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J. J. Thomson and the Structure of Light

Published online by Cambridge University Press:  05 January 2009

Russell McCormmach
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Synopsis

This essay concerns an aspect of the speculative contributions of J. J. Thomson to a field of physics somewhat removed from that upon which his popular fame and scientific eminence were alike founded. He published a number of statements in the period 1903–1910 advocating a discontinuous structure of the electromagnetic field. His unorthodox conception of the field was based upon the presumed discreteness of Faraday's physical lines of electric force. While his ideas led to significant experimental work, they were not brought together in the form of a completed theory. It was at this same time that the quantum theory was independently evolving notions of a structure of the field, and Thomson's efforts at developing a theory of light were diverted into a protracted criticism of the hypothesis of quanta. In 1924–1936 he returned to the subject of the structure of light, but these latter speculations no longer had much relevance to contemporary physical thought.

Type
Research Article
Information
The British Journal for the History of Science , Volume 3 , Issue 4 , December 1967 , pp. 362 - 387
Copyright
Copyright © British Society for the History of Science 1967

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References

1 Thomson did not argue against Maxwell's theory but rather against the way in which it it was usually presented. He claimed in fact to have “adopted exclusively” the point of view of Maxwell. Thomson, J. J., Notes on Recent Researches in Electricity and Magnetism (Oxford, 1893), v.Google Scholar

2 Ibid., v.

3 The “geometrical”, as opposed to the “analytic”, approach was the “physical” one; it was exemplified in the concept of tubes of electric force. Ibid., v.

4 Ibid., vi.

5 Ibid., vii.

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7 Ibid., 202.

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10 European physicists, who did not characterize a theory as Thomson did, spoke of his “hypothesis” of the structure of light. See, for example: Lorentz, H. A., “Alte und neue Fragen der Physik”, Phys. Zeit., xi (1910), 1250Google Scholar; Stark, J., Die Prinzipien der Atomdynamik: II. Die elementare Strahlung (Leipzig, 1910–1911), 265Google Scholar; Planck, M., “La loi du rayonnement noir” in La Théorie da Rayonnement et les Quanta, ed. Langevin, P. and de Broglie, M. (Paris, 1912), 101.Google Scholar

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12 Ibid., 3. The “unit” tubes are all of the same strength. They are distributed throughout space, and not confined to places of non-vanishing electromotive force. The electromotive force is not a measure of the number of tubes present, but of the net excess of tubes pointing in one direction over those pointing in the opposite direction. Ibid., 4.

13 Ibid., 4. Thomson had already pointed to the analogy between kinetic theory and the Faraday field concept. “On the Illustration of the Properties of the Electric Field by Means of Tubes of Electrostatic Induction”, Phil. Mag., ser. 5, xxxi (1891), 149–171.Google Scholar

14 Recent Researches, 43Google Scholar. Thomson pictured a plane wave as constituted of a series of Faraday tubes moving at the speed of light. Their numbers emitted per unit time from the plane source vary in a harmonic fashion. The moving tubes produce a calculable magnetic force at right angles to the direction of the tubes and also to the direction of their motion. Ibid., 42.

Poynting, J. H., a close friend of Thomson, had noted in 1884 a resemblance to the old emission theory in the energy characteristics of light. “The Growth of the Modern Doctrine of Energy”, Collected Scientific Papers (Cambridge, 1920), 574–575.Google Scholar

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17 Ibid., 63.

18 Ibid., 63.

19 Ibid., 63.

20 Ibid., 65.

21 Ibid., 63. Very soon after their discovery Thomson convinced himself that Röntgen rays were electromagnetic pulses. Thomson, J. J., “A Theory of the Connexion between Cathode and Röntgen Rays”, Phil. Mag., ser. 5, xlv (1898), 172–183.CrossRefGoogle Scholar

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33 Ibid., 421. Thomson is using “unit” in a different sense. It does not represent a tube itself, but a bundle of energy travelling along a tube.

