Editing Light cone (section)

==In general relativity==
[[File:light_cones_near_black_hole.svg|thumb|200px|Light cones near a black hole resulting from a collapsing star. The purple (dashed) line shows the path of a photon emitted from the surface of a collapsing star. The green (dot-dash) line shows the path of another photon shining at the singularity.]]
In flat spacetime, the future light cone of an event is the boundary of its [[causal future]] and its past light cone is the boundary of its [[causal past]].

In a curved spacetime, assuming spacetime is [[globally hyperbolic]], it is still true that the future light cone of an event [[superset|includes]] the boundary of its causal future (and similarly for the past). However [[gravitational lensing]] can cause part of the light cone to fold in on itself, in such a way that part of the cone is strictly inside the causal future (or past), and not on the boundary.

Light cones also cannot all be tilted so that they are 'parallel'; this reflects the fact that the spacetime is curved and is essentially different from Minkowski space. In vacuum regions (those points of [[spacetime]] free of matter), this inability to tilt all the light cones so that they are all parallel is reflected in the non-vanishing of the [[Weyl tensor]].