Historic Dispute : Is space filled with a medium known as the aether

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PHYSICAL SCIENCE

Historic Dispute : Is space filled with a medium known as the aether?

Viewpoint: Yes, prior to the twentieth century, many scientists believed in the existence of the aether.

Viewpoint: No, only with Albert Einstein's work in the early twentieth century did most scientists accept that the aether did not exist.

The nature of light and the possible existence of a vacuum have been subjects of philosophical speculation since the time of the ancient Greeks. Aristotle argued for the impossibility of a true vacuum, and the first efficient air pump was not developed until the time of Robert Boyle (1627-1691) While the ancients had acquired some practical knowledge of optics, the modern theory of light and color begin with the work of the Dutch scientist Christiaan Huygens (1629-1693) and the English scientist Sir Isaac Newton (1642-1727).

Huygens demonstrated how the reflection of light could be understood geometrically if light were treated as a wave traveling at finite speed. Newton claimed that light was the flow of particles. Boyle had demonstrated that sound could not travel through a vacuum, though light passed without difficulty. That observation, together with Newton's immense scientific prestige, worked against the wave hypothesis.

The wave character of light became more credible after Thomas Young (1733-1829) established that light waves could interfere with each other, and after the discovery of the polarization of light—which proved that light was a transverse wave—by the young French experimenter Etienne-Louis Malus (1775-1812) in 1808. Since sound waves, water waves, and waves traveling down a taught string all clearly require the vibration of a medium, it was only natural to assume that light waves were of the same character. Since light travels through a vacuum and through interplanetary space, it was thought that the light-carrying medium or luminiferous aether (sometimes spelled "ether") had to permeate all space. Attempts to assign mechanical properties to the aether were beset with problems. Because light traveled with immense speed and was a transverse wave, the aether had to be extremely rigid and solid rather than fluid. On the other hand, it apparently offered no resistance to the motions of the moon and planets.

The connection between light and electromagnetic phenomena was established by the Scottish mathematical physicist James Clerk Maxwell (1831-1879) in his Treatise on Electricity and Magnetism, published in 1873. Maxwell showed that the experimental results observed in the laboratory for systems of stationary charges and electrical currents could be summarized in a set of differential equations satisfied by the components of the electric and magnetic field. Maxwell's equations had solutions that described mutually transverse electric and magnetic waves traveling together through space with a speed that could be calculated from the force constants for the electrostatic and magnetic force, which in turn could be determined in the laboratory. Maxwell's theory received wide acceptance and underlined for physicists the importance of better understanding the aether, which was now apparently involved in electrical and magnetic phenomena as well as optical ones.

If an aether existed, it would also provide an absolute frame of reference for motion. As Earth completes its nearly circular orbit of the Sun, it should experience an "ether wind," the velocity of which would vary from one season to another. Light traveling in the direction of this wind and light traveling at right angles to it would then move at different speeds. Detecting the difference in speed was the basis of the Michelson experiment of 1881, later repeated more carefully as the Michelson-Morley experiment. The latter showed at most a small effect.

The inconclusive result of the Michelson-Morley experiment did not in itself spell the demise of the aether theory. Instead of the aether being perfectly stationary, perhaps Earth dragged a certain amount with it as it moved. Possibly the act of moving through the aether affected the measuring instruments.

It is generally considered that the special theory of relativity, as published by Albert Einstein in 1905, demonstrated the aether did not exist. What Einstein showed, however, was that an entirely consistent physics could be developed in which all observers in uniform motion with respect to each other would measure the same value for the speed of light, regardless of the speed with which they observed the light source to be moving. Einstein's theory met with a certain amount of opposition, in part because it did away with any need for a space-filling aether, but it gained general acceptance after observations made during a solar eclipse confirmed the effect the Sun's motion had on gravitational fields, as predicted by the expanded form of the theory published in 1912.

