The greatest physics discovery of the 20^th century

Abstract This brief paper is based on a transcript of the late Steven Weinberg’s talk on his choice of quantum mechanics as the greatest physics discovery of the 20 th century (edited by William Phillips) and focusses on the dramatic break that quantum mechanics makes with the classical mechanics that preceded it. Quantum mechanics successfully explains the behaviour of matter, including phenomena like chemistry that 19 th century physicists did not even believe fell within the province of physics. Weinberg speculates about the possibility that quantum mechanics as we know it may only be a good approximation to some more complete theory.