I thought it’d be nice, for people who wanted to delve a bit more into the science, if I said a little bit more about the stories behind the tracks on “…Songs about Space…”. This is my simplified version of the Luminiferous Aether story -  if there are any astronomers reading who want to point out factual inaccuracies, they are most welcome to do so†.

At the end of the 19th century, James Clerk Maxwell had successfully created equations describing the wave motion of light, confirming the observations and theories of Christaan Huygens two centuries earlier. Scientists naturally wanted to know what the medium or material was that supported these waves; water waves travel through the sea, sound waves through air (or other fluid) and surely light must be the same? This hypothetical medium that supports the motion of light was named the luminiferous aether*, and would have some odd properties. For example, it must be completely invisible, or light would be refracted or absorbed by it rather than travelling freely. It also must be very widely present in the universe, because everywhere we look we see light propogating – but it cannot interact with matter (atoms and so on) or we would sense its effects on the things we see around us. So how do you detect something invisible and intangible?

Physicists Michelson and Morley designed an experiment to answer this question and detect the luminiferous aether. They used a device called an interferometer. Their interferometer worked by taking a beam of light and splitting it in two. Each beam then travelled along a separate ‘arm’, and were recombined at the end. Because of the wave nature of light, the recombined light is very sensitive to things like how far the light in each arm had travelled and how fast; any differences would produce interference in the final signal, hence interferometer.

Michelson and Morley ensured that both arms were exactly the same length, so if the speed of light is the same in both arms, there would be no interference. Even very small changes in the speed of light, however, would produce detectable interference. They reasoned that as the earth revolves around the sun, we must be experiencing some sort of motion through the aether, which varies with time. If the aether is ‘flowing’ in the direction the light is travelling, it will tend to slow down or speed up how quickly the light travels, and the interferometer will be able to detect that change. They designed their experiment so that one arm would lie in the direction of motion, and be subject to the ‘aether wind’, and the other arm perpendicular and would not be affected by the motion of the earth through the aether.

Without delving any further into the details of the experiment (which gets more technical and maths heavy from here), we can cut straight to the result – there was no detected change of the speed of light as a result of our motion through the aether. This effectively disproved the aether theory, although many people have tried unsuccessfully to find it with successively more sensitive equipment. More interestingly, the Michelson-Morley result is closely allied to the observation that ‘the speed of light is the same, independent of observer motion**’, the fundamental tenet of Einstein’s Special Theory of Relativity, and the one which leads to all the wacky stuff with trains and clocks. This makes the Michelson and Morley result one of the great null results in scientific history.

†The full explanation of how the interferometry experiment worked is rather in-depth, so I have chosen not to elucidate it here. Wikipedia and the netwebs in general has more detailed explanations if your appetite is whetted…

*I’m afraid I mis-spelled it in my song. It should have been Luminiferous Aether and NOT Lumineferous Aether. They sound about the same out loud.

**In other words, no matter how fast you’re going, and no matter what direction the light is arriving from, it always appears to be travelling towards you at three hundred thousand km per second (approx). There are all sorts of caveats relating to inertial frames, but you can look those up if you’d like to find out more about Special Relativity and, indeed, general relativity, Einstein’s theory of gravitation.