Twentieth-century physics was also forced to revisit the question as to the wave or particle nature of light. In 1901 Max Planck (1858-1947) proposed that the energy of a light wave could only increase or decrease by finite amounts, which came to be called quanta. In 1905 Einstein explained the characteristics of the photoelectric effect, the emission of electrons from a metal surface when exposed to light, as the transfer of quanta of energy from the electromagnetic field to single electrons. The quanta of light energy came to be thought of as particle-like "photons." Further insight into the particle properties of light was gained in the discovery of the Compton effect, in which electrons and x-ray photons collide, exchanging energy and momentum; and in the process of pair production, in which a high-energy photon is converted into an electron-positron pair.

While relativity theory made it possible to consider the existence of a totally empty region of space, the quantum field theory—developed to explain the interaction of high-energy particles with the electromagnetic field—requires that the vacuum be thought of as anything but empty on the shortest time scales. In fact, the vacuum is treated as a place where electron-positron and other particle-antiparticle pairs come momentarily into existence and disappear. The propagation of electromagnetic fields through empty space is aided by the so-called vacuum polarization that results from the behavior of these "virtual" pairs, which might in a sense constitute the latest incarnation of the aether.

—DONALD R. FRANCESCHETTI

Viewpoint: Yes, prior to the twentieth century, many scientists believed in the existence of the aether.

The belief in the aether (or ether) that was prominent in the nineteenth century is often described in scornful and derisive tones. The search for the aether, and the disbelief when it was not found, is often ridiculed, and portrayed as one of the follies of modern science. Yet there were very good scientific reasons for supposing the existence of a substance through which light could travel. The famous 1887 Michelson-Morley experiment was not as cut and dried as is often portrayed in the literature, and the controversy over the existence of the aether raged for decades after it was performed.

Early Studies of the Aether

The aether is a hypothesized substance that allows the transmission of light throughout the universe. Its existence was postulated as far back as the ancient Greek philosophers, who offered a number of speculative theories on the nature of light and sight. The existence of the aether was seemingly set in stone when both René Descartes (1596-1650) and Isaac Newton (1642-1727) used it in their competing scientific philosophies. Descartes proposed a mechanical universe filled with matter, which he called the plenum, which was for all purposes the aether by another name. Newton was more cautious about the existence of the aether, but speculated that it filled the whole universe, and acted as "God's Sensorium," the means by which the universe was controlled. Newton's followers considered the aether essential, as it provided a stationary frame of reference for the laws of motion.

While both Descartes and Newton had used the aether mainly for philosophical reasons, there was some experimental support for the substance. In the 1660s Robert Boyle (1627-1691) succeeded in showing that the ringing of a bell could not be heard inside a vacuum. However, while sound was silenced in a vacuum, you could still see through it. Obviously, while sound required air to propagate, light did not. This suggested a more subtle substance than air was inside the supposed vacuum, and it was naturally assumed that it was the aether. This theory, by removing the notion of a complete vacuum, something that was unthinkable to many, found strong support.

Newton claimed that light was corpuscular in nature, consisting of tiny particles moving at infinite speed. However, at about the same time Christian Huygens (1629-1695) proposed that light was composed of waves, and that it had a finite speed. Waves travel through a medium, for example, a ripple in a pond moves through the water, and sound results from the vibrations of a body disturbing the air. It seemed only natural to assume that if light was a wave it must also travel through some sort of medium. Newton's corpuscular theory was dominant at first, but in the nineteenth century there were many experimental results that could only be explained if light was indeed a wave. The existence of the aether seemed more logical than ever.

Many properties of a wave are determined by the medium through which it travels. The mathematical relationship between pitch and tension in a piece of string has been known since the time of Pythagoras (sixth century b.c.). Galileo (1564-1642) refined the concept to relate to specific frequencies of vibration. The speed of the vibrations passing through a substance move more rapidly if the substance is stiffer. The speed a wave travels down a rope depends on the rope's tension and mass. The more tension or lighter the rope, the faster the wave travels. As details about the nature of light began to be found experimentally it became possible to work backwards and deduce the nature of the aether.

