Isaac Newton contended that light is made up of numerous small particles. In addition, Newton rejected light as waves in a medium because such a medium would have to extend everywhere in space, and would thereby "disturb and retard the Motions of those great Bodies" (the planets and comets) and thus "as it is of no use, and hinders the Operation of Nature, and makes her languish, so there is no evidence for its Existence, and therefore it ought to be rejected". Thus, longitudinal waves can not explain birefringence, in which two polarizations of light are refracted differently by a crystal. However, longitudinal waves necessarily have only one form for a given propagation direction, rather than two polarizations like a transverse wave. He and Isaac Newton could only envision light waves as being longitudinal, propagating like sound and other mechanical waves in fluids. Ĭhristiaan Huygens's Treatise on Light (1690) hypothesized that light is a wave propagating through an aether.
According to Boyle, the aether consists of subtle particles, one sort of which explains the absence of vacuum and the mechanical interactions between bodies, and the other sort of which explains phenomena such as magnetism (and possibly gravity) that are, otherwise, inexplicable on the basis of purely mechanical interactions of macroscopic bodies, "though in the ether of the ancients there was nothing taken notice of but a diffused and very subtle substance yet we are at present content to allow that there is always in the air a swarm of streams moving in a determinate course between the north pole and the south". In the 17th century, Robert Boyle was a proponent of an aether hypothesis. The Michelson-Morley experiment, along with the blackbody radiator and photoelectric effect, was a key experiment in the development of modern physics, which includes both relativity and quantum theory, the latter of which explains the particle-like nature of light. A major breakthrough was the theory of relativity, which could explain why the experiment failed to see aether, but was more broadly interpreted to suggest that it was not needed. This led to considerable theoretical work to explain the propagation of light without an aether. The negative outcome of the Michelson–Morley experiment (1887) suggested that the aether did not exist, a finding that was confirmed in subsequent experiments through the 1920s. By the late 1800s, the existence of the aether was being questioned, although there was no physical theory to replace it. As the nature of light was explored, especially in the 19th century, the physical qualities required of an aether became increasingly contradictory. The aether hypothesis was the topic of considerable debate throughout its history, as it required the existence of an invisible and infinite material with no interaction with physical objects.
The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space (a vacuum), something that waves should not be able to do. Luminiferous aether or ether ("luminiferous", meaning "light-bearing") was the postulated medium for the propagation of light. The luminiferous aether: it was hypothesised that the Earth moves through a "medium" of aether that carries light
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