The historic achieve-ments brought forth by physics in the 20th century:

1. The historic achieve-ments brought forth by physics in the 20th century:

2. Man discovered, for the first time since our ancestors discovered fire, the second and the vastly stronger source of energy: nuclear power.

3. Man learned to manipu-late electrons to create the transistor which led to the modern computer, there-by greatly increasing human productivity.

4. Man learned how to probe into structures of atomic dimensions which led to the double-helix, thereby ushering in bioengineer-ing technology.

5. Man take first steps on the moon.

6. However, from the viewpoint of physicists, the most important advances are the profound revolutions in our understanding of the basic concepts of physics.

7. Space • Time • Motion • Energy • Force

8. There were three themes that, singly and together, underlie the chief new ideas in the 20th century physics. We may call them:

9. Thematic Melodies of • Twentieth Century • Theoretical Physics: • Quantization • Symmetry • Phase Factor

10. 1. Quantization

11. Max Planck (1858-1947)

14. Quantization • 1900 Planck • 1905 Einstein • 1913 Bohr

15. Albert Einstein (1879-1955)

16. Niels Bohr (1885-1962)

17. It was the spring of hope, it was the winter of despair

18. At present I am myself most optimistic as regards the future of the theory. Bohr to Rutherford 1918

19. Physics is once again at a dead end at this time. For me, at any rate. It is much too difficult. Pauli to Kronig, May 21, 1925

20. Heisenberg’s mechanics has restored my zest for life. Pauli to Kronig, October 9, 1925

21. Wolftgang Pauli (1900-1958)

22. Do not enter into this conflict, we are both much too kind and gentle to participate in that kind of struggle. Both Bohr and Heisenberg are tough, hard nosed, uncompromising and

23. indefatigable. We would just be crushed in that juggernaut. Kramers to Klein 1927 Quoted in Pais’ <Genius of Science>, p.159 (2000)

24. J.R. Oppenheimer (1904-1967)

25. It was a period of patient work in the laboratory, of crucial experiments and daring action, of many false starts and many untenable conjectures. It was a time of earnest correspondence and

26. hurried conferences, of debate, criticism, and brilliant mathematical improvisation. • For those who partici-pated, it was a time of creation; there was terror as well as exaltation in their new insight . It will probably not

27. be recorded very completely as history. As history, its recreation would call for an art as high as the story of Oedipus or the story of Cromwell, yet in a realm ofaction so remote from our common experience that

28. it is unlikely to be known to any poet or any historian.” • J.R. Oppenheimer • Reith Lectures 1953

29. Werner Heisenberg (1901-1976)

30. P.A.M. Dirac (1902-1984)

31. Erwin Schrödinger (1887-1961)

32. Enrico Fermi (1901-1954)

33. Pauli — PowerFermi — Solidity, StrengthHeisenberg — Deep InsightDirac — Cartesian Purity

34. 2.Symmetry • (= invariance)

35. The five regular solids with maximum symmetry. Reprinted from A.V. Shubnikov and V.A. Koptsik, Symmetry in Science and Art (Plenum, 1974).

36. Symmetry • 1905 Einstein • 1908Minkowski

37. “Superfluous learnedness”

38. … that the basic demand of the special theory of relativity (invariance of the laws under Lorentz-transformations) is too narrow, i.e. that an invariance of the laws must be postulated also relative to non-linear transformations of

39. the coordinates in the four-dimensional continuum. • This happened in 1908. • Einstein: Autobiographical Notes • in <Albert Einstein>, ed. P.A. • Schilpp, p.67

40. With the introduction of quantum mechanics in 1925, symmetry became very important. The mathematical language for symmetry is groups.

41. It has been rumored that the “group pest” is gradually being cut out of quantum physics. H. Weyl, Nov. 1930

42. Symmetry gradually became the thematic melody (1927-1970) • atomic, molecular physics • nuclear physics • elementary particle physics

43. A great shock created by Prof. C. S. Wu in 1957 • Parity Nonconservation • in Weak Interactions

44. C.S. Wu (1912-1997)

45. Now, where shall I start? It is good that I did not make a bet. It would have resulted in a heavy loss of money (which I cannot afford); I did make a fool of myself, however (which I think I can afford) • Pauli 1957

46. Never before or afterward have I seen him so excited about physics. Heisenberg 1978

47. 3. Phase Factor

48. So if one asks what is the main feature of quantum mechanics, I feel inclined now to say that it is not non-commutative algebra, it is the phase. • Dirac 1972

49. phase factor =

50. Phase factor became particularly important through the proposal of Weyl in 1918.