As early as 1746 Leonhard Euler (1707-1783) calculated that the density of the aether must be at least a hundred million times less than that of air, but its elasticity had to be a thousand times greater, making it more rigid than steel. In the 1840s it was proposed that the aether was an elastic-fluid, a theory that helped explain the phenomenon of double refraction, as well as circular polarization, and some thermodynamics problems. However, the work of Thomas Young (1773-1829), Augustin Fresnel (1788-1827), and François Arago (1786-1853) showed that light waves were transverse, not longitudinal as previously supposed. Transverse waves were unknown in any fluid medium, so the aether was therefore logically limited to being a solid rigid enough to allow the high speed of light waves to be transmitted, yet somehow offering no resistance to the motion of larger objects such as the planets, as the Newtonian laws of mechanics satisfactorily explained the motion of the planets. It was suggested that the aether was a stiff jelly-like substance that would act as solid for vibrations, but allow the easy passage of large objects.

Despite the growing uneasiness in the strangeness of the aether, there was further support for its existence in 1850 when Armand Fizeau (1819-1896) and Jean Leon Foucault (1819-1868) revealed that light travels more slowly in water than in air, confirming that it travels as a wave, and that its wavelength is decreased in the medium of water. Fizeau also showed that the velocity of light can be sped up or slowed down by making the water flow with or against the light. The work of Michael Faraday (1791-1867) and James Clerk Maxwell (1831-1879) united electricity, magnetism, and light, and the aether became the proposed medium for magnetic force as well, yet appeared to have no other properties.

Michelson-Morley Experiment

Towards the end of the nineteenth century a number of experiments were performed to measure the relative motion of Earth to the aether. In 1881 Albert Michelson (1852-1931) attempted to measure Earth's speed through the aether using a sensitive measuring device developed by Michelson called an interferometer. It is often erroneously stated that the purpose of the experiment was to determine the existence of the aether. Michelson assumed the aether existed, and was only attempting to measure the expected effects it would produce. However, the experiment failed to produce the expected results.

It is also erroneously reported in many histories that this experiment, and the later refinements, showed no changes in speed in any direction. However, this first "null" result did actually find some change in the interference pattern, as did all the later tests, but the effect was so small as to fall within experimental error. Various reasons were proposed for the lack of a result, from errors in the experimental setup, to the possibility that the jelly-like aether was dragged along with Earth as it moved. No one seriously suggested that the aether might not exist.

While Michelson was satisfied that his interferometer was extremely precise, he nevertheless attempted to refine his technique. Teaming up with Edward Morley (1838-1923), who had a background in precise measurements, they repeated the interferometer experiment, and also confirmed Fizeau's experiment regarding the change in the speed of light in a flow of water. In 1887 they published another "null" result, stating that the change in the interference pattern was far too small to account for a stationary aether.

While many histories of science hail the 1887 result as a turning point in physics, and see it as the death of the aether, neither Michelson, Morley, or the wider scientific community of the time perceived the results in that way. Again, many plausible reasons were given for the result, all of which assumed the existence of the aether. There was simply too much previous evidence for the aether to allow one experiment to overthrow it. That same year Heinrich Hertz (1857-1894) demonstrated the first wireless transmission of electronic waves, helping to begin the radio revolution and providing strong support to the existence of the aether as a combined electromagnetic medium for light, magnetic, and electric waves.

The most promising explanation for the Michelson-Morley result was that Earth dragged a portion of the aether along with it as it traveled, hereby reducing the effective relative motion. However, Sir Oliver Lodge (1851-1940) performed an experiment that showed that this was not the case. Others, such as Sir George Stokes, insisted the aether-drag hypothesis was correct, as it explained a number of other optical phenomena.

Another explanation was provided by George Fitz Gerald (1851-1901) and Hendrick Lorentz (1853-1928) who both independently suggested that the experimental setup might shrink in the direction of Earth's movement. Maxwell had shown earlier that the force between two electric charges depends on how they are moving. FitzGerald and Lorentz suggested that the electromagnetic forces inside a solid may therefore contract by one part in a hundred million if moving in the direction of Earth's orbit. In the 1890s Lorentz calculated the transformations, which describe the length of a moving object, and its contraction when viewed by an observer at various speeds.

In 1905 Morley collaborated with Dayton Miller (1866-1941) to refine the aether experiment. While it returned a "very definite positive effect," it was still far too small to match the expected values of Newtonian physics. Then, that same year, Albert Einstein (1879-1955) published his special theory of relativity, which implied that there was no need for the mysterious and undetectable aether. All of the current experimental contradictions could be explained away by the new theory, and the aether could be forgotten.

