However, longitudinal waves by necessity have only one form for a given propagation direction, rather than two polarizations as in a transverse wave, and thus they were unable to explain the phenomenon of birefringence (where two polarizations of light are refracted differently by a crystal). The main reason for his rejection stemmed from the fact that both men could apparently only envision light to be a longitudinal wave, like sound and other mechanical waves in gases and fluids. He wrote that "I do not know what this Aether is", but that if it consists of particles then they must be "exceedingly smaller than those of Air, or even than those of Light: The exceeding smallness of its Particles may contribute to the greatness of the force by which those Particles may recede from one another, and thereby make that Medium exceedingly more rare and elastick than Air, and by consequence exceedingly less able to resist the motions of Projectiles, and exceedingly more able to press upon gross Bodies, by endeavoring to expand itself."Ĭhristiaan Huygens, prior to Newton, had hypothesized that light itself was a wave propagating through an Aether, but Newton rejected this idea. The modern understanding, of course, is that heat radiation is light, but Newton considered them two different phenomena (believing heat vibrations to be excited "when a Ray of Light falls upon the Surface of any pellucid Body"). Newton believed that these vibrations were related to things like heat radiation, saying: Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission? In order to explain refraction, in fact, Newton's Opticks (1704) postulated an "Aethereal Medium" transmitting vibrations faster than light, by which light (when overtaken) is put into "Fits of easy Reflexion and easy Transmission" (causing refraction and diffraction).
This theory was known to have its problems although it explained reflection well, its explanation of refraction and diffraction was less pleasing. Isaac Newton had assumed that light was made up of numerous small particles, in order to explain features such as its ability to travel in straight lines and reflect off surfaces. The word "aether" stems via Latin from the Greek αἰθήρ, from a root meaning "to kindle/burn/shine", which signified the substance thought in ancient times to fill the upper regions of space, beyond the clouds. Later theories including special relativity were formulated without the aether concept, and today the aether is considered to be a superseded scientific theory. In the late 19th century luminiferous aether ("light-bearing aether") was the term used to describe a 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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