In studying
Nature,
physicists have learned that certain
quantities never change. These were given names like the total
energy and the total
momentum of a system, and the fact that they never change was expressed as conservation laws.
In time it was learned that every conservation law corresponds to a symmetry of the laws of physics. The extension of this idea into quantum mechanics has made it possible to derive the laws governing elementary particles from a knowledge only of what quantities are conserved.