¹ Yukawa, H., ‘On the interaction of elementary particles. I’, Proc, Phys.-Math. Soc. Japan, (1935), 17, pp. 48–57Google Scholar. (Hereafter we abbreviate the society as PMSJ and the paper as Interaction I.) For Yukawa and his introduction of the U-field, see Darrigol, O., ‘The quantum electrodynamical analogy in early nuclear theory of the roots of Yukawa's theory’, Rev. Hist. Sci. (1988), 41, pp. 226–97CrossRefGoogle Scholar, and the following papers by Brown, L. M.: ‘Yukawa's prediction of the meson’, Centaurus, (1981), 25, pp. 71–132CrossRefGoogle Scholar; ‘How Yukawa arrived at the meson theory’, Prog. Theor. Phys. Suppl. 85, (1985), pp. 13–19Google Scholar; ‘Hideki Yukawa and the meson theory’, Physics Today, 12 1986, pp. 1–8Google Scholar; ‘Yukawa in the 1930s: a gentle revolutionary’, Historia Scientiarum, (1989), 36, 1–21Google Scholar; ‘Hideki Yukawa’, DSB, Suppl. 2.Google Scholar

² The most influential of the experimental papers was Neddermeyer, S. H. and Anderson, C. D., ‘Note on the nature of cosmic ray particles’, Phys. Rev. (1937), 51, 884–6CrossRefGoogle Scholar. For the history of this development see Brown, L. M. and Rechenberg, H., ‘Quantum field theories, nuclear forces, and the cosmic rays (1934–1938)’, American Journal of Physics, (1991), 59, pp. 595–605CrossRefGoogle Scholar (hereafter referred to as Part III) and Rechenberg, H. and Brown, L. M., ‘Yukawa's heavy quantum and the mesotron (1935–1937)’ Centaurus, (1990), 33, pp. 214–52CrossRefGoogle Scholar (hereafter referred to as Part IV).

³ Mukherji, V., ‘A history of the meson theory of nuclear forces from 1935 to 1952’, Archive for the History of the Exact Sciences, (1974), 13, pp. 27–102CrossRefGoogle Scholar; Hayakawa, S., ‘The development of meson physics in Japan’, in The Birth of Particle Physics (ed. Brown, L. M. and Hoddeson, L.), Cambridge: Cambridge University Press, 1983, pp. 82–107.Google Scholar

⁴ The published recollections include: Kemmer, N., ‘The impact of Yukawa's meson theory on workers in Europe – a reminiscence’, Suppl. Prog. Theor. Phys. Commemoration issue for the 30th Anniversary of the Meson Theory by Dr. H. Yukawa, (1965). pp. 602–8Google Scholar; Sakata, S., ‘Reminiscences of research on meson theory’, in Yukawa, H., Sakata, S. and Taketani, M., Quest for Elementary Particles (tr. Eguchi, Noriko), Tokyo: Keiso-Shobo, 1965Google Scholar; Taketani, M., ‘Methodological approaches to the development of the meson theory of Yukawa’, Suppl. Prog. Theor. Phys. No. 50, (1971), pp. 12–24CrossRefGoogle Scholar. A number of relevant recollections are to be found in: Early History of Cosmic Ray Studies (ed. Sekido, Yataro and Elliot, Harry), Dordrecht: D. Reidel, 1985CrossRefGoogle Scholar; Particle Physics in Japan, 1930–1950. vols. 1–2 (ed. Brown, L. M., Konuma, M. and Maki, Z.) Kyoto: Research Institute for Fundamental Physics, 1980 (hereafter referred to as PPJ)Google Scholar; Proc. of the Japan-USA Collaborative Workshops on the History of Particle Theory in Japan (ed. Brown, L. M., Kawabe, R., Konuma, M. and Maki, Z.) Kyoto: RIFP, 1988 (hereafter referred to as PJCW)Google Scholar; Colloque International sur I'Histoire de la Physique des Particules. J. Phys. (Paris) Suppl. 12 (1982), 43.Google Scholar

⁵ Personal interviews have been taped by L. M. Brown with the following physicists: Wick, G. C., in Geneva, on 29 10 1979Google Scholar; Kemmer, N., in Edinburgh, on 25 and 26 07 1984Google Scholar; Heitler, W., in Zürich, on 29 09 1979Google Scholar; Taketani, M., in Tokyo, on 13 10 1978 and 27 10 1984Google Scholar; Husimi, K., in Tokyo, on 13 10 1978Google Scholar; Tomonaga, S., in Tokyo, on 12 10 1978Google Scholar; Fröhlich, H., in Stuttgart, on 26 07 1989.Google Scholar

⁶ Archival sources include: Archive for the History of Quantum Physics [AHQP] (New York, etc.); Yukawa Hall Archival Library [YHAL], Kyoto; Werner Heisenberg Archive (Munich); Dirac Archive, Churchill College (Cambridge).

