Lord Kelvin vs. the Aether! (1901)

I have a few questions about the aether, and what it means in terms of density. The aether seems to defy everything that I find rational about the Universe, especially in terms of density. The fluid of the aether is deemed to be the most solid substance in the Universe, yet it passes through matter virtually unaffected - as if we were barely here. The following exerpts are taken from a great little essay I read on a blog named "Skulls in the Stars". The author's summary has managed to pretty much sum up some of my own frustration in trying to understand aether theory.

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.William Thomson, aka Lord Kelvin (1824-1907) is one of those curious physicists whose name is everywhere, but whose exact achievements in science are hard to pin down. The reality is that his influence can be found in almost every aspect of 19th century physics, and often made very subtle but fundamental contributions to the foundations and methodology of physics. An excellent biography of Thomson and his work can be found at PhysicsWorld, though it requires a (free) registration to read.

He is perhaps most known for his contributions to thermodynamics. When Thomson approached the subject, most physicists believed that heat was a physical substance, dubbed “caloric”. James Joule, another great of the era, was a lonely champion of the idea that heat is the result of the motion and vibrations of atoms and molecules, and the inevitable conclusion that there exists an absolute minimum of temperature (“absolute zero”). His work was mostly ignored by the community until Thomson heard one of his talks in 1947. Here I quote the PhysicsWorld article,

But Joule was not wrong, and Thomson – through careful thought – came to agree with him. Along the way, he connected Joule’s work with that of Carnot on heat engines. In doing so, he devised a more fundamental way of defining the absolute zero of temperature, independent of any particular material substance. It is for this reason that the fundamental unit of temperature was later called the Kelvin – the name Thomson adopted after being made a Lord in 1892. Thomson also saw the idea of conservation of energy as a great unifying principle in science, and introduced the ideas of “statical” and “dynamical” energy – or what we now call potential and kinetic energy.

It is difficult to disentangle Thomson’s work on heat and the conservation of energy from that of other scientists of the time, including Clausius, Helmholtz, Joule, Liebig and Rankine. All of them can take some of the credit for the first and second laws of thermodynamics – ideas that are so important to modern science that each contributor should be held in high regard.

Thomson played a large role in establishing the formulation of physics in terms of energy! This gives some idea of what I mean when I say that Thomson made subtle but fundamental contributions. He did not invent the idea of conservation of energy, but was instrumental in shaping its use and emphasizing its importance in all physical problems.

The PhysicsWorld article also suggests that it was Thomson, in correspondence with Stokes, who actually first stated the fundamental result of vector calculus known as Stokes’ theorem. He even proposed one of the first unified theories of atomic structure, proposing that atoms are in essence swirling vortices in the aether (I’ll have to post about this again in the near future).

By Lord Kelvin’s estimate, the hypothetical aether is orders of magnitude less dense than hydrogen gas, which is a real problem because the aether was also assumed to be a solid material; liquids and gases cannot support transverse waves.


The one intellectual leap that Lord Kelvin fails to make, however, is to doubt the existence of the aether itself. As we have noted, Kelvin’s conclusions make for a very strange material: it is a solid, but incredibly less dense than hydrogen gas; it has inertial mass, but not gravitational mass; it is a massive solid that does not interact with ordinary matter. With hindsight, these conclusions scream out against the existence of the aether, and it is telling that such a brilliant scientist such as Lord Kelvin did not realize that something was wrong. The notion of the aether was so ingrained in the minds of the physicists of the time that they never even considered questioning it.

Kelvin’s musings do illustrate nicely that the evidence was piling up against the aether; in only a few short years, in 1905, Einstein would, in essence, find the smoking gun against it.
http://skullsinthestars.com/2009/12/29/lord-kelvin-vs-the-aether-1901/