The Book of Nothing: Vacuums, Voids, and the Latest Ideas about the Origins of the Universe - John D. Barrow (2002)
Chapter 4. The Drift Towards the Ether
“The idea of an omnipresent medium has considerable attractions for the scientist. It enables him, for example, to explain how such familiar phenomena as light, heat, sound and magnetism can operate over great distances and travel through a seemingly empty space.”
NEWTON AND THE ETHER: TO BE OR NOT TO BE?
“Nothing is enough for the man to whom enough is too little.”
Newton’s studies of motion and gravity in the second half of the seventeenth century followed a trajectory that would lead to staggering success. He could explain the motions of the Moon and the planets, the shape of the Earth, the tides, the paths of projectiles, the variation of gravity with altitude and depth below the Earth, the motion of bodies when resisted by air pressure, and much else besides. Newton did this by making a spectacular imaginative leap. He formulated laws of motion in terms of an idealised situation. His first law of motion states that ‘Bodies acted upon by no forces remain at rest or in motion at constant velocity.’ No one had ever seen (or will ever see) a body acted upon by no forces, but Newton saw that such an idea provided the benchmark against which one could reliably gauge what was seen. Whilst others had thought that bodies acted upon by no forces just slowed down and stopped, Newton identified all the forces that were acting in any given situation, and thought otherwise. When no motion occurred it was because different forces were in balance, leaving zero net force acting on the body.
Despite the power and simplicity of Newton’s ideas, there was an awkward assumption at their heart. Newton had to suppose that there existed something that he called ‘absolute space’, a sort of fixed background stage in the Universe upon which all the observed motions that his laws governed were played out. Newton’s famous laws of motion applied only to motions that were not accelerating relative to this imaginary arena of absolute space.2 Today, we might approximate it by mapping out an imaginary scaffolding using the most distant, most slowly changing astronomical objects that we can see, the quasars.
Absolute space was a tricky notion. It was the linchpin of Newton’s theory but you couldn’t observe it, you couldn’t feel it and you couldn’t do anything to it. It begins to sound as mysterious and elusive as the vacuum itself. It carried with it the added difficulty of not explaining how gravity or light could be propagated through it. One answer to this riddle was to give up the notion that space was empty in between the solid objects dotted around within it and instead imagine that the ‘empty’ space between contained an extremely dilute fluid that filled its every nook and cranny like a uniform motionless sea. This fluid begins to look like a candidate to replace the entirely mathematical concept of ‘absolute space’ because motion can always be described as taking place relative to the tenuous fluid.
This great unchanging sea filling all of space became known as the ether. It is reminiscent of the elastic substance, or pneuma, that the ancient Stoic philosophers proposed as a space filler, which played an active role in their attempts to understand the world. The spreading of sound outwards from its source was interpreted as a motion through the pneuma, like a wave through water. The familiarity of this analogy did much to encourage its adoption as a model for the permeability of all space. Its removal of the need to worry about real vacua ever again was an added attraction.
Newton never displayed any great enthusiasm for the idea and adopted it with some reluctance for want of something more compelling. He recognised that the ether provided a convenient vehicle to understand some of the properties of light and the propagation of its effects through space, but the presence of a fluid would play havoc with the motions of the Moon and the planets. He understood the motions of bodies in liquids and other resisting media very thoroughly and protested that the presence of an all-pervading resisting medium would just retard the motions of the celestial bodies. Eventually, they would grind to a halt.
Torricelli, Pascal and Boyle had investigated a number of properties of the local vacua they could apparently create in their mercury columns. They had shone light through them, and so deduced that light could penetrate the evacuated space; magnetic attraction was not inhibited either; radiative heat passed unimpeded through the jars of empty space; and bodies fell to the Earth under gravity just as they did in air.3 Newton was well aware of these features of ‘empty’ space and so wondered if perhaps it was not so empty after all, so that the heat and light could be propagated by the ‘vibrations of a much subtler medium than air, which after the air was drawn out remained in the vacuum’.4
In trying to sustain this idea, Newton got himself in a very complicated tangle. His first thought was to view light as a stream of minute particles (which we would now call ‘photons’) that bounced off reflecting surfaces and behaved like the tiniest of perfectly elastic billiard balls. Unfortunately, both he and the Dutch physicist, Christiaan Huygens, had discovered that under some circumstances light did not behave like a stream of little billiard balls at all. Two light beams slightly out of phase with one another could be made to interfere and produce an alternation of dark and light bands. This behaviour is characteristic of waves but not of particles. It can be explained by adding two waves so that the peaks of one wave match the troughs of the other. Newton had observed more colourful consequences of the wavelike behaviour of light, like those we see in the colours created when light passes through oil on the surface of water or scatters off a peacock’s tail.
The most useful guide to the issue was the behaviour of sound. Sound is propagated from one point to another by means of undulations in the intervening medium. When we shout across the room it is the vibrations of the molecules of air that carry the energy that we call sound from one place to another. This picture was one that physicists focused upon when thinking about how light moved through empty space. Unfortunately, unlike heat and light, sound was not something that was transmitted through the jars of vacuum that Boyle and others had been producing. The extraction of the air from the vacuum tube removed the very medium whose vibration could convey its effects to distant places. Although we see the Sun and feel the heat that it radiates to us through the intervening ‘empty’ space we can’t hear anything that happens on the surface of the Sun, despite the fantastic violence of those events.
