# Dirac operator

In mathematics and quantum mechanics, a Dirac operator is a differential operator that is a formal square root, or half-iterate, of a second-order operator such as a Laplacian. The original case which concerned Paul Dirac was to factorise formally an operator for Minkowski space, to get a form of quantum theory compatible with special relativity; to get the relevant Laplacian as a product of first-order operators he introduced spinors.

## Formal definition

In general, let D be a first-order differential operator acting on a vector bundle V over a Riemannian manifold M. If

$D^2=\Delta, \,$

where ∆ is the Laplacian of V, then D is called a Dirac operator.

In high-energy physics, this requirement is often relaxed: only the second-order part of D2 must equal the Laplacian.

## Examples

Example 1: D=-ix is a Dirac operator on the tangent bundle over a line.

Example 2: We now consider a simple bundle of importance in physics: The configuration space of a particle with spin ½ confined to a plane, which is also the base manifold. It's represented by a wavefunction ψ: R2C2

$\psi(x,y) = \begin{bmatrix}\chi(x,y) \\ \eta(x,y)\end{bmatrix}$

where x and y are the usual coordinate functions on R2. χ specifies the probability amplitude for the particle to be in the spin-up state, and similarly for η. The so-called spin-Dirac operator can then be written

$D=-i\sigma_x\partial_x-i\sigma_y\partial_y,\,$

where σi are the Pauli matrices. Note that the anticommutation relations for the Pauli matrices make the proof of the above defining property trivial. Those relations define the notion of a Clifford algebra.

Solutions to the Dirac equation for spinor fields are often called harmonic spinors[1].

Example 3: The most famous Dirac operator describes the propagation of a free fermion in three dimensions and is elegantly written

$D=\gamma^\mu\partial_\mu\ \equiv \partial\!\!\!/,$

using the Feynman slash notation.

Example 4: There is also the Dirac operator arising in Clifford analysis. In euclidean n-space this is

$D=\sum_{j=1}^{n}e_{j}\frac{\partial}{\partial x_{j}}$

where {ej: j = 1, ..., n} is an orthonormal basis for euclidean n-space, and Rn is considered to be embedded in a Clifford algebra.

This is a special case of the Atiyah-Singer-Dirac operator acting on sections of a spinor bundle.

Example 5: For a spin manifold, M, the Atiyah-Singer-Dirac operator is locally defined as follows: For xM and e1(x), ..., ej(x) a local orthonormal basis for the tangent space of M at x, the Atiyah-Singer-Dirac operator is

$\sum_{j=1}^{n}e_{j}(x)\tilde{\Gamma}_{e_{j}(x)}$,

where $\tilde{\Gamma}$ is a lifting of the Levi-Civita connection on M to the spinor bundle over M.

## Generalisations

In Clifford analysis, the operator D: C(RkRn, S) → C(RkRn, CkS) acting on spinor valued functions defined by

$f(x_1,\ldots,x_k)\mapsto \begin{pmatrix} \partial_{\underline{x_1}}f\\ \partial_{\underline{x_2}}f\\ \ldots\\ \partial_{\underline{x_k}}f\\ \end{pmatrix}$

is sometimes called Dirac operator in k Clifford variables. In the notation, S is the space of spinors, $x_i=(x_{i1},x_{i2},\ldots,x_{in})$ are n-dimensional variables and $\partial_{\underline{x_i}}=\sum_j e_j\cdot \partial_{x_{ij}}$ is the Dirac operator in the i-th variable. This is a common generalization of the Dirac operator (k=1) and the Dolbeault operator (n=2, k arbitrary). It is an invariant differential operator, invariant under the action of the group SL(k) × Spin(n). The resolution of D is known only in some special cases.