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杨纲凯教授主讲：「最小作用量原理」
2014 年 5 月 14 日 This video is a recording of a two-hour lecture given by K. Young on 15 May 2014, to the Physics Department of The Chinese University of Hong Kong; the audience consisted mainly of undergraduates.
An introduction to the least action principle is given, first of all as an alternative to Newton's second law. The advantages of using the action S over Newton's laws are explained, through examples such as generalized coordinates, special relativity and magnetism, and bringing in the concepts of the Lagrangian and the Hamiltonian. Quantum amplitudes along a path have phases proportional to S and summing such amplitudes over all possible paths leads to Schrodinger's equation. The geodesic equation in general relativity is also quickly sketched as a simple extension of the least action principle.
Inevitably in such a lecture, there were some errors in the formulas on the boards: obviously harmless ones as well as a couple of signs. The audience might want to watch out for these.
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