6. Mysteries of Light - Part 1
This is the sixth article in the series From Particles to Angels. If you are interested in this article you should read the previous articles in the series in order, beginning with the first (On Happiness).
Despite all we know about it, light is still one of the great unsolved mysteries of science, such that it cannot be coherently described. But the mystery of light is also part of the magic of light. The reader of popular science books may find much that is familiar here, while some may come as a surprise. For anything I say that is wrong, I apologise in advance, and similarly for anything that is incomprehensible due to being poorly explained.
"But what is light really? Is it a wave or a shower of photons? .... There seems no likelihood of forming a consistent description of the phenomena of light by a choice of only one of the two possible languages. It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do!"
To get an impression of the magnitude of the contradiction, consider a familiar wave like a wave in water. As a wave travels across the ocean from one side to the other, nothing is actually crossing the ocean. There are no particles of water from one side of the ocean that arrive at the other side with the arrival of the wave. You may have noticed, when you are swimming in the surf, that just before the oncoming wave reaches you, you are pulled in toward the face of the oncoming wave, then when it reaches you, the wave lifts you up, you move forward a little bit with the wave, and assuming you don't surf the wave in to shore, and are not caught in a rip, it will drop you down again and leave you more or less where you were originally. And this is all the particles of water are doing when the wave passes by. They go back a little bit, up a little bit, forward a little, and then down a little to return to their original positions, only travelling in a small circle. What is being transported across the ocean is information, message passed from hand to hand just like a Mexican Wave, as each particle of water tells the next "Hey, do this!" That is what a wave is, whether it is in water, or a sound wave in air, or an earthquake, or the AC current from your power outlet. In some of these cases, such as the case of a sound wave, the particles do not travel in a circle, but only forward and back, in what is called a "compression wave". Actually the "up and down" component of the water wave is something that happens only near the surface, as the water attempts to relieve the pressure.
A wave requires a medium to carry the signal, the behaviour, something that does the wave motion. Without the water there is no wave. The wave is just water doing something.
Now compare this to a particle. We have a cannon and we shoot a cannonball out of it. The cannonball sails over the field in a low arc and strikes a barn, smashing through the wooden wall and into piles of straw and alarmed chickens. A cannonball is not a signal. A cannonball is a cannonball. We can shoot a cannonball through the air, or through the vacuum of space, because it doesn't require a medium to transmit the message. A particle of light we call a "photon", and it travels through space like a cannonball.
So which is it, a particle or a wave? How can it be both?
"As to that which occurs in the production of Sound, one knows that it is occasioned by the agitation undergone by an entire body, or by a considerable part of one, which shakes all the contiguous air. But the movement of the Light must originate as from each point of the luminous object, else we should not be able to perceive all the different parts of that object, as will be more evident in that which follows. And I do not believe that this movement can be better explained than by supposing that all those of the luminous bodies which are liquid, such as flames, and apparently the sun and the stars, are composed of particles which float in a much more subtle medium which agitates them with great rapidity, and makes them strike against the particles of the ether which surrounds them, and which are much smaller than they."
"If in two large tall cylindrical Vessels of Glass inverted, two little Thermometers be suspended so as not to touch the Vessels, and the Air be drawn out of one of these Vessels, and these Vessels thus prepared be carried out of a cold place into a warm one; the Thermometer in vacuo will grow warm as much, and almost as soon as the Thermometer which is not in vacuo. And when the Vessels are carried back into the cold place, the Thermometer in vacuo will grow cold almost as soon as the other Thermometer. Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission? And do not the Vibrations of this Medium in hot Bodies contribute to the intenseness and duration of their Heat? And do not hot Bodies communicate their Heat to contiguous cold ones, by the Vibrations of this Medium propagated from them into the cold ones? And is not this Medium exceedingly more rare and subtile than the Air, and exceedingly more elastick and active? And doth it not readily pervade all Bodies? And is it not (by its elastick force) expanded through all the Heavens?"
The scientist James Clerk Maxwell was one of the founders of electromagnetic theory and considerably fleshed out the vague intuitions of his predecessors, giving formal mathematical descriptions of light and electricity.
"In several parts of this treatise an attempt has been made to explain electromagnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The undulatory theory of light also assumes the existence of a medium. We have now to shew that the properties of the electromagnetic medium are identical with those of the luminiferous medium."
The Ether (also spelled "Aether" or "Aither") was a concept of the ancient Greeks. It should not be confused with the gas used by dentists as an anaesthetic. Aether was one of the primordial deities (Protogonoi (Πρωτογόνος = "first-born")) of the ancient Greeks, that is, one of the first born among the gods. According to Hesiod, Aether was born of Erebus (Darkness) and Nyx (Night).
"From Chaos came forth Erebus and black Night; but of Night were born Aether and Day, whom she conceived and bare from union in love with Erebus."