Newton’s first attempt to draw these two properties of light together was to imagine that bullets of light must create waves by hitting the ether, just as throwing a stone into water creates a train of waves moving outwards from the impact point. The light would be able to set up an undulatory motion in the ether fluid. Gravity would accelerate them until the accelerating force became equal to the resisting force of the ether and then they would move with constant speed. However, light moves so rapidly that the accelerating force would need to be unrealistically large in order to accelerate the light particles up to 186,000 miles per second so quickly.
Newton was never fully persuaded of the cogency of this ethereal picture and continued to pose questions about the propagation of light and gravity through space without ever convincingly answering them. Newton would not allow himself to lapse into the ancient delusion that some innate property of things called ‘gravity’ was responsible for the distant action of one mass on another (for this would explain nothing). In his famous correspondence with Richard Bentley5 about the ways in which his work on gravity and motion could lend support to a new form of Design Argument for the existence of God based upon the precision and invariance of the laws of Nature themselves, rather than the fortuitous outworkings of those laws, Newton revealed his puzzlement at the way gravity could apparently act through a vacuum:6
“It is inconceivable, that inanimate brute matter should, without the mediation of something else, which is not material, operate upon, and affect other matter without mutual contact; as it must do if gravitation, in the sense of Epicurus be essential and inherent in it … That gravity should be innate, inherent, and essential to matter, so 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 and force may be conveyed from one to another, 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 must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers.”
One can imagine how problematic Newton’s picture of forces acting instantaneously at a distance must have been for many of his contemporaries to accept. The rival theory of planetary motion in Newton’s time was the vortex theory of Descartes. It viewed the Universe as a great whirlpool of swirling particles whose actions upon one another were conveyed by physical contact (Figure 4.1). Descartes denied that a vacuum existed in space and filled it with a transparent fluid, matière subtile, which became a key part of the Cartesian world view.
This picturesque swirling image of the Universe had far more popular appeal than Newton’s austere mathematical clockwork. Everyone had seen eddies of turbulent water. The analogy was familiar and convincing: stirring water in one part of the bath tub would propagate effects across the surface to other parts of the water. Descartes appeared to offer a plausible mechanism whereby the effects of gravity could be communicated through space. Yet Descartes’ theory failed. It could not explain the observed motions of the planets, enshrined in Kepler’s famous ‘laws’. It was a lesson on the difference between human conceptions of what looks ‘natural’ and what is natural.8
Figure 4.1 René Descartes’ system of vortices (1636).7 Each vortex represents a solar system in a never-ending expanse of solar systems. The centres of the vortices (at the points marked S, E and A) are stars that are shining because of the turbulent motions of the vortices. The sinuous tube passing across the top of the picture is a comet that is moving too fast to be captured by any of the solar systems.
The ebb and flow of Newton’s views about the ether make interesting reading. In the 1670s he was seeking to persuade Boyle that there existed a subtle ethereal spirit of air (aere) because in a vacuum a swinging pendulum continued oscillating for so little longer than it did in air. Newton argued that there must exist another fluid, playing a similar role to that of air, which slows the pendulum even if it is placed in one of Boyle’s vacua. He also claimed that some metals could fuse and become heavier even when sealed within a glass container. This, he suggested, meant that some sparse fluid must be passing through the pores in the glass container in order to increase the mass of the metal. A little later he tried to use the ether as a device to explain the reflection and refraction of light, and to persuade Boyle that the non-uniformity of an ether could explain the existence of gravity.
By the 1680s Newton had lost his enthusiasm for the ether. In the Principia (1687), he excludes the existence of such a medium permeating masses because it would have an incalculable disturbing influence upon the motion of celestial bodies. Then, in the second book of the Principia, he considers directly ‘the opinion of some that there is a certain ethereal medium extremely rare and subtle, which freely pervades the pores of all bodies’ and seeks to find experiments which might test the idea. He returns to its effects on the swinging pendulum, now interpreting the evidence as indicating that there is no discernible difference in the damping of the pendulum’s motion in air or vacuum. Thus he concludes that if an ether exists, its effects must be so subtle as to be indiscernible and so it can safely be ignored for the purposes of explaining gravity and other observable phenomena – a complete about-turn.
Six years later Newton was trying to convince Bentley of the impossibility of an influence like gravity acting instantaneously over great distances, whilst writing to Leibniz that a fine form of matter did indeed fill the heavens above. By the time the second edition of the Principia appeared in 1713, Newton had added to the text of the first edition that there was indeed a ‘subtle spirit which pervades and lies hid in all gross bodies’ and it was this that allowed him to understand the forces of Nature: gravity, heat, light and sound. In what way it did so was not revealed, because it ‘cannot be explained in a few words’.
