Lacking the eloquence of those three, I might seem to have chosen a particularly unpromising topic for this lecture: the contents of an empty box!
Let us lift the lid and look inside. While we see nothing we all know very well that the box is not really empty. It is filled with air. But ever since Robert Boyle made his wonderful air pump, we have known how to seal up the box and remove the air, more or less entirely - and what then remains? Is there anything left, that we cannot see?
If the answer was to be no, this lecture would be distinctly vacuous, but happily we have believed since ancient times that there remains a hidden world in this apparently empty space. It has been the task of the physicist to reveal the invisible - and sometimes to speculate about it, before it could be revealed. The physicist can sing with Porgy & Bess "Ah've got plenty of nothing and nothin's plenty for me".
So what else does our box contain?
For many centuries the official answer was the ether, but this could mean many things. So this lecture is about the age-old quest for an understanding of the ether, especially in the nineteenth century.
First we should trace the word back to its origins in Greece. Let me quote Sir Oliver Lodge, who wrote many books on the ether around the turn of the century.
"Appollonius of Tyana is said to have asked the Brahmins of what they
supposed the cosmos to be comprised."
"Of the five elements"
"How can there be a fifth" demanded Appollonius "beside water, and air and
earth and fire?"
"There is the ether" replied the Brahmin, which we
must regard as the element of which the Gods are made: for just as all
mortal creatures inhale the air, so do immortals and divine natures inhale
the ether".
This ether was associated with a fiery heaven in which souls and gods resided. For the natural philosopher it also made up a nice matched set of fundamental constituents of nature, and could serve to account for whatever could not be handled by the standard ones - rather like the sand wedge in a golf bag.
The Newtonian ether
In throwing off the fanciful science of the middle ages, and concentrating
on what could be observed, Isaac Newton and his contemporaries constructed a
new view of the world in which the contact and collision of solid bodies was
the dominant theme. But even at the heart of Newton's greatest triumph -
accounting for the planetary orbits in terms of a new law of gravitation -
there lay an uncomfortable paradox. The law of gravitation is one of
action-at-distance, between bodies across empty space. As Newton himself
said:
"That one body may act upon another at a distance, through a vacuum, without the mediation of anything else by and through which their action may be conveyed from one to the other, is to me so great an absurdity that I believe no man, who has in philosophical matters a competent faculty of thinking, can ever fall into it".
Gravity and other forces which act at a distance were strongly at odds with the new outlook, so it was necessary to retain the ether in one form or another, as a fluid medium through which such interactions could be passed. The ether could also carry light, which was already recognised as a sort of wave or vibration. It was natural then to think of light waves in ether as the analogue of sound waves in air.
In Dublin, Newton's philosophy was taught by Dr. Richard Helsham, who was a physician as well as a physicist. He attended Dean Swift and enjoyed many a good dinner party with him and other Dublin intellectuals. His lectures on Natural Philosophy (published posthumously in 1739) contain an interesting problem, which was to be properly solved a century later by another Irishman, George Gabriel Stokes: what is the drag force on a sphere which moves through a fluid? Helsham's motivation for including this was the recognition that the Earth should move through the ether and might be subject to a drag force, like a soccer ball moving through the air.
There was no evidence of such a drag, nor indeed of any effects of the ether other than the physical properties which it was invented to rationalise. At that stage, arguments about the ether debate were more ad hoc philosophy than physics.
One hundred years later, mathematicians such as Stokes had made such progress in describing elastic solids and fluids that they felt ready to construct a full theory of the ether. The ensuing debate occupied the whole of the 19th century, and it is intertwined with two of the greatest achievements of that century. They were the theory of heat, and the development of an understanding of light waves.
The many varieties of material ether
Although formidable mathematics was brought to bear on the ether, it remained elusive. Light waves do not quite correspond to the vibrations of any simple solid or liquid that we know. In an effort to fit the facts, several attempts were made to make analogies with unusual materials.