34 In an earlier paper of the same year Thomson showed that the radiation from heated bodies does not conform to the second law of thermodynamics unless the time of collision of an electron with a molecule—the mechanism producing the thermal radiation—is inversely proportional to the kinetic energy which the electron had before the collision. Thomson, J. J., “On the Electrical Origin of the Radiation from Hot Bodies”, Phil. Mag., ser. 6, xiv (1907), 217–231.CrossRefGoogle Scholar

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38 Taylor, G. I., “Interference fringes with feeble light”, Proc. Camb. Phil. Soc., xv (1909), 114–115.Google Scholar

39 For an excellent exposition and analysis of Einstein's light-quantum hypothesis, see Klein, M. J., “Einstein's First Paper on Quanta”, Natural Philosopher, ii (1963), 59–86Google Scholar. For further discussion, see also Klein, M. J., “Ehrenfest's Contributions to the Development of Quantum Statistics. I”, Koninkl. Nederl. Akademie van Wetenshappen, Proceedings, ser. B, lxii (1959), 41–62Google Scholar; “Einstein and the Wave-Particle Duality”, Natural Philosopher, iii (1964), 1–49Google Scholar; “Einstein, Specific Heats, and the Early Quantum Theory”, Science, cxlviii (1965), 173–180.Google Scholar

40 Millikan associated Einstein's with Thomson's ideas on a number of occasions. See, for example, Millikan, R. A., The Electron, 222Google Scholar. The unlikelihood that Einstein was acquainted with Thomson's theory is pointed out in Klein, M. J., “Einstein's First Paper on Quanta”, loc. cit. (39), 80.Google Scholar

41 According to Thomson the ether has mass but no weight. He pictured the moving lines of electric force as “gripping” the neighbouring ether and carrying it with them, the mass of the transported ether being calculable by the laws of electricity. Though the mass of light is small, its momentum and energy are considerable due to its great velocity. The main reason why Thomson endowed the “invisible universe” of the ether with mass and motion is that otherwise the interaction of electrical bodies would not obey Newton's third law of motion. For his own account of the connection between mechanics, electricity, and the ether, see Thomson, J. J., “On the light thrown by recent investigations on electricity on the relation between matter and ether”, Annual Report of the Smithsonian Institution, 1908, 233–244.Google Scholar

42 Lord Rayleigh quoted G. F. C. Searle, a lifelong colleague of Thomson, who had observed that Thomson “could not always remember how an idea had got into his mind … He would be told by someone or would read somewhere some new idea. Later on he would find the idea floating in his mind and he would suppose that the idea was original to himself and would treat it as if it were.” Rayleigh, Lord, J. J. Thomson, 118–119.Google Scholar

43 Ibid., 136.

44 Ibid., 219.

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46 Given his interest in the subject, Thomson undoubtedly followed Jeans' and Rayleigh's discussion of the energy distribution in the radiation from heated bodies. In a letter to Nature in 1905 Rayleigh quoted Planck's theoretical formula, but confessed that he had not “succeeded in following Planck's reasoning”. Rayleigh, Lord, “The Dynamical Theory of Gases and of Radiation”, Nature, lxxii (1905), 54–55.CrossRefGoogle Scholar

In his 1907 paper on the radiation from heated bodies, Thomson wrote down Planck's final formula for black-body radiation in its empirical form, i.e. without the new constants h and k. He did not say anything about Planck's ideas but only remarked that at low (Thomson slipped and said “high”) temperatures Planck's formula agreed with experiment better than Rayleigh's. Thomson, J. J., op. cit. (34), 230.Google Scholar

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55 Ibid., 255.

56 Thomson spoke both of molecular energy and of molecular radiant energy in his discussions of the quantum theory, and it is not clear that he made a distinction between them. If energy is always discrete, then radiant energy is necessarily discrete; the reverse implication, however, does not hold. This difference, which was seldom explicitly recognized, was noted in Wilson, H. A., “On the Statistical Theory of Heat Radiation”, Phil. Mag., ser. 6, xx (1910), 121.CrossRefGoogle Scholar

57 It appears that the first public response to the quantum theory in Britain was that of Larmor. At the meeting of the British Association in 1902 he rederived Planck's radiation formula, “discarding the vibrators, and considering the random distribution of the permanent elements of the radiation itself, among the differential elements of volume of the enclosure, somewhat on the analogy of the Newtonian corpuscular theory of optics”, Report of the British Association, 1902, 546Google Scholar. In his Bakerian Lecture in 1908 Larmor put forward a new statistical basis for Planck's theory, concluding that it was “now without any implication that energy is itself constituted on an atomic basis”, Larmor, J., “On the Statistical and Thermodynamical Relations of Radiant Energy”, Proc. Roy. Soc., ser. A, lxxxiii (1909), 90.Google Scholar

