Light
Color
In 1666, at the age of 23, Newton began his experiments with sunlight falling on triangular glass prisms and showed for the first time that "colours are not qualifications of light derived from refractions or reflections of natural bodies, but original and connate properties." "White light is not homogenial but consists of rays, some of which are more refrangible than others." By refraction, white light, up till then believed to be a pure substance, had been unfolded into the solar spectrum, from the least refracted scarlet, to the most refracted violet.
Color is an intrinsic property of light. It's confirmed in Newton's conclusions, derived from his experiments with prisms : "All homogeneal light has its proper colour answering to its degree of refrangibility. Colour cannot be changed by reflections and refractions".
Here we have an operational definition of homogeneal, now called monochromatic, light. It is a mode of light that cannot be decomposed further by prisms. Distinct kinds of monochromatic light correspond to distinct colors. It was clear to Newton that color is to light what pitch is to sound. Objects exposed to sunlight, a mixture of colors, appear to have a specific color because of selective action. A flower is red because it absorbs all but red light and reflects the rest. An object is white because it reflects everything, black because it absorbs nearly all colors, and so on.
The speed of light
Rasmus Bartholin, a member of the Bartholin family which played a dominant role in the University of Copenhagen for about a hundred and fifty years, was a professor of mathematics and medicine there. In 1669, he published his observation of a phenomenon. Studying the transition of a beam from air into a crystal of Icelandic spear, he discovered double refraction. Upon entering that crystal, light suffers not one deflection but two at once, the beam splits into two parts.
Ole Romer, Bartholin's amanuensis, later his son-in-law, working at the recently established Royal Observatory in Paris, measured for the first time the velocity of light. Training a telescope on Io, Jupiter's innermost moon, Romer found, in 1676, that this satellite shows a peculiar variation in its motion around Jupiter, from which the value 214,300 kilometers per second for the light velocity could be deduced -- about two-thirds the modern value --. Romer's work provides example of the marvels revealed by the new 17th century instrument, the telescope, a culmination of the development of lenses that had begun in the late 13th century.
Wave
Newton recorded his findings in his "Opticks", a book which also full of qualitative ideas dealing with such subjects as the origins of the rainbow, double refraction and the meaning of Romer observations. Perhaps most interesting of all are his conjectures on the constitution of light. Newton conjectured that light consists of material bodies, moving in straight lines through a homogeneous medium, which velocities independent of colour (since otherwise an aging beam of white light would change colour), but with sizes (weights) that are different for different colors.
Meanwhile, almost simultaneously with Newton's suggestion of light as small bullets, a quite distinct proposal about the nature of light had been put forward. In 1690, Christian Huyghens had published his "Traite de la Lumiere". Huyghens, proposed that light "spreads, as sound does, by spherical surfaces and waves. For I call them waves from their resemblance to those of which are seen to be found in water when a stone is thrown into it." In 1665 Grimaldi had already stated that light is propagated not only by directly, by refraction, or by reflection, but also in still a fourth way : by diffraction, a process in which light bends a bit when it passes through a hole in a barrier, so that the transition between light and shadow on a screen is slightly fuzzy. Through experimentation he was able to demonstrate that the observed passage of light could not be reconciled with the idea that it moved in a straight line. Rather, the light that passed through the hole took on the shape of a cone. Later physicists used his work as evidence that light was a wave.
In the closing years of the 17th century, the two views about the nature of light had just emerged : the corpuscular theory and the wave theory. However, these two theories had to be considered incompatible. A particle is at a given place at a given time, while a wave is spread out in space at one time (think of a wave in water). Particles are localized, waves are not. Hence, the two theories were mutually exclusive : they could not both be right. There were Hooke, Leibnitz, Euler and Huyghens, who were strongly critical of the corpuscular picture. However, Newton's authority was so immense, that, all told, his views held the upper hand all through the 18th century. It remained until the beginning of the 19th century, when real progress was brought about by two young men, neither of whom belonged to the academic world. One was an English physician, Thomas Young, and the other a French government civil engineer, Augustin Fresnel. Thanks to the extraordinary ideas of these two intruders, the corpuscular theory had become, within a few decades, of historical interest only, while the wave theory acquired fundamental importance in the physics of the 19th century.
The decisive turn began with Thomas Young interpretation of light interference (1801), a phenomenon that had been known to Newton as well as Huyghens, but not understood by them. To illustrate what is at issue, consider what happens when we drop two stones -- not too far apart -- into a pond of water. "Neither series of waves will destroy the other, but their effects will be combined. If the elevations of one series coincides with those of the other, they must together produce a series of greater joint elevations. We say, the waves are in phase. But if the elevations of one series are so situation as to correspond to the depression of the other, they must exactly fill up those depressions, and the surface of the water must remain smooth. We say, the waves are out of phase. Now I maintain that similar effects take place whenever two portion of light are thus mixed. This I call the general law of the interference of light".