Aether and the Theory of Relativity

Relativity was not widely accepted at first, and one of the reasons for the strong resistance to it was the cherished notion of the aether. There were many eminent scientists of the time who refused to believe that waves could travel without a medium to propagate through. Such an idea went against centuries of scientific authority and experimental evidence. It was not until after World War I that a direct observational experiment of relativity could be made, when the light of stars behind the Sun was shown to be bent by the Sun's gravity during a solar eclipse. Relativity was pronounced triumphant, and support for the undetectable aether dwindled.

Einstein had been greatly influenced by the Lorentz transformations, but he admitted that he did not seriously consider the Michelson-Morley experiment, as it was only one of a whole series of puzzling aether experiments that had been done in the nineteenth century. Yet the Michelson-Morley experiment became one of the most celebrated means of explaining the ideas of relativity theory to the wider scientific community. Over time it became accepted that the experiments had been about proving or disproving the existence of the aether, and had formed a crucial logical step towards the theory of relativity.

In 1920 Einstein gave a speech titled "Aether and Relativity Theory" to an audience in Germany. Surprisingly the speech assumed the existence of the aether, even going so far as to say "space without ether is unthinkable." However, Einstein's aether was a far different substance than the strange solid-fluid substance that had proved to be impossible to find. Yet that old concept of the aether survived in many circles, and in 1920 Miller began a further series of aether experiments. In 1925 Miller announced that he had found a consistent, but small, positive result over many years, and he concluded that "there is a relative motion of the earth through the ether," contrary to relativity. Michelson and many others immediately began experiments of their own, none of which found positive results, and by 1929 even Miller was conceding that his results may have been in error.

The aether was consigned to the same fate of such other hypothesized substances as phlogiston and caloric and magnetic effluvia. Over time those who had championed the aether have become something of a laughing stock, and the story of the search for the aether is told as though it was folly. However, there were many good logical and scientific reasons for thinking the aether existed. Throughout the nineteenth century a number of key experimental results were obtained that seemed most easily explained by a electromagnetic medium. The controversial nature of many of the so-called "null" results in the aether-drift experiments did not immediately suggest the non-existence of the aether, and a number of innovative, but logical, explanations for the Michelson-Morley results were given, some of which helped inspire relativity theory. The aether was shown to be undetectable and unnecessary, however, that does not imply that it may not exist. There may indeed be a substance that acts as a medium for the transmission of light that has no other properties. John Bell (1928-1990), a mathematical physicist, even proposed that the theory of the aether be revived as a solution to a dilemma in quantum physics—although perhaps only half-seriously. The aether is still occasionally invoked by inventors of impossible perpetual motion machines and free-energy devices, but it should be remembered that it once had the support of solid evidence and logical arguments, and it only disappeared from scientific thought after many years of debate and experiment showed that it was simpler to abandon the medium than explain its weird properties and lack of detectability.

—DAVID TULLOCH

Viewpoint: No, only with Albert Einstein's work in the early twentieth century did most scientists accept that the aether did not exist.

In the opening years of the twentieth century, the study of light was the central concern of most leading physicists. Mainstream physics agreed that light consisted of electro-magnetic waves, moving through a substance called the aether. However, the precise nature of the aether had not been identified and the resolution of this problem was regarded as the major task for physics in the new century. Then, due to his musings on the significance of the speed of light, Albert Einstein (1879-1955) developed a revolutionary framework for thinking about the relationship between space and time. In a few years, the special theory of relativity, as this framework was called, had changed the focus of physics. This transformed the understanding of light. Einstein showed that the aether, and the notion of absolute space and time that it represented, did not exist. The aether gradually ceased to be of any interest to the mainstream of inquiry in the physical sciences. Einstein's work on the photoelectric effect also transformed the understanding of light waves. This led to the idea of wave-particle duality as a way of characterizing the nature of light in different situations.

