Niels Bohr's Times, in Physics, Philosophy, and Polity

Niels Bohr's Times, In Physics, Philosophy, and Polity

ABRAHAM PAIS

CLARENDON PRESS • OXFORD 1991 Contents

If you find those sections marked with an asterisk (*) too technical, then just skip them and read on.

1 A Dane for all seasons 1 (a) Themes 1 (b) Some personal recollections 4 (c) A tour through this book 14

2 'In Denmark I was born . . .' 32

3 Boyhood 42

4 Toward the twentieth Century: from ancient optics to relativity theory 52 (a) 1903 52 (b) The natureof light; beginnings 53 (c) Particles or waves? 56 (d) Color, visible and invisible 60 (e) Of Maxwell's theory, Hertz's experiment, and the definition of classical physics 63 (f) Trouble with the aether: the Michelson-Morley experiment 66 (g) In which classical physics comes to an end and Einstein makes his first appearance 68

5 Natura facit saltum: the roots of quantum physics 74 (a) The age of continuity 74 (b) Kirchhoff's law 75 (c) 1860-1896 77 (d) 1896: physics takes a bizarre turn 78 (e) Introducing Max Planck 79 (f) A brief digression on Statistical mechanics 80 (g) In which Planck stumbles on a new law that ushered in the physics of the twentieth Century 82 (h) Particles or waves? 87

6 Student days 92 (a) Physics in Denmark, from a College for the clergy to the epoch of0rsted 92 (b) In which Bohr begins his university studies and Starts mobiliz- ing help in writing 97 (c) The atom: Status in 1909 103 (d) Niels Bohr, M.Sc, Ph.D. 107 (e) Death of father. Bohr becomes engaged 111 XIV CONTENTS 7 In which Bohr goes to England for postdoctoral research 117 (a) Cambridge: Thomson, father of the electron 117 (b) Manchester: Rutherford, father of the nucleus 121

8 Bohr, father of the atom 132 (a) Young man in a hurry 132 (b) In which Bohr leaves the church and gets married 133 (c) The Rutherford memorandum 135 (d) 'The language of spectra ... a true atomic music of the spheres' 139 (e) In which Bohr hears about the Balmer formula 143 (f) Triumph over logic: the hydrogen atom(*) 146 (g) Reactions, including Bohr's own 152

9 How Bohr secured his permanent base of Operations 160 (a) The early schools in quantum physics 160 (b) In which Bohr returns to Manchester and then becomes Denmark's first professor of theoretical physics 163 (c) In which Bohr acquires his own institute 166

10 'It was the spring of hope, it was the winter of despair' 176 (a) Mathematics in physics 176 (b) The old quantum theory 1913-1916: sketches(*) 179 1. Introductory. 2. How order was brought in the periodic table of elements. 3. The Stark effect. 4. The Franck-Hertz experiment. 5. New quantum numbers; the fine structure of the hydrogen spectrum (c) In pursuit of principles: Ehrenfest, Einstein, and Bohr 189 1. Ehrenfest on adiabatics. 2. Einstein on probability. 3. Bohr on correspondence (d) Thecrisis 196 1. Helium. 2. The Zeeman effect. 3. The fourth quantum number. 4. Enter Pauli (e) Bohr and the periodic table of elements 202 1. From electron rings to electron shells. 2. The mystery of the rare earths. 3. Bohr's quantum number assignments. 4. The exclusion principle. 5. The discovery of hafnium (f) The Nobel Prize 210 1. The Prize and the press. 2. Who nominated Bohr? 3. The ceremonies. 4. Who did Bohr nominate?

11 Bohr and Einstein 224 (a) Comparisons 224 (b) First encounters 227 (c) More on Einstein and the light-quantum 230 (d) 'The culmination of the crisis': the BKS proposal 232 (e) The new era dawns: de Broglie 239 (f) Spin 241 CONTENTS xv

12 'A modern Viking who comes on a great errand' 249 (a) Bohr & Sons 249 (b) International recognition 251 (c) First trip to America 253 (d) Bohr as fund raiser 255 (e) The institute up tili mid-1925. Introducing Heisenberg 260

13 'Then the whole picture changes completely': the discovery of quantum mechanics 267 (a) A last look back: Bohr as 'director of atomic theory' 267 (b) Kramers in 1924 270 (c) Heisenberg in 1924 272 (d) 1925: how quantum mechanics emerged 'quite vaguely from the fog'(*) 275 (e) Bohr's earliest reactions 279 (f) Early 1926: the second Coming of quantum mechanics 280 (g) The summer of 1926: Born on probability, causality, and determinism 284 (h) Appendix, c- and q-numbers for pedestrians 289

