An interferometer is a device constructed to split a beam of light into two perpendicular beams and then to bring the two beams together again. Any difference between the beams creates a pattern of interference that manifests itself as a group of bright spectral lines called fringes. Originally designed to measure the fluctuations in the velocity of light, interferometers are now widely used in spectroscopy, chemistry, and precision metrology and inspection.

In 1881, Albert Michelson (1852-1931) was conducting experiments to determine the speed of light. Trained in classical physics, Michelson was a firm believer in the existence of an aether-wind; at that time, light was imagined as an undulating wave, like ripples on a body of water. Just as those ripples need water to move through, light also required a medium for travel, which scientists at the time called the aether.

The intent of Michelson's experiment was to detect...