Editing World line (section)

==World lines in general relativity==
The use of world lines in [[general relativity]] is basically the same as in [[special relativity]], with the difference that [[spacetime]] can be [[curvature|curved]]. A [[metric tensor|metric]] exists and its dynamics are determined by the [[Einstein field equations]] and are dependent on the mass-energy distribution in spacetime. Again the metric defines [[lightlike]] (null), [[spacelike]], and [[timelike]] curves. Also, in general relativity, world lines include [[timelike]] curves and null curves in spacetime, where [[timelike]] curves fall within the lightcone. However, a lightcone is not necessarily inclined at 45 degrees to the time axis. However, this is an artifact of the chosen coordinate system, and reflects the coordinate freedom ([[diffeomorphism invariance]]) of general relativity. Any [[timelike]] curve admits a [[Proper frame|comoving observer]] whose "time axis" corresponds to that curve, and, since no observer is privileged, we can always find a local coordinate system in which lightcones are inclined at 45 degrees to the time axis. See also for example [[Eddington-Finkelstein coordinates]].

World lines of free-falling particles or objects (such as planets around the Sun or an astronaut in space) are called [[geodesic]]s.