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Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends

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In May 1918 Paul Ehrenfest received a monograph from Niels Bohr in which Bohr had used Ehrenfest’s adiabatic principle as an essential assumption for understanding atomic structure. Ehrenfest responded by inviting Bohr, whom he had never met, to give a talk at a meeting in Leiden in late April 1919, which Bohr accepted; he lived with Ehrenfest, his mathematician wife Tatyana, and their young family for two weeks. Albert Einstein was unable to attend this meeting, but in October 1919 he visited his old friend Ehrenfest and his family in Leiden, where Ehrenfest told him how much he had enjoyed and profited from Bohr’s visit. Einstein first met Bohr when Bohr gave a lecture in Berlin at the end of April 1920, and the two immediately proclaimed unbounded admiration for each other as physicists and as human beings. Ehrenfest hoped that he and they would meet at the Third Solvay Conference in Brussels in early April 1921, but his hope was unfulfilled. Einstein, the only physicist from Germany who was invited to it in this bitter postwar atmosphere, decided instead to accompany Chaim Weizmann on a trip to the United States to help raise money for the new Hebrew University in Jerusalem. Bohr became so overworked with the planning and construction of his new Institute for Theoretical Physics in Copenhagen that he could only draft the first part of his Solvay report and ask Ehrenfest to present it, which Ehrenfest agreed to do following the presentation of his own report. After recovering his strength, Bohr invited Ehrenfest to give a lecture in Copenhagen that fall, and Ehrenfest, battling his deep-seated self-doubts, spent three weeks in Copenhagen in December 1921 accompanied by his daughter Tanya and her future husband, the two Ehrenfests staying with the Bohrs in their apartment in Bohr’s new Institute for Theoretical Physics. Immediately after leaving Copenhagen, Ehrenfest wrote to Einstein, telling him once again that Bohr was a prodigious physicist, and again expressing the hope that he soon would see both of them in Leiden.

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Notes

  1. Editor’s Note: Einstein had to cancel a trip to Copenhagen planned for January 1921; see Bohr to Einstein, June 22, 1921, The Collected Papers of Albert Einstein. Vol. 12 (ref. 54), pp. 53–54; on p. 53 [translated by Hentschel (ref. 54), pp. 24–25; on p. 24]. In fact, however, he and Ilse Einstein had visited Copenhagen seven months earlier, on June 24, 1920, where he gave a lecture the next day on gravitation and geometry to the Danish Astronomical Society; see ibid., p. 54 [p. 25]. The two stayed with Elis Strömgren in the observatory; see ibid., Vol. 10 (ref. 35), p. 580.

References

Note on Archival Sources: The Ehrenfest Scientific Correspondence (ESC) and the Ehrenfest Research Notebooks, are deposited in the Museum Boerhaave (Rijksmuseum voor de Geschiedenis van de Natuurwetenschappen en van de Geneeskunde, Leiden, The Netherlands); hereafter Museum Boerhaave. Most but not all of the letters in the Ehrenfest Scientific Correspondence are listed in Bruce Wheaton, Catalogue of the Paul Ehrenfest Archive at the Museum Boerhaave Leiden (Leiden: Communication 151 of the National Museum for the History of Science and Medicine "Museum Boerhaave," 1977). A letter that is listed is designated, for example, by the notation ESC9, S9, 356, which means that it is located on ESC Microfilm 9, in Section 9, and is Number 356.

  1. Bohr to Ehrenfest, May 5, 1918, Museum Boerhaave, ESC1, S5, 152.

  2. Ehrenfest to Lorentz, August 25, 1913, Museum Boerhaave, ESC7, S4, 198.

  3. J. M. Burgers, Het Atoommodel van Rutherford-Bohr (Haarlem: De Erven Loosjes. 1918).

  4. N. Bohr, “On the Quantum Theory of Line-Spectra,” Det Kongelige Danske Videnskabernes Se1skab. Skrifter. Naturvidenskabelig og mathematisk Afdeling 8. Raekke, IV. 1, Parts 1–2 (1918), 1–100; reprinted in Niels Bohr Collected Works. Vol. 3. The Correspondence Principle (1918–1923) (Amsterdam, New York, Oxford: North-Holland Publishing Company, 1976), pp. 67–166.

  5. Ehrenfest to Einstein, May 8, 1918; reprinted in The Collected Papers of Albert Einstein. Vol. 8. The Berlin Years: Correspondence, 1914–1918. Part B. 1918, edited by Robert Schulmann, A.J. Kox, Michel Janssen, and József Illy (Princeton: Princeton University Press, 1998), pp. 756–767; translated by Ann M. Hentschel (Princeton: Princeton University Press, 1998), pp. 555–556.

