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A hypothetical medium once believed to be necessary to support the propagation of electromagnetic radiation. It is now regarded as unnecessary and in modern theory electromagnetic radiation can be propagated through empty space. The existence of the ether was first called into question as a result of the Michelson–Morley experiment. In Kaluza–Klein theory the fifth dimension of space–time corresponds to the ether.

Subjects: Astronomy and Astrophysics — Physics.

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