éther

Æther, also known as ether, is a hypothetical substance once believed to fill all of space. In classical physics, it was thought to be the medium through which light waves propagated, similar to how sound waves travel through air. This concept arose from the observation that waves require a medium to travel. Early experiments, such as the Michelson-Morley experiment, attempted to detect the presence and motion of the Earth through this æther. These experiments, however, failed to find any evidence of its existence, leading to significant challenges to the æther theory.

The advent of Albert Einstein's theory of special relativity in 1905 revolutionized the understanding of light and space. Relativity proposed that light is an electromagnetic wave and does not require a medium to propagate, a concept now widely accepted. The concept of æther was thus largely abandoned by mainstream physics. However, the term æther has seen a resurgence in some pseudoscientific or esoteric contexts, where it is used to describe various non-physical energies or spiritual substances. Within these contexts, its properties and meanings can vary significantly and are not aligned with scientific understanding.