Newton’s last views on the ether appear in some of the questions posed at the end of the second edition of his Opticks (1717). Here, he claims fresh experimental evidence for the ether’s existence by comparing the behaviour of thermometers in air with those sealed in an evacuated tube.9 Again, the lack of discernible differences in their response to heat convinced Newton that a medium ‘more rare and subtle than air’ must still be present in the evacuated container in order to transmit the heat from outside. Harking back to his first speculations about the existence of an ether, he then suggests that this subtle medium must be much sparser within dense bodies like the Sun and the planets than it is in the interplanetary space between them. Thus gravity arises because bodies attempt to move from where the ether is denser to where it is sparser10 – ‘every Body endeavouring to go from the denser parts of the Medium towards the rarer’– seeking to even out the distribution.11 Finally, Newton tried to offer some mechanical explanation for the elusivity of the ether. It was made of particles that are ‘exceedingly small’ and its elasticity arose from the fact that these particles repel one another. The forces are stronger in small bodies than in large ones in proportion to their mass. The result is12
“… a Medium exceedingly more rare and elastick than Air, and by consequence exceedingly less able to resist the motion of Projectiles, and exceedingly more able to press upon gross Bodies, by endeavouring to expand itself.”
Newton’s speculations on the links between the elusivity and the elasticity of the ether ended with these questions. He never published a detailed theory of the quantitative properties of the ether and its role in mediating the force of gravity. The clarity of his predictions of the effects of gravity and motion stood in contrast to his continual attempts to come to a satisfactory conclusion about the true nature of the vacuum and the way in which forces traverse it. Newton was ahead of his time in almost everything he deduced about the workings of the world, but in the matters of the ether and the vacuum, the jump into the future was too far even for him.
DARKNESS IN THE ETHER
“I have not had a moment’s peace or happiness in respect to electromagnetic theory since November 28, 1846. All this time I have been liable to fits of ether dipsomania, kept away at intervals only by rigorous abstention from thought on the subject.”
The problem of empty space was entwined with another long-standing riddle: the darkness of the night sky. Descartes’ philosophy had rested firmly upon a belief in the impossibility of empty space. He believed in a universe of unending extent. Only matter could have spatial extent and so where there was no matter there could be no space. Everything was moved by forces arising out of direct physical contact. There was no spooky action at a distance across the vacuum. His picture of celestial vortex motions which permitted interactions to occur only by contact (see Figure 4.1) led him to refute the atomists’ picture of ‘atoms’ of matter separated by void. Matter must be continuous and free from voids or other discontinuities. If atoms were introduced into his theory then they would necessarily be in continuous contact with one another and so necessarily extended rather than isolated points of matter as the atomists had imagined.
Descartes’ Newtonian opponents rejected his conception of matter moved by purely mechanical laws. Many assumed that the darkness of the night sky between the stars was direct evidence of the infinite and eternal extracosmic void which the ancients maintained existed beyond the edge of a material world of finite size and age. We were seeing through the finite celestial world into the dark void beyond. Thus we see that the Cartesians combined Aristotelian and Epicurean ideas: like Aristotle, they rejected both the vacuum as a physical reality and the atomic nature of matter, but like Epicurus they believed that space had no limit. The Newtonians, by contrast, merged Stoic and Epicurean philosophies: like the Stoics they rejected the idea that the stars were infinite in number and extent, but like the Epicureans they accepted the existence of the vacuum and the basic atomic structure of matter. Later, the Newtonian picture would dispense with its Stoic aspect and use only the Epicurean picture of the boundless population of stars, pictured in Figure 4.2.
Anyone who believed that the Universe contained an infinite distribution of stars was faced with explaining the darkness of the night sky.14 If one looked out into such an infinite array of stars then it would be like looking into a never-ending forest: one’s line of sight would always end on a tree. We should see the entire sky as if it were a single bright starry surface. Evidently, this was not the case.
The hypothesis that space was filled with a tenuous ether created new possibilities for explaining the darkness of the night sky. In the nineteenth century, the Irish astronomer John Gore suggested16 that the darkness of interstellar space might be evidence for regions of total vacuum devoid of both matter and ether:
Figure 4.2 Isaac Newton’s view of the Universe in 1667, during his early years in Cambridge.15 This picture combines ancient Epicurean and Stoic conceptions of the cosmos.
“It has been argued by some astronomers that the number of the stars must be limited, or on the supposition of an infinite number uniformly scattered through space, it would follow that the whole heavens should shine with a uniform light, probably equal to that of the sun.”17
Gore and the Canadian astronomer Simon Newcomb18 both believed that the puzzle of the dark night sky would be solved if our Milky Way galaxy were shielded from the stars and nebulae beyond by a perfect vacuum region across which starlight could not travel. Thermodynamically, this sounds rather odd. What happens to the starlight when it impinges upon this impervious vacuum region? They suggested that it was reflected back so that
“we may consider … the reflecting vacuum as forming the internal surface of a hollow sphere.”
In their scenario each galaxy of stars and ordinary material is surrounded by a spherical ‘halo’ of ether, but the intergalactic region between the ether halos is a perfect vacuum which light cannot penetrate (see Figure 4.3).
One can see that for all practical purposes, the other galaxies, with their ethereal halos, might as well not exist. They are unobservable in principle. The darkness of the night sky is really being explained by supposing that the Universe is astronomically finite and contains very few stars. All the rest are an optical illusion. Unfortunately, this does not work. If each galaxy is surrounded by a mirror of perfect vacuum then the starlight from the stars it contains will be bounced back and forth across the galaxy and end up contributing a similar amount of light to the visible sky as would be incoming from the other galaxies.