The aether (αἰθήρ) was the upper, purer air where the gods reside ("Zeus, who sits on high and dwells in the aether" (Hesiod's Theogony)), as opposed to aer (αέρας), the lower air that mortals breathe. What is now called "Physics" was originally considered a part of philosophy, the part called "natural philosophy" (that is, philosophy about nature). Because the Greek gods were personified aspects of nature, these were also the subjects of natural philosophy. The ancient Greek philosophers described the ether in the following terms.
"For they determined in their minds to name two forms, one of which they should not – and that is where they have erred. And they distinguished them as opposite in kind and set up signs for them separately from one another here: the ethereal fire of flame, gentle, very light, in every direction the same as itself and not the same as the other; and that too, by itself, opposite – unknowing night, dense in kind and heavy."
"The dense and the wet and the cold and the dark congregated here where now is the earth, and the rare and the hot and the dry moved out to the farther part of the ether." (Anaxagoras (500-428 BC).)
"The ethereal power pursues souls to the sea, the sea spits them up onto the threshold of the earth, the earth into the light of the bright sun, and the sun hurls them into the whirls of the ether: the one receives them from the other: all hate them."
"And so, implying that the primary body is something else beyond earth, fire, air, and water, they gave the highest place a name of its own, aither, derived from the fact that it 'runs always' for an eternity of time."
The word "quintessence" means literally "fifth essence", and refers to Aristotle's fifth element beyond the four traditionally proposed by the ancients (earth, air, fire and water). The ether was the pure and perfect substance of the heavens. From ether comes our word "ethereal".
"In the sphere there are five elements, those inside the sphere: fire and water and earth and air, and what is the hull of the sphere, the fifth." (Philolaos (c.470 – c.385 BC), fragment 12)
"There was yet a fifth combination which God used in the delineation of the universe."
It was not only the ancient Greeks who formed this five-fold division of the elements of nature. It has a long and broad history. Hinduism described the 5 mahābhūta ("great" or "gross" elements): space (or "ether"), air, fire, water and earth. In Sanskrit, aether is "akasha" (आकाश).
"From this very self (ātman) did space come into being; from space, air; from air, fire; from fire, the waters, from the waters, the earth; from the earth, plants; from plants, food; and from food, man."
"Some wise men say it is inherent nature, while others say it is time – all totally deluded. It is rather the greatness of God present in the world by means of which this wheel of brahman goes around. Who always encompasses this whole world – the knower, the architect of time, the one without qualities, and the all-knowing one – it is at his command that the work of creation, to be conceived of as earth, water, fire, air, and space, unfolds itself."
In Japanese Buddhism it took the form of the godai (五大), the "five great" elements: earth (地 chi), water (水 sui), fire (火 ka), wind (風 fu), and void (空 ku).
When scientists adopted the ether as the medium for the propagation of light it became known as the "luminiferous aether".
"In 19th-century Europe, not every scientist saw such novelties as developments to be cheered. Boltzmann encountered opposition. Many of his fellow physicists did not believe that his goals were worthwhile, or even qualified as science. They had measured the expansion of gases and could write down a simple law relating temperature, pressure, and volume. Boltzmann's alleged atoms, by contrast, were invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen? This was why Ernst Mach proclaimed that he didn't believe that atoms existed."
Ludwig Boltzmann (1844–1906) eventually committed suicide, but a few other scientists were also proposing the idea of atoms. In 1827, the British botanist Robert Brown (1773-1858), while looking at pollen grains under a microscope, had watched tiny particles moving around randomly in the water. The movement was not a smooth flowing as one might expect from tiny currents in the water, but rather an erratic jiggling with many sudden changes in direction.
"While examining the form of these particles immersed in water, I observed many of them very evidently in motion.... These motions were such as to satisfy me, after frequently repeated observation, that they arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself. Grains of pollen of the same plant taken from antherae immediately after bursting, contained similar subcylindrical particles, in reduced numbers, however, and mixed with other particles, at least as numerous, of much smaller size, apparently spherical, and in rapid oscillatory motion."
He could not account for the motion, named "Brownian motion" in his honour, but in 1905 Albert Einstein (1879-1955) suggested that Brownian motion was caused by molecules of water colliding with the particles, and this conclusion was supported by others. After this atoms became a commonly accepted part of science. Not so very long ago.
But in that same eventful year of 1905 Einstein showed that a phenomenon known as the "photoelectric effect" could be explained if light consisted of discrete particles he called “light quantum”. The name "photon" was coined in 1926 by the American physical chemist Gilbert Lewis (1875-1946). So that now there were atoms of light as well. The word "atom" comes from another ancient Greek concept, but that is a conversation for another time. The photon was here to stay, but it did not displace the wave model of light, but instead merely sat uncomfortably along side it.
