beamsplitter, where it is reflected and sent to the detector. When the two beams combine at the detector, they interfere and produce a pattern of fringes that depends upon the path they traveled and the time it took them to travel this path.
In 1887, Michelson, along with the physicist Edward W. Morley (1838-1923), set out to determine if the speed of light was dependent on the speed of the observer. According to the accepted theory of the time, light had to propagate in a medium called the ether. The motion of the earth traveling through the ether would affect the fringe pattern on Michelson's interferometer, because it would take the light longer to travel over one arm than the other arm. When the interferometer was rotated through 90°, the fringes would shift if the speed of light was not constant. In this most celebrated null experiment in the history of science, Michelson and Morley observed no changes in the fringes after many repetitions. The speed of light appears constant, regardless of the speed of its source, which was later explained by Einstein's theory of relativity.