"What is there in places empty of matter?" asked Isaac Newton in 1706. Nature abhors a vacuum, and so do people. For as long as we have considered the universe, we have sought to fill its emptiness.
In the fourth century BC, Aristotle proposed "aether" as a name for the fifth element postulated by his teacher Plato. Aether, or ether, made up the worlds of the outer celestial spheres believed to surround our own.
By the middle of the 17th century, René Descartes was using the word to describe the medium of space. Vortices in this aether, he felt, were responsible for the accumulation of the particles that formed matter and shaped solid objects from pebbles to planets.
Later, the word encompassed a number of related ideas about the subtle matter keeping the stars and planets in place. Many 18th-century astronomers would invoke the aether to account for the variations and discrepancies in the motions of celestial bodies, or the way light travelled through space.
Newton's version of the aether - a strong, subtle and elastic medium - was responsible for what we now recognise as gravity and electromagnetism, as well as our own physical movements and sensations. He wondered if this aether wasn't a living force - essentially, spirit - though he knew he could never prove this to be the case.
The aether was still very much alive in the 19th century. Following James Clerk Maxwell's unification of electricity and magnetism, visible light was revealed to be another wavelength in the electromagnetic spectrum. Maxwell's "undulations" still needed a medium through which to travel - emptiness was not an option. The Luminifeous Aether was proposed to answer that need.
For the most part, the aether vanished after the Michelson-Morley experiment of 1887, which unsuccessfully sought to record "aether wind" as a measurable effect of the Earth moving through the ether. Their failure was taken as proof the aether had never existed.
Subsequently, modern theories of subatomic physics and Einstein's space-time continuum have relegated the aether to the domains of mysticism and pseudoscience. Its spirit lives on in dark energy, a modern manifestation of this most ancient idea.
