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Tuesday, October 02, 2012

Data remains, but theories come and go: Luminiferous ether

Project Gutenberg, a wonderful organization, publishes a few public domain books, mostly old ones with an expired copyright, every day. I often check to see what's newly available. Today, the offerings included a title which intrigued me, namely The Ether of Space, by Sir Oliver Lodge. (London and New York: Harper, 1909)

I have copied the last sentence of the book, and the summary:

I regard the non-disturbance of the ether of space by moving matter as established.


The estimates of this book . . . are that the ether of space is a continuous, incompressible, stationary, fundamental substance or perfect fluid, with what is equivalent to an inertia-coefficient of 1012 grammes per c.c.; that matter is composed of modified and electrified specks, or minute structures of ether, which are amenable to mechanical as well as to electrical force and add to the optical or electric density of the medium; and that elastic-rigidity and all potential energy are due to excessively fine-grained etherial circulation, with an intrinsic kinetic energy of the order 1033 ergs per cubic centimetre.

You are probably wondering what on (or off) of earth the "ether of space" might be. The short answer is that there is no such thing.

Why, then, you may ask, was Sir Oliver Lodge, who must have been a man of some importance, since he had received a knighthood, writing about it? Why? Because he (and most of his contemporary physicists) had believed that, if light was a wave, something must be waving -- there had to be a medium to carry the light waves. They postulated the existence of a luminiferous aether, or luminiferous ether, as the material which constituted that medium. The proposal for such a substance, permeating space, goes back at least to Newton. ("luminiferous" means "light-bearing.")

By 1887, the Michelson-Morley experiment had sounded the death knell of the idea -- Michelson and Morley had found no evidence that light traveled faster in one direction than another, whereas, if it was traveling through ether, you would expect that it would travel faster if it was going in the same direction that the ether was. Lodge was well aware of the experiment. He wrote:

The experiment thus seems to prove that there is no motion through the ether at all, that there is no etherial drift past the earth, that the ether immediately in contact with the earth is stagnant—or that the earth to that extent carries all neighbouring ether with it.

 Why not just say that there is no evidence that ether exists, and that we might as well discard the idea? (Other physicists had begun to do so by 1909.) Because Lodge had a mistaken world-view. His data were good and useful. His theory was wrong, or at least unnecessary.

Science changes over time, usually in this manner. An experiment comes along that casts doubt on the prevailing theory, and a few brave souls suggest that the prevailing theory may be wrong. But most of the scientists in that field don't change. Eventually, they become irrelevant, or die, and the replacement theory is accepted. (For more on this, see The Structure of Scientific Revolutions by Thomas S. Kuhn.)

Einstein's view of light required no luminiferous ether, as Newton's had. Newton's data were good, and so were Lodge's. But their theories weren't able to explain everything relevant, and eventually, they were displaced. Einstein's may be, too. We'll see, eventually.

There are parallels in many areas, such as religion and politics, but I'll refrain from going further in those directions. Thanks for reading. If you could, you were using light.

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