Imagine you are in a fast car, fleeing from the police. If they are 50 miles behind you, and you are going 50 miles an hour, while they are going 100 miles an hour, you expect that they will reach you in an hour. If they are going at 100 miles an hour and you are parked at the side of the road, you expect they will reach you in half an hour. If they are going at 100 miles an hour, while you are fleeing from them at the same speed, you expect that you can maintain the same distance from them, so that they will never catch you. If they are travelling at 100 miles an hour while you speed away at 200 miles an hour, you expect to leave them far behind. But that is not the way it works with light. It doesn't matter how fast or slow you are going, or in what direction, light will still come at you at exactly the same speed: 186,000 miles per second (300,000 km/s). Explaining this created a whole new crisis in physics that ultimately led to Einstein's Special Theory of Relativity.
A lot of what we associate with the Special Theory of Relativity predates it. The physicist Hendrik Lorentz (1853–1928) attempted to account for the constancy of the speed of light by positing that when objects are in relative motion their lengths contract and time moves more slowly. Lorentz developed his model in the context of an ether theory.
"The ions were supposed to be perfectly permeable to the aether, so that they can move while the aether remains at rest. I applied to the aether the ordinary electromagnetic equations, and to the ions certain other equations which seemed to present themselves rather naturally. In this way I arrived at a system of formulae which were found sufficient to account for a number of phenomena."
The equations used in Special Relativity are called "Lorentz Transformations" in honour of their originator.
You may read that the existence of the ether was disproved by the Michelson-Morley experiment in 1887, but this statement is a little misleading. The Michelson-Morley experiment attempted to check if the speed of light was different in different directions. Consider waves in water, such as out in the middle of the ocean. Your little boat is motionless in the water, while the distant waves 10 nautical miles away, approach at speed of 10 nautical miles per hour (1 knot). You expect they will reach you in an hour. If you start your engine and flee in the opposite direction, at 10 knots, you expect you can maintain the same distance between yourself and them.
So Albert A. Michelson and Edward W. Morley figured, since the Earth was flying through space at some speed, it was moving relative to the ether in some direction, and so the speed of light should be less in the direction opposite to the direction it was moving relative to the ether, and more in the direction it was travelling relative to the ether. When they found that the speed was the same in every direction, it was concluded that there was no "quaint" ether.
"Between 1887 and 1905 there were several attempts, most notably by the Dutch physicist Hendrik Lorentz, to explain the result of the Michelson-Morley experiment in terms of objects contracting and clocks slowing down when they moved through the ether. However, in a famous paper in 1905, a hitherto unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary, providing one was willing to abandon the idea of absolute time."
"They found no such effect. The light travelled at the same speed in both directions, no matter in which direction they pointed the two beams. There was no evidence that the light was propagating in any background material through which the earth was moving. Since that time, Einstein's special theory of relativity has been tested innumerable times. Today many predictions of observers, are observed daily in the high-energy accelerators built by particle physicists to study the fundamental structure of matter. Since this theory is incompatible with the requirement that the light travels in an aether, no large-scale effort has been exerted to extend the pioneering work of Michelson and Morley in probing for such a medium. One might thus date 1905 as the end of the aether story."
"My actual account of that history is somewhat more elaborate. It begins with brief remarks on the nineteenth century concept of the aether ..., that quaint, hypothetical medium which was introduced for the purpose of explaining the transmission of light waves and which was abolished by Einstein."
"But Michelson and Morley found no time difference at all. They even repeated the experiment at various times in the year in case the direction of the aether wind varied as the Earth rotated round the Sun. But no matter what they tried, no time lag was observable. There was no aether."
But what the Michelson-Morley experiment confirmed was the constancy of the speed of light, as predicted by James Clerk Maxwell. Whether we posit photons flying through a void, or a luminiferous aether serving as a medium for "light waves", the constancy of the speed of light poses much the same problem. Sometimes, instead of the Michelson-Morley experiment being credited with the death of the ether theory, the emphasis is laid on the Special Theory of Relativity.
"However, in a famous paper in 1905, a hitherto unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary."
The contribution of Einstein's Special Theory of Relativity was a single, very simple idea, not a whole new theory, but a final piece that provided completion to what had gone before.
That idea was that the laws of physics were invariant under constant relative motion. The job of science is to provide the what and the why. Maxwell had provided the what: "the speed of light is constant for all observers". Now Einstein had provided the why: "because if it wasn't, the laws of physics would appear different for observers in relative motion." If you are in a spaceship travelling at a constant velocity, you can't tell that you're moving. If you are in a train travelling at night through the desert on a straight stretch of track, you can't tell that you're moving. If you pour tea into a cup, it behaves the same as if you were motionless. The great beauty and appeal of Einstein's theory was that it didn't require any mention of an ether. Nobody had to provide a physical model for "how" it happened, there was just a "rule", a universal law that was the "cause". The theorizing part was finished, it had been explained, and special relativity was added to the physicist's arsenal of measuring and predicting tools.
