Paper
Einstein and the quantum hypothesis
Published Feb 1, 2005 · D. Cassidy
Annalen der Physik
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Abstract
In 1905 Einstein was the first to propose that light behaves in some circumstances as if it consists of localized units, or quanta, of energy–light quanta. He showed that this hypothesis could account for several phenomena, including in particular the photoelectric effect [1]. In 1922 Einstein was awarded the Nobel Prize for Physics for the year 1921, “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”. [2] In 1900 Max Planck had derived a law for the frequency distribution of the energy of thermal, or blackbody, radiation contained in a material cavity of perfectly reflecting walls. He did so by assuming that the energy density is divided into discrete energy elements distributed over a large number of assumed charged harmonic oscillators in equilibrium with the thermal radiation [3]. In his subsequent papers, Einstein showed in 1906 that Planck’s theory agreed implicitly with the light-quantum hypothesis, since it required the energy quantization of the charged oscillators in amounts corresponding to emitted and absorbed light quanta [4]. In 1907 Einstein demonstrated the necessity of quantizing the harmonic oscillators in a derivation of Planck’s law using methods of statistical mechanics, and he showed that the quantization of the atomic oscillators in a solid crystal lattice results in an equation for the specific heat of solids as a function of temperature that deviates from the classical Dulong-Petit law with decreasing temperature [5]. Together, these papers demonstrated that Planck’s radiation law, which had been experimentally confirmed, could not be reconciled with either of the two pillars of classical theory, Maxwell’s electromagnetic theory or the kinetic-molecular theory of matter. Fundamental revisions of both electromagnetic and mechanical theory were required in order to encompass the quantum nature of both radiation and matter. In a letter to his friend Conrad Habicht in 1905, Einstein called his forthcoming light-quantum paper “very revolutionary, as you will see”. [6] The quantum hypothesis always remained for Einstein a problem and a puzzle, rather than a solution. He called his first proposal of the hypothesis, for the behavior of light, only “a heuristic viewpoint” that enabled natural explanations of certain phenomena. In 1907 he declared that the “new conception of the phenomena of light emission and light absorption. . . still in no way possesses the character of a complete theory, but it is remarkable in so far as it enables the understanding of a series of regularities.” [7] In 1909 Einstein wrote to his first collaborator Jakob Laub of his intense struggle with the problem of light quanta: “I am ceaselessly occupied with the problem of the constitution of radiation. . . This question of quanta is so extraordinarily important and difficult that everyone should be concerned with it.” [8] Einstein struggled with the quantum hypothesis for the rest of his life. He was never satisfied with quantum mechanics, even though he had contributed greatly to bringing it about. He never regarded it as the fundamental theory that was needed. He pointed to Max Born’s later statistical interpretation of the wave function as an indication of this. In 1926 he wrote to Born: “The quantum mechanics is very worthy of
Einstein proposed the light-quantum hypothesis in 1905, which led to fundamental revisions in electromagnetic and mechanical theory, but remained a problem and a puzzle for the rest of his life.
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