The ether

Æther or luminiferous ether

An example of an historical scientific conception

The word 'ether' has a number of scientific meanings. In chemistry the term ether refers to both a class of chemical compounds (an ether being one of the ethers) and a particular substance that is one member of that class (ether being diethyl ether). (The same applies to terms like alcohol, sugar and salt which can all sometimes be used to refer to a particular substance, but sometimes as class of compounds.)

This is unhelpful as it requires authors to make particular effort to avoid ambiguity:

"He [Herbert C. Brown] and Subba Rao demonstrated that they could indeed produce diborane from sodium borohydride and aluminium chloride in an ether solution (the ether they had been using was a nonvolatile commercially available solvent, not the ordinary diethyl ether that is an anaesthetic) under mild conditions, and that diborane would add to simple alkenes."

Roberts, 1989

However the term ether, or æther (aether) was used long before its adoption in chemistry for a kind of subtle, tenuous matter that was conjectured to fill space.

Aether as a special element

Even here, strictly, the term has more than one meaning. Originally, the ether was a name sometimes given to the fifth element or quintessence that was conjectured to complement the four elements found on planet earth proposed by some of the Greek philosophers. It was though that all materials found on earth were comprised of these four elements in various combinations. The elements were called earth, water, air and fire, and a considerable theory or model was developed in terms of how transformations of matter involved changes in elemental composition. The four elements were considered to have a natural ordering (fire above air above water above earth – a stone would tend to fall downwards as it was mostly earth) but as most materials were mixtures and stuff got agitated the elements never settled out completely.

"Anaxagoras of Clazomenae (c. 500-428 BC) postulated another element called the aether, which was in constant rotation and carried with it the celestial bodies."

Freely (2012)

Ether, or æther, was positioned to be an additional element that was not found on earth, but was the substance of the heavens (a term that at that time still suggested an association with deities). It was assumed that the heavens would be made of a different substance to things down here on earth (an assumption that was slowly replaced by our modern view that the earth is just one planet in one solar system in one galaxy and is made of the same basic stuff as the rest of the universe).

For many centuries it was widely accepted in many cultures (for example in Christian tradition) that the heavens were perfect and unchanging unlike on earth where change and decay were the norm.

It was for this reason that it was long assumed that comets were atmospheric phenomena as strange new bodies could not appear in the heavens. (Chinese astronomers however had recognized novae, new stars, many centuries earlier). This is an example of how metaphysical commitments – ideas that arose from philosophical rather than empirical investigation – worked against scientific discoveries as they excluded possibilities such as comets being bodies moving in space beyond the moon's orbit.

There was sometimes less consensus in scientific ideas before the adoption of modern institutions (learned scientific societies; research journals; university chairs in science), so not everyone thought comets were atmospheric:

Testing for the ether

Isaac Newton actually set up an experiment to test whether a subtle aethereal medium quite freely permeates all the pores and passages of all bodies. His method reflected the technology and materials available at the time, and involved swinging an empty wooden box as a pendulum which was allowed to come to rest. He then filled the box with metal which increased the weight of the pendulum 'bob' (but not the external volume, or the external surface area in contact with the air) by 78 times. After adjusting for the stretching of the cord suspending the pendulum he repeated his observations.

Newton's hypothesis was that there was no ether and so the only resistance would be with the air, and so the box should be impeded by the same amount in both cases.

Of course, this is less a test of the aether than its 'subtelty'- if it was a totally massless fluid with no viscosity it may flow through materials without any impact. (A modern parallel may be the difficulty in detecting neutrinos aor gravitation weaves.) One of Newton's editors however suggested that an ether that had no effect on matter may as well just be an empty vacuum (see: very subtle aether pervades and fills all things).

Newton (1999) found a small discrepancy (which we might consider within 'experimental error' given the set up used), so rather than exclude the possibility of an aether interacting with matter he concluded: "The resistance encountered by the empty box on its internal parts is therefore more than 5,000 times smaller than the similar resistance on the external surface". That is, he put a limit on the relative effect of the ether, if it existed.

Elsewhere in the Principia, however, he argued that

"…the celestial spaces, through which the globes of the planets and comets move continually in all directions very freely and without any sensible diminution of motion, are devoid of any corporeal fluid, except perhaps the very rarest vapours and rays of light transmitted through those spaces."

The luminiferous ether

Whilst the idea of the need for an unearthly element to make up heavenly bodies was slowly abandoned – as it became recognised that the Earth itself was another body in space and indeed a planet like Mercury, Mars, Jupiter, Saturn and Neptune (even if their names still reflect the ancient gods they were once associated, even identified, with) – scientists did not immediately give up on the aether.

Ether was not needed to construct other worlds, but was seen as necessary as a medium. Light travelled form the Sun (and indeed other stars) to the Earth, through space. Space was considered to be largely empty of normal matter – a vacuum. But if light was considered to be a kind of wave motion, then it clearly needed a medium. Waves transfer energy by causing some medium (water, air, even a solid like wood) to oscillate – so if light, which could be reflected, refracted, diffracted and give rise to interference like other waves, was a kind of wave then there must be some kind of universal medium that vibrated as it was transmitted through: the luminiferous (light bearing) ether.

The great visionary scientist James Clerk Maxwell, who showed how electricity, magnetism and light (and other kinds of what became known as electromagnetic radiation) were closely related, assumed the existence of the luminiferous ether. Lord Kelvin even suggested atoms might be excitations of the ether:

The luminiferous ether seemed to be needed, and scientists conjectured what kind of material properties it must have to function as a medium for light without seeming to retard planets.

Scientists even tried to measure how quickly the earth was moving through the ether.

The idea of the luminiferous ether was only finally abandoned in the twentieth century (due to the acceptance of Einstein's theory of special relativity), along with the notion of space being an absolute reference frame,

"However, the conception of a universal ether, which was so useful in the development of the electromagnetic theory, appeared in this theory as an absolute frame of reference for the space-time description. The unsatisfactory nature of this conceptions, from a philosophical point of view, was strongly emphasised by the failure of all attempts to demonstrate the motion of the earth relative to this hypothetical universal ether…"

Bohr, 1934

Work cited:
  • Bohr, Neils (1934) Atomic theory and the description of nature. Cambridge University Press
  • Freely, John (2012). Before Galileo. The birth of modern science in Medieval Europe.
  • Newton, Isaac (1999) Mathematical Principles of Natural Philosophy (3rd edition, 1726): The authoritative translation (I. Bernard Cohen and Anne Whitman), University of California Press
  • Roberts, Royston M. (1989) Serendipity. Accidental discoveries in science.

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