We identify the lift to M theory of the four types of orientifold points, and show that they involve a chiral fermion on an orbifold fixed circle. From this lift, we compute the number of normalizable ground states for the SO(7V) and Sp(7V) supersymmetric quantum mechanics with sixteen supercharges. The results agree with known results obtained by the mass deformation method. The mass of the orientifold is identified with the Casimir energy.
- M(atrix) Theories