Wednesday, 31 January 2007

Relativity 3: The Lumeniferous Aether

Continued from –

Relativity 1: The Speed of Light,-by-James-OFee.html

and Relativity 2: Particle or Wave?

The last time I mentioned how opinion in the nineteenth century solidified on the theory that light was a wave. Waves normally consist of vibrations in a medium. Two people holding a rope between them can create a wave if one pulls the rope rhythmically up and down. Waves in the sea come about by water particles tumbling in a circle. And sound occurs when air particles vibrate. Pump out the air to create a vacuum, and the sound disappears.

So what is the medium that transmits light? Scientists christened the supposed medium the ‘Lumeniferous Aether’, or simply the ‘Aether’.

‘Lumeriferous’ = producing or transmitting light; ‘aether’, from Greek aither, the root of aitho = to burn, shine. Later the word became written as ‘ether’, although it then ran the danger of confusion with the chemical widely used as an anaesthetic.

It was not possible to observe the aether directly – its existence was inferred from the behaviour of light. Scientists are familiar with the process of inference, and through it there have benn many famous scientific discoveries. For example, the planet Uranus was discovered through observing perturbations in the orbit of Neptune, for which, it was inferred, an unknown planet was responsible.

Nonethless, as the nineteenth century proceeded it became evident that the aether needed to be a very odd substance, with some remarkable properties. The aether had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. It had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale. Furthermore, in 1864 James Clerk Maxwell published his remarkable equations, which showed that light was an electromagnetic wave, but they carried the implication that the aether must be "still" universally, otherwise the speed of light ‘c’ would vary from place to place.

In 1887 there was carried out one of the most famous experiments in the history of science. The ‘Michelson-Morley’ experiment

was the culmination of co-operation between two notable scientists. For his contribution, Michelson received the Nobel Prize in 1908. Today the experiment is considered to represent the first strong evidence against the existence of the aether as traditionally believed.

I’ve mentioned before the amazingly sensitive phenomenon of interference. Michelson and Morley split a beam of light into two parts, which they sent at right angles to each other over long distances. Had the earth been travelling through the aether, the effect should have been to produce interference patterns when the two half-beams were combined. None was ever observed, in spite of the increasing sophisticaltion of the apparatus. In 1887 Michelson and Morley announced their conclusion. It was the most famous ‘null result’ in the history of science.

Even after the experiment, Michelson and Morley continued to work in the field, yet the Aether Hypothesis continued to hold sway during the remainder of the century – there appeared no alternative. And there was one important effort to ‘save the appearances’ and explain the Michelson-Morley result while preserving the aether hypothesis.

The Fitzgerald–Lorentz contraction proposed that all objects physically contract along the line of motion relative to the aether, so while the light may indeed transit slower on that arm, it also ends up travelling a shorter distance that exactly cancels out the drift. The factor proposed by Lorenz was [1- (u/c)**2]**(-1/2) where u is the velocity of an object relative to the observer, and c is the speed of light.

Despite Lorenz's ingenious contribution, the aether hypothesis was fatally damaged and ready for a blow which would dispatch it forever. This came from a quite unexpected quarter, but the breakthrough would revolutionise the way man thought about the universe.

To be continued

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