The "Young experiment", light from a source passes through a slit that generates a beam moving in various directions, then hits a screen with two narrow slits. The waves emerging from each slit hit a photographic plate on which they produce an interference pattern : lines of strongest intensity (waves in phase), the dark region (totally out of phase) and regions of weaker illumination (partially out of phase). When, at a given point, two light waves are out of phase, they produce no light at all at that point. Light superposed on light can yield darkness, a behavior evidently at variance with a corpuscular theory.
Nevertheless, since Young had not expressed his correct interpretation of interference in rigorous mathematical term, he left room for ridicule by the proponents of the corpuscular theory. It was Fresnel who formulated these ideas in a precise mathematical way (1815), and so was able for the first time to account quantitatively for such a wealth of available experimental data, that the defenders of the particle theory were silenced. Fresnel's theoretical considerations successfully reproduced a quantitative description of patterns in Young's experiment, and also Grimaldi's diffraction processes.
Infrared & Ultraviolet
I had been known since time immemorial that sunlight heats up bodies. In 1800, William Herschel asked, "How does this property depend on color?" To find out, he first generated a solar color spectrum, then exposed a thermometer successively to various small spectral portions. Moving from violet to red, he found an increase in heating. However, when placing his thermometer beyond the red, he found an even higher reading. Herschel had discovered infrared light, the invisible light.
If, however, infrared radiation truly deserves to be called light, one should demand that it reflect, refract, interfere, etc, in a way qualitatively similar to visible light. The first to confirm experimentally the reflective and refractive properties with the help of prisms, mirrors, and thermometers was Herschel himself. He reported on 219 experiments -- all done in 1800 --, some with sunlight, some with "red hot iron, cooled till it can no longer be seen in the dark", some with "culinary heat" (stoves), others with chimney fires. Herschel nevertheless remained curiously reluctant to accept his rays as an extension of the visible spectrum, but others rapidly took the point.
Johann Ritter from Jena, repeated and confirmed Herschel's discovery and next raised a new question : "If there is radiation beyond the red, why should the same not also be true beyond the violet?"
Thermometry would be of no help there, but there was another criterion. It had been known since the days of the alchemists that sunlight blackens certain silver salts. Moreover, Ritter also knew from others that violet is the most effective blackener among the visible rays. Accordingly, he exposed a strip of white paper covered with silver chloride to the solar spectrum and found (1801) that the blackening was even more pronounced beyond the violet. He had "seen" ultraviolet radiation by means of what is, essentially, a photographic plate. As time went by, others confirmed that these rays too are light.
Electromagnetism
“”The theory I propose may therefore be called a theory of the Electromagnetic Field
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| — Maxwell in A Dynamical Theory of the Electromagnetic Field (1864) |
The description of forces in terms of fields was new. Earlier, it had generally been assumed that forces such as those between two electrically charge particles exerted themselves instantaneously between them. On the other hand, Maxwell considered this action to be transported by an electric field that is present at all points in space time. The strength of the field and the direction in which it acts can be measured by placing a test body, a tiny charged particle, at the place and time of one's choice. Likewise, a magnetic filed is measured by its action on a tiny magnet. A familiar example is a compass, which measures the direction of earth's magnetic field at some point in space and time. Even though the compass does not measure the field's strength.
Maxwell's finding represents the synthesis and culmination of contributions dating back to the eighteenth century of men like Coulomb, Volta, Orsted, Ampere and above all Faraday, the greatest of the 19th century experimentalists.
In 1820, Orsted discovered that an electric current -- a charge in motion -- generates magnetic action, as is seen from the experimental fact that a compass needle changes direction when an electric current is made to pass through a nearby wire. He also coined the term "electromagnetism" to express that electric and magnetic phenomena are inseparably intertwined. This became even more evident when in 1831 Faraday found the converse : a moving magnet generates electric action, inducing electric currents to flow in a nearby metallic wire.
Electric and magnetic phenomena are only separable when the charges and/or magnets are at rest. Motion mixes them.
Maxwell showed that all these, and many more experimental results from those earlier days, can be seen as consequences of his field equations, that describe the evolution in space and time of electric and magnetic fields generated by charges and magnets, at rest or in motion. In his language, the Orsted-Faraday findings can simply be phrased "an electric field changing with time generates a magnetic field, and vice versa."
Not only that, Maxwell went much further, by showing that his electromagnetic equations also form the theoretical basis for optical phenomena. His argument proceeds in three steps.
Step one. It had been shown by Ampere that Orsted's results can be put in the following quantitative form. A moving electric charge e creates a magnetic field with strength B. The resulting magnetic force acting on nearby compass needle equals evB/c, where v is the charge's velocity which is perpendicular to B, and c is some other velocity. Also, c is a universal constant. It is always the same velocity regardless how big e or v or B. The magnitude and universality of c can be determined by experiments that measure the magnetic force for various choices of e, v and B.