⁷ See Brown, and Rechenberg, , Part IV, op. cit. (2).Google Scholar

⁸ These new contributions to field quantization included: Pauli, W. and Weisskopf, V., ‘Über die Quantizierung der skalaren relativistischen Wellengleichung’, Helv. Phys. Acta, (1934), 7, pp. 709–31Google Scholar; Dirac, P. A. M., ‘Relativistic wave equations’, Proc. Roy. Soc. London, (1936), A155, pp. 447–59CrossRefGoogle Scholar; Yukawa, H. and Sakata, S., ‘Note on Dirac's generalized wave equations’, Proc. PMSJ, (1937), 19, pp. 91–5Google Scholar; Proca, A., ‘Sur la théorie undulatoire des électrons positifs et négatifs’, J. Phys. Rad. (1936), 7, pp. 347–53.CrossRefGoogle Scholar

⁹ Document YHAL: E020 60 P12.

¹⁰ Document YHAL: FO 50 30. The material in this document of about eighteen pages is extensive, and it appears suitable for a good deal more than one lecture.

¹¹ Anderson, Carl D. and Neddermeyer, Seth H., ‘Cloud chamber observations of cosmic rays at 4300 meters elevation and near sea level’ Phys. Rev., (1936), 50, pp. 263–71.CrossRefGoogle Scholar

¹² Op. cit. (10). For the translation of the contents of this (and many other) documents, we are very grateful to Professor Rokuo Kawabe.

¹³ These are: 6 January, YHAL: E03 060 P13; 11 January, YHAL: E02 080 P12. See Brown, and Rechenberg, , Part IV, op. cit. (2)Google Scholar; for a summary of the talk at PMSJ, op. cit. (9), see Hayakawa, S., ‘The development of meson physics in Japan’, in The Birth of Particle Physics (ed. Brown, L. M. and Hoddeson, L.), Cambridge: Cambridge University Press, 1983, pp. 82–107, esp. pp. 88–9.Google Scholar

¹⁴ These papers are: Yukawa, H. and Sakata, S., ‘On the interaction of elementary particles. II’, Proc. PMSJ, (1937), 19, pp. 1084–93Google Scholar; Yukawa, H., Sakata, S. and Taketani, M., ‘On the interaction of elementary particles. III’Google Scholar, ibid. (1938), 20, pp. 319–40; hereafter we shall refer to these papers as Interaction II and III.

¹⁵ Document YHAL: E06 U02. This letter and a second one, sent to the editor of the Physical Review on 4 10 1937Google Scholar and also rejected, are reproduced and discussed by Kawabe, Rokuo: ‘Two unpublished manuscripts of Yukawa on the meson theory – Hideki Yukawa in 1937’, in PJCW, op. cit. (4), pp. 175–93Google Scholar. They are also discussed in part IV, op. cit. (2).

¹⁶ See the article on Nishina in DSB. Also, articles on Sakata, Tomonaga and Yukawa in DSB, Suppl. 11 (1990).Google Scholar

¹⁷ We have discussed some of the early collaborative work of Yukawa and Sakata in our Part IV, op. cit. (2).

¹⁸ There is a manuscript (YHAL: E02 130 P12), in Yukawa's hand and in English, entitled ‘On the interaction of elementary particles. II. Generalization of the Mathematical Scheme’. It is marked ‘Read Sept [blank], 1937’, but that date is overwritten as Nov. 28, 1936. In it Sakata is cited as having in press a ‘detailed discussion’ of the magnetic moments of proton and neutron, based on the theory of heavy quanta. This manuscript could have been written by Yukawa no earlier than the summer of 1937, as he cites two letters to the Physical Review which were published in June and July, respectively: Oppenheimer, J. R. and Serber, R., ‘Note on the nature of cosmic ray particles’, Phys. Rev. (1937), 51, p. 1113CrossRefGoogle Scholar and Stueckelberg, E. C. G., ‘On the existence of heavy electrons’Google Scholar, ibid. (1937), 52, pp. 41–2.

