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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:
- the invariance of physical systems with respect to translation (when simply stated, it is just that the laws of physics don't vary with location in
space) translates into the law of conservation of linear
momentum;
- invariance with respect to rotation gives law of conservation of angular momentum;
- invariance with respect to time gives the well known law of conservation of energy, et cetera.
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.
Proof
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:
- In classical mechanics, M is the one-dimensional manifold
, representing time and the target space is
the tangent bundle of space of
generalized positions.
- In field theory, M is the spacetime manifold and the target space is the set of values the fields can take at any given point. For example,
if there are m real-valued scalar
fields, φ1,...,φm, then the target manifold is . If the field is a real vector field, then the target manifold
is isomorphic to . There's actually a much more elegant way using tangent bundles over M, but for the purposes of this proof, we'd just stick to
this version.
Now suppose there is 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 space-like 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).
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