"What 'laws of mechanics' is it necessary that light obey in order to explain the results of the Michelson-Morley experiment? Only the principle of relativity. If the apparatus is at rest with respect to the ether, the light takes equal times to traverse the two paths of the apparatus. If we could apply the principle of relativity, we would find the same result in any inertial frame of reference, i.e., with the apparatus moving uniformly with respect to the ether. This is, of course, just the result of the Michelson-Morley experiment. Einstein understood the Michelson-Morley experiment in just these basic terms and proposed that: ... Light, and in fact all electromagnetic phenomena, obey the principle of relativity. We now believe that this is the proper 'explanation' of the Michelson-Morley experiment."
It was not Einstein's intention to kill the ether theory, and he himself did not believe he had. Writing in 1920 he had the following to say.
"Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."
Some others of the founding fathers of Quantum Mechanics were of a like mind. In 1951 Paul Dirac wrote the following.
"We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an aether."
Louis de Broglie had the following to say in 1972.
"However, in the case of a particle which does not appear as subjected to perturbations, such as an electron in a hydrogen atom, what could be the origin of these assumed perturbations? To answer this question, any particle, even isolated, has to be imagined as in continuous “energetic contact” with a hidden medium, which constitutes a concealed thermostat. This hypothesis was brought forward some fifteen years ago by Bohm and Vigier, who named this invisible thermostat the “subquantum medium”. As a further assumption, the particle is considered as continuously exchanging energy and momentum with such a hidden thermostat. These exchanges would happen regularly, in a well defined manner, if the guided motion existed alone, but a random energy exchange is superposed, which has a fluctuation character of well known kind in statistical thermodynamics. If a hidden sub-quantum medium is assumed, knowledge of its nature would seem desirable. It certainly is of quite complex character. It could not serve as a universal reference medium, as this would be contrary to relativity theory. Moreover, it does not behave as a unique thermostat, but rather as an ensemble of thermostats, the temperatures of which are related to the proper energies M0c2 of various kinds of molecules. Although interesting explanations have been proposed for this sub-quantum medium’s nature, it seems premature to discuss the problem in the present paper."
But the general view had become that the ether hypothesis was dead and buried, after all: "They had measured [this, that and the other] and could write down a simple law relating [all and sundry]. [The] alleged [ether was], by contrast, ... invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen?"
The ether hypothesis became an embarrassment, and its long mystical tradition may have had something to do with it. The ether was the darling of nuts, and ether theories lived on at the fringe of society alongside Ether Technology and Akashic Records, so that science became hostile towards it. In 1998 Robert B. Laughlin won a Nobel Prize for Physics. In 2005 he had the following to say on the ether.
"About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo."
In 2004 Frank Wilczek won a Nobel Prize for Physics. In 1999 he had the following to say on the ether.
"Quite undeservedly, the ether has acquired a bad name. There is a myth, repeated in many popular presentations and textbooks, that Albert Einstein swept it into the dustbin of history. The real story is more complicated and interesting. I argue here that the truth is more nearly the opposite: Einstein first purified, and then enthroned, the ether concept. As the 20th century has progressed, its role in fundamental physics has only expanded. At present, renamed and thinly disguised, it dominates the accepted laws of physics. And yet, there is serious reason to suspect it may not be the last word."
Our story of light is not quite over yet, but perhaps we will pause a moment here. If you want more, check out Mysteries of Light - Part 2.
Any comments welcome.
Despite all we know about it, light is still one of the great unsolved mysteries of science, such that it cannot be coherently described. But the mystery of light is also part of the magic of light. The reader of popular science books may find much that is familiar here, while some may come as a surprise. For anything I say that is wrong, I apologise in advance, and similarly for anything that is incomprehensible due to being poorly explained.
Particles and Waves
You may have heard of "the particle-wave duality of light". Albert Einstein had the following to say of it."But what is light really? Is it a wave or a shower of photons? .... There seems no likelihood of forming a consistent description of the phenomena of light by a choice of only one of the two possible languages. It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do!"
(The Evolution of Physics, by Albert Einstein and Leopold Infeld, p.278.)
To get an impression of the magnitude of the contradiction, consider a familiar wave like a wave in water. As a wave travels across the ocean from one side to the other, nothing is actually crossing the ocean. There are no particles of water from one side of the ocean that arrive at the other side with the arrival of the wave. You may have noticed, when you are swimming in the surf, that just before the oncoming wave reaches you, you are pulled in toward the face of the oncoming wave, then when it reaches you, the wave lifts you up, you move forward a little bit with the wave, and assuming you don't surf the wave in to shore, and are not caught in a rip, it will drop you down again and leave you more or less where you were originally. And this is all the particles of water are doing when the wave passes by. They go back a little bit, up a little bit, forward a little, and then down a little to return to their original positions, only travelling in a small circle. What is being transported across the ocean is information, message passed from hand to hand just like a Mexican Wave, as each particle of water tells the next "Hey, do this!" That is what a wave is, whether it is in water, or a sound wave in air, or an earthquake, or the AC current from your power outlet. In some of these cases, such as the case of a sound wave, the particles do not travel in a circle, but only forward and back, in what is called a "compression wave". Actually the "up and down" component of the water wave is something that happens only near the surface, as the water attempts to relieve the pressure.
