Wednesday, April 02, 2014
There Is No Aether Either
Science achieves progress largely through consensus. That consensus is never that we are exactly right but that our assumption is probable. We proceed with it as an accepted theory and dominant paradigm. It's something that the religious right has difficulty with— the lack of absolute certainty and static stability. But it's been a pretty good system for the last several centuries and brought our species a great number of technological advances. But very often to step forward, we must discard an erroneous assumption. In 1887 the Michelson–Morley experiment proved that there was no aether. Hertz discovered radio waves in 1888. the timing was no coincidence. More here.
In the case of "luminiferous aether" this was a medieval, pseudo-scientific alchemists idea that had less evidence behind it than dark matter. In the 17th century they called it æther, ether, aether wind, and even an "aethereal Medium." The æther was descended from the idea of the classical elements like earth, air, water and fire. It was referred to in Plato's dialogs "there is the most translucent kind which is called by the name of aether." He considered it a sub-type of air. But that was more than 400 B.C.!
Sadly we were still in the depths of that aether idea more than two thousand years later. In the 1880s Sir Oliver Lodge and James Clerk Maxwell were waxing poetic about the damn aether. Maxwell believed that the propagation of electric and magnetic waves required a physical medium like waves in water. He discussed his scientific basis for aether in two papers in particular. In 1856, he published the paper On Faraday's Lines of Force and in 1862 he followed it up with On Physical Lines of Force which suggested that light, electric fields and magnetic fields could be explained in a unified electromagnetic theory. In 1864 he wrote a paper debuting that theory: A Dynamical Theory of the Electromagnetic Field. He downplayed the aether in the third one and focused on the waves movement through space.
Michelson and Morley put it to rest in 1887. If the aether was real it was thought that it should be possible to measure it's effects or motion. But since it was assumed to move at the speed of light it was also assumed it's perturbations would be also impossible to measure. Maxwell pointed out in 1878 that it would have to be inferred from second order effects. In 1881 Albert Michelson used a half-silvered mirror to split a light beam and recombined them in an eyepiece to examine transverse displacement. He assumed that a beam reflecting parallel to the flow of aether would take longer than a beam reflecting perpendicular to the aether. His results were negative. He took this as a confirmation of Stokes' hypothesis of complete aether dragging. There is a lot more detail on this in the book The Ethereal Aether by Loyd Swenson.
In 1885 Michelson began working with Ed Morley at Western Reserve University to improve on the 1881 experiment. A similar experiment was constructed but with extreme efforts applied to reduce vibration. it sat on a block of sandstone in a trough of mercury (above). They rotated the interferometer trying to find the flow of the aether. There was nothing. Instead of reporting the null result Michaelson reported the observation of one-fortieth of the expected displacement. but more importantly that this was within the range of experimental error that would allow the speed to be zero. Others confirmed the null result and thus radio waves were free to travel through a vacuum when Hertz discovered them the following year.