A. Schuster, having argued that certain ideas of the quantum theory “could only be true if energy had, like matter, an atomic constitution”, thought that he had “thus finally disposed of the matter by a reductio ad absurdum”. But he discovered that this view was not considered absurd by everyone, and that “such atomic constitution” came to be “openly advocated”, Schuster, A., The Progress of Physics During 33 Years (1875–1908): Four Lectures Delivered to the University of Calcutta During March 1908 (Cambridge, 1911), 111.Google Scholar

Jeans wrote in 1910 that Planck's law demanded “something more” than what was contained in Planck's original papers, namely, that the “energy in the aether itself must also be atomic”, and that it should be “physically impossible to divide these atoms” of energy. The only alternative, if one “agreed that these conditions do not hold in nature”, and Jeans believed they did not, was to suppose that the “state of the aether represented by Planck's law is not a final steady state”, Jeans, J. H., “On Non-Newtonian Mechanical Systems, and Planck's Theory of Radiation”, Phil. Mag., ser. 6, xx (1910), 953.Google Scholar

58 The response of the British and Continental physicists to invitations to the first Solvay congress presents a striking comment on the respective reactions of British and Europeans to the quantum theory. The congress, an international gathering of leading physicists, was held in Brussels in 1911. Its purpose was to discuss the crisis in physics brought about by the quantum theory. Of the list of twenty-five to whom invitations were sent, nineteen were European and six British. Every European but one accepted his invitation, while of the six British invited only two attended. The four who declined were Larmor, Rayleigh, Schuster, and Thomson. The two who attended were Jeans and Rutherford. Five of the six were unsympathetic to the quantum theory, and the sixth, Rutherford, while not unsympathetic, was not deeply involved with quanta. The names of those invited are included in the invitational letter from E. Solvay, 15 June 1911, Lorentz Collection, Algemeen Rijksarchief, The Hague.

59 Thomson's contemporaries frequently applied the adjective “bold” to his ideas. In reference to a paper delivered at the British Association, the reporter for Nature spoke of the “boldness now always expected from Sir J. J. Thomson”, Nature, xcii (1913), 305Google Scholar. Millikan remarked on the “boldness” of Thomson's proposal of a structure of light, Millikan, R. A., The Electron, 222Google Scholar. Arguing that a certain atomic model of Thomson should be taken seriously, M. Born observed that such “bold concrete ideas have often led to surprising consequences”, Born, M., “Über das Thomsonsche Atom-modell”, Phys. Zeit., x (1909), 1031Google Scholar. The same atomic model was described by Lorentz as a “bold hypothesis”, Lorentz, H. A., “Alte und neue Fragen der Physik”, Phys. Zeit., xi (1910), 1251.Google Scholar

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61 Thomson's concern was with the preservation of Newtonian physics, that group of laws of matter and motion in terms of which he believed the field to be wholly explicable. A more common view was that electromagnetic theory and mechanics were individually challenged by the notion of discontinuous physical processes. The British physicists J. W. Nicholson, Jeans, and S. B. McLaren all believed that Newton's laws, or more generally, any laws of motion in mechanics expressible in terms of differential equations, would have to be replaced by new and as yet undiscovered laws in the description of atomic-scale phenomena. See Nicholson, J. W., “The Constitution of the Solar Corona. II”, Mon. Not. Roy. Astr. Soc., lxxii (1912), 677CrossRefGoogle Scholar; Jeans, J. H.' opening address in the “Discussion on Radiation”, Report of the British Association, 1913, 378Google Scholar; McLaren, S. B., “The Theory of Radiation”, Phil. Mag., ser. 6, xxv (1913), 43–44CrossRefGoogle Scholar. However, two of the three, Nicholson and McLaren, believed that the classical theory of light was correct. Only Jeans thought that the nature of light was an open question. McLaren forcefully expressed his sense of the relative importance of the laws of mechanics and light: it would be a “small thing to sacrifice the ordinary mechanical notions of matter” in order to “save the classical view of radiation as a continuous wave motion”. Ibid., 43.

61 Stark had no illusions about the standing of the quantum theory among his contemporaries generally. By revealing that the light-quantum hypothesis was behind his experiments on canal rays he said that he was aware that he risked “discrediting the experimental results”. Stark, J., “Neue Beobachtungen an Kanalstrahlen in Beziehung zur Lichtquantenhypothese” Phys. Zeit., ix (1908), 768.Google Scholar

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66 Campbell, N. R., “Discontinuities in Light Emission”, Proc. Camb. Phil. Soc., xv (1909), 310–328Google Scholar and xv (1910), 513–525.

