AKA: Wibbly, insubstantial space gas that is stronger than steel
Theory born: 4th century BCE
Parent(s): Aristotle
Description: The birth of the idea of aether was not due to a desire to explain the behaviour of waves. Rather, it was an attempt by Aristotle to explain the separation of the world into layers of elements. He proposed that aether was an element lighter than air (which is it naturally rose to the heavens); water was denser (explaining why rain fell through the air) and earth was denser still (explaining why rocks sank in water).
Aristotle
In the late 17th century, Isaac Newton, Robert Boyle and Christiaan Huygens were all grappling with ideas of propagation of forces and energy through a vacuum (Newton and Huygens considering the transmission of light and Boyle the action at distance of forces like magnetism). All agreed that there must be Although Huygens and Newton would eventually disagree on the form of light (Huygens believing light to be a wave; Newton believing it to be made of particles or 'corpuscles'), they agreed that there must be a medium for light to propagate through. This conclusion made sense for Huygens to reach, given scientific understanding at the time: sound required air as a medium for the disturbance to pass through and ripples required water, Newton's belief in aether was more of a fudge to compensate for the failure of his own theory to explain refraction (why exactly would a particle of light change its direction moving from air to glass, for example?)
Wave or particle?
However, it was the properties that aether would require to fulfil this role that would eventually lead to its collapse: it had to be rigid to electromagnetic radiation such as light (allowing light to propagate through it), fluidic (to fill space), massless and with zero viscosity (so there would be no drag on planets as they orbited the sun - essentially it had to be transparent to matter), incompressible and non-dispersive. A magical substance, in essence. With so many predictions as to its properties, it would not take much evidence to bring the whole theory crashing down.
Theory death: circa 1887
Cause of theory death: A null result in the Michelson-Morley experiment.
It's difficult to pinpoint exactly when luminiferous aether died. As more evidence about the nature of light was gathered, the properties of aether became more and more fantastical. For example, as the tiny wavelengths of light became clear in the 19th century, calculations revealed that aether, if it existed, would need to have rigidity millions of times that of steel. It is, however, generally agreed that the death blow was dealt by the Michelson-Morley experiment in 1887, even if the theory itself managed to stagger on a while longer.
By this point, two competing theories were in vogue: that the motion of the Earth partially dragged aether behind it or that it completely did (that there is relative motion between Earth and aether or that there is not). Michelson (and later Morley) set out to discover which was the case by investigating the effect (or not) of 'aether wind' on the behaviour of light (if the Earth and the surrounding aether were moving at different speeds, there would be a wind-like effect in a particular direction).
In essence, the first experiment by Michelson (in 1881) involved splitting a light ray into two beams and then bouncing those beams backwards and forwards along paths at right-angles to each other before recombining them. This should have lead to one beam of light travelling in parallel with the direction of the aether wind and the other travelling perpendicular to it.
The Michelson-Morley experiments
The beam travelling parallel to the aether wind - into the wind - should have taken longer to complete the journey than the one that was not. When the rays recombined, there should have been a noticeable interference pattern caused by the shifting of these rays. There wasn't. It seemed then that the Earth wasn't dragging aether behind it - there was no 'aether wind'. A second experiment in 1887 (Michelson and Morley this time) with significantly more sensitivity proved this yet again. Partial aether dragging did not exist.
So the aether had to be completely moving along with the Earth, right?
The problem was that in 1886 Michelson and Morley had repeated an experiment by Hippolyte Fizeau looking into the behaviour of light waves passing through flowing water. In brief, light passing with the flow of water (and the aether being dragged with the water) should have been accelerated and light passing against the flow (and against the aether being dragged with the water) should have been decelerated. It was predicted that the speed of light would be a simple sum of the velocity of light and the velocity of the flowing medium. This turned out to be true, albeit with some variations that would remain unexplained until Einstein. It strongly matched theoretical predictions that aether was being completely dragged along with the Earth.
Simplified diagram of the Fizeau Experiment
(bs - beam splitter, m - mirror)
Suddenly, there were two incompatible sets of experimental data about the aether - it both was and was not moving relative to the Earth - and the field of study was thrown into absolute chaos as these irreconcilable results lead to more and more complicated models being suggested to resolve them.
Ultimately, the very final gasp of the theory of luminiferous aether came with Einstein's special theory of relativity in 1905, which managed to explain all the experimental results seen thus far, but without needing to go to all the bother of inventing space gas with ludicrously magical properties.
Occam's Razor strikes again.
Occam's Razor: good for science and for shaving
Attribution
Michaelson-Morley experiment image - Stigmatella aurantiaca, Wiki Commons
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