The more I study the history of aether physics, the more I feel that modern physicists underappreciate both the huge influence the theory had on the development of physics and how it indirectly spurred many positive scientific discoveries, even though it is an incorrect theory. The "aether", for those not familiar with it, was a hypothetical substance theorized in the early 1800s to be the medium in which light waves propagate, just as water waves travel through water and sound waves travel through air. Many papers were written speculating on the nature of the aether before Einstein's special theory of relativity (1905) argued convincingly that the aether was unnecessary.
Nevertheless, these speculations resulted in a number of interesting results. For instance, we have noted previously that Earnshaw's theorem (1839), an important result in electromagnetic theory, arose from an attempt to determine the forces that hold the aether together. In 1902, Lord Rayleigh attempted to detect the aether-induced "length contraction" by measuring the birefringence of moving objects, an ingenious attempt that gave a negative result.
In the broadest sense, a "good" theory is one which raises interesting questions that may inevitably be tested by experiment. Even if it proves to be fundamentally incorrect in the end, it has spurred numerous theoretical and experimental results. This can be contrasted with sham "theories" such as intelligent design (the "theory" that living creatures are too complex to have developed without the aid of a creator), which has resulted in no testable predictions and exists only as a way to push religion into the classroom.
By 1900, the aether remained a vexing mystery, and perhaps the foremost scientific problem, for the physicists of the era. It is not surprising that many famous scientists expended considerable energy to try and elucidate its properties. In 1901, a paper appeared in the Philosophical Magazine (Ser. 6, vol. 2, 161-177) by the famous (even infamous) Lord Kelvin, entitled, "On Ether and Gravitational Matter through Infinite Space." It is not, in fact, an original publication; as Kelvin puts it,
This is an amplification of Lecture XVI, Baltimore, Oct. 15, 1884, now being prepared for print in a volume on Molecular Dynamics and the Wave Theory of Light, which I hope may be published within a year from the present time.
In fact, the article begins with a reprinting of material from 1854, nearly fifty years old! This is, if nothing else, a measure of how baffling the aether was to physicists of the time -- material fifty years old was still, in some sense, "state of the art".
The 1901 paper, as a whole, summarizes Kelvin's theoretical musings on the nature of the aether, and highlights how perplexing the topic remained before Einstein's wonderful theory came along and shattered the aether hypothesis once and for all.