ætere

The term "ætere" is an archaic or poetic term for ether, specifically referring to the hypothetical substance once believed to fill the upper regions of space, distinct from the air we breathe. In classical and medieval thought, this celestial ether was considered the element of which the heavens and celestial bodies were made. It was thought to be a highly refined, invisible, and unchangeable substance, unlike the four terrestrial elements of earth, water, air, and fire.

This concept of a luminiferous aether was later revived in the 19th century as a medium for the propagation of light. Scientists hypothesized that light waves, like sound waves, required a medium to travel through. The luminiferous aether was proposed as this universal medium that permeated all of space, even vacuum. Experiments such as the Michelson-Morley experiment in 1887 were designed to detect the Earth's motion through this aether, but famously failed to find any evidence of its existence.

The advent of Albert Einstein's theory of special relativity in 1905 ultimately rendered the concept of the luminiferous aether obsolete. Relativity demonstrated that light, as an electromagnetic wave, does not require a medium for propagation and can travel through the vacuum of space. Therefore, the aether, in both its classical and 19th-century scientific contexts, is no longer considered a valid scientific concept. The word itself persists primarily in historical and literary contexts.