14 The Spirit of Copenhagen 295 (a) The Copenhagen team in 1926. Heisenberg resolves the helium puzzle 295 (b) In which Schrödinger comes on a visit 298 (c) Prelude to complementarity. The Bohr-Heisenberg dialog 300 (d) The uncertainty relations, with a look back at the correspondence principle 304 (e) Complementarity: anew kind ofrelativity 309 (f) Solvay 1927. The Bohr-Einstein dialog begins 316^

15 Looking into the atomic nucleus 324 (a) Beginnings of a new direction for Bohr and his school 324 (b) Theoretical nuclear physics: the prehistoric era 325 (c) Great progress: the first artificial transmutation of chemical elements and the first signs of a new force. Great confusion: the proton-electron model of the nucleus 327 (d) In which quantum mechanics reveals nuclear paradoxes and the neutron is discovered 330 (e) In which the Bohrs move to the Residence of Honor 332 (f) In which Bohr takes nuclear matters in hand 335 (g) Being a brief prelude to the war and the years thereafter 341

16 Toward the edge of physics in the Bohr style, and a bit beyond 346 (a) Particles and fields 346 (b) QED(*) 350 (c) Spin (continued). The positron. The meson(*) 352 (d) BohronQED(*) 358 (e) Bohr and the crisis of 1929. The neutrino 364 XVI CONTENTS

17 How Bohr orchestrated experimental progress in the 1930s, in physics and in biology 375 (a) Four fateful factors 375 (b) The first accelerators 375 (c) Weaver at the heim 379 (d) Troubles in Germany 381 (e) Bohr and the Rockefeiler foundation's emergency program 383 (f) The discovery of induced radioactivity 386 (g) Four fateful factors fit 387 (h) How Hevesy introduced isotopic tracers in biology 388 (i) Bohr as fund raiser (continued) 394 (j) Denmark's first accelerators and the fifth fateful factor 398

18 Of sad events and of major journeys 407 (a) Days of sorrow 407 (b) Times of travel 413 19 'We are suspended in language' 420 (a) Bohr and philosophy: 'It was, in a way, my life' 420 (b) Complementarity (continued). More on the Bohr-Einstein dialog. A new definition of 'phenomenon' 425 (c) Bohr on Statistical mechanics 436 (d) Complementarism 438 1. Introductory. 2. Psychology. 3. Biology. 4. Human cultures. 5. Conclusion: language

20 Fission 452 (a) The early days, including Bohr's discovery of the role of uranium 235 452 (b) Fission in Copenhagen 458 (c) Atomic energy? Atomic weapons? 460 (d) Bohr as President of the Kongelige Danske Videnskahernes Selskab 464 21 Bohr, pioneer of 'glasnost' 473 (a) Introduction 473 (b) Denmark and Germany, from 16 November 1864 until 4 May 1945 474 (c) Bohr's war years, the Scandinavian episode 479 1. Keeping the work going. 2. Heisenberg's visit. 3. A letter from England. 4. The Swedish interlude. 5. The fate of the institute (d) Bohr's war years, the Anglo-American episode 490 1. On to Britain. 2. Anglo-American efforts up tili October 1943. 3. From London to New York. 4. Bohr's role in the weapons program (e) Bohr, Churchill, Roosevelt, and the atomic bomb 497 1. Glasnost 1944. 2. Meetings with Ambassador Halifax and Justice Frankfurter. 3. The Kapitza letter. 4. Bohr meets Churchill. 5. Bohr meets Roosevelt. 6. Coda. 7. Going home CONTENTS xvii

22 In which Bohr moves füll steam into his later years 509 (a) Prolog 509 (b) The later writings, 1945-1962 510 1. Physics, research. 2. Physics, discussions of complementarity. 3. Complementarity outside physics. 4. Occasional addresses. 5. Writings in memoriam (c) Glasnost 1950: Bohr's open letters to the United Nations 513 (d) CERN 519 (e) Nordita 521 (f) Ris0 523 1. The National Laboratory. 2. A new part of the Niels Bohr Institute. 3. Riso in 1989 (g) The later travels 528 (h) The final half year 529 23 Epilog 534

Appendix A Synopsis of this book in the form of a chrono- logy 538

Index of names 547 Index of subjects 553