It seemed only natural to James Clerk Maxwell and his colleagues that electromagnetic radiation, including light, would consist of vibrations in a lumeniferous (“Light-bearing”) aether. The mechanism was considered analogous to sound which, since ancient times, had been known to consist of vibrations in air and other media. Another assumption was that this aether was stationary or perhaps drifted at a uniform speed and direction with respect to space.
In the late nineteenth century it was known that the earth traveled rapidly through space, its motion a complex result of its orbit about the sun and the sun’s absolute motion. Moreover, because the earth rotates about its axis, an observer at an apparently fixed location would have that motion to factor in as well. Accordingly, the thinking was that if the speed of light is constant, then the transit time for light traveling between two points situated along the direction of motion should differ depending on the direction. It would be possible to measure a perceptible wind speed, so to speak, due to the observer’s motion with respect to the lumeniferous aether.
Two quintessential nineteenth century American researchers, Albert Michelson (theoretician, planner, conceptualizer) and Edward Morley (technician, mechanic, skilled builder of instruments) joined to devise and perform experiments that would definitively measure the aether wind and by implication verify its existence.
Direct measurement of the speed of light back then was not accurate enough to detect the calculated difference in upstream and downstream elapsed travel time. To meet the challenge, Michelson conceived and with Morley built an interferometer consisting of a light source, half-silvered glass plate, mirrors and optics. To isolate the apparatus from ambient vibration and thermal effects, they mounted it on a heavy stone slab that floated in a pool of mercury. It could be easily rotated by hand to change the orientation of the instrument with respect to the linear motion.
The experiment split a beam of light in two, part reflected by and part transmitted through the half-silvered mirror. The now-separate beams went to mirrors at the far ends of two perpendicular arms, whereupon they were reflected back to recombine in an eyepiece. Constructive and destructive interference between the out-of-phase light waves would result in observable fringing.
The degree of interference between the two beams of light was much less than expected. They and other experimenters repeated the procedure with increasingly accurate and vibration-proof instrumentation. In 1887, Michelson wrote:
The experiments on the relative motion of the earth and aether have been completed and the result decidedly negative. The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe – the maximum displacement was 0.02 and the average much less than 0.01 – and then not in the right place. As displacement is proportional to squares of the relative velocities it follows that if the aether does slip past the relative velocity is less than one sixth of the earth’s velocity.
This null result constitutes the most knowledge-altering failed experiment in history. Many expedients were proposed including the notion that the earth was somehow dragging the lumeniferous aether along with it, though this would appear impossible because the same phenomena should be observable at other locations in the universe.
A final attempt to hold the theoretical line was the proposed Lorentz-Fitzgerald contraction (1889), which asserted that the Michelson-Morley problem was, in reality, due to the shortening of an object along its direction of motion. This idea brings new meaning to the word “heuristic!”
It remained for Albert Einstein to find a way out of the Michelson-Morley contradiction. This didn’t happen until the beginning of a new century.