A wave requires a medium to carry the signal, the behaviour, something that does the wave motion. Without the water there is no wave. The wave is just water doing something.
Now compare this to a particle. We have a cannon and we shoot a cannonball out of it. The cannonball sails over the field in a low arc and strikes a barn, smashing through the wooden wall and into piles of straw and alarmed chickens. A cannonball is not a signal. A cannonball is a cannonball. We can shoot a cannonball through the air, or through the vacuum of space, because it doesn't require a medium to transmit the message. A particle of light we call a "photon", and it travels through space like a cannonball.
So which is it, a particle or a wave? How can it be both?
Ether (Aether or Aither)
For a long time scientists accepted the wave model of light and assumed there was some fine medium in space that served to transmit light and heat from one place to another."As to that which occurs in the production of Sound, one knows that it is occasioned by the agitation undergone by an entire body, or by a considerable part of one, which shakes all the contiguous air. But the movement of the Light must originate as from each point of the luminous object, else we should not be able to perceive all the different parts of that object, as will be more evident in that which follows. And I do not believe that this movement can be better explained than by supposing that all those of the luminous bodies which are liquid, such as flames, and apparently the sun and the stars, are composed of particles which float in a much more subtle medium which agitates them with great rapidity, and makes them strike against the particles of the ether which surrounds them, and which are much smaller than they."
(Treatise on Light by Christiaan Huygens (1629–1695) Chapter 1.)
"If in two large tall cylindrical Vessels of Glass inverted, two little Thermometers be suspended so as not to touch the Vessels, and the Air be drawn out of one of these Vessels, and these Vessels thus prepared be carried out of a cold place into a warm one; the Thermometer in vacuo will grow warm as much, and almost as soon as the Thermometer which is not in vacuo. And when the Vessels are carried back into the cold place, the Thermometer in vacuo will grow cold almost as soon as the other Thermometer. Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission? And do not the Vibrations of this Medium in hot Bodies contribute to the intenseness and duration of their Heat? And do not hot Bodies communicate their Heat to contiguous cold ones, by the Vibrations of this Medium propagated from them into the cold ones? And is not this Medium exceedingly more rare and subtile than the Air, and exceedingly more elastick and active? And doth it not readily pervade all Bodies? And is it not (by its elastick force) expanded through all the Heavens?"
(Optics by Sir Isaac Newton (1642–1727), p.349.)
The scientist James Clerk Maxwell was one of the founders of electromagnetic theory and considerably fleshed out the vague intuitions of his predecessors, giving formal mathematical descriptions of light and electricity.
"In several parts of this treatise an attempt has been made to explain electromagnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The undulatory theory of light also assumes the existence of a medium. We have now to shew that the properties of the electromagnetic medium are identical with those of the luminiferous medium."
(A Treatise on Electricity and Magnetism by James Clerk Maxwell (1831–1879), Part IV, Chapter XX.)
The Ether (also spelled "Aether" or "Aither") was a concept of the ancient Greeks. It should not be confused with the gas used by dentists as an anaesthetic. Aether was one of the primordial deities (Protogonoi (Πρωτογόνος = "first-born")) of the ancient Greeks, that is, one of the first born among the gods. According to Hesiod, Aether was born of Erebus (Darkness) and Nyx (Night).
"From Chaos came forth Erebus and black Night; but of Night were born Aether and Day, whom she conceived and bare from union in love with Erebus."
(Theogony, by Hesiod (c. 700 BC).)
The aether (αἰθήρ) was the upper, purer air where the gods reside ("Zeus, who sits on high and dwells in the aether" (Hesiod's Theogony)), as opposed to aer (αέρας), the lower air that mortals breathe. What is now called "Physics" was originally considered a part of philosophy, the part called "natural philosophy" (that is, philosophy about nature). Because the Greek gods were personified aspects of nature, these were also the subjects of natural philosophy. The ancient Greek philosophers described the ether in the following terms.
"For they determined in their minds to name two forms, one of which they should not – and that is where they have erred. And they distinguished them as opposite in kind and set up signs for them separately from one another here: the ethereal fire of flame, gentle, very light, in every direction the same as itself and not the same as the other; and that too, by itself, opposite – unknowing night, dense in kind and heavy."
(Parmenides (early 5th century BC).)
