(Note: This is an attempt to get myself rolling on my long-ignored series of posts explaining Einstein's theories of relativity. It's also a really cool experiment in the history of science.)
One of the most fascinating aspects of 19th century physics is that many remarkable ideas and ingenious experiments were motivated by a physical hypothesis which we now know to be incorrect: namely, the aether. When light was demonstrated to have wavelike properties in the early 1800s, it was natural to reason that, like other types of waves, light must result from the excitation of some medium: after all, water waves arise from the oscillations of water, sound waves arise from the oscillation of air, and string vibrations are of course the oscillations of string. The hypothetical medium which carries light vibrations was dubbed the "aether", due to its unknown, "aetherial" nature.
A lot of scientists speculated on the physical properties of the aether, and sometimes this speculation produced lasting results in other fields; for instance, Earnshaw's theorem was originally conceived to try and describe the forces involved in the aether's oscillation.
By the late 1800s, however, more and more research cast doubt on the very existence of the aether, notably the Michelson-Morley experiment (to be discussed below). In response, theoreticians produced more and more "patches" to the aether theory, until at last Einstein published his special theory of relativity, which eliminated the need for an aether and in fact suggested that the idea of an aether was incompatible with the experimental evidence.
Before this happened, however, at least one brilliant researcher took up the challenge of testing one of the "patches" to the aether. Lord Rayleigh (1842-1919), distinguished physicist and eventual Nobel Prize winner, conceived of and carried out a very clever optical experiment to see whether objects shrink in the direction of motion, a phenomenon known as length contraction.
As is often the case, even though the experiment was unsuccessful, we can still learn many useful lessons about the workings of science from it!
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