Figure 4.3 Newcomb and Gore’s solution to the puzzle of the darkness of the night sky.19 Each galaxy of stars is surrounded by a sphere of ether. The space between the galaxies contains no ether and so cannot transmit light. The spheres of ether act as if they are reflecting mirrors and prevent observers within them receiving light from other spheres.
NATURAL THEOLOGY OF THE ETHER
“If God had meant us to do philosophy he would have created us.”
During the eighteenth and nineteenth centuries theologians were greatly impressed by arguments for the existence of God which cast Him in the role of cosmic Designer. The existence of such a Designer, they argued, was evident from the structure of the world around us. This structure had two telling strands. First, there were the apparent contrivances of the living world. Animals seemed to inhabit environments that were tailor-made for their needs. What more perfect design could be found than the camouflage markings on the coat of an animal that merged so exactly with its surroundings? These arguments about the ways in which the outcomes of Nature’s laws are in mutual harmony had been supplemented by the more sophisticated Design Argument based upon the success of Newton’s elucidation of simple laws of Nature that Richard Bentley promulgated. This second version of the Design Argument pointed to the simplicity and mathematical power of those simple, all-encompassing, Newtonian laws as the primary evidence for a Cosmic Lawgiver who framed them.21
In all of these natural theological discussions the Universe was viewed as a harmonious whole in which all its components were wisely and optimally integrated into a grand cosmic scheme. Humanity was a beneficiary of this scheme, but only the most naive pictures of Design would insist on making humanity’s well-being the goal or final cause of the whole creation. The ether fitted into this teleological conception of the Universe because it resolved the old objection that a void space serves no purpose and implies that the Deity was responsible for making things that were a waste of space. The ether plugged this critical gap by doing away with the purposeless emptiness. It found itself elevated in the minds of some theologians to play a role just a little lower than the angels as the main secondary cause by which God regulated the motions of the celestial bodies. For example, John Cook argues that
“Ether is the Rudder of the Universe, or as the Rod, or whatever you will liken it to, in the Hand of the Almighty, by which he naturally rules and governs all material created Beings … Now how beautiful is this Contrivance in God.”22
This line of argument was developed most elaborately by William Whewell in his contribution to the famous Bridgewater Treatises on Natural Theology, a series of works by distinguished nineteenth-century scholars seeking to provide support for Christian religious belief by appeal to scientific discovery. Whewell’s volume23 dealt with the contribution of astronomy and physics to the argument. Since he was a strong supporter of Huygens’ wave theory of light, the ether played a key role in his conception of the physical universe and he was greatly persuaded of its crucial role in the theological scheme of things as well. He argued that ether was providentially designed by the Almighty in order to enable us to see the Universe with our visible sense. It was one of the three fundamental substances in the Universe, beside matter and fluid.24 Without it, the Universe would be dead, inert and unknowable. Its very existence was thus evidence for the wisdom, goodness and anthropocentric good intentions of God.
Amongst notable scientists, the most speculative views on the ether are to be found in the works of the Scottish physicist Peter Guthrie Tait, who is famous for his joint work with Lord Kelvin and his pioneering ideas in the mathematical theory of knots. In 1875, Tait co-authored a popular science book with Balfour Stewart which bore the title The Unseen universe; or, physical speculations on a future state.25 Its purpose was to demonstrate the harmony of religion and science and, in seeking to do this, it had some remarkable things to say about the ether.
Stewart and Tait suggested that all matter was composed of particles of ether, but these ether particles were composed of an even subtler collection of ether particles, and so on, ad infinitum. This hierarchy of ethers was arranged in an ascending one-way street of energies, so that lower-order ethers could always form from a higher, but not vice versa. Stewart and Tait imagined their staircase of ethers rising, like Jacob’s ladder, to attain infinite energy and ultimately becoming eternal and co-equal with God. The creation of the world was simply the cascade of energy down the spectrum of ethers so that it became localised in matter at the lowest levels, those we see around us and in which we have our being.
A DECISIVE EXPERIMENT
“Now the sirens have a still more fatal weapon than their song, namely their silence … someone might possibly have escaped from their singing; but from their silence never.”
In the middle of the nineteenth century, it was the accepted view of almost all scientists that space was filled with a ubiquitous ethereal fluid. There was no vacuum. All forces and interactions were mediated by the presence of ether, either by waves of ether or by vortices. The favoured scenario was one in which the ether was, on average, stationary; others suggested that it was dragged around by the daily rotation of the Earth and by its annual orbit of the Sun. To question this picture seemed rather foolish, a little like questioning whether the Earth possessed an atmosphere of air. The existence of the ether was fast becoming one of those scientific truths we hold to be self-evident. Yet, whilst its existence was not doubted, its physical characteristics were the subject of lively debate.27 Some held it to be thin and tenuous, others argued it was an elastic solid, and others still that its properties changed according to the ambient conditions. In such a confused atmosphere speculative theories abound, and all manner of contrived additional properties are easily invented to modify the favoured hypothesis in the face of new objections or awkward facts. What is needed is a decisive experiment. Just as Torricelli cut through the convoluted debates about the possibility of the physical vacuum by providing an experimental window into the question, so it would be with the ether. The impetus was to come from a side of the Atlantic where few would have expected the next great step in our understanding of motion to be taken.
