Clifford Algebras and Spinors III: Bochner identity

Let $M$ be a Riemannian manifold, and let $Cl(M)$ be its Clifford bundle. Let $E \to M$ be any vector bundle with connection, and assume that $C^\infty(M, E)$ is a $Cl(M)$-module. We can define a Dirac operator $\mathcal{D}$ acting on sections of $E$ via the formula
$$\mathcal{D} \sigma = \sum_{i=1}^n e_i \cdot \nabla_i \sigma$$
for any orthonormal frame $\{e_1, \dots, e_n\}$ on $M$, and where $\cdot$ denotes the Clifford module action. We demand that the connection on $E$ is compatible with Clifford multiplication in the following sense:
$$\nabla_j (e_i \cdot \sigma) = (\nabla_j e_i) \cdot \sigma + e_i \cdot \nabla_j \sigma.$$

Let $R$ denote the curvature of $E$, i.e. we have
$$[\nabla_i, \nabla_j] \sigma =  R(e_i, e_j) \sigma+ \nabla_{[e_i, e_j]} \sigma$$

We can define an endomorphism $\mathcal{R}$ on $E$ by
$$\mathcal{R} = \frac{1}{2} \sum_{ij} R(e_i, e_j).$$

Theorem. We have the identity $\mathcal{D}^2 = -\Delta + \mathcal{R}$.

Proof. We compute
$$\begin{align} \mathcal{D}^2 \sigma &= \sum_{ij} e_i \nabla_i \left( e_j \nabla_j \sigma \right) \\ &= \sum_{ij} e_i e_j \nabla_i \nabla_j \sigma + e_i ( \nabla_i e_j ) \nabla_j \sigma \\ &= -\Delta \sigma + \frac{1}{2}\sum_{ij}e_i e_j [\nabla_i, \nabla_j] \sigma + \sum_{ij} e_i ( \nabla_i e_j) \nabla_j \sigma \\ &= -\Delta \sigma + \frac{1}{2}\sum_{ij}e_i e_j R(e_i, e_j) \sigma + \frac{1}{2}\sum_{ij} e_i e_j \nabla_{[e_i, e_j]} \sigma+ \sum_{ij} e_i ( \nabla_i e_j) \nabla_j \sigma \\ &= (-\Delta + \mathcal{R})\sigma + \frac{1}{2} \sum_{ij} \left( e_i e_j \nabla_{[e_i, e_j]}\sigma +  e_i (\nabla_i e_j) \nabla_j + e_j (\nabla_j e_i) \nabla_i \right)\sigma \end{align}$$
We will be done provided we can show that the last term vanishes. Notice that it is fully tensorial, since it can be expressed as $\mathcal{D}^2 + \Delta - \mathcal{R}$. On the other hand, the terms $[e_i, e_j]$ and $\nabla_j e_i$ are (by definition!) proportional to Christoffel symbols. Since we can always choose a frame so that these vanish at a point, these terms must vanish identically. Hence we have $0 = \mathcal{D}^2 + \Delta - \mathcal{R}$, as desired.

Clifford Algebras and Spinors, Part II: Spin Structures and Dirac Operators

A very good reference for today's material is Dan Freed's (unpublished) notes on Dirac operators, available here.

Spin(n)

Consider the Clifford algebra $Cl(\mathbb E^n)$ as constructed in yesterday's post. Define maps $t, \beta: Cl(\mathbb E^n) \to Cl(\mathbb E^n)$ via
$$(e_1 \cdots e_k)^t = e_k \cdots e_1, \ \beta(e_1 \dots e_k) = (-1)^k e_k \dots e_2 e_1$$
 There is a natural inclusion $\mathbb E^n \hookrightarrow Cl(\mathbb E^n)$. Given $x \in Cl(\mathbb E^n)$ and $v \in \mathbb E^n$, we can consider the product $x v x^t$. In general, this might not be contained in $\mathbb E^n \subset Cl(\mathbb E^n)$.

