Aether no more than a metaphysical construct

Steve Carlip and Philip Gibbs summarize the history of special relativity:

In 1879 it was thought that light must propagate through a medium in space just as sound propagates through the air and other substances. The two scientists Michelson and Morley set up an experiment to attempt to detect the ether, by observing relative changes in the speed of light as the Earth changed its direction of travel relative to the sun during the year. To their surprise, they failed to detect any change in the speed of light.

Fitzgerald then suggested that this might be because the experimental apparatus contracted as it passed through the ether, in such a way as to countermand the attempt to detect the change in velocity. Lorentz extended this idea to changes in the rates of clocks to ensure complete undetectability of the ether. Einstein then argued that those transformations should be understood as changes of space and time rather than of physical objects, and that the absoluteness of space and time introduced by Newton should be discarded. Just after that, the mathematician Minkowski showed that Einstein's theory of relativity could be understood in terms of a four dimensional non-euclidean geometry that considered space and time as one entity, ever after called spacetime.

This refers to Fitzgerald 1889, Lorentz 1895, Einstein 1905, and Minkowski 1908. It ignores the 1899-1905 work of Lorentz and Poincare, where they perfected the Lorentz transformations and applied them to space and time more generally than Einstein, and before Einstein.

But what if we pursued the original theory of Fitzgerald and Lorentz, who proposed that the ether is there, but is undetectable because of physical changes in the lengths of material objects and the rates of clocks, rather than changes in space and time? For such a theory to be consistent with observation, the ether would need to be completely undetectable using clocks and rulers. Everything, including the observer, would have to contract and slow down by just the right amounts. Such a theory could make exactly the same prediction in all experiments as the theory of relativity; but in that case the ether would be no more than a metaphysical construct unless there was some other way of detecting it — which nobody has found. In the view of Einstein, such a construct would be an unnecessary complication, to be best eliminated from the theory.

That is correct about Lorentz aether theory, and it is what Poincare always said. As early as 1889, he said:

Whether the ether exists or not matters little - let us leave that to the metaphysicians; what is essential for us is, that everything happens as if it existed, and that this hypothesis is found to be suitable for the explanation of phenomena. After all, have we any other reason for believing in the existence of material objects? That, too, is only a convenient hypothesis; only, it will never cease to be so, while some day, no doubt, the ether will be thrown aside as useless.

Poincare also said this in his popular 1902 book, which Einstein read with his physicist friends. They say that he was greatly fascinated by it.

Sometimes Einstein fans imply that Lorentz and Poincare had an inferior understanding of relativity because they did not realize that experiments had reduced the aether to an undetectable metaphysical construct. But that is exactly how Poincare described it. Einstein was not as clear about it, and used language similar to Lorentz.

I explain these points in my book.