¹⁹ These are an eight-page memorandum in Japanese on the scalar meson theory (YHAL: E02 110 P12) and a second, entitled ‘Quantization of the Yukawa field’ (YHAL: E02 122 P12).

²⁰ Sakata, S., ‘Reminiscences of research on meson theory’, in Yukawa, H., Sakaca, S. and Taketani, M., Quest for Elementary Particles (tr. Eguchi, Noriko), Tokyo: Keiso-Shobo, 1965.Google Scholar

²¹ See also Taketani's account of their conversations at Osaka in Taketani, M., ‘Methodological approaches to the development of the meson theory of Yukawa in Japan’, Prog. Theor. Phys., Suppl. 50, (1971), pp. 1–24, esp. p. 16.Google Scholar

²² ‘The militarists…sanctioned and encouraged a veritable witchhunt, for all persons whose slightest word or deed could be considered to be lèse majesté…Even the two great Imperial Universities at Tokyo and Kyoto, which had always enjoyed great prestige and considerable academic freedom were condemned for harboring “red” professors and were subjected to purges’, Reischauer, Edwin O., Japan Past and Present, 3rd edn, Tokyo: Charles E. Turtle, 1964, pp. 174–5.Google Scholar

²³ Sakata, , op. cit. (20).Google Scholar

²⁴ See Oppenheimer, and Serber, , op. cit. (18)Google Scholar, and Stueckelberg, , op. cit. (18).Google Scholar

²⁵ See Sakata, , op. cit. (20)Google Scholar. Pauli and Weisskopf did not, of course, expect their spin 0 theory to describe the electron, but Pauli was delighted to find a self-consistent theory that did not satisfy the postulates that Dirac had assumed to derive his electron theory (and which had led many physicists to believe that all elementary particles must have spin ½). Pauli liked to call his new theory the ‘anti-Dirac’ theory. See Weisskopf, V. F., ‘Growing up with field theory: the development of QED’Google Scholar, in Brown, and Hoddeson, , op. cit. (3), pp. 56–81, esp. p. 70.Google Scholar

²⁶ Interaction II. op. cit. (14).Google Scholar

²⁷ Stueckelberg, , op. cit. (18)Google Scholar, had stated explicitly that Yukawa's quanta were unstable, in view of their weak interaction with the electron and neutrino.

²⁸ Anderson, and Neddermeyer, , op. cit. (11)Google Scholar and Phys. Rev. (1937), 51, pp. 884–6Google Scholar; Street, J. C. and Stevenson, E. C., Phys. Rev. (1937), 51, p. 1005Google Scholar. They quote also ‘preliminary results of Nishina and others’, giving the mass of the new particle as about 1/10 of the proton mass. This letter had been submitted to the Physical Review, but was delayed in publication by a referee report. It appeared finally as Nishina, Y., Takeuchi, M. and Ichimiya, T., Phys. Rev. (1937), 52, pp. 1198–9.CrossRefGoogle Scholar

²⁹ Stueckelberg, , op. cit. (18)Google Scholar and Oppenheimer, and Serber, , op. cit. (18)Google Scholar. The latter authors had stated that ‘the reconciliation of the approximate saturation character of nuclear forces with the apparent equality of like and unlike particle forces and with the magnetic moments of neutron and proton could be achieved [in Yukawa's theory] only by an extreme artificiality’.

³⁰ Interaction II, op. cit. (14), p. 1084.Google Scholar

³¹ Interaction II, op. cit. (14), p. 1088.Google Scholar

³² Interaction II, op. cit. (14), p. 1090.Google Scholar

³³ It is perhaps worth noting here that Nishina was an author of the first correct calculation of the Compton scattering cross-section using Dirac's relativistic theory of the electron: Klein, O. and Nishina, Y., ‘Über die Streuung von Strahlen durch freie Elektronen nach der neuen relativistischen Quantendynamik von Dirac’, Z. Phys. (1929), 52, pp. 853–68.CrossRefGoogle Scholar

³⁴ Sakata, , op. cit. (20).Google Scholar

³⁵ Yukawa, H., ‘On the interaction of elementary particles. III. (01 6, 1937)’Google Scholar, YHAL: E03 060 P13, 22 pp., and an undated addendum, YHAL: E03 070 P13, 3 pp. The contents are as follows: (1) Generalized Maxwellian linear equations for the new field. Vertauschungsrelationen. Supplementary conditions. (2) Interaction of heavy quanta with the electromagnetic field. (3) Linear field equations of Proca type. (4) Interaction of the U-field with the heavy particle.

