In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. The following list is a partial listing of conservation laws that have never been shown to be inexact (though the latest results in General relativity and cosmology seem to break this rule):
Noether's theorem expresses the equivalence which exists between conservation laws and the invariance of physical laws with respect to certain transformations (typically called "symmetries") (This only applies to systems describable by a Lagrangian). There is an analogous theorem for Hamiltonian mechanics. For instance, time-invariance implies that energy is conserved, translation-invariance implies that momentum is conserved, and rotation-invariance implies that angular momentum is conserved.
Some conservation laws hold in many circumstances, but exceptions to them have been observed. Such is the violation of parity conservation; apparently the universe has "handedness" (right versus left).
In fact, quantities that are conserved, the invariants, seem to preserve what one would like to call some kind of a 'physical reality' and seem to have a more meaningful existence than many other physical quantities. These laws bring a great deal of simplicity into the structure of a physical theory. They are the ultimate basis for most solutions of the equations of physics.
