Main Page | See live article | Alphabetical index

Noether's theorem

Noether's theorem is a central result in theoretical physics that expresses the equivalence of two different properties of physical laws. It is named after the early 20th century mathematician Emmy Noether.

Noether's theorem relates pairs of basic ideas of physics, one being the invariance of the form that a physical law takes with respect to any (generalized) transformation that preserves the coordinate system (both spatial and temporal aspects taken into consideration), and the other being a conservation law of a physical quantity.

Informally, Noether's theorem can be stated as:

To every symmetry, there corresponds a conservation law and vice versa.

The formal statement of the theorem derives an expression for the physical quantity that is conserved (and hence also defines it), from the condition of invariance alone. For example:

When it comes to quantum field theory, the invariance with respect to general gauge transformations gives the law of conservation of electric charge and so on. Thus, the result is a very important contribution to physics in general, as it helps to provide powerful insights into any general theory in physics, by just analyzing the various transformations that would make the form of the laws involved, invariant.


Suppose we have an n-dimensional manifold, M and a target manifold T. Let be the configuration space of smooth functions from M to T.

Before we go on, let's give some examples:

Now suppose there's a functional


called the action. (Note that it takes values in to , rather than ; this is for physical reasons, and doesn't really matter for this proof.)

To get to the usual version of Noether's theorem, we need additional restrictions on the action. We assume S(φ) is the integral over M of a function

called the Lagrangian, depending on φ, its derivative and the position. In other words, for φ in

Suppose given boundary conditions, which are basically a specification of the value of φ at the boundary of M is compact, or some limit on φ as x approaches ; this will help in doing integration by parts). We can denote by N the subset of consisting of functions, φ such that all functional derivatives of S at φ are zero and φ satisfies the given boundary conditions.

Now, suppose we have an infinitesimal transformation on , given by a functional derivative, δ such that

for all compact submanifolds N. Then, we say δ is a generator of a 1-parameter symmetry Lie group.

Now, for any N, because of the Euler-Lagrange theorem, we have


Since this is true for any N, we have


You might immediately recognize this as the continuity equation for the current
which is called the Noether current associated with the symmetry. The continuity equation tells us if we integrate this current over a spacelike slice, we get a conserved quantity called the Noether charge (provided, of course, if M is noncompact, the currents fall off sufficiently fast at infinity).

External links