A bit of history: Michelson

When Clerk Maxwell wrote to D.P. Todd of the U.S. Nautical Almanac Office in Washington in 1879, he inquired about the possibility of measuring the velocity of the solar system through the ether by observing the eclipses of Jupiter's moons. Roemer had used measurements of the eclipse times to obtain a number for the speed of light. Maxwell concluded that the effects he sought were too small to measure - but that assertion came to the attention of a young naval instructor named A. A. Michelson who had just been transferred to that office. In 1878, Michelson had made an excellent measurement of the speed of light at the age of 25, and he thought the detection of motion through the ether might be measurable.

Michelson proceeded to invent a new instrument with accuracy far exceeding that which had been attained to that date, and that instrument is now universally called the Michelson interferometer. In trying to measure the speed of the Earth through the supposed "ether", you could depend upon one component of that velocity being known - the velocity of the Earth around the sun, about 30 km/s. Using a wavelength of about 600 nm, there should be a shift of about 0.04 fringes as the spectrometer was rotated 360°. Though small, this was well within Michelson's capability. Michelson, and everyone else, was surprised that there was no shift. Michelson's terse description of the experiment: "The interpretation of these results is that there is no displacement of the interference bands. ... The result of the hypothesis of a stationary ether is thus shown to be incorrect." (A. A. Michelson, Am. J. Sci, 122, 120 (1881))

The proponents of an "ether" as a propagating medium for the light were not ready to give up the idea, and proposed that the Earth dragged the ether along with it in its orbit, thus accounting for the negative result of the interferometer experiment. Lord Rayleigh wrote to Michelson, urging him to repeat the experiment with greater accuracy to test these hypotheses. Michelson, with the collaboration of E. W. Morley, constructed a new interferometer with multiple mirrors and a pathlength about 10 times longer. This device should have given a fringe shift of about 0.4, but they observed less than 0.005 fringe (A. A. Michelson and E. W. Morley, Am. J. Sci., 134, 333 (1887)). Although repeated over the next 40 years with ever greater precision and the same negative result, this 1887 experiment is pointed to as one of the experimental foundations of relativity, and earned Michelson the Nobel Prize in 1907.

The Mysterious Ether

When we reached the point where we could demonstrate that light was a wave, then it was presumed that the wave must have a medium in which to travel. All the other waves we knew about required a medium. Since no medium was apparent between the earth and the sun, it was presumed that this medium was transparent and therefore not readily observable - it was called the "ether". The popular presumption was that this ether was stationary and filled all of space. This involved the presumption that there was an absolute reference frame in the universe, and that all the movement of planets and stars was through this ether.

These presumptions were part of the historical setting of the Michelson-Morley Experiment. With the interferometer which he invented, Michelson found no evidence of the ether, to his and everyone else's surprise. Michelson's terse description of the experiment: "The interpretation of these results is that there is no displacement of the interference bands. ... The result of the hypothesis of a stationary ether is thus shown to be incorrect." (A. A. Michelson, Am. J. Sci, 122, 120 (1881))

The proponents of an "ether" as a propagating medium for the light were not ready to give up the idea, and proposed that the Earth dragged the ether along with it in its orbit, thus accounting for the negative result of the interferometer experiment. Lord Rayleigh wrote to Michelson, urging him to repeat the experiment with greater accuracy to test these hypotheses. Repeated over the next 40 years with ever greater precision and the same negative result, this 1887 experiment is pointed to as one of the experimental foundations of relativity, and earned Michelson the Nobel Prize in 1907.

While proponents for the existence of a medium in space, an "ether", still exist, the standard position is that there is no medium in space. One of the reasons for this position is that there was no direct experimental evidence for the existence of the ether - everything can be explained without it, hence the Ockham's razor approach. Another involves the preposterous mechanical properties required of a medium which supports a wave at 3 x 10⁸ m/s. The velocity of any medium-dependent wave has the nature of the square root of an elastic property divided by an inertial or density property. To support a wave speed of the speed of light would require an incredibly high "stiffness" for space which has near zero density. A medium so tenuous that it produced no detectable drag on the planets which moved through it must yet have an incredibly high restoring force to bring it back to equilibrium once the planet passed.