Conservation law (physics)
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves.
One particularly important physical result concerning laws of conservation is Noether's theorem, which states that there is a onetoone correspondence between laws of conservation and differentiable symmetries of physical systems. For example, the conservation of energy follows from the timeinvariance of physical systems, and the fact that physical systems behave the same regardless of how they are oriented in space gives rise to the conservation of angular momentum.
Exact laws
A partial listing of physical laws of conservation due to symmetry that are said to be exact laws, or more precisely have never been [proven to be] violated:Conservation Law  Respective Noether symmetry invariance  Number of dimensions  

Conservation of massenergy  Time invariance  Lorentz invariance symmetry  1  translation about time axis 
Conservation of linear momentum  Galilean invariance  3  translation about x,y,z position  
Conservation of angular momentum  Rotation invariance  3  rotation about x,y,z axes  
CPT symmetry (combining charge, parity and time conjugation)  Lorentz invariance  1+1+1  (charge inversion q→q) + (position inversion r→r) + (time inversion t→t)  
Conservation of electric charge  Gauge invariance  1⊗4  scalar field (1D) in 4D spacetime (x,y,z + time evolution)  
Conservation of color charge  SU(3) Gauge invariance  3  r,g,b  
Conservation of weak isospin  SU(2)_{L} Gauge invariance  1  weak charge  
Conservation of probability  Probability invariance  1⊗4  total probability always=1 in whole x,y,z space, during time evolution 
Approximate 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 (applies for nonrelativistic speeds and when there are no nuclear reactions)
 Conservation of baryon number (See chiral anomaly)
 Conservation of lepton number (In the Standard Model)
 Conservation of flavor (violated by the weak interaction)
 Conservation of parity
 Invariance under Charge conjugation
 Invariance under time reversal
 CP symmetry, the combination of charge and parity conjugation (equivalent to time reversal if CPT holds)
See also
 Charge conservation

Conserved quantity
 Some kinds of helicity are conserved in dissipationless limit: hydrodynamical helicity, magnetic helicity, crosshelicity.
 Continuity equation
 Noether's theorem
 Philosophy of physics
 Symmetry in physics
 Totalitarian principle
References
 Victor J. Stenger, 2000. Timeless Reality: Symmetry, Simplicity, and Multiple Universes. Buffalo NY: Prometheus Books. Chpt. 12 is a gentle introduction to symmetry, invariance, and conservation laws.
External links
 Conservation Laws — an online textbook