³⁶ Proca, , op. cit. (8).Google Scholar

³⁷ Diracy, , op. cit. (8)Google Scholar, and Yukawa, and Sakata, , op. cit. (8).Google Scholar

³⁸ Yukawa, , op. cit. (35)Google Scholar: also Yukawa, H., ‘Interaction of the heavy quanta with the electromagnetic field’Google Scholar, YHAL E03 080 P13 and ‘Interaction of the neutron and the proton’, YHAL: E03 090 P13. Yukawa, H., Sakata, S. and Taketani, M., ‘On the interaction of elementary particles. III. (Read September 25, 1937)’Google Scholar, YHAL: E03 100 P13, manuscript of five pages, dated November, 1937.

³⁹ For example, colloquium talk by Yukawa at Osaka University on ‘The magnetic moment of the neutron’, 10 06 1937Google Scholar (YHAL: F05 140 Til); also, a talk by Taketani at the Kyoto branch of PMSJ on 25 November 1937, mentioned in a postcard of Taketani to Sakata, 26 September 1937 (YHAL: E02 120 P13). Taketani had published separately in Japanese: ‘Quantization of proton and neutron fields and their magnetic moments’, Kagaku, (1937), 7, pp. 532–3.Google Scholar

⁴⁰ YHAL: E03 100 P13 and E03 110 P13. The published paper is Interaction III, op. cit. (14).

⁴¹ For these new developments in field quantization, see note 8 above. They also refer to other relevant works that uses ‘Proca's scheme’, namely: Kemmer, N., ‘Nature of the nuclear field’, Nature, (1938), 141, 116–17CrossRefGoogle Scholar and Bhabha, H. J., ‘Nuclear forces, heavy electrons, and the β-decay’Google Scholar, ibid., (1938), pp. 117–18. In fact, Bhabha does not refer to Proca, apparently considering his vector field equations to be an obvious generalization of the spin 0 theory of Pauli and Weisskopf. In a recent letter to one of us (Kemmer, N. to , L. M. B., 24 05 1989)Google Scholar, Professor Kemmer has written that he had ‘lived through the birth of the Pauli Weisskopf paper’, and so Proca's equation was already known to him when he saw the paper cited in note 8. Although Kemmer realized that it could never be applied to the electron (as Proca attempted), he felt that he should nevertheless cite ‘Proca's otherwise quite misguided paper’. Kemmer thinks he may be responsible for that now-common attribution. While the Yukawa group said in the quoted paragraph that it was only later that Proca's work ‘came to our notice’, it should be noted that the earliest partial draft of Interaction III, dated 6 01 1937Google Scholar, has a section headed ‘Linear field equations of Proca type’. See note 35. Interaction III also quotes a Russian spin 1 paper: Durandin, E. and Erschow, A., ‘Über einige Anwendungen der Supraquantelung in der Wellenmechanik des Elektrons’, Phys. Zeit. Sowj. (1937), 12, pp. 466–71.Google Scholar

⁴² Fröhlich, H. and Heitler, W., ‘Magnetic moments of the proton and the neutron’, Nature, (1938), 141, pp. 37–8CrossRefGoogle Scholar. See also Taketani, note 39.

⁴³ Heitler, W. and Fröhlich, H. to Yukawa, H., 5 03 1938 (YHAL).Google Scholar

⁴⁴ The quotations are all from Interaction III, pp. 319–21Google Scholar. The Bhabha reference is to the paper cited in note 41.

⁴⁵ See note 42; also see Bhabha, , op. cit. (41).Google Scholar

⁴⁶ Actually, parity conservation prevents the mixing of states of even and odd orbital angular momentum, but mixing of S and D states does occur as a result of non-central forces; it is responsible, e.g., for the quadrupole moment of the deuteron.

