Clifford module

In mathematics, a Clifford module is a representation of a Clifford algebra. In general a Clifford algebra C is a central simple algebra over some field extension L of the field K over which the quadratic form Q defining C is defined.

The abstract theory of Clifford modules was founded by a paper of M. F. Atiyah, R. Bott and A. Shapiro. A fundamental result on Clifford modules is that the Morita equivalence class of a Clifford algebra (the equivalence class of the category of Clifford modules over it) depends only on the signature p − q mod 8. This is an algebraic form of Bott periodicity.

Matrix representations of real Clifford algebras

We will need to study anticommuting matrices (AB = −BA) because in Clifford algebras orthogonal vectors anticommute

$A \cdot B = \frac{1}{2}( AB + BA ) = 0.$

For the real Clifford algebra $\mathbb{R}_{p,q}\,$, we need p + q mutually anticommuting matrices, of which p have +1 as square and q have −1 as square.

$\begin{matrix} \gamma_a^2 &=& +1 &\mbox{if} &1 \le a \le p \\ \gamma_a^2 &=& -1 &\mbox{if} &p+1 \le a \le p+q\\ \gamma_a \gamma_b &=& -\gamma_b \gamma_a &\mbox{if} &a \ne b. \ \\ \end{matrix}$

Such a basis of gamma matrices is not unique. One can always obtain another set of gamma matrices satisfying the same Clifford algebra by means of a similarity transformation.

$\begin{matrix} \gamma_{a'} &=& S &\gamma_{a } &S^{-1} \end{matrix}$

where S is a non-singular matrix. The sets γ _a' and γ _a belong to the same equivalence class.

Real Clifford algebra R_3,1

Developed by Ettore Majorana, this Clifford module enables the construction of a Dirac-like equation without complex numbers, and its elements are called Majorana spinors.

The four basis vectors are the three Pauli matrices and a fourth antihermitian matrix. The signature is (+++-). For the signatures (+---) and (---+) often used in physics, 4x4 complex matrices or 8x8 real matrices are needed.

See also

• Weyl–Brauer matrices
• Higher-dimensional gamma matrices

References

• Atiyah, Michael; Bott, Raoul; Shapiro, Michael (1964), "Clifford Modules", Topology 3 ((Suppl. 1)): 3–38, doi:10.1016/0040-9383(64)90003-5, http://www.ma.utexas.edu/users/dafr/Index/ABS.pdf 
• Deligne, Pierre (1999), "Notes on spinors", in P. Deligne, P. Etingof, D. S. Freed, L. C. Jeffrey, D. Kazhdan, J. W. Morgan, D. R. Morrison, E. Witten, Quantum Fields and Strings: A Course for Mathematicians, Providence: American Mathematical Society, pp. 99–135 . See also the programme website for a preliminary version.
• Harvey, F. Reese (1990), Spinors and Calibrations, Academic Press, ISBN 978-0123296504 .
• Lawson, H. Blaine; Michelsohn, Marie-Louise (1989), Spin Geometry, Princeton University Press, ISBN 0-691-08542-0 .