For example, Osborne Reynolds got very excited by the notion that the ether might have the properties of sand. It was to be granular. He recognised that this kind of material had been overlooked by the elasticity specialists and had strange properties. They are indeed very strange - if you put a large stick into a jar of sand you may easily pull it out, but if you simply tap the jar sharply, the sand will instantly settle in such a way that the whole jar can be raised by lifting the stick. If you step on wet sand at the beach, you will see as Reynolds did that sand becomes dry around your foot, when common sense says it should become wetter. Such observations drew great admiration from the likes of Lord Kelvin (who shared with Reynolds his birthplace of Belfast), but only bemusement from Reynolds' colleagues as regards the nature of the ether. His rather undisciplined ideas are well regarded today, for granular materials are a hot topic of research and - to be fair to Reynolds - we don't understand them much better than he did.
Stokes thought the ether was more like a jelly or a wax, or like the cup of thick chocolat au lait that Sir Gabriel enjoyed one day in a Paris café, when he wrote to Lord Kelvin in Glasgow about his idea.
Kelvin himself thrashed around with ether models for fifty years. In one of these he conceived the ether as a special kind of liquid foam, and again this has a resonance in materials research today. The hypothesis he made about the ideal structure of a foam of equal-sized bubbles remained controversial for a hundred years. It was overthrown by my research student, Robert Phelan in 1994, when he was the first to find a structure of lower energy - 0.3% less. A headline at the bottom of the front page of the Irish Times read "Throwing shapes at Trinity". I have regretted ever since that I had not fed the paper a better headline "Ireland beats Scotland by 0.3%" It was the morning of the international rugby match against that country.
The end of the ether
When Kelvin conceived his foam model, lying in bed in his country house, the idea of a material ether was already in decline. Its death warrant had been signed by James Clerk Maxwell when he produced a combined theory of electricity and magnetism, out of which light waves emerged naturally as fluctuations of electric and magnetic fields.
But even Maxwell himself did not at once discard the idea of an ether. Indeed he described it as follows:
"The vast interplanetary and interstellar regions will no longer be regarded as waste places in the Universe. We shall find them to be already full of this wonderful medium; so that no human power can remove it from the smallest portion of space or produce the slightest flaw in its infinite continuity".
Only after fifty years of refinement and familiarisation of Maxwell's work did its leading proponents - the Maxwellians - firmly insist that all the properties of light could be found in Maxwell's theory.
It was against that background that Kelvin maintained his personal determination that the ether was a "real thing". Your models, said George Francis Fitzgerald, provide at best an allegory of the ether. "Certainly not an allegory on the banks of the Nile " replied Kelvin in a fitting joke for two Irishmen to share.
And even the Maxwellians kept the word ether to stand, at least poetically, for empty space endowed with Maxwell's properties, and perhaps a little more. Listen, for example, to the triumphant George Francis Fitzgerald of TCD in 1888, the acknowledged leader of the Maxwellians, telling the world the significance of the experiment of Henrich Hertz. (This experiment generated electromagnetic waves, similar to light waves but of long wavelength, by means of an electrical circuit, in accordance with Maxwellian ideas. As well as that fundamental significance it may be regarded as the invention of radio transmission).
Fitzgerald:
"It was a great step in human progress when man learnt to make
material machines
when he used the elasticity of his bow and the rigidity of his arrow to
provide food and defeat his enemies.
It was a great advance when he learnt to use the chemical action of fire, when he learnt to use water to float his boats and air to drive them.
When he used artificial selection to provide himself with food and domestic animals.
For two hundred years he has made heat his slave to drive his machinery.
Fire, water, earth and air have long been his slaves,
But it is only within the last few years that man has won the battle lost by the giants of old.
Has snatched the thunderbolt from Jove himself.
And enslaved the all-pervading ether!"