Albert Michelson was born on 19 December 1852 in the small town of Strelno near the Polish-German border.28 Technically, Strelno had been in Germany since the time of Frederick the Great but its traditions were Polish, like its citizens, and it was less than eighty miles from Copernicus’ birthplace. In the face of political upheaval and persecution, the Michelson family joined thousands of other Polish emigrants to the United States when Albert was just two years old. After working as a jeweller for a time in New York, Albert’s father Samuel joined the gold rush to California to seek his fortune. Soon afterwards, California became a State of the Union and grew rather prosperous. Samuel Michelson prospered as well and set up his own store in Calaveras County. The rest of the family eventually joined him after a formidable sea voyage to Panama followed by a dangerous overland trek across the neck of the continent (in the days before the canal) to the Pacific, where they took another boat to San Francisco before the final overland journey to the Gold Towns. There, in a wild-west frontier atmosphere, far from the world of learning and traditional culture, the young Michelson spent his early formative years. As a child, he was exceptionally gifted at constructing mechanical devices and showed an early aptitude for mathematics, together with a fascination for the rocks and minerals that the miners dug out of the ground. On reaching his thirteenth birthday, he was sent away to high school in San Francisco, and after graduating successfully three years later he entered a fierce competition for a place at the US Naval Academy in Annapolis, Maryland. Alas, he didn’t get the place. He tied in the examinations with a younger candidate from a poor background who was given the casting vote by the selection board despite the mountain of letters written in support of Michelson.
Michelson didn’t give up. Such was his determination to be admitted to the Academy that he appealed directly to President Grant for a further place to be created. Learning of the President’s daily routine of walking his dog, he travelled to Washington and waited on the White House steps for him to return. Grant listened patiently to the teenager’s request but said there was really nothing he could do. All the places at the college were filled. But then he remembered a letter he had received from Michelson’s congressman arguing his case on the ground of his father’s great contribution commercially and politically to the Republican cause: to reward young Michelson would bring further support to the President in his home state. For whatever reason, the President decided to intervene and sent the young Michelson to see the Superintendent of the Naval Academy in person. Interviews followed and after just a few days Michelson heard that an extra place had been created at the Academy for new entrants that year and it had been awarded to him. He entered as a cadet midshipman and gradually distinguished himself at the college in all the science courses, less so in military matters.29 After graduating, and spending a short spell at sea, he was appointed instructor in physics and chemistry at the Academy and began to develop his expertise in optics and experimental physics. His first distinguished contribution to science was a precision measurement of the speed of light. After this work was completed, in 1880, Michelson took a period of leave from the Navy and took his family to Europe. It was a trip that was to change the direction of science.
Michelson spent two years moving between some of the leading European universities, learning about the new developments in physics and, inevitably, listening to some of the foremost theoretical physicists expound their theories of the ether – the great puzzle of the day. It became a source of continuing fascination for Michelson. Did this strangely elusive medium exist or not? Was there a way of measuring it?
James Clerk Maxwell had suggested30 that by checking whether the speed of light was the same in different directions we would learn something about the motion of the ether stream through which the light had to propagate; for
“If it were possible to determine the velocity of light by observing the time it takes to travel between one station and another on the earth’s surface, we might by comparing the observed velocity in opposite directions determine the velocity of the ether with respect to these terrestrial stations.”
But Maxwell doubted whether it would be possible to conduct this experiment and discover the answer. Michelson ignored these pessimistic predictions. He saw that there was a straightforward way to realise Maxwell’s inspired suggestion. Suppose that the ether is not moving, so it specifies some state of absolute rest. Then we must be moving through it, as the Earth spins on its axis and orbits the Sun. A suitable detector might be able to measure the wind of ether that our movement through it would create, just like a cyclist feels the wind on his face as he rides through still air. If the ether was moving, then we should feel different effects when we move upstream and downstream within it.
Michelson began to develop his ideas by drawing up simple analogies. Moving through the ether should be like swimming in a river. The flow of the river is the flow of ether past us caused by the Earth’s motion through the stationary sea of ether. Now imagine a swimmer who makes two return trips in the river. The first is across the river at right angles to the flow; the second is downstream and then back upstream. In both cases he ends up at the same point at which he began; the two round-trip paths are shown in Figure 4.4. If the same total distance is swum in each case then it is always quicker to swim the cross-river circuit than the down-and-upstream circuit. To see this, let’s do a simple example. Suppose the river flows at a speed of 0.4 metres per second and the swimmer can swim at a speed of 0.5 metres per second in still water. Each leg to be swum will be 90 metres in length.
Figure 4.4 A swimmer makes two round trips of equal distance: one across the river and back, the other upstream and downstream.
The swim downstream followed by the return upstream has the swimmer moving at 0.5 + 0.4 = 0.9 metres per second relative to the bank on the downward trip. The time to swim 90 metres is therefore 90 ÷ 0.9 = 100 seconds. Returning upstream his speed relative to the bank is only 0.5 − 0.4 = 0.1 metres per second and he takes 90 ÷ 0.1 = 900 seconds to make it back to the start. The total round-trip time is therefore 900 + 100 = 1000 seconds.
