Quaternions, Lorentz Group and the Dirac Theory

It is shown that a subgroup of SL(2, H), denoted Spin(2, H) in this paper, which is defined by two conditions in addition to unit quaternionic determinant, is locally iso-morphic to the restricted Lorentz group, L ↑ +. On the basis of the Dirac theory using the spinor group Spin(2, H), in which the charge conjugation transformation becomes linear in the quaternionic Dirac spinor, it is shown that the Hermiticity requirement of the Dirac Lagrangian, together with the persistent presence of the Pauli-Gürsey SU(2) group, requires an additional imaginary unit (taken to be the ordinary one, i) that commutes with Hamilton's units, in the theory. A second quantization is performed with this i incorporated into the theory, and we recover the conventional Dirac theory with an automatic 'anti-symmetrization' of the field operators. It is also pointed out that we are naturally led to the scheme of complex quaternions, H c , in which a space-time point is represented by a Hermitian quaternion, and that the isomorphism SL(1, H c)/Z 2 ∼ = L ↑ + is a direct consequence of the fact Spin(2, H)/Z 2 ∼ = L ↑ +. Using SL(1, H c) ∼ = SL(2, C), we make explicit the Weyl spinor indices of the spinor-quaternion, which is the Dirac spinor defined over H c .