Around the same time the material ether was dealt another blow by the
experiment of Michelson and Morley, which echoes that old problem in
Helsham's textbook. This failed to detect any effect of the bodily notion
of the ether relative to the earth, upon light waves propagating in that
ether. In today's physics textbooks, this is given a decisive role in
killing off the ether, but it was in reality only one small chapter in its
gradual demise. Incidentally, it was not crucial to the inspiration of
Einstein's relativity either - but of such convenient myths is school and
undergraduate teaching constructed.
Voices from beyond
There is another side to this story which is both amusing and sad. The
mysterious ether was eagerly adopted by the spiritualists who became
fashionable in the Victorian period, as a pseudoscientific justification of
their claims.
By 1870 spiritualism, transplanted from the United States, had taken firm root in England. Mediums, professional and amateur, proliferated. The upper classes delighted in their performances and the leading exponents were national celebrities.
The movement found an early and influential champion from the first rank of the scientific establishment in the person of Sir William Crookes. He was impelled into that dark circle by the tragic loss of a brother. Gradually he was attracted by another emotion - he spoke of "peculiar temptations". These were embodied in the shapely form of Miss Florence Cook. Her seances featured the materialisation of another young girl, Katie King.
The scene is comic. A trivial piece of trickery, practised in the half-light, deceived an eminent man of science, whose hormones must have ruled his head. No wonder that a Hollywood movie has been considered.
Some scientists remained staunchly resistant to the new fashion and the constant invocation of the ether to support it: Faraday, Tyndall and Kelvin were all outspoken against it. But many others - such as Rayleigh, J.J. Thomson, Ramsay, Crookes and Lodge took what Kipling called "the oldest road, the craziest road of all" leading to nothing but "sorrow in store".
Remember, in order to understand their astonishing credulity, that this was the time when all sorts of new rays emerged in the laboratory. These were both real - in the case of x-rays and various emanations from radioactive substances - and imaginary, the products of self-delusion. The spurious N-rays, discovered in France by Blondlot, were and observed in Dublin by Felix Hackett, who published his findings. It seems that only the UCD students refused to believe him!
From this to the world of occult phenomena was a small step. Of the scientists who took up took it, and became devotees of spiritualism, Oliver Lodge was the most steadfast. He was more of a heavyweight academic physicist than Crookes, and indeed he tempered his advocacy with caution most of the time. In fact he was a Maxwellian, and a great admirer of his colleague across the Irish Sea, George Francis Fitzgerald. One day when walking along the Dublin Quays, I happened to step into Lafayette's old photographic studio, which happily remains there. There, to my amazement, stood on an easel a magnificent photographic portrait of Lodge.
"Do you know who that is?" I said. " Yes", said the Manager confidently, "it is Sir Oliver Lodge " and then he smiled. "But who was Sir Oliver Lodge?" Now he knows. For the studio it was just their prize example of turn-of-the-century work. For me it was like suddenly meeting an old friend.
Despite his fervent support for Maxwell's theory, Lodge still believed that, as an Irish comedian used to say "There's more". The ether was, he said, "the primary instrument of mind, the vehicle of soul, the habitation of spirit".
But like Kelvin, eventually he found himself struggling against a flood tide of scepticism as the new century dawned.
Just when it seemed that it had all been a waste of time, the paroxysm of grief engendered by the Great War created a new clientele of eager believers. In 1915, Lodge's youngest son Raymond was killed in Flanders. In his anguish he turned again to spiritualism and soon made contact with his lost, loved son.
He recounted the whole story in his book "Raymond", with the now customary chapters on life, death and the ether. It was a huge success. As the war drew to a close, the tenth edition was already being printed.
Spiritualism has since declined, but it exerts a powerful hold on a dedicated minority. The Society for Psychical Research, founded in 1882, still exists. It presumably meets regularly to engage in earnest discussions of the ether.
And perhaps this will always be so as long as we yearn for something more than a brief life and bereavement. As Yeats said :
Though grave-diggers' toil is long,
Sharp their spades, their muscles strong,
They but thrust their buried men
Back in the human mind again
The twentieth century
In science we no longer speak of the ether in the empty box. Instead we picture Maxwell's fluctuating electric and magnetic fields - we are as at home with those once-abstract fields as we are with solid matter. We may well then ask: what particular form do these fields take? In particular, how is energy distributed among the electromagnetic waves that bounce around inside the box, if we leave it alone?