  6. Ehrenfest, Research Notebook XXIV, May 12, 1918, Museum Boerhaave.

  7. Bohr, “Quantum Theory” (ref. 4), p. 4; 70.

  8. Martin J. Klein, Paul Ehrenfest. Vol. 1. The Making of a Theoretical Physicist (Amsterdam, London: North-Holland Publishing Company and New York: American Elsevier Publishing Company, 1970), pp. 284–287.

  9. Ehrenfest to Bohr, May 10, 1918, Niels Bohr Archive, Copenhagen, Denmark.

  10. Ehrenfest to Bohr, January 13. 1919, Niels Bohr Archive, Copenhagen, Denmark.

  11. Ehrenfest to Kramers, October 1916 (undated), Museum Boerhaave.

  12. Einstein to Ehrenfest, March 22, 1919; reprinted in The Collected Papers of Albert Einstein. Vol. 9. The Berlin Years: Correspondence, January 1919–April 1920, edited by Diana Kormos Buchwald, Robert Schulmann, József Illy, Daniel J. Kennefick, and Tilman Sauer (Princeton: Princeton University Press, 2004), pp. 15–16, on pp. 15–16; translated somewhat differently by Ann M. Hentschel (Princeton: Princeton University Press, 2004), pp. 9–10, on p. 9. See also Ronald W. Clark, Einstein: The Life and Times (New York and Cleveland: The World Publishing Company, 1971) pp. 219–221.

  13. Bohr to Ehrenfest, March 12, 1919, Museum Boerhaave, ESC1, S5, 154.

  14. Bohr to Ehrenfest, January 25, 1919, Museum Boerhaave, ESC1, S5, 153.

  15. Bohr to Ehrenfest, April 1, 1919, Museum Boerhaave, ESC1, S5, 155.

  16. Ehrenfest to Bohr, February 6, 1919, Niels Bohr Archive, Copenhagen, Denmark.

  17. Ehrenfest, Notes for a lecture at Leiden, Spring 1926, Museum Boerhaave.

  18. Ehrenfest, Research Notebook XXV, April 25, 1919, Museum Boerhaave.

  19. See, for example, Léon Rosenfeld, “On Quantum Electrodynamics,” in W. Pauli, ed., Niels Bohr and the Development of Physics (London: Pergamon Press and New York: McGraw-Hill Book Co., 1955), pp. 70–95, on p. 77; reprinted in Robert S. Cohen and John J. Stachel, ed., Selected Papers of Léon Rosenfeld (Dordrecht, Boston, London: D. Reidel Publishing Company, 1979), pp. 413–441, on p. 420; see also Stefan Rozenta1, “The Forties and the Fifties,” in S. Rozental, ed., Niels Bohr: His life and work as seen by his friends and colleagues (Amsterdam: North-Holland Publishing Company and New York: John Wiley & Sons, 1967), pp. 149–190, on pp. 161–164, 179–183.

  20. Bohr to Ehrenfest, May 10, 1919, Museum Boerhaave, ESC1, S5, 161.

  21. Ehrenfest to Bohr, February 28, 1919, Niels Bohr Archive, Copenhagen, Denmark.

  22. Ehrenfest, Research Notebook XXV, April 29-May 1, 1919, Museum Boerhaave.

  23. Ehrenfest to Einstein, September 21, 1919; reprinted in The Collected Papers of Albert Einstein. Vol. 9 (ref. 12), pp. 164–166, on p. 165; translated somewhat differently by Hentschel (ref. 12), pp. 94–95, on p. 94.

  24. Ehrenfest to Bohr, May 14, 1919, Niels Bohr Archive, Copenhagen, Denmark.

  25. Bohr to Ehrenfest, June 1, 1919, Museum Boerhaave, ESC1, S6, 163.

  26. Einstein to Ehrenfest, [November 8, 1919]; reprinted in The Collected Papers of Albert Einstein. Vol. 9 (ref. 12), pp. 227–228, on p. 227; translated somewhat differently by Hentschel (ref. 12), pp. 135–136, on p. 135.

  27. Ehrenfest to Bohr, June 4, 1919, Niels Bohr Archive, Copenhagen, Denmark.

  28. Bohr to Ehrenfest, October 22, 1919, Museum Boerhaave, ESC1, S6, 164.

  29. George de Hevesy to Ernest Rutherford, October 14, 1913; quoted in A.S. Eve, Rutherford: Being the Life and Letters of the Rt. Hon. Lord Rutherford, O.M. (New York: The Macmillan Company and Cambridge: At the University Press, 1939), p. 226; also quoted in Léon Rosenfeld, “Introduction,” to Niels Bohr, On the Constitution of Atoms and Molecules (Copenhagen: Munksgaard and New York: W.A. Benjamin, 1963), p. xlii.