67 Ibid., 311.

68 Ibid., 521.

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70 Einstein's ideas on this subject are thoroughly analysed in M. J. Klein, “Einstein and the Wave-Particle Duality.” Einstein thought that the difficulties of combining wave and particle characteristics were not insuperable. At a discussion in 1909, in which Planck and Stark took part, Einstein explained that he imagined a light quantum as a “singularity surrounded by a large vector field”. He thought that such an interpretation could explain interference phenomena. Einstein, A., “Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung”, Phys. Zeit., x (1909), 826.Google Scholar

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72 Ibid., 312.

73 Ibid., 312.

74 Ibid., 309.

75 Ibid., 302.

76 Thomson, J. J., “On the Theory of Radiation”, Phil. Mag., xx (1910), 238–247.CrossRefGoogle Scholar

77 Ibid., 246.

78 Ibid., 246.

79 It is an ironic note that the major role of Thomson in the development of the quantum theory was as an unwitting producer of quantizable atomic models. Thomson's atomic mechanisms of 1904 and 1912 were made the basis of Nicholson's quantum theory of atoms, the earliest attempt (1912) by a British physicist to extend the quantum theory in new directions. In Europe, too, A. E. Haas and others imposed quantum conditions on Thomson's atomic models.

80 Planck, Lorentz, and Sommerfeld all spoke out publicly against light quanta in 1910.

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So far as I know, Planck's theory was referred to as the “quantum theory” for the first time in Britain in 1912. This terminology occurs in Nicholson, J. W., lac. cit. (61).Google Scholar

84 Thomson, J. J., op. cit. (82), 643.Google Scholar

85 Ibid., 650.

86 Ibid., 643.

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88 For example, in 1913 Campbell said that Thomson's ideas on the structure of light were “notable as the only attempt that has been made to visualize the mechanism by which the energy of radiation is divisible into quanta of finite amount which can be absorbed almost instantaneously by a system on which they fall”, Campbell, N. R., Modern Electrical Theory, 2nd ed. (Cambridge, 1913), 251.Google Scholar

89 Stark, too, at about the same time as Thomson, attempted to unify the wave and particle properties. He imagined light quanta to be capable of forming aggregates. In explaining partial reflection, he supposed that an aggregate of quanta undergoes division, each of the resulting halves increasing its porosity so as to take up the same volume as the original undivided aggregate. Stark's explanation of optical phenomena is discussed in Lorentz, , op. cit. (59), 1250.Google Scholar

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90 Einstein used this expression in a letter to his friend C. Habicht, quoted in Seelig, C., Albert Einstein, A Documentary Biography, trans. Savill, M. (London, 1956), 74–75.Google Scholar

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92 In 1914 Hughes observed that the “prevalent” view then was that Einstein's hypothesis “gives the correct mathematical expression for the energy transformations, but gives no indication of the phenomena to which it is applied”, Hughes, A. L., Photoelectricity (Cambridge, 1914), 6.Google Scholar

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98 Thomson, J. J., “The Nature of Light”, Nature, cxxxvii (1936), 232–233CrossRefGoogle Scholar. Thomson also discusses this letter in his autobiography, Recollections and Reflections (New York, 1937), 410.Google Scholar

99 Thomson did introduce some mathematical analysis into his theory in 1910 (and again from 1925 on). He calculated the energy and momentum in an elementary conical tube of force when the electron to which it is attached is set in motion. The results, however, did not really lead anywhere.

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102 Dirac was attracted by the possibility of representing spatially discrete objects by Faraday's lines of force, exactly as Thomson had been. Dirac observed that “when we go over to quantum theory, we bring a kind of discreteness into our basic picture. We can suppose that the continuous distribution of Faraday lines of force that we have in the classical picture is replaced by just a few discrete lines offeree with no lines efforce between them”, Dirac, P. A. M., “The Evolution of the Physicist's Picture of Nature”, Scientific American, ccviii, No. 5 (05 1963), 51.Google Scholar

103 Campbell, N. R., Modern Electrical Theory, and ed. (Cambridge, 1913).Google Scholar

104 Thomson, J. J., Recollections and Reflections, 410.Google Scholar