In 1957 the Chinese-American theoretical physicist TD (Tsung-Dao) Lee, who has died aged 97, became the second-youngest scientist to win a Nobel prize. He did this with another Chinese émigré to the US, Chen Ning Yang, who was four years older than he was. They became Nobel laureates for physics for work that overthrew the widely accepted “parity laws” – that the forces acting on the fundamental subatomic particles are symmetric between left and right. In the popular description, they overthrew the concept of “mirror symmetry”.
Before Lee and Yang questioned this fundamental principle, it was believed that the mirror image of any process displays a sequence of events that could equally well occur in the real world. In effect, there is no way to tell whether you are viewing a real event or its mirror image. This was well established in the familiar case of electromagnetic forces and the strong force that binds atomic nuclei. For example, the mirror image of an electrically charged particle being steered in one direction by electric or magnetic forces reveals a sequence that is realised in the real world simply by reversing the direction of the said forces.
