# Noether's Theorem

% Remco Bloemen % 2015-08-26


To every differentiable symmetry generated by local actions, there corresponds a conserved current.

## Discrete Langrangian Mechanics

In Langrangian mechanics a system is described by a set of generalized parameters $\vec x(t)$, a time depended vector in the state space of the system.

The physics of the system is contained in a Lagrangian function:

$$L(t, \vec x, \vec v): ℝ × \mathrm{T}\mkern2mu X → ℝ$$

where $\vec x ∈ X$ and $\vec v ∈ \mathrm{T}_{\vec x} X$.

Hamilton’s principle than states that:

$$δ \vec x(t) \∫ t L(\vec x(t), (\d t {\vec x}) (t), t) = 0$$

this can be restated as the Euler-Lagrange equations:

$$\d t \∂ {v_k} L = \∂{x_k} L$$

## Noether’s theorem

Given a symmetry $T_r$, $\vec Q_r$, the quantity

$$\g{ \∂ {\vec v} L · \vec v - L } T_r - \∂ {\vec v} L · \vec Q_r$$

is conserved.

## In reality

### The standard model

Symmetry Generators Conserved quantity

Time shift 1 Energy Translation 3 Momentum Rotation 3 Angular Momentum Boosts 3 ?? Gauge 6 Color charge, Weak isospin, Electric charge, Weak hypercharge Phase 4 Baryon, electron, muon and tau numbers

### General Relativity

The Einstein-Hilbert Langrangian:

$$L = \frac{c^4}{16 π G} R_j^j \sqrt{-\det g_{μν}}$$

Killing vector fields are symmetries of the metric tensor.

Remco Bloemen
Math & Engineering
https://2π.com