ABSTRACT
One of the most elegant achievements of theoretical physics was to realize that behind every conservation law there is a symmetry involved. There is a profound reason for symmetries being so frequently found in physics: the isotropy of space and time for the absolute physical laws. The appearance of intensity pulsations at the output of a laser was noted immediately after the first successful operation of the ruby laser. A qualitative change in the understanding of laser physics occurred when it was realized that those oscillations were due to the nonlinear interaction between the electromagnetic field and the atoms in the cavity. One of the most elegant achievements of theoretical physics was to realize that behind every conservation law there is a symmetry involved. There is a profound reason for symmetries being so frequently found in physics: the isotropy of space and time for the absolute physical laws.