Waves of Light

Prior to the nineteenth century, the mainstream of scientific thought regarded light as streams of tiny particles. This was the position of Sir Isaac Newton (1642-1727), whose authority had an enormous influence on all fields of scientific endeavor. It was not until the nineteenth century that serious consideration began to be given to the idea that light might consist of waves rather than particles. Successive experiments by physicists found that the wave theory best explained the behavior of light. The work of the British physicist James Maxwell (1831-1879) on electro-magnetic phenomena further strengthened this approach. Maxwell developed equations to describe the behavior of electro-magnetic phenomena. Thus, he was able to calculate the speed at which electro-magnetic waves moved in a vacuum, which coincided with the velocity of light. From this, Maxwell concluded that light was waves of oscillating electro-magnetic charges.

The Luminiferous Aether

The triumph of the wave theory of light made the aether the center of attention. The aether had a long history in philosophical thought about the nature of the universe. It was believed to be the substance that filled up the realms of celestial space and was often identified as the mysterious fifth element, along with air, water, earth, and fire, the basis of all matter. The aether acquired additional significance in the nineteenth century, with its emphasis upon a mechanical approach in which all events could be attributed to local causes. Nineteenth-century physicists were opposed to the idea of action at a distance, the idea that an object at one point could affect an object at another point with no medium for the effect. For a lighted candle to influence another object, such as the human eye, there had to be something that could transmit the influence from one point to another. If light was a wave, it required a medium to move through. As it had been observed that light traveled through a vacuum, it was concluded that whatever the medium of light was, it was even thinner than air. The aether, the mysterious substance that filled space, was identified as the luminiferous medium.

Therefore, to understand light, it was necessary to understand the special characteristics of the light medium. Many physicists developed complex models in order to try and describe the aether, but none of these were able to capture its special and apparently contradictory qualities. The aether had to be very thin and elastic, so that solid objects, such as the earth, could pass through it without resistance. At the same time, it had to be very dense to allow for the transmission of the vibration of light from one part of the aether to the next at such high speeds. For the last two decades of the nineteenth century, the attempt to accurately describe the aether occupied some of the best minds in physics.

The Aether and Absolute Space

One of the most crucial issues regarding the aether was the question of its immobility. Most physicists regarded the aether as fixed and stagnant, completely stationary. Objects might move through it, but no part of the aether was moving, relative to any other part. It stretched throughout space, acting as a framework against which the movement of light could be studied and measured. In this sense, the aether can be identified with the concept of absolute space. Most of the statements made about the movement of objects in the universe were relative; Earth was moving at a certain speed relative to the Moon, or another moving planetary object. Yet nineteenth-century science had at its foundation the idea of absolute space and time, a fixed reference system in nature from which Earth's absolute velocity could be measured. All movement in the universe was therefore movement relative to this stationary system. However, given that we participate in Earth's movement through the universe, some people were skeptical about the possibility of being able to step outside this and gain knowledge of our absolute position in the universe. As James Maxwell so eloquently wrote: "There are no landmarks in space; one portion of space is exactly like every other portion so that we cannot tell where we are. We are, as it were, on an unruffled sea, without stars, compass, soundings, wind or tide, and we cannot tell in what directions we are going. We have no log which we can cast out to take a reckoning by; we may compare our rate of motion with respect to the neighboring bodies, but we do not know how these bodies may be moving in space."

The Michelson-Morley Experiment

However, not everyone shared this vision of humanity hurtling blindly through space. Many believed that the aether provided a fixed reference system against which all motion could be measured. Two American scientists, Albert Michelson (1852-1931) and Edward Morley (1838-1923), constructed a machine that could theoretically discern the speed of Earth through the aether. The Michelson-Morley experiment was set up to measure the impact of the aether drift on the velocity of light. The aether drift was the wind that was created by the velocity of Earth through the motionless aether, just as a person traveling in a car on a still day feels a wind if he or she put a hand out the window. Michelson reasoned that this wind should have an effect on the speed of light. He set up a machine that constructed a race between two rays of light, one moving in the same direction that Earth was moving, and the other in a perpendicular direction. The effect of the aether drift meant that one ray of light should reach the finish line before the other. The difference in time would be incredibly small, but Michelson devised a machine, based on earlier experiments, called an interferometer, that was sensitive enough to be able to measure the difference.