Now consider the cross-river route. He will find it just as hard to swim each way as he is always swimming at right angles to the current. His speed perpendicular to the flow of the river is given by an application of Pythagoras’ theorem for triangles, applied to the velocities. The actual speed he can swim across the river will be equal to the square root of 0.52 − 0.42 = 0.09, which is 0.3 metres per second (see Figure 4.5). Thus he can swim 90 metres in 90 ÷ 0.3 = 300 seconds. The total time he takes to swim 90 metres across the river and 90 metres back is therefore 600 seconds. This is different from the round-trip time up and downstream because of the speed of flow of the river. Only if the speed of flow of the river is zero will the two round-trip times be the same.
Figure 4.5 The speed that the swimmer can achieve across the river is given by Pythagoras’ theorem applied to the triangle of velocities.
Michelson concluded that the same should happen if light was ‘swimming’ through the ether. Two light rays emitted in perpendicular directions and reflected back to their starting point should take different times to complete their round trips over the same distance because, like the swimmer in the river, they would experience different total amounts of drag from the flowing ether. Most important of all, if there was no ether then the two round-trip travel times by the different light rays should be exactly the same.
Michelson conceived of a beautiful experiment to test the ether hypothesis. He sent two beams of light simultaneously in directions at right angles to each other and then reflected them back along the directions they had come. The ether hypothesis could then be tested by checking if they both returned to the starting point at the same moment. The experimental set-up is shown in Figure 4.6. Very high precision was possible by exploiting the wavelike character of light. If the light waves arrived back at the same point slightly out of phase with the source waves then there would a slight darkening caused by the overlap of peaks of one light wave with troughs of the other and a brightening where peaks overlap peaks and troughs overlap troughs. The phenomenon, known as interference, creates an alternating sequence of dark and light bands.31 In Michelson’s experiment, seeing no interference pattern of alternating fringes would mean that there could be no effect of the ether slowing the light in one direction but not in a direction perpendicular to it.32
Although the concept of the experiment was simple, the execution was a considerable challenge. The speed of light is 186,000 miles per second whilst the speed of the Earth in its annual orbit around the Sun is only about 18 miles per second. Extraordinary care and accuracy was required if the experimental measurements were going to be accurate and not disrupted by measurement errors and other fluctuations in the experimental set-up. To get some idea of the challenge, if the ether did exist then the tiny difference that should be detected in the light-travel time for rays moving in the two directions would be just one-half of the square of the ratio of the speed of the Earth to the speed of light – less than one part in one hundred million! In order to convince scientists that there was no etherinduced time difference, the accuracy of the measurement had to be better than that.
Figure 4.6 A simplified sketch of Michelson’s experiment. A light beam is divided by a partially reflecting glass plate at G into two beams at right angles. One moves along the path of length L, the other along the path of length K. Each is reflected back by mirrors at M and N and the two light beams are recombined at G and observed. If the times required by the two light beams to travel their two paths are different they will be out of phase when they recombine at G and interference bands will be created.
Fortunately for Michelson, the famous telephone engineer Alexander Graham Bell put up the money to fund the experiment and the interferometer was built in Berlin in 1881. Michelson made his first attempt at the experiment at the University of Berlin, in the laboratory of the famous German physicist Hermann von Helmholtz. He immediately encountered problems. He had to make sure that his array of mirrors was kept at constant temperature by surrounding the entire experiment with melting ice at zero degrees Centigrade, then he had to deal with the vibrations created by the Berlin traffic thundering past outside. The traffic noise ultimately proved unbeatable in Berlin, so he dismantled his apparatus and moved it to the Astrophysical Observatory nearby in Potsdam. Faced now with only the modest vibrations caused by pedestrians, and with his apparatus firmly mounted on the rigid base of the telescope, Michelson finally succeeded in creating the quiet conditions needed to carry out the measurement to the accuracy he needed. The experiment was repeated many times with the apparatus in different orientations, and also at different times of the year, so that the Earth’s motion relative to the Sun would be different. The result was entirely unexpected. With an accuracy that was easily able to detect the Earth’s motion through the ether, it was found that there was no interference pattern. The Earth was not ploughing its way through a ubiquitous ether at all. Michelson’s momentous paper reported his results in August 1881 and concluded that ‘The hypothesis of a stationary ether is erroneous’.33
The responses to Michelson’s discovery fell into two camps. Some concluded that the ether must be non-stationary and dragged around by the Earth as it orbits the Sun so that there is no relative motion between the ether and Earth; others simply concluded that the ether didn’t exist after all. Michelson remained agnostic about the theoretical interpretation of his result.
Michelson returned to a new position at the Case Institute of Technology in Cleveland.34 There he gained a new collaborator, an American chemistry professor fifteen years his senior, called Edward Morley. Morley was deeply religious. His original training had been in theology and he only turned to chemistry, a self-taught hobby, when he was unable to enter the ministry. Michelson, by contrast, was a religious agnostic. But what they had in common was great skill and ingenuity with scientific instruments and experimental design. Together they repeated Michelson’s experiment to discover if the speed of light was the same in different directions of space. When they finished analysing their results in June 1887 there again were no interference fringes. Light was travelling at the same speed in different directions irrespective of the speed of its source through space. There was no stationary ether.35 This was an incredible conclusion. It meant that if you fired a light beam from a moving source it would be found to have the same speed relative to the ground that it would have if the source were stationary. Light moved like nothing else that had ever been seen.