  30. Einstein to Sommerfeld, August 3, 1916; reprinted in The Collected Papers of Albert Einstein. Vol. 8 (ref. 5). Part A. 1914–1917, edited by Robert Schulmann, A.J. Kox, Michel Janssen, and József Illy (Princeton: Princeton University Press, 1998), p. 326; translated slightly differently by Ann M. Hentschel (Princeton: Princeton University Press, 1998), p. 241; also reprinted in Arnold Sommerfeld, Wissenschaftlicher Briefwechsel. Band 1. 1892–1918, edited by Michael Eckert and Karl Märker (Berlin, Diepholz, München: Deutsches Museum, Verlag für Geschichte der Naturwissenschaften und der Technik, 2000), p. 563; facsimilie reprinted in Armin Hermann, ed., Albert EinsteinArnold Sommerfeld Briefwechsel (Basel and Stuttgart: Schwabe & Co., 1968), p. 42.

  31. Einstein to Planck, October 23, 1919; reprinted in The Collected Papers of Albert Einstein. Vol. 9 (ref. 12), p. 216; translated somewhat differently by Hentschel (ref. 12), pp. 128–129, on p. 129. See also Carl Seelig, Albert Einstein. A Documentary Biography, translated by Mervyn Savill (London: Staples Press, 1956), p. 160.

  32. Einstein to Ehrenfest, [November 8, 1919] (ref. 26), pp. 227–228, on p. 228; translated somewhat differently by Hentschel (ref. 26), pp. 135–136, on 136.

  33. Ehrenfest to Bohr, January 30, 1920, Niels Bohr Archive, Copenhagen, Denmark.

  34. Niels Bohr, The Theory of Spectra and Atomic Constitution: Three Essays (Cambridge: At the University Press, 1922), Essay II, pp. 20–60.

  35. Einstein to Bohr, May 2, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), p. 634; translated somewhat differently by J. Rud Nielsen in “Introduction,” p. 22; also reprinted in The Collected Papers of Albert Einstein. Vol. 10. The Berlin Years: Correspondence, May–December 1920 and Supplementary Correspondence, 1909–1920, edited by Diana Kormos Buchwald, Tilman Sauer, Ze’ev Rosenkranz, József Illy, and Virginia Iris Holmes (Princeton: Princeton University Press, 2006), p. 244; translated somewhat differently by Ann M. Hentschel (Princeton: Princeton University Press, 2006), p. 151.

  36. Einstein to Ehrenfest, May 4, 1920; reprinted in The Collected Papers of Albert Einstein. Vol. 10 (ref. 35), p. 246; translated somewhat differently by Hentschel (ref. 35), p. 152; also quoted in Seelig, Albert Einstein (ref. 31), p. 185.

  37. Bohr to Einstein, June 24, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 634–635; also reprinted in The Collected Papers of Albert Einstein. Vol. 10 (ref. 35), p. 321; translated somewhat differently by Hentschel (ref. 35), p. 200.

  38. P. Langevin and M. de Broglie, ed., La Théorie du Rayonnement et les Quanta. Rapports et Discussions de la Réunion tenue a Bruxelles, du 30 octobre au 3 novembre 1911. Sous les Auspices de M. E. Solvay (Paris: Gauthier-Villars, 1912); see also Maurice de Broglie, Les Premiers Congrès de Physique Solvay et l’Orientation de la Physique depuis 1911 (Paris: Albin Michel, 1951); Jagdish Mehra, The Solvay Conferences on Physics: Aspects of the Development of Physics since 1911 (Dordrecht and Boston: D. Reidel, 1975), pp. 13–72; Pierre Marage and Grégoire Wallenborn, ed., The Solvay Councils and the Birth of Modern Physics (Basel, Boston, Berlin: Birkhäuser Verlag, 1999), pp. 95–110.

  39. Institut International de Physique Solvay, La Structure de la Matière. Rapports et Discussions du Conseil de Physique tenu a Bruxelles du 27 au 31Octobre 1913. Sous les Auspices de l’Institut International de Physique Solvay (Paris: Gauthier-Villars, 1921); see also Mehra, Solvay Conferences (ref. 38), pp. 75–92; Marage and Wallenborn, Solvay Councils (ref. 38), pp. 112–132.