However, the experiment produced a null result. The rays of light were unaffected by the aether drift. This suggested that, relative to the aether, Earth was not moving at all. It was as if the car you were in was traveling at 40 mph (70 kph), but you could not feel the wind that should have been created by the velocity of the vehicle. This result was puzzling, because so much of what scientists knew about the movement of light depended upon the existence of the aether drift. Once again, the mysterious aether had appeared to evade detection. A variety of different explanations were developed that tried to account for Michelson and Morley's results. However, until Einstein, none directly questioned the existence of the aether itself. Conceptually, it was too fundamental to the fabric of physics as a medium for the movement of electro-magnetic phenomena.

One physicist who was particularly interested in the significance of the result of the Michelson-Morley experiment was H. A. Lorentz. (1853-1928) Through his calculations on the behavior of small electrically charged particles, Lorentz observed that their mass changed as they moved through the aether. Therefore, Earth's velocity through the aether caused a contraction in matter that was positioned parallel to Earth's movement. In the case of the Michelson-Morley experiment, everything that was traveling into the aether wind was contracted. This contraction canceled out the disadvantage of racing into the wind, producing the null result. Lorentz admitted that this theory was "very startling." However, he managed to incorporate these conclusions within the traditional framework of physics, retaining the importance of the aether as the fixed reference system against which to measure the movement of electro-magnetic phenomena.

Einstein and the Aether

In 1905, Albert Einstein considered the problem from a different angle. The issue of the movement through the aether was not his prime concern. Instead, Einstein's theories were produced out of his consideration on the contradictions between two apparently valid principles. The first was the Galilean principle of relativity, which says that the laws regarding motion are the same regardless of the frame of reference. In other words, whether you are in a car traveling at a uniform speed of 100 kph, or in a train traveling at 10 kph, the laws of motion are the same in both cases. The second principle stems from Maxwell's equations, which established the law that light in a vacuum always moves with a velocity c = 300,000km/sec. According to the principle of relativity then, this must be the speed of light in all circumstances, whether it is measured in a car or on a train, regardless of the speed at which these are moving. Within the existing framework of physics, this was impossible, because the speed of anything always depended upon the framework from which it was being measured. For instance, the speed of the car might be 100 kph as measured from the roadside, but it will only be 90 kph measured from a train traveling parallel in the same direction at 10 kph. The same should apply to light. Therefore, either the principle that states the speed of light, or the principle of relativity, had to be incorrect.

However, Einstein found that the problem lay with the concept of absolute space and time. The old approach assumed that space and time were the same for all observers. Between any two events, say the firing of a pistol and the bullet striking its target, there would be a spatial separation, perhaps 80 ft (25 m), and a time interval, perhaps 0.04 seconds, which all observers would agree on. Einstein was able to show that this was not the case; the observed spatial separation of and time interval between the events depend on the reference frame of the observer. By eliminating the assumptions of absolute space and time, Einstein was able to develop a consistent physics in which the speed of light was always c, regardless of the frame of reference. This resolved the apparent contradiction that had so troubled him. These calculations exactly mirrored those that Lorentz had earlier devised in his observations, but Einstein was the first to recognize their true significance for ideas about space and time. These conclusions form the basis of what is known as the special theory of relativity.

The relativity of space and time had enormous consequences for the role of the aether in physics. Einstein's statement at the beginning of his 1905 paper "On the Electrodynamics of Moving Bodies" indicates this. "The introduction of a Light aether will prove to be superfluous, for according to the conceptions which will be developed, we shall introduce neither a space absolutely at rest, and endowed with special properties…." There was no fixed point, no stationary reference system within this new vision of the universe; everything was in movement relative to each other. As Einstein points out, the result of the Michelson-Morley experiment will always be null. "According to this theory there is no such thing as a 'specially favored' (unique) co-ordinate system to occasion the introduction of the aether-idea, and hence there can be no aether-drift, nor any experiment with which to demonstrate it." In other words, the Michelson-Morley experiment kept getting a null result because they were measuring the velocity of Earth against something that did not exist. Because of this shift away from notions of absolute space and time, it was gradually recognized that the aether, or any kind of medium for light, was unnecessary.

