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ScienceWeek
HISTORY OF PHYSICS: EINSTEIN AND RADIATION
The following points are made by Daniel Kleppner (Physics Today 2005 February):
1) Albert Einstein had a genius for extracting revolutionary theory from simple considerations: From the postulate of a universal velocity he created special relativity; from the equivalence principle he created general relativity; from elementary arguments based on statistics he discovered energy quanta. His 1905 paper on quantization of the radiation field (often referred to, inaccurately, as the photoelectric-effect paper) was built on simple statistical arguments, and in subsequent years he returned repeatedly to questions centered on statistics and thermal fluctuations.
2) In 1909, Einstein showed that statistical fluctuations in thermal radiation fields display both particle-like and wave-like behavior. His was the first demonstration of what would later become the principle of complementarity. In 1916, when he turned to the interplay of matter and radiation to create a quantum theory of radiation, he once again based his arguments on statistics and fluctuations.
3) Einstein's theory of radiation is a treasure trove of physics, for in it one can discern the seeds of quantum electrodynamics and quantum optics, the invention of masers and lasers, and later developments such as atom-cooling, Bose-Einstein condensation, and cavity quantum electrodynamics. Our understanding of the Cosmos comes almost entirely from images brought to us by radiation across the electromagnetic spectrum. Einstein's theory of radiation describes the fundamental processes by which those images are created.
4) Einstein's 1905 paper on quantization endowed Max Planck's quantum hypothesis with physical reality. The oscillators for which Planck proposed energy quantization were fictitious, and his theory for blackbody radiation lacked obvious physical consequences. But the radiation field for which Einstein proposed energy quantization was real, and his theory had immediate physical consequences. His paper, published in March 1905, was the first of his wonder year. In rapid succession he published papers on Brownian motion, special relativity, and his quantum theory of the specific heat of solids.
5) In 1907, his interest shifted to gravity, and he took the first tentative steps toward the theory of general relativity. His struggle with gravitational theory became all-consuming until November 1915, when he finally obtained satisfactory gravitational field equations. During those years of struggle, however, Einstein apparently had a simmering discontent with his understanding of thermal radiation, for in July 1916, he turned to the problem of how matter and radiation can achieve thermal equilibrium. One could argue that 1916 was too soon to deal with that problem because there were serious conceptual obstacles to the creation of a consistent theory. Einstein, in his Olympian fashion, simply ignored them. In the next eight months, he wrote three papers on the subject, publishing the third, and best known, in 1917.[1,2]
References (abridged):
1. A. Einstein, Phys. Z. 18, 121 (1917); English translation On the Quantum Theory of Radiation, by D. ter Haar, The Old Quantum Theory, Pergamon Press, New York (1967), p. 167
2. A. Pais, Rev. Mod. Phys. 49, 925 (1977)
Physics Today http://www.physicstoday.org
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HISTORY OF PHYSICS: ON THE BIRTH OF THE QUANTUM
The following points are made by Peter Atkins (citation below):
1) The virus that was to destroy classical physics was first identified in the late 19th century by physicists studying a somewhat recondite problem to do with the light emitted from a hot body. To understand what happened, we need to know that light is a form of electromagnetic radiation, which means that it consists of waves of electric and magnetic fields propagating at the speed of light. The wavelength of the radiation is the distance between peaks of the waves, and for visible light is about 5 ten-thousandths of a millimeter. Everyone says that is very small: it is, but it is almost imaginable -- just think of a millimeter divided into a thousand slices, and then slice one of those slices in half. Different colors of light correspond to different wavelengths of radiation, with red light having a relatively long wavelength and blue light a relatively short wavelength. White light is a mixture of all colors of light. Small changes in wave-length have considerable consequences: as traffic lights change from red, through yellow, to green, the wavelength decreases from 7.0 to 5.8 and then to 5.3 ten-thousandths of a millimeter, and car drivers respond accordingly to these minuscule differences. The microwave radiation used in microwave ovens is also electromagnetic radiation, but its wavelength is several centimeters, so that's easy to imagine.
2) We also need to be aware of the term "frequency": if you imagine standing at a point with a wave rushing past, then the frequency is the number of peaks that pass you per second. Long wavelength light has a low frequency, because only a few peaks pass you per second; short wavelength light has a high frequency, because many peaks pass you per second. For visible light, about 600 trillion (6 x 10^[14]) peaks rush by per second, so the frequency is reported as 6 x 10^[14] cycles per second (6 x 10^[14] hertz, Hz). Red light is relatively low frequency, blue light is relatively high frequency radiation. We perceive this radiation as different colors because different receptors in our eyes respond to different frequencies.
