Ether
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 the publication of Maxwell's equations. There was such a strong sentiment that such an ether was at least necessary to define absolute rest that the quest to identify it continued until the start of the twentieth century. In 1887 the experiments of American scientists Albert Michelson and Edward Morley demonstrated the constancy of the speed of light regardless of the direction the light traveled relative to Earth's revolution about the Sun. The findings were ultimately seized upon by German American physicist Albert Einstein to develop the special theory of relativity. Einstein realized that the Michelson-Morley experiments demonstrated that there was no discernible ether or propagation medium to alter the measured velocity of light.
Michelson and Morley devised an experiment that used light interference to measure effects that might result from Earth's motion through the ether. In particular, the Michelson-Morley experiments of 1887 indicated that no ether-related effects were detected. Irish physicist George Francis FitzGerald (1851-1901) and Dutch physicist Hendrik Antoon Lorentz attempted to explain this negative result by postulating that the lengths of objects were changed slightly by their motion through the ether; the problem, however, was not finally solved until 1905, when Einstein published his special theory of relativity. Einstein's theory abandoned the notion of the ether entirely while retaining the Lorentz-FitzGerald contraction effect in a different form.
Postulating that the speed of light is constant for all observers, and that all motion of constant velocity is relative, there was no longer a need for an ether to provide a standard to measure all motion against because the concept of absolute motion was discarded after the emergence of relativity theory.
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