Maxwell's equations incorporate this Ampere's law. The value of the constant c remained a piece of experimental input. In his memoir, Maxwell quoted the best experimental value known at that time : c equals 3 x 1010 cm/s.
Step two. Measurements of the velocity of light had considerably improved since Romer's time. Maxwell quoted two experimental answers : 3.14 and 2.98 x 1010 cm/s.. The obvious questions arose : "Why should the velocity of light be practically the same as Ampere's c?"
Step three. Maxwell was ready for this challenge. Consider, a region of space very far removed from charges or magnets. In that region there either are no electric and magnetic fields, or else there may be superpositions of pure electromagnetic waves propagating with the velocity of c. "This seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws".
This was just a prediction. It had yet to be demonstrated experimentally. That was done in 1887, after Maxwell's death, by Heinrich Hertz. By means of oscillatory electric spark discharges, he managed to generate electromagnetic waves with frequencies of about 108 cycle-per-second (wavelength 3 meters), a typical VHF frequency used for TV. In a series of papers he showed that, like visible light, these waves reflect, diffract, interfere, are transverse and propagate with the velocity c.
Relativity
Even before the 20th century had begun, there were indications that Maxwell's theory was flawed. Not his electromagnetic field equations, but his accompanying picture for the mechanism of wave transmission.
Maxwell assumed that an aether was necessary for understanding the propagation of electromagnetic waves through space. This aether transmits vibrations. Then, came a time when the existence of aether at last became a subject of experimental inquiry. The year was 1887, the same in which Hertz had confirmed Maxwell's electromagnetic theory of light. The place was Cleveland, Ohio. The scientists involved were Albert Michelson and Edward Morley (MM).
The experiment they performed was delicate and difficult. In order to appreciate their strategy, it is necessary to state in more detail what Maxwell meant by aether and to define more precisely the meaning he attached to the velocity of light c.
The aether, through which light was assumed to be propagated, is an all-pervasive medium in a state of absolute rest. Maxwell's "c" is the speed of light relative to this resting aether. Stars -- not planets -- were believed to be at rest relative to this resting aether. Thus the velocity c is the speed of light as measured by a hypothetical observer standing on a fixed star.
The need for specifying a velocity relative to an observer is familiar from daily experience. Example : two men sit in a train. One of them gets up and walks forward with a velocity of 3 mph, as seen by the other man. A third man standing on a platform sees the train go by with a speed of 60 mph. To him, the same walker moves forward with a speed of 60+3 mph.
Back to light as seen by Maxwell. As said, the velocity c is the one measured by an observer on a fixed star ("on the platform"). An earthling actually measures the light produced by some source at rest, relative to the earth ("the other man on the train"). But the earth moves relative to the fixed stars, just as the train rushes past the platform. Hence, our earthling will expect to observe a light velocity different from c. In fact, he will anticipate that the light velocity is different for different directions of a light beam relative to the direction of the earth's motion.
Michelson had attempted to measure the light velocity differences mentioned above and found none. His subsequent experiment jointly with Morley, performed with much improved precision, led to the same result : the velocity of light is independent of the speed with the light source moves relative to observer.
The MM result defied logic. Classical logic, that is. Something had to be wrong with the extrapolation of the 60 + 3 example to light. Some have tried to save the situation, but all such attempts have been to no avail. The correct answer, given by Einstein, is that classical logic itself needs modification in terms of a radically new doctrine : the special theory of relativity.
Postulate 1. All laws of physics take the same form for all observers in uniform motion (velocities constant in size and direction) relative to each other. This postulate was nothing new for mechanics. Consider the basic Newtonian law of mechanics : a force acting on a particle equals the particle's mass times its acceleration. Acceleration means change of velocity with time. Acceleration therefore remains unaltered if we add a constant velocity to the observer's motion.
But this situation is utterly different for Maxwell's equations. In Ampere's law, magnetic forces do depend on velocity. More generally, in Maxwell's equations, the light velocity c appears. The MM experiment implies that Maxwell's definition of c in terms of motion relative to the fixed stars will not do. Einstein's first postulate asserts that we should in fact refrain from giving a preferred status to the one type of observer who rests relative to the fixed stars. "The unsuccessful attempts to discovery any motion of the earth relative to the 'light medium' lead to the conjecture that to the concept of absolute rest, there correspond no properties of the phenomena."
Postulate 2. The velocity of light in a vacuum is the same whether the light be emitted by a body at rest or by a body in uniform motion. In special relativity, the negative outcome of the MM experiment is elevated to the status of a postulate which it is in conflict with classical physics and violates everyday intuition.