Chronology of Milestone Events in Particle Physics - EINSTEIN 1905
Chronology of Milestone Events in Particle Physics

  Nobel prize to A. Einstein awarded in 1921 "for services to Theoretical Physics, and especially of the law of the photoelectric effect''  

EINSTEIN 1905

Einstein, A.;
Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt / On a Heuristic Point of View about the Creation and Conversion of Light
Annalen der Physik. Leipzig 17 (1905) 132;

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Reprinted in
(translation into English) A. B. Arons and M. B. Peppard, Am. J. Phys. 33 (1965) 367.
(translation into English) The Old Quantum Theory, ed. by D. ter Haar, Pergamon Press (1967) 91.
Albert Einstein: Die Hypothese der Lichtquanten. Dokumenten der Naturwissenschaften - Abteilung Physik, V.7 ed. A. Hermann, Stuttgart, 26.
The collected papers of Albert Einstein, v.2 The Swiss years: writings 1900-1909, edited by J. Stachel, Princeton Univ. Press, (1989) 150.
(translation into English) The WORLD of the ATOM, editors H. A. Boorse and L. Motz, Basic Books, Inc. Publishers, New York - London, v.I (1966) 544.

Introduction
A profound formal distinction exists between the theoretical concepts which physicists have formed regarding gases and other ponderable bodies and the Maxwellian theory of electromagnetic process in so-called empty space. While we consider the state of a body to be completely determined by the positions and velocities of a very large, yet finite, number of atoms and electrons, we make use of continuous spatial functions to describe the electromagnetic state of a given volume, and a finite number of parameters cannot be regarded as sufficient for the complete determination of such a state. According to Maxwellian theory, energy is to be considered a continuous spatial function in the case of all purely electromagnetic phenomena including light, while the energy of a ponderable object should, according to the present conceptions of physicists, be represented as a sum carried over the atoms and electrons. The energy of a ponderable body cannot be subdivided into arbitrarily many or arbitrarily small parts, while the energy of a beam of light from a point source (according to the Maxwellian theory of light or, more generally, according to any wave theory) is continuously spread over an ever increasing volume.
The wave theory of light, which operates with continuous spatial functions, has worked well in the representation of purely optical phenomena and will probably never be replaced by another theory. It should be kept in mind, however, that the optical observations refer to time averages rather than instantaneous values. In spite of the complete experimental confirmation of the theory as applied to diffraction, reflection, refraction, dispersion, etc., it is still conceivable that the theory of light which operates with continuous spatial functions may lead to contradictions with experience when it is applied to the phenomena of emission and transformation of light.
It seems to me that the observations associated with blackbody radiation, fluorescence, the production of cathode rays by ultraviolet light, and other related phenomena connected with the emission or transformation of light are more readily understood if one assumes that the energy of light is discontinuously distributed in space. In accordance with the assumption to be considered here, the energy of a light ray spreading out from a point source is not continuously distributed over an increasing space but consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.
In the following I wish to present the line of thought and the facts which have led me to this point of view, hoping that this approach may be useful to some investigators in their research.

Related references
See also
M. Planck, Annalen der Physik. Leipzig 1 (1900) 99;
M. Planck, Annalen der Physik. Leipzig 4 (1901) 561;
Analyse data from
P. Lenard, Annalen der Physik. Leipzig 8 (1902) 150;
P. Lenard, Annalen der Physik. Leipzig 12 (1903) 469;

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Explanation of the photoelectric effect with use of the quantum hypothesis of Planck. Light is a flow of corpuscular objects with definite energies - Planck's quanta of energy.
    
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