⁴⁷ Bhabha, , op. cit. (41)Google Scholar. However, see also Stueckelberg, , op. cit. (18).Google Scholar

⁴⁸ Pauli, W. to Weisskopf, V., 13 01 1938Google Scholar; published in Meyenn, Karl von (ed.) Wolfgang, Pauli: Scientific Correspondence. Volume II: 1930–1939, Berlin: Springer-Verlag, 1985, pp. 547–50, esp. p. 548.Google Scholar

⁴⁹ Hoch, P., ‘Flight into self absorption and xenophobia. The plight of refugee theorists amongst British and American experimentalists in the 1930s highlights cultural and natural differences in science’, Physics World, 01 1990, pp. 23–6. esp. pp. 24–5Google Scholar. Hoff's remarks about Cambridge physics are borne out by Dirac, Britain's premier theorist, who occupied a chair in applied mathematics. However, it does not take note of the strong ties between Ralph Fowler, Stokes Lecturer in Mathematical Physics and later occupant of the Plummer chair, and Britain's foremost experimental physicist, his father-in-law Ernest Rutherford. See also Nevil Mott, ‘Theory and experiment in the Cavendish circa 1932’, in Hendry, John (ed.) Cambridge Physics in the Thirties, Bristol: Adam Hilger, 1984, pp. 125–32Google Scholar. On p. 126, Mott writes, ‘At that time theory had no recognised place in the Cavendish. No space was assigned to theorists, who were supposed to work in their college rooms or in their lodgings. Theorists were members of the faculty of Mathematics.’

⁵⁰ In addition to Hoch, , op. cit. (49)Google Scholar, see the excellent article by Stuewer, R. H., ‘Nuclear physicists in a new world. The émigrés of the 1930s in America’, Berichte Wissenschaftsgesch, (1984), 7, pp. 23–40CrossRefGoogle Scholar, which deals with the refugees' rather different reception in the United States.

⁵¹ Heitler, W., The Quantum Theory of Radiation, Oxford: Clarendon Press, 1936Google Scholar. Biographical information on Heitler can be found in Jost, R., ‘Walter Heitler. Nekrolog’, Vierteljahrschrift des Naturf. Ges. in Zurich, (1983), 128, pp. 139–41Google Scholar; Rasche, E., ‘Laudatio auf Prof. Walter Heitler’, Arch. Int. Histoire des Sciences, (1980), 30, pp. 159–66.Google Scholar

⁵² Biographical information on Kemmer was obtained by one of us (L.M.B.) in interviews with him during July 1984.

⁵³ See Haken, M., ‘Herbert Fröhlich 70 Jahre alt’, Phys. Blätter, (1975), 31, pp. 664–5CrossRefGoogle Scholar. Information also from interview by L. M. Brown in Stuttgart, July 1989.

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⁵⁶ Bhabha, H. J. and Heitler, W., ‘The passage of fast electrons and the theory of cosmic showers’, Proc. Roy. Soc. London, (1937), A159, pp. 432–58CrossRefGoogle Scholar. This work was considered in Brown, and Rechenberg, , Part III, op. cit. (2)Google Scholar, where other historical treatments are cited.

⁵⁷ It should be noted that German refugees in the United States met stronger competition in phenomenological theoretical physics. American theorists were oriented toward treating problems arising from the latest research in nuclear and cosmic ray physics (in which American experimental physics was outstanding). For the competitiveness issue, see, e.g., Steuwer, , op. cit. (50)Google Scholar, and Wheeler, John Archibald, ‘Some men and moments in the history of nuclear physics’, in Stuewer, R. H. (ed.) Nuclear Physics in Retrospect, Minneapolis: University of Minnesota Press, 1979, pp. 213–322.Google Scholar

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⁶¹ Kellogg, J. B., Rabi, I. I. and Zacharias, J. R., ‘The gyromagnetic properties of hydrogen’, Phys. Rev. (1936), 56, pp. 472–76CrossRefGoogle Scholar. These authors gave their results as: μ_p = (2.85 ± 0.15) μ₀, and μ_d = (0.85 ± 0.03) μ₀, where μ₀ is the nuclear magneton of the proton. Hence, μ_n = ( – 2.00 ± 0.18) μ₀. Estermann, I., Simpson, O. C. and Stern, O., ‘The magnetic moment of the proton’, Phys. Rev. (1937), 57, p. 1004Google Scholar. In 1937, Otto Frisch also returned to the problem of measuring the magnetic moment of the neutron.

