# 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

${\displaystyle D^{2}=\Delta ,\,}$

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

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

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

${\displaystyle D=-i\sigma _{x}\partial _{x}-i\sigma _{y}\partial _{y},\,}$

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: Feynman's Dirac operator[which?] describes the propagation of a free fermion in three dimensions and is elegantly written

${\displaystyle D=\gamma ^{\mu }\partial _{\mu }\ \equiv \partial \!\!\!/,}$

using the Feynman slash notation.

Example 4: Another Dirac operator[which?] arises in Clifford analysis. In euclidean n-space this is

${\displaystyle D=\sum _{j=1}^{n}e_{j}{\frac {\partial }{\partial x_{j}}}}$

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

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

where ${\displaystyle {\tilde {\Gamma }}}$ 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

${\displaystyle f(x_{1},\ldots ,x_{k})\mapsto {\begin{pmatrix}\partial _{\underline {x_{1}}}f\\\partial _{\underline {x_{2}}}f\\\ldots \\\partial _{\underline {x_{k}}}f\\\end{pmatrix}}}$

is sometimes called Dirac operator in k Clifford variables. In the notation, S is the space of spinors, ${\displaystyle x_{i}=(x_{i1},x_{i2},\ldots ,x_{in})}$ are n-dimensional variables and ${\displaystyle \partial _{\underline {x_{i}}}=\sum _{j}e_{j}\cdot \partial _{x_{ij}}}$ 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.