"The dense and the wet and the cold and the dark congregated here where now is the earth, and the rare and the hot and the dry moved out to the farther part of the ether." (Anaxagoras (500-428 BC).)
"The ethereal power pursues souls to the sea, the sea spits them up onto the threshold of the earth, the earth into the light of the bright sun, and the sun hurls them into the whirls of the ether: the one receives them from the other: all hate them."
(Empedocles (c. 490 – 430 BC).)
"And so, implying that the primary body is something else beyond earth, fire, air, and water, they gave the highest place a name of its own, aither, derived from the fact that it 'runs always' for an eternity of time."
(On the Heavens by Aristotle (384–322 BC), Book 1, part 3.)
The word "quintessence" means literally "fifth essence", and refers to Aristotle's fifth element beyond the four traditionally proposed by the ancients (earth, air, fire and water). The ether was the pure and perfect substance of the heavens. From ether comes our word "ethereal".
"In the sphere there are five elements, those inside the sphere: fire and water and earth and air, and what is the hull of the sphere, the fifth." (Philolaos (c.470 – c.385 BC), fragment 12)
"There was yet a fifth combination which God used in the delineation of the universe."
("Timaeus" by Plato (428 - 348 BC))
It was not only the ancient Greeks who formed this five-fold division of the elements of nature. It has a long and broad history. Hinduism described the 5 mahābhūta ("great" or "gross" elements): space (or "ether"), air, fire, water and earth. In Sanskrit, aether is "akasha" (आकाश).
"From this very self (ātman) did space come into being; from space, air; from air, fire; from fire, the waters, from the waters, the earth; from the earth, plants; from plants, food; and from food, man."
(Taittirīya Upaniṣad c. 6th century BC.)
"Some wise men say it is inherent nature, while others say it is time – all totally deluded. It is rather the greatness of God present in the world by means of which this wheel of brahman goes around. Who always encompasses this whole world – the knower, the architect of time, the one without qualities, and the all-knowing one – it is at his command that the work of creation, to be conceived of as earth, water, fire, air, and space, unfolds itself."
(Śvetāśvatara Upaniṣad c. 5th century BC.)
In Japanese Buddhism it took the form of the godai (五大), the "five great" elements: earth (地 chi), water (水 sui), fire (火 ka), wind (風 fu), and void (空 ku).
When scientists adopted the ether as the medium for the propagation of light it became known as the "luminiferous aether".
Photons
But in 1905 a young Albert Einstein threw a spanner in the works of the wave model of light. In the 19th-century, most scientists did not believe in atoms."In 19th-century Europe, not every scientist saw such novelties as developments to be cheered. Boltzmann encountered opposition. Many of his fellow physicists did not believe that his goals were worthwhile, or even qualified as science. They had measured the expansion of gases and could write down a simple law relating temperature, pressure, and volume. Boltzmann's alleged atoms, by contrast, were invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen? This was why Ernst Mach proclaimed that he didn't believe that atoms existed."
(Boltzmanns Atom: The Great Debate That Launched A Revolution In Physics, by David Lindley.)
Ludwig Boltzmann (1844–1906) eventually committed suicide, but a few other scientists were also proposing the idea of atoms. In 1827, the British botanist Robert Brown (1773-1858), while looking at pollen grains under a microscope, had watched tiny particles moving around randomly in the water. The movement was not a smooth flowing as one might expect from tiny currents in the water, but rather an erratic jiggling with many sudden changes in direction.
"While examining the form of these particles immersed in water, I observed many of them very evidently in motion.... These motions were such as to satisfy me, after frequently repeated observation, that they arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself. Grains of pollen of the same plant taken from antherae immediately after bursting, contained similar subcylindrical particles, in reduced numbers, however, and mixed with other particles, at least as numerous, of much smaller size, apparently spherical, and in rapid oscillatory motion."
(Robert Brown (1828))
He could not account for the motion, named "Brownian motion" in his honour, but in 1905 Albert Einstein (1879-1955) suggested that Brownian motion was caused by molecules of water colliding with the particles, and this conclusion was supported by others. After this atoms became a commonly accepted part of science. Not so very long ago.
But in that same eventful year of 1905 Einstein showed that a phenomenon known as the "photoelectric effect" could be explained if light consisted of discrete particles he called “light quantum”. The name "photon" was coined in 1926 by the American physical chemist Gilbert Lewis (1875-1946). So that now there were atoms of light as well. The word "atom" comes from another ancient Greek concept, but that is a conversation for another time. The photon was here to stay, but it did not displace the wave model of light, but instead merely sat uncomfortably along side it.
