量子力学与实验4.pdf

CN Chapter 4 CT Symmetries in Physics Symmetry is a fundamental attribute of the natural world that enables an investigator to study particular aspects of physical systems by themselves. For example, the assumption that space is homogeneous, or possesses translational symmetry, leads to the conclusion that the linear momentum of a closed isolated system does not change as the system moves. This makes it possible to study separately the motion of the center of mass and the internal motion of the system. A systematic treatment of the symmetry properties and the conservation law is useful in solving more complicated problems. It provides a deeper insight into the structure of physics. In this chapter we consider the geometrical symmetries that may be associated with the displacements of a physical system in space and time, with its rotation and inversion in space, and with the reversal of the sense of progression of time. 125 CHAPTER 4 – MANUSCRIPT A 4.1 Symmetries and Conservation Laws B 4.1.1 Symmetry in Classical Physics An alternative way to describe a classical system is to introduce Lagrangian L: d ∂L ∂L − = 0 dt ∂q˙i ∂qi which is called the Lagrange equation. For a conservative system, L = T − V where V = V (q , q , ...q ). 1 2 N For