3) Two characteristics of light emitted from an incandescent object, so-called "black-body radiation", had been identified in the late 19th century and expressed as laws. In 1896, the German physicist Wilhelm Wien (1864-1928) noticed that the intensity of black-body radiation -- the brightness of the incandescent body -- was greatest at a wavelength that depended in a simple way on the temperature. This feature is qualitatively familiar to us in everyday life, because we know how an object glows first red hot as it is heated and then white hot as its temperature is raised still further. This shift in color indicates that more and more blue (short wavelength) light contributes to the initially red (long wavelength) incandescence as the temperature is raised, so the maximum in the intensity shifts to shorter wavelengths. In 1879, the Austrian physicist Josef Stefan (1835-93) investigated another familiar everyday feature, how the total intensity of the emitted light increases sharply with temperature, and expressed the dependence quantitatively as a law.
4) Neither Wien's nor Stefan's law could be explained in terms of classical physics despite strenuous efforts by highly talented theoreticians. In a lecture at the Royal Institution on 27 April 1900 Lord Kelvin (William Thomson) (1824-1907) identified the failure to account for black-body radiation as one of the two little black clouds then apparent on the horizon of classical physics (the other black cloud was the failure to detect motion through the ether). Kelvin's two little clouds were to grow into a tumultuous storm that would sweep away our conceptions of the world, the manner in which we carry out our calculations and interpret our observations, and our understanding of the deep structure of reality.
5) In exasperation, Max Planck (1858-1947) unwittingly and unwillingly gave birth to quantum theory. On 19 October 1900 he proposed an equation that seemed to account for Wien's and Stefan's laws, and struggled for the following weeks to provide a theoretical foundation for his expression. In a lecture given before the German Physical Society on 14 December 1900, now regarded as the official birthday of quantum theory, he presented his solution. First, he pictured the radiation as being driven by the vibration of oscillators -- atoms and electrons -- in the hot body, with each frequency of vibration corresponding to the presence of a particular color of light in the radiation. That was the standard view, and his contemporaries had all done the same. His contemporaries had also tacitly assumed that the energy of each of these oscillators was continuously variable, just as the swing of a pendulum can (they supposed) have any amplitude. Planck, however, took a radically different view. He proposed that the energy of each oscillator could be changed only in discrete steps, a staircase of energy rather than a ramp. Specifically, he proposed that the energy of an oscillator of a given frequency is an integral multiple of (h) x frequency, where (h) is a new universal constant, now called Planck's constant. That is, he proposed that the staircase of allowed energies of any given oscillator is 0, 1, 2,... times the quantity h x frequency.
6) The value of (h) is so small that the steps in energy for most forms of electromagnetic radiation (especially the radiation we call visible light) are so small that they are undetectable except by very sophisticated methods, so it is easy to understand how physicists were led to think that energies could be varied continuously. Look at a pendulum: Can you see that its amplitude of swing can be changed only stepwise? The stepwise variation of energy, however, is the only way in which the properties of black-body radiation can be explained, and the stepwise variation of energy -- its quantization -- is now an established fact.
7) In private, Planck confided to his son that he thought he had made a discovery comparable to those of Newton. Nevertheless, for much of the rest of his life he tried desperately but fruitlessly to explain quantization in the context of classical physics. There are two lessons here for our comprehension of the scientific method. One is that revolutionary ideas gather strength from resistance to continuous attack. Unlike in some other fields of human endeavor, where crazy ideas are embraced unquestioningly as engaging and welcome friends, in science a crazy idea is subject to constant attack, especially -- really especially -- if it overthrows an established paradigm. The second lesson is that old men (and old women, although for them there is perforce and regrettably currently less empirical evidence) are not the best evangelists of radical science, for deeply imbued as they are in their conventional upbringing they commonly resent the passing of their learning. Like new mores, new paradigms become accepted only as old generations die.
8) Be that as it may, Planck's revolutionary, crazy idea that energy came in lumps, that it is granular rather than smooth, that it is as sand rather than as water, an idea that was to transform our perception of reality, was met by silence. At first it was regarded as a mathematical ruse. The physical reality of his proposal emerged only about 1905 when Albert Einstein (1879-1955) stepped into the arena.