The Michelson-Morley Experiment
But light was to hold more problems than the particle wave duality. When James Clerk Maxwell formed his mathematical model of light and electricity he found that his equations required a strange new property, specifically that the speed of light be a universal constant. This may not sound too dramatic, but let's pause to consider it.Imagine you are in a fast car, fleeing from the police. If they are 50 miles behind you, and you are going 50 miles an hour, while they are going 100 miles an hour, you expect that they will reach you in an hour. If they are going at 100 miles an hour and you are parked at the side of the road, you expect they will reach you in half an hour. If they are going at 100 miles an hour, while you are fleeing from them at the same speed, you expect that you can maintain the same distance from them, so that they will never catch you. If they are travelling at 100 miles an hour while you speed away at 200 miles an hour, you expect to leave them far behind. But that is not the way it works with light. It doesn't matter how fast or slow you are going, or in what direction, light will still come at you at exactly the same speed: 186,000 miles per second (300,000 km/s). Explaining this created a whole new crisis in physics that ultimately led to Einstein's Special Theory of Relativity.
A lot of what we associate with the Special Theory of Relativity predates it. The physicist Hendrik Lorentz (1853–1928) attempted to account for the constancy of the speed of light by positing that when objects are in relative motion their lengths contract and time moves more slowly. Lorentz developed his model in the context of an ether theory.
"The ions were supposed to be perfectly permeable to the aether, so that they can move while the aether remains at rest. I applied to the aether the ordinary electromagnetic equations, and to the ions certain other equations which seemed to present themselves rather naturally. In this way I arrived at a system of formulae which were found sufficient to account for a number of phenomena."
(Simplified Theory of Electrical and Optical Phenomena in Moving Systems, by H. A. Lorentz)
The equations used in Special Relativity are called "Lorentz Transformations" in honour of their originator.
You may read that the existence of the ether was disproved by the Michelson-Morley experiment in 1887, but this statement is a little misleading. The Michelson-Morley experiment attempted to check if the speed of light was different in different directions. Consider waves in water, such as out in the middle of the ocean. Your little boat is motionless in the water, while the distant waves 10 nautical miles away, approach at speed of 10 nautical miles per hour (1 knot). You expect they will reach you in an hour. If you start your engine and flee in the opposite direction, at 10 knots, you expect you can maintain the same distance between yourself and them.
So Albert A. Michelson and Edward W. Morley figured, since the Earth was flying through space at some speed, it was moving relative to the ether in some direction, and so the speed of light should be less in the direction opposite to the direction it was moving relative to the ether, and more in the direction it was travelling relative to the ether. When they found that the speed was the same in every direction, it was concluded that there was no "quaint" ether.
"Between 1887 and 1905 there were several attempts, most notably by the Dutch physicist Hendrik Lorentz, to explain the result of the Michelson-Morley experiment in terms of objects contracting and clocks slowing down when they moved through the ether. However, in a famous paper in 1905, a hitherto unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary, providing one was willing to abandon the idea of absolute time."
(A Brief History of Time: from the Big Bang to Black Holes by Stephen W. Hawking, p.20.)
"They found no such effect. The light travelled at the same speed in both directions, no matter in which direction they pointed the two beams. There was no evidence that the light was propagating in any background material through which the earth was moving. Since that time, Einstein's special theory of relativity has been tested innumerable times. Today many predictions of observers, are observed daily in the high-energy accelerators built by particle physicists to study the fundamental structure of matter. Since this theory is incompatible with the requirement that the light travels in an aether, no large-scale effort has been exerted to extend the pioneering work of Michelson and Morley in probing for such a medium. One might thus date 1905 as the end of the aether story."
(The Fifth Essence: The Search for Dark Matter in the Universe by Lawrence Krauss, pp.26-7.)
"My actual account of that history is somewhat more elaborate. It begins with brief remarks on the nineteenth century concept of the aether ..., that quaint, hypothetical medium which was introduced for the purpose of explaining the transmission of light waves and which was abolished by Einstein."
(Subtle is the Lord: The science and the life of Albert Einstein, by Abraham Pais, pp.20-1.)
"But Michelson and Morley found no time difference at all. They even repeated the experiment at various times in the year in case the direction of the aether wind varied as the Earth rotated round the Sun. But no matter what they tried, no time lag was observable. There was no aether."
(The Arrow of Time: The quest to solve science's greatest mystery by Peter Coveney and Roger Highfield, p.72.)
But what the Michelson-Morley experiment confirmed was the constancy of the speed of light, as predicted by James Clerk Maxwell. Whether we posit photons flying through a void, or a luminiferous aether serving as a medium for "light waves", the constancy of the speed of light poses much the same problem. Sometimes, instead of the Michelson-Morley experiment being credited with the death of the ether theory, the emphasis is laid on the Special Theory of Relativity.
"However, in a famous paper in 1905, a hitherto unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary."
The contribution of Einstein's Special Theory of Relativity was a single, very simple idea, not a whole new theory, but a final piece that provided completion to what had gone before.