THE AMAZING SHRINKING MAN
“A mathematician may say anything he pleases, but a physicist must be at least partially sane.”
Josiah Willard Gibbs
How could the ether still exist in the face of the null result of Michelson and Morley? An answer was suggested first in 1889 by George FitzGerald at Trinity College Dublin and then developed independently a little later by the Dutch physicist Hendrik Lorentz at Leiden. They suggested that the length of an object will be seen to diminish if it moves at increasing speeds.36 If we take two rulers and hold one still on the Earth but let the other fly past at high speed parallel to it then, as the moving ruler passes by, it will be seen to be shorter than the stationary one. This sounded crazy, even to physicists, but FitzGerald and Lorentz derived their claim from the properties of Maxwell’s theory of light and electromagnetism. FitzGerald even tried to explain the basis of the contraction by arguing that the inter-molecular forces holding solid bodies together are probably electromagnetic in origin and so were likely to be affected if they moved through the ether. He thought that an increase in their attractiveness could be responsible for drawing molecules closer together and reducing the length of any chain they formed.
The amount of the FitzGerald-Lorentz shrinkage was predicted to be very small. Lengths of moving objects would contract by a factor equal to √(1–v2/c2), where v is their speed and c is the speed of light. For a speed of 500 km per hour, we are looking at a contraction that is not much bigger than one hundred billionth of one per cent.
FitzGerald had noticed that if this √(1–v2/c2) correction factor was applied to the analysis of Michelson’s apparatus fixed on the Earth’s surface as it moved around the Sun, it could explain why Michelson measured no effect from the ether. The arm of the interferometer contracts by a factor √(1–v2/c2) in the direction of its motion through the ether at a speed v. At an orbital speed of 29 kilometres per second this results in a contraction of only one part in 200,000,000 in the direction of the Earth’s orbital motion. The length of the arm perpendicular to the ether’s motion is unaffected. This small contraction effect exactly counterbalances the time delay expected from the presence of a stationary ether. If the FitzGerald-Lorentz contraction occurred then it allowed the existence of a stationary ether to be reconciled with the null result of the Michelson-Morley experiment. Space need not be empty after all.
The ideas of FitzGerald and Lorentz37 were regarded as extremely speculative by most physicists of their day, and not taken very seriously as a defence of the ether. They were considered to be purely mathematical excursions devoid of real physical motivation. Attitudes began to change in 1901 when a young German physicist, Walter Kaufmann, studied the fast-moving electrons, called beta particles, emitted by radioactive elements, and showed that the measured masses of these electrons were also dependent on their speeds, just as Lorentz had predicted. Their masses increased with increasing speed, v, to a value equal to their mass when at rest divided by the FitzGerald-Lorentz factor √(1–v2/c2).
The most awkward feature of these attempts to evade casting out the ether was the need to distinguish between a system that was moving and one that was not in some absolute sense. It is all very well to enter a value for v which corresponds to the Earth’s orbital velocity around the Sun in the FitzGerald contraction formula, but what if the Sun and its local group of stars are themselves in motion? What velocity do we use for v and with respect to what do we measure it?
EINSTEIN AND THE END OF THE OLD ETHER
“Navy: Please divert your course 15 degrees to the North to avoid a collision.
Civilian: Recommend you divert your course 15 degrees to South to avoid a collision.
Navy: This is the Captain of a US Navy ship. I say again, divert your course.
Civilian: No, I say again, divert your course.
Navy: This is the aircraft carrier Enterprise. We are a large warship of the US Navy. Divert your course now!!
Civilian: This is a lighthouse. Your call.”
Canadian naval radio conversation38
The nineteenth century ended with a confusing collection of loose ends dangling from Michelson and Morley’s crucial experiment: the absence of the expected ether effect, the need to know the absolute value of velocity, the possibility that motion affects length and mass, and the significance of the speed of light. Albert Einstein made his first appearance on the scientific stage in 1905, at twenty-six years of age, by solving all of these problems at once in an announcement39 of what has become known as the ‘special theory of relativity’. The English translation of his famous paper has the innocuous-sounding title ‘On the Electrodynamics of Moving Bodies’.
Einstein abandoned the idea that there was any such thing as absolute motion, absolute space or absolute time. All motion was relative and two postulates, that the laws of motion and those of electromagnetism must be found to be the same by all experimenters moving at constant velocities relative to one another, and that the velocity of light in empty space must be measured to be the same by all observers regardless of their motion, sufficed to explain everything. This enabled him to deduce as a simple consequence the precise laws for length, mass and time change proposed by FitzGerald and Lorentz. This theory reduced to Newton’s classical theory of motion when the motions occurred at velocities far less than that of light but behaved in quite different ways as velocities approached that of light in empty space. Newton’s theory was seen to be a limiting case of Einstein’s.
This feature of a successful new theory of physics is worth dwelling on as it is overlooked by many commentators. Recently, there have been many newspaper polls to pick the most influential thinkers of the millennium. Newton has topped some polls, but finished behind Shakespeare, Einstein and Darwin in others. On one occasion, Newton’s lower position was justified on the grounds that some of his laws of motion had been shown to be ‘wrong’ by the work of Einstein. Indeed, the outsider might be tempted to think that the whole progression of our knowledge about the workings of Nature is replacing wrong theories by new ones which we think are right for a while but which will eventually be found to be wrong as well. Thus, the only sure thing about the currently favoured theory is that it will prove to be as wrong as its predecessors.