Definition. We define the group $Pin(n)$ to consist of all those $g \in Cl(\mathbb E^n)$ such that
$$g \beta(g) = 1, \ \ g v \beta(g) \subset \mathbb E^n \ \forall\ v \in \mathbb E^n.$$
Similarly, we define the group $Spin(n)$ to be the subgroup of $Pin(n)$ such that $gg^t = 1$.

Theorem. The natural action of $Pin(n)$ on $\mathbb E^n$ is by othogonal transformations, giving a natural map $Pin(n) \to O(n)$. This map is a double cover. Similarly, $Spin(n)$ is a double cover of $SO(n)$. If $n \geq 2$, $Spin(n)$ is simply connected.

The importance of the spin groups is due to the following basic fact. Suppose that $G$ is a Lie group with Lie algebra $\mathfrak{g}$. Any representation of $G$ induces a representation of $\mathfrak{g}$. However,  given a representation of $\mathfrak{g}$, it is not always possible to integrate it to a representation of $G$. But it is always possible to integrate a representation of $\mathfrak{g}$ to produce a representation of the universal cover of $G$. For $n \geq 2$, $Spin(n)$ is the universal cover of $SO(n)$.

Spin Structures

Let $(M^n, g)$ be a Riemannian manifold. Recall that the frame bundle $O(M)$ is the manifold consisting of pairs $(x, \mathbb{e})$ where $x \in M$ and $\mathbb{e} = \{e_1, \dots, e_n\}$ is an orthonormal frame in $T_x M$. Since the orthogonal group $O(n)$ acts on the set of orthonormal frames, this makes $F(M)$ into a principal $O(n)$ bundle over $M$. Let us assume that $M$ is oriented, so that we may reduce its structure group to $SO(n)$.

Suppose that $V$ is a representation of $SO(n)$. Then we may form the associated bundle $SO(M) \times_{SO(n)} V$, which is a vector bundle over $M$ with structure group $SO(n)$. If we take the defining representation then we obtain the tangent bundle, but of course there are many others. Unfortunately, since $SO(n)$ is not simply connected, not every representation of $\mathfrak{so}_n$ can be integrated to a representation of $SO(n)$. At the level of geometry, this means that in a certain sense there are certain vector bundles over $M$ that are "missing"! Even more disturbing, is that these "missing" bundles appear to be necessary to describe many of the fundamental particles that appear in the standard model--so this has real world consequences. The solution is to equip $M$ with a spin structure.

Definition. A spin structure on $M$ is a principal $Spin(n)$-bundle $Spin(M)$ over $M$ together with a bundle morphism $Spin(M) \to SO(M)$ which is a reduction of structure (i.e., satisfies the obvious axioms).

As you might expect, not every manifold admits a spin structure, and spin structures may not be unique. Loosely speaking, a spin structure is a slightly stronger notion of orientability. Spin structures may always be chosen locally, and the obstruction to consistent gluing is not too difficult to characters as a certain $\mathbb Z_2$ cohomology class, called the second Stiefel-Whitney class.

Spin Connection and Dirac Operators

The reduction of structure $Spin(M) \to SO(M)$ allows us to pull back the Levi-Civita connection on $SO(M)$ to obtain a connection on $Spin(M)$, called the spin connection. Let $S_0$ be the spinor module described in the previous post. Then we may construct the associated bundle
$$S = Spin(M) \times_{Spin(n)} S_0$$
which is called the spinor bundle. Moreover, since $S_0$ is a Clifford module, there is well-defined notion of Clifford multiplication on sections of $S$. We may then define the Dirac operator $\mathcal{D}$ by
$$\mathcal{D} = \sum_{a=1}^n c(e_a) \nabla_{e_a}$$
where $\{e_a\}$ is any orthonormal frame, $\nabla$ is the spin connection, and $c$ denotes Clifford multiplication.

Next time: the Weitzenböck formula, and maybe a vanishing theorem.

Clifford Algebras and Spinors

Clifford Algebras

Today I'd like to write some brief notes about Clifford algebras and spinors. A classic reference is the paper "Clifford Modules" by Atiyah-Bott-Shapiro. Clifford algebras not only useful in algebra and geometry, but are essential for the construction of theories with fermions. Let $V$ be a vector space with a non-degenerate symmetric bilinear form $B$. We define the Clifford algebra $Cl(V, B)$ to be the unital associative algebra generated by $v \in V$ subject to the relation
$$vw + wv = -2B(v,w)$$
Equivalently, the definiting relation is $v^2 = -B(v,v)$.

