In general relativity, the geometric setting for physical phenomena is a Lorentzian manifold, which is interpreted as a curved spacetime, and which is specified by defining a metric tensor g a b (or by defining a frame field). The Riemann curvature tensor R a b c d of this manifold and associated quantities such as the Einstein tensor G a b , are well-defined. In general relativity, they can be interpreted as geometric manifestations (curvature and forces) of the gravitational field.