⁶² Wick, G. C., ‘Teoria dei raggi β e memento magnetico del protone’, Rend. Acad. Lincei, (1935), 21, pp. 170–3.Google Scholar

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⁶⁴ Bhabha, and Heitler, , op. cit. (56)Google Scholar. A similar work appeared at nearly the same time: Carlson, J. F. and Oppenheimer, J. R., ‘On multiplicative showers’, Phys. Rev. (1937), 51, pp. 220–31.CrossRefGoogle Scholar

⁶⁵ Heitler, W., ‘On the analysis of cosmic rays’, Proc. Roy. Soc. London, (1937), A161, pp. 261–83.CrossRefGoogle Scholar

⁶⁶ Ibid., p. 282.

⁶⁷ See the correspondence in Meyenn, von, op. cit. (48), esp. pp. 522–6Google Scholar: Pauli, to Heisenberg, , 10 and 14 06 1937Google Scholar and Heisenberg, to Pauli, , 12 and 17 06 1937.Google Scholar

⁶⁸ See Pauli, (Zürich) to Weisskopf, (Copenhagen), 3 08 1937Google Scholar, in Meyenn, von, op. cit. (48), p. 533Google Scholar: ‘By the way, will Heitler come? This would be desirable because of the discussion on cosmic radiation.’

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⁷³ Fröhlich, H. and Heitler, W., ‘Magnetic moments of the proton and the neutron’, Nature, (1938), 141, pp. 37–8CrossRefGoogle Scholar. Their argument does not really rule out integer spins other than zero.

⁷⁴ Ibid., p. 38. One can also obtain the fractional dissociation time α from this argument (not given by the authors) as about 8 per cent.

⁷⁵ Kemmer, N., ‘Nature of the nuclear field’ Nature, (1938), 141, pp. 116–17.CrossRefGoogle Scholar

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⁸² Interaction II, op. cit. (14).Google Scholar

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⁸⁴ Fröhlich, and Heitler, , op. cit. (73).Google Scholar

⁸⁵ Bhabha, and Heitler, , op. cit. (56).Google Scholar

⁸⁶ Bhabha, H. J., ‘On the penetrating component of cosmic radiation’, Proc. Roy. Soc. London, (1938), A164, pp. 257–93CrossRefGoogle Scholar. Heitler, 's paper was ‘On the analysis of cosmic rays’, Proc. Roy. Soc. London, (1937), A161, pp. 261–83CrossRefGoogle Scholar, which appeared in the spring of that year.

⁸⁷ Blackett, P. M. S. and Wilson, J. G., ‘The energy loss of cosmic ray particles in metal plates’, Proc. Roy. Soc. London, (1937), A160, pp. 304–22, esp. p. 322.CrossRefGoogle Scholar

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⁹⁰ Bhabha, , op. cit. (76).Google Scholar

⁹¹ Bhabha, H. J., ‘On the theory of heavy electrons and nuclear forces’, Proc. Roy. Soc. London, (1938), A166, pp. 501–27CrossRefGoogle Scholar, footnote on p. 504.

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¹⁰⁰ Kemmer, , op. cit. (94).Google Scholar

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¹⁰³ These are respectively (in modern terminology) scalar and pseudoscalar for spin 0, and vector and pseudovector (or axial vector) for spin 1. In terms of relativistic tensors, the spin 0 fields are zero rank and completely antisymmetric fourth-rank tensors, respectively, while the spin 1 fields are first rank and antisymmetric third-rank tensors, an antisymmetric second-rank tensor, and its dual. These are not all independent, so that there are four cases of free fields. In interaction, however, there are six cases to consider. For example, in relativistic electrodynamics we can consider interaction with the six-vector (antisymmetric tensor) field strength or with the four-vector potential.

¹⁰⁴ Again, as in note 101, these are classified according to the spin and parity of (in modern terminology) the nucleon current.

¹⁰⁵ Kemmer, , op. cit. (94), p. 128.Google Scholar

¹⁰⁶ In his interview (note 52 above), Kemmer mentioned that he had decided to call the generalized d'Alembert equation describing the four-vector field. □øα– k²øα = 0, the ‘Proca equation’, although he was already familiar with the theory of the equation, and even though Proca had ‘misguidedly’ written that it could be applied to the electron.