Einstein and Photons

Einstein's work in 1905 was also significant for light in other respects. The wave theory of light appeared to have triumphed over the particle theory in the nineteenth century. But Einstein's work on the photoelectric effect showed that the wave theory was by itself insufficient to explain the behavior of light. He reintroduced the notion of particles of light into the vocabulary of physics, but this time in a much more sophisticated way. In terms of its interaction with matter, Einstein suggested light was composed of particles of energy. These particles interacted with matter, and could eject electrons from their original positions. The development of the idea of light photons, as they came to be known, challenged the earlier dichotomy between theories of light based on particles or waves. Whether the concept of light as a wave or as a particle is more appropriate depends on the context of what is being observed. The two models complement each other, and this is referred to as wave-particle duality.

Conclusion

Einstein's work on light went beyond classical physics in two respects. He eliminated the need for the aether as the light medium, and he demonstrated that the description of light as a wave was inadequate to explain all the characteristics of light. As in most cases, the publication of Einstein's work in 1905 did not cause a sudden rupture with earlier ideas. The full ramifications were not immediately apparent to all, and many physicists ignored his work and carried on with their own research into the aether. But gradually, the impact began to filter through the profession of physics, and the cutting edge of physics was inquiry based on these new ideas. The aether was firmly tied to the most basic assumptions of nineteenth-century physics. With the death of absolute space, it ceased to be of any interest to most mainstream physicists.

—KATRINA FORD

KEY TERMS

AETHER DRIFT:

The relative movement of Earth and the aether. The velocity of Earth through the stagnant aether creates an aether drift.

ELECTROMAGNETIC WAVE:

A system of electric and magnetic fields traveling together in space.

LONGITUDINAL WAVE:

The medium is displaced in line or parallel to the direction of travel, causing compressions and rarefactions. Sound waves in air are longitudinal.

PHOTOELECTRIC EFFECT:

Release of electrons from an object exposed to electromagnetic radiation.

PHOTON:

Particle of electromagnetic radiation.

RELATIVITY:

The theory that the distances and time intervals measured by an observer depend on the state of motion of the observer in such a way that all observers measure the speed of light in vacuum to have the same value, regardless of the motion of the light source.

TRANSVERSE WAVE:

Where the displacement of the medium occurs at right angles to the direction of travel of the wave motion. For example, a vibrating string, or ripples in a pond.

VACUUM:

Space in which there is very few atoms or molecules.

WAVE-PARTICLE DUALITY:

Light can be described as both consisting of electromagnetic waves, and particles of energy. Which description is more appropriate depends upon what aspect of light is being described.

ALBERT EINSTEIN AND THE NOBEL PRIZE

Albert Einstein was awarded the Nobel Prize for physics in 1921. That he received the prize was not surprising. By this time, the general theory of relativity, which applied his ideas about the relativity of space and time to the problem of gravity, had been confirmed by observational data. As a result, Einstein had achieved worldwide fame and was recognized as one of the greatest minds in the history of science. Indeed, the fact that he was going to receive a Nobel Prize was so widely anticipated that in 1919 the expected prize money for the award was included as part of his divorce settlement with his wife.

However, what is surprising is that Einstein was not awarded the Nobel Prize for his most famous achievement. His citation states that he received the prize "for services to theoretical physics, and especially for his discovery of the photoelectric effect." There was no direct mention of the special and general theory of relativity, which had gained him celebrity status and for which he is still best known today. The reason for this indicates something about the nature of the Nobel Prize and about the wider reception of Einstein's theory. In 1895, the Swedish industrialist Alfred Nobel had left provision for the Nobel Prize in his will, which specified that part of his fortune should be "distributed in the form of prizes to those who, during the preceding years, shall have conferred the greatest benefit on mankind." Theoretical physics was not strong in Sweden at this time, and to those judging the prize, the significance of the theory of relativity to "the greatest benefit of mankind" was not clear. It seemed more in keeping with the spirit of Nobel's instructions to cite a part of Einstein's work, such as his discovery of the photoelectric effect, that appeared to have obvious practical applications. Therefore, while it is the special and general theory of relativity that has granted Einstein a degree of immortality, at the time it was not regarded as appropriate for a Nobel Prize.

—KatrinaFord

Science in Dispute