Null hypersurface

A null hypersurface is a concept from the field of differential geometry and general relativity, relating to the geometry of spacetime. In general, a hypersurface is a submanifold of one dimension less than its ambient manifold. For example, in a four-dimensional spacetime (which typically includes three spatial dimensions and one time dimension), a hypersurface is a three-dimensional surface. A **null hypersurface** specifically refers to a hypersurface where the normal vector at each point is a null vector.