One of the greatest topics for debate in the late nineteenth century was the nature of light: classical physicists believed that light was an undulatory wave--that it moved through a medium, just as sound does through air and tides through water. They proposed the existence of drifting ether, an invisible substance that filled the universe, and that this ether-wind carried light along with it. A new school of thought disputed the ether-drift theory, claiming that light acted as a particle and required no medium through which to travel. The American physicist Albert Abraham Michelson was a staunch subscriber to the ether theory, and in 1881 he conducted a series of experiments to prove its existence.
Michelson hypothesized that the ether-wind was strong enough to alter the speed of light--a beam of light moving with the current would move faster than a beam moving against it. He built a device that would split a beam of light in two, send the split beams along two different paths of slightly different lengths, and then recombine them. The difference, if any, between the two would reveal whether or not an ether-wind was present. His 1881 experiment showed no evidence of an ether-wind, and so in 1887 he recruited the help of fellow scientist Edward Morley, and they, armed with an improved measuring device, repeated his efforts to detect the ether. Once again, no evidence was found.
The failure of the Michelson-Morley experiment served to unite physicists worldwide against the old ether theory; once it had been accepted that the velocity of light was a constant, new hypotheses were required to explain its nature. Among the prominent spokesmen for the optical revolution was Hendrik Lorentz, whose research on light formed the basis for Albert Einstein's development of the special theory of relativity in 1905.
The other happy accident stemming from Michelson's efforts was the new science of interferometry. The improved device used in the Michelson-Morley experiment was the first interferometer, and Michelson, who had always harbored an intense passion for accuracy in measurement, quickly recognized his invention's potential as a precision instrument.
The principle of interferometry is a simple one: two beams of light split from a single source are bounced along mirrors and joined again at a screen or piece of film. If the two beams have a slightly different path length, the recombined light will be slightly out of phase. This results in a pattern of so-called intereference fringes, which can be analyzed to make very precise measurements of the light and the nature of the object emitting it. Though originally designed to measure the difference in velocity between the two beams, it became evident that the interferometer could be used to measure wavelength down to a single length. A wide variety of applications presented themselves, from measuring very small objects (with an accuracy of less than a millionth of an inch) to determining the size and separation of distant stars.
Interferometry has since become an important tool for scientists, who constantly find new uses for the unprecedented precision it affords. One of the most recent discoveries is the ability of holograms to produce an interference fringe when placed in a system with a comparison wave. This process, called holographic interferometry, provides scientists with the ability to store a wave as a hologram in order to more easily repeat their research, or to save the wave over time--a significant advantage over conventional interferometry.
Because interferometry involves measuring differences in two beams of light directed along slightly different paths, it is easier to perform interferometry on long-wavelength light, such as radio waves, than on shorter-wavelength light. Conversely, if one could do interferometry on shorter-wavelength light, the potential for measuring the distances, positions, and sizes of faraway objects such as stars in enormous. In the late 1980s, an optical interferometer, the Mark III interferometer was constructed at the Mount Wilson Observatory, in Pasadena, California, to make the first attempts at performing interferometry of optical light, which has much shorter wavelengths than radio light. In the late 1990s, the descendant of this instrument, the Navy Prototype Optical Interferometer, was constructed near Flagstaff, Arizona. With this generation of optical interferometers becoming fully functional, interferometry stood in 1998 poised on the edge of a renaissance.
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