The physics definition of symmetry is only slightly more precise than our everyday definition; it is any kind of transformation that leaves the laws of physics that apply to a system unchanged. Another term is often used to describe systems that have symmetry--a system is said to be invariant under a certain transformation if that transformation is a symmetry.

The role of symmetries in physics has changed greatly since the time of Isaac Newton. Until the twentieth century, they were mainly invoked to solve the complicated differential equations of classical mechanics and electromagnetism. However, with the discovery of special relativity by Albert Einstein in 1905, a new era of symmetry usage began. Any fundamental theory of physics had to satisfy the constraints placed on it by the symmetries of special relativity. When physicists began to tackle the problems of the quantum mechanics of...