  40. Clark, Einstein (ref. 12), p. 223.

  41. Rutherford to B.B. Bo1twood, February 28, 1921; reprinted in Lawrence Badash, ed., Rutherford and Bo1twood: Letters on Radioactivity (New Haven and London: Yale University Press, 1969), pp. 341–344, on p. 342; quoted by Clark, Einstein (ref. 12), p. 223.

  42. Lorentz to Ehrenfest, July 12, 1920, Museum Boerhaave, ESC7, S7, 293.

  43. Ehrenfest to Bohr, August 19, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 609–610, p. 609.

  44. Ehrenfest to Bohr, October 17, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), p. 610.

  45. Bohr to Ehrenfest, November 22, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 611–612, on p. 611.

  46. Lorentz to Ehrenfest, December 17, 1920, Museum Boerhaave, ESC7, S7, 302.

  47. Ehrenfest to Bohr, December 27, 1920; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 612–613.

  48. L. Rosenfeld, Niels Bohr: An Essay Dedicated to Him on the Occasion of His Sixtieth Birthday, October 7, 1945 (Amsterdam: North-Holland, 1961), p. 5; in the second, corrected edition the quotation is somewhat changed; see Cohen and Stachel, Selected Papers of Léon Rosenfeld (ref. 19), pp. 313–326, on p. 315.

  49. W. Pauli, “Niels Bohr on His 60th Birthday,” Reviews of Modern Physics 17 (1945), 97-101, on 99; reprinted in Wolfgang Pauli, Collected Scientific Papers. Vol. 2, edited by R. Kronig and V.F. Weisskopf (New York, London, Sydney: Interscience Publishers, a division of John Wiley & Sons, Inc., 1964), pp. 1048–1052, on p. 1050.

  50. Ehrenfest to Bohr, December 27, 1920 (ref. 47); Bohr Collected Works (ref. 47), p. 613.

  51. Bohr to Ehrenfest, January 20, 1921; translated by J. Rud Nielsen in “Introduction,” Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 29–30.

  52. Kramers to Ehrenfest, February 18, 1921, Museum Boerhaave.

  53. Ehrenfest to Bohr, December 27, 1920 (ref. 48); Bohr Collected Works (ref. 48), p. 612.

  54. Einstein to Lorentz, February 22, 1921; reprinted in The Collected Papers of Albert Einstein. Vol. 12. The Berlin Years: Correspondence, January–December 1921, edited by Diana Kormos Buchwald, Ze’ev Rosenkranz, Tilman Sauer, József Illy, and Virginia Iris Holmes (Princeton: Princeton University Press, 2009), pp. 53–54; translated by Ann M. Hentschel (Princeton: Princeton University Press, 2009), pp. 95–96, and in The Scientific Correspondence of H.A. Lorentz. Vol. 1, edited by A.J. Kox (New York: Springer, 2008), p. 541.

  55. Clark, Einstein (ref. 12), pp. 223, 382–383; see also Einstein to Maurice Solovine, March 8, 1921, Solovine to Einstein, March 11, 1921, Einstein to Solovine, March 16, 1921, Collected Papers of Albert Einstein. Vol. 12 (ref. 54), pp. 122–123, 135–136, 144; translated by Hentschel (ref. 54), pp. 68–69, 74–75, 81.

  56. Bohr to Ehrenfest, March 23, 1921; translated by J. Rud Nielsen in “Introduction,” Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 30–31.

  57. Ehrenfest to Kramers, March 25, 1921; reprinted and quoted in Niels Bohr Collected Works. Vol. 3 (ref. 4), p. 615; Ehrenfest to Bohr, March 25, 1921; reprinted in ibid., pp. 615–616.

  58. Bohr to Ehrenfest, March 28, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 616–620.

  59. Ehrenfest, Research Notebook XXVI, March 31-April 6, 1921, Museum Boerhaave.

  60. Ehrenfest to Bohr, September 27, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 627–629, on p. 629.

  61. Bohr to Ehrenfest, April 12, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), p. 620.

  62. Paul Langevin, in Institut International de Physique Solvay, Structure et Propriétés des Noyaux Atomiques. Rapports et Discussions du Septième Conseil de Physique tenu a Bruxelles du 22 au 29 Octobre 1933. Sous les Auspices de l’Institut International de Physique Solvay (Paris: Gauthier-Villars, 1934), pp. vii–ix, on p. viii.

