Strong accomplishment with weak force: C.N. Yang turns 85

Prof. Chen Ning Yang, one of the most remarkable physicists of our time, celebrates his 85th birthday on September 22nd, 2007. In his honour, a conference in Singapore is taking place [1], which will condignly appreciate his person, his work and his influence on today’s physics. Born in 1922 in China, he first studied physics at the Tsinghua University in Kunming. As a scholarship holder of this University, he then went to the University of Chicago in 1946 to continue with his physics studies with E. Teller. After his promotion in 1948, he joined a working group around E. Fermi, before moving to the Institute for Advanced Studies, Princeton, NJ, where he accepted a professorship in 1955. From 1965 on, he then taught at the State University of New York, Stony Brook, until his retirement in 1999.

C. N. Yang became popular mainly due to his works on weak interaction. It led to this as follows: Until 1956 it was taken for granted that all physical laws conserve the parity, i.e. the mirror symmetry. Also the experiments of the arising nuclear physics confirmed this rule. However, first doubts arose when, with the help of accelerators, new elementary particles were generated. In 1955, investigations by R. H. Dalitz showed that, at their disintegration, some of these particles feature an unexplicable emission behaviour. In 1956 at a conference on high energy physics in Rochester, it was also dealt with this problem in one of C. N. Yang’s presentations.  R. Feynman who was attending that conference, brought the assumption of the experimenter M. Block to the discussion that the implicit accepted parity conservation by Dalitz had to be challenged. He was supported by E. Wigner, who, in 1927, formulated the parity conservation for electromagnetic interaction and was now ready for considering the painful step of a break of symmetry. Yang and his colleague T. D. Lee evinced that they would have considered it as well, but would not have gotten any further.

In [2] it is lively described how both of them succeeded: In 1956, as Yang went to see his colleague Lee at the Columbia University, both of them were looking for a parking place in front of a Chinese restaurant in despair at lunch time and had to park their cars in violation of the traffic regulations. The double trouble of the unsolved parking place search and the parity question (in this order?) seemed to have been of such a  great impact that, when sitting down in the restaurant, the breakthrough of a  conclusion succeeded: Only processes of the strong and electromagnetic interaction obtain the parity, but not the one of the weak interaction. Lee and Yang then proposed a series of experiments and, eventually, were able to win Miss Chien-Shiung Wu at the National Bureau of Standards (in 1956) for her famous experiment, which measures the electron emission at the β-decay of Co60. Those Co60 nucleii, which are polarized in a strong magnetic field at a temperature at milli-kelvin range, are supposed to emit electrons to the same intensity into and contrary to the alignment of the spin, if the β-decay was symmetric. The experiment, though, produced a stronger emission towards the created field, which was verified by a pole change of the field. A later experiment with Co58, a positron emitter, showed reconfirming the contrary behaviour of emission.

Series of further exclusive experiments, which approved the parity breaking at weak interaction, followed at frequent intervals. For their work, Lee and Yang were awarded with the Nobel prize in 1957.

The SPS presents its best wishes to Prof. Yang to his great day and for all coming years for the good of the physics, to which he has contributed so much.

B. Braunecker


[1] Conference in Honor of CN Yang’s 85th Birthday: Statistical Physics, High Energy, Condensed Matter and Mathematical Physics (31 October-3 November 2007),
[2] Anthony Zee: „Magische Symmetrie, Die Ästhetik der modernen Physik“, Insel Taschenbuch 1501, 1990


[Released: October 2007]