Clifford Algebras and Their Representations

Introductory and historical remarks Clifford (1878) introduced his ‘geometric algebras’ as a generalization of Grassmann algebras, complex numbers and quaternions. Lipschitz (1886) was the first to define groups constructed from ‘Clifford numbers’ and use them to represent rotations in a Euclidean space. É. Cartan discovered representations of the Lie algebras son(C) and son(R), n > 2, that do not lift to representations of the orthogonal groups. In physics, Clifford algebras and spinors appear for the first time in Pauli’s nonrelativistic theory of the ‘magnetic electron’. Dirac (1928), in his work on the relativistic wave equation of the electron, introduced matrices that provide a representation of the Clifford algebra of Minkowski space. Brauer and Weyl (1935) connected the Clifford and Dirac ideas with Cartan’s spinorial representations of Lie algebras; they found, in any number of dimensions, the spinorial, projective representations of the orthogonal groups. Clifford algebras and spinors are implicit in Euclid’s solution of the Pythagorean equation x − y + z = 0 which is equivalent to ( y − x z z y + x ) = 2 ( p q )( p q ) (1)