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Biography continued

Light Quanta and Wave-Particle Duality
The most common misconceptions concerning Einstein's introduction of his revolutionary light quantum (light particle) hypothesis in 1905 are that he simply applied Planck's quantum hypothesis of 1900 to radiation and that he introduced light quanta to "explain" the photoelectric effect discovered in 1887 by Heinrich Hertz and thoroughly investigated in 1902 by Philipp Lenard. Neither of these assertions is accurate. Einstein's arguments for his light quantum hypothesis-that under certain circumstances ra­diant energy (light) behaves as if it consists not of waves but of particles of energy proportional to their frequencies were absolutely fundamental and, as in the case of his theory of Brownian motion, based on his own insights into the foundations of thermodynamics and statistical mechan­ics. Furthermore, it was only after presenting strong argu­ments for the necessity of his light-quantum hypothesis that Einstein pursued its experimental consequences. One of several such consequences was the photoelectric effect, the experiment in which high-frequency ultraviolet light is used to eject electrons from thin metal plates. In particular, Ein­stein assumed that a single quantum of light transfers its entire energy to a single electron in the metal plate. The famous. equation he derived was fully consistent with Lenard's observation that the energy of the ejected electrons depends only on the frequency of the ultraviolet light and not on its intensity. Einstein was not disturbed by the fact that this apparently contradicts James Clerk Maxwell's clas­sic electromagnetic wave theory of light, because he realized that there were good reasons to doubt the universal validity of Maxwell's theory.

Although Einstein's famous equation for the photoelec­tric effect-for which he won the Nobel Prize of 1921- appears so natural today, it was an extremely bold predic­tion in 1905. Not until a decade later did R.A. Millikan finally succeed in experimentally verifying it to everyone's satisfaction. But while Einstein's equation was bold, his light quantum hypothesis was revolutionary: it amounted to re­viving Newton's centuries-old idea that light consists of particles. No one tried harder than Einstein to overcome opposi­tion to this hypothesis. Thus, in 1907 he proved the fruit­fulness of the entire quantum hypothesis by showing it could at least qualitatively account for the low-temperature behavior of the specific heats of solids. Two years later he proved that Planck's radiation law of 1900 demands the coexistence of particles and waves in blackbody radiation, a proof that represents the birth of the wave-particle duality. In 1917 Einstein presented a very simple and very important derivation of Planck's radiation law (the modern laser, for example, is based on the concepts Einstein introduced here), and he also proved that light quanta must carry mo­mentum as well as energy.

Meanwhile, Einstein had become involved in another series of researches having a direct bearing on the wave-­particle duality. In mid-1924 S.N. Bose produced a very insightful derivation of Planck's radiation law-the origin of Bose-Einstein statistics-which Einstein soon developed into his famous quantum theory of an ideal gas. Shortly thereafter, he became acquainted with Louis de Broglie's revolutionary new idea that ordinary material particles, such as electrons and gas molecules, should under certain circumstances exhibit wave behavior. Einstein saw immedi­ately that De Broglie's idea was intimately related to the Bose-Einstein statistics: both indicate that material particles can .at times behave like waves. Einstein told Erwin' Schrodinger of De Broglie's work, and in 1926 Schrodinger made the extraordinarily important discovery of wave me­chanics. Schrodinger's (as well as C. Eckart) then proved that Schrodinger's wave mechanics and Werner Heisen­berg's matrix mechanics are mathematically equivalent: they are now collectively known as quantum mechanics, 'one of the two most fruitful physical theories of the 2Oth_ century. Since Einstein's insights formed much of the back­ground to both Schrodinger's and Heisenberg's discoveries, the debt quantum physicists owe to Einstein can hardly be exaggerated.

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