Probably the biggest single misconception I come across in popular discussions of dark matter and dark energy is the accusation that these concepts are a return to the discredited idea of the aether. They are not -- in fact, they are precisely the opposite.I do not agree with this. The main experimental prediction of the aether was that light has wave properties, and has the same properties everywhere.
Back in the later years of the 19th century, physicists had put together an incredibly successful synthesis of electricity and magnetism, topped by the work of James Clerk Maxwell. They had managed to show that these two apparently distinct phenomena were different manifestations of a single underlying "electromagnetism." One of Maxwell's personal triumphs was to show that this new theory implied the existence of waves traveling at the speed of light -- indeed, these waves are light, not to mention radio waves and X-rays and the rest of the electromagnetic radiation spectrum.
The puzzle was that waves were supposed to represent oscillations in some underlying substance, like water waves on an ocean. If light was an electromagnetic wave, what was "waving"? The proposed answer was the aether, sometimes called the "luminiferous aether" to distinguish it from the classical element. This idea had a direct implication: that Maxwell's description of electromagnetism would be appropriate as long as we were at rest with respect to the aether, but that its predictions (for the speed of light, for example) would change as we moved through the aether. The hunt was to find experimental evidence for this idea, but attempts came up short. The Michelson-Morley experiment, in particular, implied that the speed of light did not change as the Earth moved through space, in apparent contradiction with the aether idea.
So the aether was a theoretical idea that never found experimental support. In 1905 Einstein pointed out how to preserve the symmetries of Maxwell's equations without referring to aether at all, in the special theory of relativity, and the idea was relegated to the trash bin of scientific history.
Aether was a concept introduced by physicists for theoretical reasons, which died because its experimental predictions were ruled out by observation. Dark matter and dark energy are the opposite: they are concepts that theoretical physicists never wanted, but which are forced on us by the observations.
Here is what Michelson concluded in his 1881 paper, The Relative Motion of the Earth and the Luminiferous Ether:
The result of the hypothesis of a stationary ether is thus shown to be incorrect, and the necessary conclusion follows that the hypothesis is erroneous.So only a particular aether hypothesis was negated. Lorentz later showed in 1895 that this conclusion was false, and that the experiment was consistent with an aether at rest. Lorentz himself said said in 1895, "It is not my intention to ... express assumptions about the nature of the aether."
This conclusion directly contradicts the explanation of the phenomenon of aberration which has been hitherto generally accepted, and which presupposes that the earth moves through the ether, the latter remaining at rest.
Contrary to Carroll, the aether was not necessarily thought to have a “frame of rest” in the 1800s. See Maxwell's 1878 essay for a survey of aether theories.
Dark energy is the discovery that the ubiquitous and uniform aether has positive energy and negative pressure. We do not know that the energy has a role in the transmission of light, so there may be more than one aether. But it fits right in with the aethers that Maxwell wrote about.
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