In 1932, Hsin P. Soh of MIT, advised by Arthur Eddington, published a theory attempting to unify gravitation and electromagnetism within a complex 4-dimensional Riemannian geometry. The line element ds is complex-valued, so that the real part corresponds to mass and gravitation, while the imaginary part with charge and electromagnetism. The usual space x, y, z and time t coordinates themselves are real and spacetime is not complex, but tangent spaces are allowed to be.