  63. Bohr to Ehrenfest, July 10, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 621–623, on p. 622.

  64. Ehrenfest to Lorentz, July 14, 1921, Museum Boerhaave.

  65. Lorentz to Ehrenfest, July 17, 1921, Museum Boerhaave, ESC7, S7, 309.

  66. Ehrenfest to Bohr, July 17, 1921, Niels Bohr Archive, Copenhagen, Denmark; partially reprinted (about the first half) in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 623–624.

  67. Bohr to Ehrenfest, July 28. 1921, Museum Boerhaave.

  68. Ehrenfest to Bohr, August 7, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 625–626.

  69. Bohr to Ehrenfest, September 1, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), p. 626.

  70. Bohr to Ehrenfest, September 16, 1921; reprinted in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 626–627.

  71. Lorentz to Ehrenfest, September 4, 1921, Museum Boerhaave, ESC7, S7, 310.

  72. Ehrenfest to Bohr, September 27, 1921 (ref. 60); Bohr Collected Works (ref. 60), p. 628.

  73. P. Ehrenfest, “Le Principe de Correspondance,” in Institut International de Physique Solvay, Atomes et Électrons. Rapports et Discussions du Conseil de Physique tenu a Bruxelles du 1 au 6 Avril 1921. Sous les Auspices de l’Institut International de Physique Solvay (Paris: Gauthier-Villars, 1923), pp. 248–254; reprinted in Paul Ehrenfest, Collected Scientific Papers, edited by Martin J. Klein (Amsterdam: North-Holland Publishing Company, 1959), pp. 436–442; N. Bohr, “L’Application de la Théorie des Quanta aux Problèmes Atomiques,” in ibid., pp. 228–247; reprinted in English in Niels Bohr Collected Works. Vol. 3 (ref. 4), pp. 364–380.

  74. Ehrenfest to Kramers, November 2, 1921, Museum Boerhaave.

  75. Bohr to Ehrenfest, September 16, 1921 (ref. 70); Bohr Collected Works (ref. 70), p. 627.

  76. Bohr to Ehrenfest, April 12, 1921 (ref. 61); Bohr Collected Works (ref. 61), p. 620.

  77. Bohr to Ehrenfest, June 28, 1921, Museum Boerhaave, ESC1, S6, 173.

  78. Ehrenfest to Bohr, July 3, 1921, Niels Bohr Archive, Copenhagen, Denmark.

  79. Bohr to Ehrenfest, July 10, 1921 (ref. 63); Bohr Collected Works (ref. 63), p. 621.

  80. Bohr to Ehrenfest, October 24, 1921, Museum Boerhaave, ESC1, S6, 179.

  81. Ehrenfest to Bohr, October 26, 1921, Niels Bohr Archive, Copenhagen, Denmark.

  82. Ehrenfest to Kramers, October 24, 1921, Museum Boerhaave.

  83. Bohr to Ehrenfest, November 11, 1921, Museum Boerhaave, ESC1, S6, 180; Margrethe Bohr to Ehrenfest, November 18, 1921, Museum Boerhaave, ESC1, S6, 181.

  84. Ehrenfest to Lorentz, February 4, 1922, Museum Boerhaave.

  85. Ehrenfest to Einstein, December 27, 1921 reprinted in The Collected Papers of Albert Einstein. Vol. 12 (ref. 54), pp. 396–397; translated somewhat differently by Hentschel (ref. 54), p. 209.

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  1. Tate Laboratory of Physics, University of Minnesota, 116 Church Street SE, Minneapolis, MN, 55455, USA

    Martin J. Klein

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Martin J. Klein, Eugene Higgins Professor Emeritus of History of Physics and Professor Emeritus of Physics at Yale University, died on March 28, 2009, at the age of 84. He intended this paper, whose topic he had broached in his paper, “Great Connections Come Alive: Bohr, Ehrenfest and Einstein,” in Jorrit de Boer, Erik Dal, and Ole Ulfbeck, ed., The Lesson of Quantum Theory (Amsterdam: North-Holland, 1986), pp. 325–342, to be the first chapter of the second volume of his renowned biography of Paul Ehrenfest. His daughter, Sarah Zaino, Executor of his Estate, has granted permission to publish it. Anne J. Kox, historian of physics and friend of Martin’s, found it among his papers while visiting Yale for Martin’s memorial service on October 16, 2009, and he has supplied the pictures for figures 1, 6, and 8. Roger H. Stuewer added the abstract and key words, brought the references up to date, and edited it. Requests for reprints should be directed to him at Tate Laboratory of Physics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 USA; e-mail: rstuewer@physics.umn.edu.

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Klein, M.J. Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends. Phys. Perspect. 12, 307–337 (2010). https://doi.org/10.1007/s00016-010-0025-6

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