Isotropic lines have been used in cosmological writing to carry light. For example, in a mathematical encyclopedia, light consists of photons : "The worldline of a zero rest mass (such as a non-quantum model of a photon and other elementary particles of mass zero) is an isotropic line." For isotropic lines through the origin, a particular point is a null vector, and the collection of all such isotropic lines forms the light cone at the origin.