The Clifford algebra inherits a $\mathbb Z$-filtration as well as a $\mathbb Z_2$-grading from the tensor algebra. In fact, we have an analogue of the Poincare-Birkhoff-Witt theorem for Lie algebras:

Theorem The associated graded algebra of $Cl(V,B)$ is naturally isomorphic to the exterior algebra on $V$.

In this way, we may view the Clifford algebra as a quantization of the exterior algebra, much in the same way that $U(\mathfrak g)$ is a quantization of the Poisson algebra of functions on $\mathfrak g^\ast$ for a Lie algebra $\mathfrak g$.

Example. Take (V,B) to be the Euclidean space $\mathbb E^1$. Then we have a single generator $e$ satisfying the relation $e^2 =  -1$. Hence
$$Cl(\mathbb R) \cong \mathbb R \cdot 1 \oplus \mathbb R \cdot e \cong \mathbb C$$
Where the isomorpism is given by $e \mapsto i = \sqrt{-1}$.

Example. Take $\mathbb E^2$. We have generators $e_1, e_2$ both squaring to -1, and additionally we have $e_1 e_2 = e_2 e_1$. We can define an isomorphism from $Cl(\mathbb E^2)$ to the quaternions $\mathbb H$ by $e_1 \mapsto i, e_2 \mapsto j$.

Spinors

Now consider the complexified Clifford algebra, denoted $\mathbb{C}l(V)$. Since we can now take square roots of negative numbers, the complex Clifford algebra is insensitive to the signature (as long as our bilinear form is non-degenerate). Denote by $C_n$ the complex Clifford algebra $Cl(\mathbb C^n)$,
where $\mathbb C^n$ is equipped with the standard bilinear form $(x,y) = \sum_{i=1}^n x_i y_i$.

Definition. A subspace $W \subset \mathbb C^n$ is isotropic if the restriction of the standard bilinear form to $W$ is identically 0. A maximal isotropic subspace is an isotropic subspace that is not properly contained in any other isotropic subspace.

Theorem. Let $W$ be a maximal isotropic subspace, and let$\{w_1, \dots, w_k\}$ be a basis of $W$. Let $\omega = w_1 \cdots w_k \in C_n$, and let $S = C_n \cdot \omega$. If n is even, then $S$ is an irreducible Clifford module. If n is odd, then $S=S^+ \oplus S^-$ is a direct sum irreducible Clifford modules, and $S^+ \cong S^-$.

Irreducible Clifford modules are called spinor modules. This description of spinor modules allows one to prove straightforwardly the following complete classification of complex Clifford algebras.

Corollary. We have $C_{2m} \cong \mathrm{End}(\mathbb C^m)$ and $C_{2m+1} \cong \mathrm{End}(\mathbb C^m) \oplus \mathrm{End}(\mathbb C^m)$.

Note that this classification depends on n mod 2, which is closely related to Bott periodicity. There is a similar classification of real Clifford algebras.

Dirac Operators

Now we come to the real importance of Clifford algebras. Consider Euclidean space $\mathbb{E}^n$ and let $S$ be a spinor module for its Clifford algebra. We define the Dirac operator acting on $S$-valued functions as
$$D f = \sum_{i=1}^n e_i \cdot \partial_i f$$
Now the amazing property of $D$ is the following:
$$D^2 = \sum_{i,j} e_i e_j \partial_i \partial_j = \sum_i e_i^2 \partial_i^2 + \sum_{i,j} e_i e_j [\partial_i, \partial_j] = -\Delta$$
hence the Dirac operator provides an algebraic (as opposed to pseudodifferential) square root of the Laplacian.

To Be Added in an Update...

Supersymmetric point particle, Dirac operators on spin manifolds, Weitzenböck formula, spinor reps of Lorentz algebra, N=1 susy.