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

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

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**The historic achieve-ments brought forth by physics in the**20th century:**Man discovered, for the first time since our ancestors**discovered fire, the second and the vastly stronger source of energy: nuclear power.**Man learned to manipu-late electrons to create the**transistor which led to the modern computer, there-by greatly increasing human productivity.**Man learned how to probe into structures of atomic**dimensions which led to the double-helix, thereby ushering in bioengineer-ing technology.**However, from the viewpoint of physicists, the most**important advances are the profound revolutions in our understanding of the basic concepts of physics.**Space**• Time • Motion • Energy • Force**There were three themes that, singly and together, underlie**the chief new ideas in the 20th century physics. We may call them:**Thematic Melodies of**• Twentieth Century • Theoretical Physics: • Quantization • Symmetry • Phase Factor**Quantization**• 1900 Planck • 1905 Einstein • 1913 Bohr**It was the spring of hope,**it was the winter of despair**At present I am myself most optimistic as regards the future**of the theory. Bohr to Rutherford 1918**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**Heisenberg’s mechanics has restored my zest for life.**Pauli to Kronig, October 9, 1925**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**indefatigable. We would just be crushed in that juggernaut.**Kramers to Klein 1927 Quoted in Pais’ <Genius of Science>, p.159 (2000)**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**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**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**it is unlikely to be known to any poet or any historian.”**• J.R. Oppenheimer • Reith Lectures 1953**Pauli — PowerFermi — Solidity,**StrengthHeisenberg — Deep InsightDirac — Cartesian Purity**2.Symmetry**• (= invariance)**The five regular solids with maximum symmetry. Reprinted**from A.V. Shubnikov and V.A. Koptsik, Symmetry in Science and Art (Plenum, 1974).**Symmetry**• 1905 Einstein • 1908Minkowski**… 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**the coordinates in the four-dimensional continuum.**• This happened in 1908. • Einstein: Autobiographical Notes • in <Albert Einstein>, ed. P.A. • Schilpp, p.67**With the introduction of quantum mechanics in 1925, symmetry**became very important. The mathematical language for symmetry is groups.**It has been rumored that the “group pest” is gradually**being cut out of quantum physics. H. Weyl, Nov. 1930**Symmetry gradually became the thematic melody (1927-1970)**• atomic, molecular physics • nuclear physics • elementary particle physics**A great shock created by Prof. C. S. Wu in 1957**• Parity Nonconservation • in Weak Interactions**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**Never before or afterward have I seen him so excited about**physics. Heisenberg 1978**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**Phase factor became particularly important through the**proposal of Weyl in 1918.