This caricature misses the key feature. When an important change takes place in science, in which a new theory takes the stage, the incoming theory is generally an extension of the old theory which has the property of becoming more and more like the old theory in some limiting situation. In effect, it reveals that the old theory was an approximation (usually a very good one) to the new one that holds under a particular range of conditions. Thus, Einstein’s special theory of relativity becomes Newton’s theory of motion when speeds are far less than that of light, Einstein’s general theory of relativity becomes Newton’s theory of gravity when gravitational fields are weak and bodies move at speeds less than that of light. In recent years we have even begun to map out what the successor to Einstein’s theory may look like. It appears that Einstein’s theory of general relativity is a limiting, low-energy case of a far deeper and wider theory, which has been dubbed M theory.40
In some respects this pattern of ‘limiting’ correspondence is to be expected. The old theory has been useful because it has explained a significant body of experimental evidence. This evidence must continue to be well explained by the new theory. So, wherever physics goes in the next millennium, if there are still high-school students learning it in a thousand years’ time, they will still be learning Newton’s laws of motion. Their application to everyday problems of low-speed motion will never cease. Although they are not the whole truth, they are a wonderful approximation at low speeds41 to a part of the whole truth. They are not ‘wrong’ unless you try to apply them to motions close to the speed of light.
Einstein’s brilliant success in bringing together all that was known about motion into a simple and mathematically precise theory was the end of the nineteenth-century ether. Einstein’s theory had no need of any ether to convey the properties of light and electricity. His postulate that the speed of light must be the same for all observers had the FitzGerald-Lorentz contraction as a direct consequence, and the non-detection of any light-delay effect in the Michelson-Morley experiment was a key prediction of his theory. Many years later, on 15 January 1931, Einstein made a speech in Pasadena to an audience containing many of the world’s greatest physicists. Michelson was there, making what would turn out to be his last public appearance before his death four months later. Einstein paid tribute to the importance of the experiment that Michelson first performed in guiding physicists to their revolutionary new picture of space, time and motion:42
“You, my honoured Dr. Michelson, began this work when I was only a little youngster, hardly three feet high. It was you who led the physicists into new paths, and through your marvellous experimental work paved the way for the development of the Theory of Relativity. You uncovered an insidious defect in the ether theory of light, as it then existed, and stimulated the ideas of H.A. Lorentz and FitzGerald, out of which the Special Theory of Relativity developed. Without your work this theory would today be scarcely more than an interesting speculation; it was your verification which first set the theory on a real basis.”
In fact, Einstein’s career intersected with the ether on many occasions. It only became known after his death that at the age of fifteen he became interested in the stationary elastic ether. He even wrote an article about what happens to the state of the ether when an electric current is turned on, which was not published until 1971.43 Later, he also contemplated carrying out experiments which would be able to verify the existence of an ether. Gradually, he began to doubt its existence. In 1899, he wrote to his girlfriend Mileva Maric of his doubts:
“I am more and more convinced that the electrodynamics of the bodies in motion, such as it is presented today, does not correspond with reality and that it will be possible to formulate it in a simpler way. The introduction of the word ‘ether’ in the theories of electricity leads to the idea of a medium about the motion of which we speak without the possibility, as I think, to attribute any physical sense to such a speech.”44
As a student he learned about Lorentz’s theory of electrodynamics, and the role played by the ether, in his course textbooks. When his thinking drove him towards his new theory of motion, he found he had no need of the ether or of a vacuum with any special properties. It was enough to be able to talk about bodies moving in space and through time. That space was empty unless one chose to add further ingredients to it. It was a matter for investigation whether one needed to include a magnetic or an electric field everywhere in the Universe. If such fields of force were ubiquitous, then his theory could handle them but, equally, it could apply itself to the movements of bodies in a completely empty space.45
The developments in our understanding of matter and motion in the first few years of the twentieth century brought to an end what is sometimes called ‘classical’ physics. Just a few years before there had been serious speculation that the work of physics was all but done. There were refinements to make, further decimal places to establish in experimental accuracy, but all the great physical principles of Nature were thought by some to have been mapped out. The details merely had to be filled in. The discoveries of the quantum theory of matter and the relativity of motion changed everything. New vistas opened up. But they were vistas that did not need a theory of the vacuum, or even a clear notion of what it was. Emphasis switched to the study of how fields and particles influenced one another. Ancient dilemmas like that of the extracosmic void or the nature of absolute space were issues that philosophers still talked about but they were not subjects that promised new insights. Physicists seemed rather relieved to be able to ignore the vacuum for a change, rather than find it like the proverbial tail wagging the dog, in steering the direction of theories of electricity, magnetism and motion.
This brief era of nothingless physics was soon ended. Within ten years of Einstein’s issue of a redundancy notice to the ether, the issue of the vacuum was back in a central and puzzling place in scientific thinking. The deeper and wider extensions of the special theory of relativity and the quantum picture of matter would reinstate the vacuum in a central position from which it has yet to be dislodged at the start of another century.