Adapted from: Peter Atkins: Galileo's Finger: The Ten Great Ideas of Science. Oxford University Press 2003, p.201. More information at: http://www.amazon.com/exec/obidos/ASIN/0198606648/scienceweek
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QUANTUM PHYSICS: ON MAX PLANCK
Notes by ScienceWeek:
Max Planck (1858-1947), the father of quantum physics, received the Nobel Prize in Physics in 1918 for his work on energy quanta. Planck's initial professional interest was thermodynamics, and he studied under Hermann Helmholtz (1821-1894), Rudolf Clausius (1822-1888), and Gustav Kirchhoff (1824-1887) at the University of Berlin, where he received his doctorate _summa cum laude_ in 1879. Of his doctorate and his illustrious mentors, Planck said Helmholtz did not read his dissertation, Kirchhoff read it but did not approve of it, and Clausius was not at all interested in it. In fact, Planck's doctoral dissertation was of only minor importance and certainly not a harbinger of things to come. Helmholtz did, however, recognize Planck's talent, and Helmholtz was later instrumental in getting Planck a professorship at the University of Berlin in 1885, where Planck remained until he retired in 1926.
In physics, an ideal radiator or absorber absorbs and thus emits radiation of all frequencies equally and fully. A radiator/absorber of this kind is called a "blackbody", and its radiation spectrum is referred to as "blackbody radiation", which depends on only one parameter, its temperature. The classical idealized experiment involves the spectral distribution of blackbody heat radiation emerging from a hole in a black box kept at a certain temperature. At the end of the 19th century, there were no satisfactory theoretical explanations of the experimental observations of blackbody radiation. Planck's theoretical explanation, which was eminently successful, was based on the totally new idea of discrete energy quanta -- and thus quantum physics was born.
The following points are made by Anton Zellinger (Nature 2000 408:639):
1) Planck began working on the problem of black body radiation during the early years of his professorship at the University of Berlin. In 1894, the general problem was how to explain the colors emitted by glowing bodies. The classical explanation of that time worked well for the short parts of the light spectrum, but did not agree with experiments for all wavelengths. Planck had the advantage of close access to the most recent experimental results on the spectral distribution of blackbody heat radiation, results obtained by Otto Lummer (1860-1925), Ernst Pringsheim (1859-1917), and Ferdinand Kurlbaum (?-?) and Heinrich Rubens (?-?) at the University of Berlin.
2) The author (Zellinger) points out that after a decade of work on the problem, Planck eventually found a full explanation of blackbody radiation only after forcing himself "to an act of despair" by assuming that energy can only be exchanged between the light field inside the blackbody box and the walls of the container in discrete quanta, multiples of the energy E = hv, where (v) is the frequency of the light and (h) is a constant (now called "Planck's constant"). For many years, Planck tried unsuccessfully to find an alternative derivation of this experimentally successful radiation law from other known laws of physics, but he gradually came to accept the realization that he had found something fundamentally new. Thus, the quantum idea, in the work of Planck, actually predated 1900, but Planck disbelieved it and refused to publish it.
3) Zellinger points out that the next important step in the early days of quantum physics occurred in 1905, when Albert Einstein (1879-1955) introduced the radical hypothesis of quanta of light to explain the photoelectric effect. For some time, this remained the only significant instance of the idea of the quantum being taken seriously. Einstein's hypothesis met with strong objections from his contemporaries, including from Planck himself. As late as 1913, Planck, together with his colleagues Heinrich Rubens, Walther Nernst (1864-1941), and Emil Warburg (?-?), wrote in a recommendation letter for Einstein's election to the Prussian Academy of Sciences: "One should not hold against him too much that in his speculations he might have occasionally overshot the goal, as for example in his hypothesis of the quanta of light." It is an irony that it was this hypothesis that gained Einstein the Nobel Prize in Physics in 1921.
4) The history of quantum physics is a classic instance of how ultimately successful revolutionary ideas in science are sometimes hardly ever accepted in the beginning, even by their originators. It is not easy to shake the mind out of a deep groove.
Nature http://www.nature.com/nature
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EINSTEIN ON PLANCK
The following points are made by Albert Einstein (citation below):
1) Many kinds of men devote themselves to science, and not all for the sake of science herself. There are some who come into her temple because it offers them the opportunity to display their particular talents. To this class of men science is a kind of sport in the practice of which they exult, just as an athlete exults in the exercise of his muscular prowess. There is another class of men who come into the temple to make an offering of their brain pulp in the hope of securing a profitable return. These men are scientists only by the chance of some circumstance which offered itself when making a choice of career. If the attending circumstances had been different, they might have become politicians or captains of business.
2) Should an angel of God descend and drive from the temple of science all those who belong to the categories I have mentioned, I fear the temple would be nearly emptied. But a few worshipers would still remain -- some from former times and some from ours. To these latter belongs our Planck. And that is why we love him.
Adapted from: Albert Einstein: from the preface to /Where is Science Going?/ by Max Planck. Original German text 1933, English text Ox Bow Press 1981.
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