We review the applications of geometric algebra geometric algebra in electromagnetism, gravitation and multiparticle quantum systems. We discuss a gauge theory formulation of gravity and its implementation in geometric algebra, and apply this to the fermion bound state problem in a black hole background. We show that a discrete energy spectrum arises in an analogous way to the hydrogen atom. A geometric algebra approach to multiparticle quantum systems is given in terms of the multiparticle spacetime algebra. This is applied to quantum information processing, multiparticle wave equations and to conformal geometry. The application to conformal geometry highlight some surprising links between relativistic quantum theory, twistor theory and de Sitter spaces.

A.N. Lasenby, C.J.L. Doran and E. Arcaute, **Applications of Geometric Algebra in Electromagnetism, Quantum Mechanics and Gravity, **In R. Ablamowicz, ed. *Sixth International Conference on Clifford Algebras and their Applications, Tennessee 2002*, p. 467, Birkhauser (2003)