¹⁰⁷ Dirac, , op. cit. (8).Google Scholar

¹⁰⁸ Kemmer, , op. cit. (94), pp. 136–7.Google Scholar

¹⁰⁹ In working out the resulting interaction, Kemmer found that there were terms linear and quadratic in the f's and g's. The quadratic terms, however, corresponded to direct pointlike interaction of the nucleons which he had shown could be eliminated. In a footnote on p. 142, Kemmer thanks Pauli for permission to state that Pauli had also proved that the pointlike interaction played no role in binding or in scattering.

¹¹⁰ Kemmer, , op. cit. (94), p. 147Google Scholar. ‘Fröhlich and others’ is Fröhlich, , Heitler, and Kemmer, , op. cit. (96)Google Scholar. Kemmer's conclusion that only case (b), the vector meson case, fitted the empirical neutron–proton interaction was no longer valid in the context of a charge-independent (also called isospin invariant or symmetric) meson theory. In that case, the correct choice would have been the pseudoscalar meson theory. This became the standard theory in the 1950s, after the discovery of the neutral pion, and the demonstration of the pseudoscalar nature and the charge-symmetric interaction of the charged and neutral pions. See Kemmer, , op. cit. (80), pp. 16–17Google Scholar, for wartime correspondence between Kemmer in Britain and Pauli in America, in which Pauli pointed out that pseudoscalar mesons were required by a charge-independent theory, and in which he stressed the importance of including the tensor force in the deuteron problem.

¹¹¹ Heitler, , op. cit. (70), pp. 422–3.Google Scholar

¹¹² Ibid.

¹¹³ Fröhlich, , Heitler, and Kemmer, , op. cit. (96), p. 155Google Scholar. The paragraph continues: ‘They seem to be absorbed strongly as soon as they reach an energy of less than 200 × 10⁶ e-volts. It is probable that this absorption is due to some sort of nuclear processes [original emphasis]… There is no need for the introduction of any neutrino.’

¹¹⁴ Curiously, however, Kemmer believed that the strong nuclear interaction will give rise to explosive showers of the type that Heisenberg inferred from the Fermi theory. On pp. 147–8 of Kemmer, , op. cit. (94)Google Scholar, he pointed out that the perturbation expansion arising from the longitudinal part of the vector field is ‘an expansion in powers of a fundamental length’, and that, according to Heisenberg, ‘thus multiple processes such as cosmic ray showers should occur…when sufficiently high energies are available’.

¹¹⁵ Fröhlich, , Heitler, and Kemmer, , op. cit. (96), pp. 155–6Google Scholar. It is clear that at least two of the authors took the magnetic moment results a bit too seriously (when we note that the divergent results required an arbitrary cutoff, and that even in 1990 no quantitative explanation of the magnetic moments has been given). At the same time, all three ignored the radioactivity of the mesotron, which played a significant role in cosmic ray phenomena. (Hindsight does work better than prescience.)

¹¹⁶ The article cited is Interaction II, op. cit. (14).

¹¹⁷ The quotations are from Fröhlich, , Heitler, and Kemmer, , op. cit. (96), p. 156.Google Scholar

¹¹⁸ Heitler, W., The Quantum Theory of Radiation, Oxford: Oxford University Press, 1936.Google Scholar

¹¹⁹ At this point they note that Bhabha has also obtained the same field equations and the relativistic interaction of type (b).

¹²⁰ Fröhlich, , Heitler, and Kemmer, , op. cit. (96), p. 166.Google Scholar

¹²¹ This information is contained in the three footnotes on p. 504 of his article, op. cit. (91).

¹²² Bhabha, , op. cit. (86).Google Scholar

¹²³ Bhabha, , op. cit. (91), p. 501.Google Scholar

¹²⁴ In this he differed from his earlier paper, op. cit. (86), in which he said on p. 290: ‘This change in the rest mass may be spontaneous, or caused by an external agency. The former possibility is not very interesting as far as cosmic radiation is concerned, for if the probability is large, the change will take place before the particle reaches earth, if small, then the chance of its taking place in the very short time taken by the particle in penetrating the earth's surface [atmosphere?] is also negligible.’ Evidently, Bhabha was assuming that the heavy electrons belonged to the primary cosmic rays.

¹²⁵ This quotation and the preceding one are from Bhabha, , op. cit. (91), p. 505.Google Scholar

¹²⁶ Heitler, , op. cit. (95).Google Scholar

¹²⁷ Heitler, , op. cit. (95), p. 529.Google Scholar

¹²⁸ Fröhlich, and Heitler, , op. cit. (73).Google Scholar