This innocent question is one of the great questions of the history of science. In December 2000 physicists congregated in Berlin to celebrate the theoretical solution of the problem by Max Planck in 1900. This is regarded as the birth of quantum theory, the principal ingredient of 20th Century physics. There is another element of myth in this notion. It was Einstein, five years later, who asserted that Planck's somewhat serendipitous formula was inconsistent with traditional physics.
One of the many consequences of the new quantum theory is that no matter how much you cool the box you cannot take all of that electromagnetic energy out of it: there is an irreducible minimum - the box cannot be emptied!.
Planck's historic formula, which describes what's in the box at any temperature, works surprisingly well if we think of the entire universe as a rather large and almost empty box. The sudden realisation that it was so came crashing in upon cosmology in the mid-sixties and is largely responsible for the current thriving industry of Big Bang Theorists. This came about in a curious way.
Two American scientists, trying to improve microwave communications, found an unexpected background hiss interfering with their efforts. They tried to attribute it to such artifacts as pigeons nesting in their antenna - but, having eliminated all such causes, finally they were driven to recognise that that it came from deep space and might have deep significance. Indeed it has.
In a further twist, cosmologists and astronomers are increasingly sure that there may also be a dark matter out there, not detectable by us in any direct way.
And meanwhile quantum field theorists insist that our box, large or small, is home to many other fields, besides Maxwell's. These represent the possible appearance and disappearance of elementary particles of all kinds.
Our box, once filled with ethereal spirits, and almost emptied again by the Maxwellians, is once more beginning to be very crowded. It is full of invisible and inevitable fields, some that we know, and more that we don't. Sir Oliver Lodge might well look down from that easel in the Lafayette Studio, wink, and say: "Well, didn't I tell you so?"
Looking forward
And what of the future?
Physicists have never been very good at futurology - for example,
Rutherford, the father of nuclear physics, insisted that the exploration of
nuclear energy was pure "moonshine". But at this millennial moment in time,
it's hard to resist the question.
On the grand scale, dark matter will be identified. But not easily, I suspect. It may turn out to be something quite novel. If so, the 21st century will begin as dramatically as did the 20th, and dark matter will then do much more than just tidy up the models of astronomers for such things as the large-scale structure of the universe - the tenuous foam-like structure in which matter is distributed.
Looking in the opposite direction, inwards to the smallest lengths we have ever contemplated - physics on the scale of 10-33 cm, we are told that our 20th century physics breaks down, that space itself becomes discontinuous, like the grains of sand of Reynolds or the foam structure that Kelvin suggested. I wish I understood what this kind of new theory really means: I am as bewildered as the contemporaries of Einstein, when he insisted that a fresh start must be made, in 1905. It will certainly lead to something very new and our grandchildren will confidently learn about it at school.
I don't believe we are on the brink of a theory of everything, as some say - it seems to me more a slogan to sell books than to tell truths. Admittedly there is a certain ennui in physics today, a sense of convergence towards finality. Not for the first time! It has always been misguided.
Every branch of physics has enjoyed a spectacular century, and like a Caesar's army returning in triumph, we are likely to suffer a bit of hangover. Been there, done that. But decline and fall are unlikely.
My own branch of the subject, condensed matter physics, has transformed our society by producing the silicon chip and letting the genie of information technology out of the bottle. It is still busy providing the means to take that revolution yet further in such directions as fibre optics, with quite unforeseeable consequences.
Let me add a final and related thought from interdisciplinary science: surely one of the greatest challenges of the 21st century will be to understand the brain. I hope that physicists can help to provide the mental agility, flexibility and the boldness necessary to comprehend it. In so doing, we may even arrive at some proper scientific theory of consciousness.
And then Sir Oliver would really be pleased.