Conservation law

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

Any particular conservation law is a mathematical identity to certain symmetry of physical system.

1 Exact laws
2 Approximate conservation laws
3 Global and local conservation laws
4 Philosophy of conservation laws
5 See also
6 External links

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Exact laws

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

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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 mass
Conservation of baryon number (See chiral anomaly)
Conservation of flavor (violated by the weak interaction)
Conservation of parity
CP symmetry

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Global and local conservation laws

A conserved property of a physical system may be conserved either locally, or just globally. To be conserved locally, the property must flow from one place to another, and not just disappear one place and reappear another. On the other hand, if the conserved quantity is allowed to appear somewhere else, but with the total amount of the conserved quantity remaining the same, then we have a global conservation law.

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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.

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