That idea was that the laws of physics were invariant under constant relative motion. The job of science is to provide the what and the why. Maxwell had provided the what: "the speed of light is constant for all observers". Now Einstein had provided the why: "because if it wasn't, the laws of physics would appear different for observers in relative motion." If you are in a spaceship travelling at a constant velocity, you can't tell that you're moving. If you are in a train travelling at night through the desert on a straight stretch of track, you can't tell that you're moving. If you pour tea into a cup, it behaves the same as if you were motionless. The great beauty and appeal of Einstein's theory was that it didn't require any mention of an ether. Nobody had to provide a physical model for "how" it happened, there was just a "rule", a universal law that was the "cause". The theorizing part was finished, it had been explained, and special relativity was added to the physicist's arsenal of measuring and predicting tools.
"What 'laws of mechanics' is it necessary that light obey in order to explain the results of the Michelson-Morley experiment? Only the principle of relativity. If the apparatus is at rest with respect to the ether, the light takes equal times to traverse the two paths of the apparatus. If we could apply the principle of relativity, we would find the same result in any inertial frame of reference, i.e., with the apparatus moving uniformly with respect to the ether. This is, of course, just the result of the Michelson-Morley experiment. Einstein understood the Michelson-Morley experiment in just these basic terms and proposed that: ... Light, and in fact all electromagnetic phenomena, obey the principle of relativity. We now believe that this is the proper 'explanation' of the Michelson-Morley experiment."
(Introduction to Special Relativity, by James H. Smith, p.40-1.)
It was not Einstein's intention to kill the ether theory, and he himself did not believe he had. Writing in 1920 he had the following to say.
"Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."
(Ether and the Theory of Relativity by Albert Einstein (1879–1955).)
Some others of the founding fathers of Quantum Mechanics were of a like mind. In 1951 Paul Dirac wrote the following.
"We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an aether."
(Is there an Aether? by P. A. M. Dirac (Nature, 24 November, 1951).)
Louis de Broglie had the following to say in 1972.
"However, in the case of a particle which does not appear as subjected to perturbations, such as an electron in a hydrogen atom, what could be the origin of these assumed perturbations? To answer this question, any particle, even isolated, has to be imagined as in continuous “energetic contact” with a hidden medium, which constitutes a concealed thermostat. This hypothesis was brought forward some fifteen years ago by Bohm and Vigier, who named this invisible thermostat the “subquantum medium”. As a further assumption, the particle is considered as continuously exchanging energy and momentum with such a hidden thermostat. These exchanges would happen regularly, in a well defined manner, if the guided motion existed alone, but a random energy exchange is superposed, which has a fluctuation character of well known kind in statistical thermodynamics. If a hidden sub-quantum medium is assumed, knowledge of its nature would seem desirable. It certainly is of quite complex character. It could not serve as a universal reference medium, as this would be contrary to relativity theory. Moreover, it does not behave as a unique thermostat, but rather as an ensemble of thermostats, the temperatures of which are related to the proper energies M0c2 of various kinds of molecules. Although interesting explanations have been proposed for this sub-quantum medium’s nature, it seems premature to discuss the problem in the present paper."
(Interpretation of quantum mechanics by the double solution theory, by Louis de Broglie.)
But the general view had become that the ether hypothesis was dead and buried, after all: "They had measured [this, that and the other] and could write down a simple law relating [all and sundry]. [The] alleged [ether was], by contrast, ... invisible, intangible, and imperceptible. What was the point of explaining a straightforward law, derived directly from experiment, in terms of hypothetical entities that could not be seen and might never be seen?"
The ether hypothesis became an embarrassment, and its long mystical tradition may have had something to do with it. The ether was the darling of nuts, and ether theories lived on at the fringe of society alongside Ether Technology and Akashic Records, so that science became hostile towards it. In 1998 Robert B. Laughlin won a Nobel Prize for Physics. In 2005 he had the following to say on the ether.
"About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo."
(A Different Universe: Reinventing Physics from the Bottom Down, by Robert B. Laughlin, pages 120–121.)
In 2004 Frank Wilczek won a Nobel Prize for Physics. In 1999 he had the following to say on the ether.
"Quite undeservedly, the ether has acquired a bad name. There is a myth, repeated in many popular presentations and textbooks, that Albert Einstein swept it into the dustbin of history. The real story is more complicated and interesting. I argue here that the truth is more nearly the opposite: Einstein first purified, and then enthroned, the ether concept. As the 20th century has progressed, its role in fundamental physics has only expanded. At present, renamed and thinly disguised, it dominates the accepted laws of physics. And yet, there is serious reason to suspect it may not be the last word."
("The Persistence of Ether" by Frank Wilczek, Physics Today, January 1999, p.11.)
Our story of light is not quite over yet, but perhaps we will pause a moment here. If you want more, check out Mysteries of Light - Part 2.
Any comments welcome.
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