The Aether is dead!  Long live the Aether!  

Since ancient Greece until the mid 20th century, thinkers regarded light as the excitation of an invisible mist, called the Aether.  Aether was also the name of a primordial god of light for the Greeks.  In Homeric dialogue, the term Aether described pure, fresh air and clear sky.  For Plato, aether was the fifth element (the other four are earth, air, water, and fire).  And unlike the other elements, aether was only capable of local motions, naturally moving in circles.  Aristotle imagined that the celestial spheres, which he suggested responsible for the orbital motion of the planets, emerged as structures of the aether.

In physics, the luminiferous aether, thought responsible for conveying light and invisible-actions-at-a-distance like gravity and magnetism, held sway for several thousand years until the rather recent epoch of Einsteinian relativity.  Much of the contemporary theoretical knowledge that we depend upon today concerning electromagnetism and even classical representations of gravity, is founded upon the notion of aether.  Indeed, Newton, Faraday, Maxwell and so many others embraced the concept as a medium of action in their physical theories.  When the aether was finally abandoned for relativity in the 20th century, Einstein himself protested that his new mathematical conception was simply a new kind of aether theory:

“To deny the aether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view.”

Lorentz, Eddington, and other relativists agreed.  Their ascription of physical properties to the space between bodies, after all, seemed a re-framing of the traditional concept of aether.  So why, then, has the aether been summarily abandoned?  It appears Einstein made a serious departure from the aether-of-old in one critical aspect:  Whereas the historic aether was a static tapestry, upon which all material processes unfolded, the aether of general relativity is a more responsive, complex material.  Einstein’s aether is not static but rather, is organized reflexively by the matter of our world.  This forms a sort of feedback loop in the dynamics of moving bodies.  Einstein attributes this transformation in his thought to Ernst Mach:

“Mach’s ether not only conditions the behaviour of inert masses, but is also conditioned in its state by them.  Mach’s idea finds its full development in the ether of the general theory of relativity.”

Einstein continues, 

“What is fundamentally new in the ether of the general theory of relativity as opposed to the ether of [the past] consists in this, that the state of the former is at every place determined by connections with the matter and the state of the ether in neighbouring places…”

It seems clear that Einstein did not mean to completely remove a mediator of invisible action with his general theory:

“the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty.”

Ultimately, however, Einstein did not believe that the invisible medium responsible for the “properties of space” could ever be directly accessed.  See, Einstein’s aether, reorganizes itself in the presence of material, and so any attempt to pin it down and characterize its structure would necessarily perturb that structure, making its examination impossible. 

Aether could therefore never be studied as a ponderable media, “consisting of parts which may be tracked through time.”  This precluded the aether from even the possibility of scientific observation and thus excluded it from the empirical descriptions of motion within it.  Einstein seems to have resigned himself to a picture of an aether that was untouchable, and so physical treatments of it could be reasonably neglected going forward.   

The rest of the world seems to have agreed.  Presently, the aether barely gets a mention in most modern physics textbooks.  This omission of scale seems astounding in light of the fact that literally every physicist before the 20th century invoked the aether in one form or another to explain the invisible phenomena such as light, magnetism, or the orbits of planets.  Yet, the term ‘aether’ was on its way out of fashion long before Einstein bellied up to the table to reframe the idea. 

The predecessor to Einstein’s general relativistic aether, the luminiferous aether, was decidedly put to down following by a series of experiments conducted throughout the 19th century.  Recall that the historic aether was a sort of static backdrop to physical existence.  Experimenters searched for indirect evidence of the aether’s motion with respect to objects on Earth but could find none.  Several experiments imagined discovery of an “aether drag” as the earth and local bodies moved against the resisting aether.  In the words of the exasperated experimenters A. Michelson and E. Morely in 1887, 

“It appears, from all that precedes, reasonably certain that if there can be any relative motion between the earth and the luniniferous aether, it must be small.”

The experimentalists held out hope that this was just an artifact of their experiments on the Earth’s surface, but surely the aether could be detected at sufficient altitude.  Eventually, this optimism, too subsided.

Over the next 50 years, the static aether that had persisted for hundreds, if not thousands, of years under wide-ranging scientific scrutiny, faded into oblivion. A universal reference frame was decidedly undetectable and so the aether of old went cold.  Over the course of the following half-century, relativists wrestled with a means of rectifying the lack of a universal backdrop with the invariable speed of light.  Einstein consummated the solution to their problem mathematically, by ignoring the structural organization of space, the aether.  Since space, the aether of general relativity, was an untouchable black box, he considered only ponderable objects as matter, and demonstrated relationally how light’s invariance does not contradict the universality of physical law.

The triumph of Einstein was to proceed in explanation of light and gravitation without working out a single universal reference frame of static aether.  Yet to understand why so many earlier minds, unlike the relativists, had become entrenched in the requirement of a static, unresponsive, aether we must travel backward to the 17th century and trace the history of the search for understanding of light, gravity, electricity, magnetism, and chemistry:  what I like to call the Big Five atomic phenomena.  

Comprehending the historic quest for understanding invisible phenomena is crucial for physically interpreting contemporary fundamental physics and so I plan to make this a multi-part series.  Next time, we’ll begin this investigation in France, with one of the aether’s first serious theorists, Rene Descartes.  Descartes advocated a vortex theory of gravity and light, which held sway in many intellectual circles for hundreds of years, even after Newton’s magnus opus, Principia, correctly described the motion of heavenly bodies without resort to hypothetical mediators.