Null coordinate
= Null coordinate
A <null coordinate> is a coordinate function $u$ whose gradient is null, $g^{ab}\partial_au\partial_bu=0$. Its level sets are <null hypersurfaces>. Pairs of such coordinates straighten radial <null directions> in two-dimensional orbit metrics; a monotone reparametrization of either function preserves its null level sets. Examples include the <retarded and advanced null coordinates> and the radial <Kruskal–Szekeres coordinates>.