# Conservation law

In physics, a **conservation law** states that a particular measurable property of an isolated physical system does not change as the system evolves.

## Exact laws

The following is a partial listing of conservation laws that have never been shown to be inexact.

- Conservation of energy
- Conservation of momentum
- Conservation of angular momentum
- Conservation of electric charge
- conservation of color charge
- CPT symmetry

## Approximate conservation laws

There are also approximate conservation laws. These are approximately true in particular situations, such as low speeds, short time scales, or certain interactions.

- Conservation of baryon number (See chiral anomaly)
- Conservation of flavor (violated by the weak interaction)
- Conservation of mass (only in nonrelativistic theories)
- Conservation of parity
- CP symmetry.

## Philosophy of conservation laws

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") for systems which obey the Principle of least action and hence having a Lagrangian and a Hamiltonian (See Classical mechanics, Hamiltonian mechanics for details). 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.

*Things that remain unchanged, in the midst of change*

The idea that some things remain unchanging throughout the evolution of the universe has been motivating philosophers and scientists alike for a long time.

In fact, quantities that are conserved, the *invariants*, seem to preserve what some 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.

## See also

de:Erhaltungssatz
el:Νόμος Διατήρησης
fr:Loi de conservation
id:Hukum kekekalan
it:Leggi di conservazione
hu:Megmaradási tétel
nl:Behoudswet
ja:保存則
pl:Prawa zachowania
ru:Закон сохранения
sk:Zákony zachovania
sl:Ohranitveni zakon
sv:Konserveringslag
th:กฎการอนุรักษ์
uk:Закони збереження
zh:守恒定律