Citation | Yang's outstanding contributions to the theory of elementary particle physics and to statistical mechanics are too many to enumerate. It is worth mentioning just three of these which have proven revolutionary in the context of physics and/or mathematics. In 1954, together with R.L. Mills, Yang proposed a modification of Maxwell's equations which could describe particle interactions beyond the electromagnetic. This was achieved by changing the gauge symmetry group from U(1) to the non-abelian group of rotations in three dimensions. Allowing greater choice for this non-abelian group the equations have become the basis for all unified gauge theories of particle interactions. Because of their geometrical significance, the study of these equations has enriched modern pure mathematics. In 1956, together with T.D. Lee, Yang suggested that the possibility that parity need not be conserved in weak interactions would resolve certain puzzles in experimental data. They further suggested that the measurement of the beta decay of polarised nuclei would detect this parity violation directly. They promptly received the Nobel Prize for physics a year later when this effect had been confirmed. In 1967, Yang s [sic] longstanding interest in statistical mechanics led him to solve the problem of a system of quantum mechanical particles moving on a line with repulsive delta-function interactions. In the course of this, he realised the importance of a system of equations now known as the "Yang-Baxter" equations which have become the cornerstone of a much wider class of solvable theories and have led to a new and important mathematical concept, namely that of the so-called "quantum groups". The extraordinarily wide range of influence shown by Yang's seminal ideas pays testimony to their power. |