Ether, sometimes spelled aether, is a hypothetical substance once thought to fill all of empty space and act as a medium for the propagation of electromagnetic waves. Ether was also known as the luminiferous ether because of its assumed association with light transmission.

Until advancement of the equations of electromagnetism put forth by Scottish physicist James Clerk Maxwell in the nineteenth century it was assumed that light required a medium of propagation just as sound pressure waves require air, water, or solid matter for propagation. Accordingly, the ether was thought to be the medium that allowed light to move through the supposed empty space between the Sun and Earth. In Newtonian physics the ether was also considered to be the basis for determining absolute rest, with all motion being absolutely defined in relation to such an ether.

The quest to identify an ether did not end abruptly with...