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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. The following is a partial listing of conservation laws that have never been shown to be inexact. (Actually, in General relativity energy, momentum and angular momentum are not conserved because a general curved spacetime manifold would not possess Killing symmetries for translations or rotations.)
There are also approximate conservation laws (i.e. true approximately for short time scales) in particle physics like those of baryon number (which is not really conserved, if for nothing else but chiral anomaly, speculations about GUT theories aside) and strangeness (which is violated by the weak interaction).
Noether's theorem expresses the equivalence which exists between conservation laws and the invariance of physical laws with respect to certain transformations (typically called " symmetriesSymmetry is a characteristic of geometrical shapes, equations and other objects; we say that such an object is symmetric with respect to a given operation if this operation, when applied to the object, does not appear to change it. The three main symmetri") (This only applies to systems describable by a LagrangianA Lagrangian of a dynamical system, named after Joseph Louis Lagrange, is a functional of the dynamical variables which concisely describes the equations of motion of the system. The equations of motion are obtained by means of an action principle, writte). There is an analogous theorem for Hamiltonian mechanicsHamiltonian mechanics is a re-formulation of classical mechanics that was invented in 1833 by William Rowan Hamilton. It arose from Lagrangian mechanics, another re-formulation of classical mechanics, introduced by Joseph Louis Lagrange in 1788. It can ho. 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. (If a quantity isn't conserved, in what sense is it a conservation law???) Such is the violation of parity conservation; apparently the universe has " handednessHandedness is an attribute of human beings defined by their unequal distribution of fine motor skill between the left and right hands. Individuals more skilled with the right hand are called right-handed', and those more skilled with the left are left-han" (right versus left).
Philosophy of Conservation Laws
- 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 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.
Physics
