Editing Time travel (section)

==== Different spacetime geometries ====
The theory of [[general relativity]] describes the universe under a system of [[Einstein field equations|field equations]] that determine the [[Metric (general relativity)|metric]], or distance function, of spacetime. There exist exact solutions to these equations that include [[closed time-like curve]]s, which are [[world line]]s that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation that can be described as time travel. Such a solution was first proposed by [[Kurt Gödel]], a solution known as the [[Gödel metric]], but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,<ref name="Thorne1994"/>{{rp|499}} such as [[Mach's principle|rotation]] and lack of [[Hubble expansion]]. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.<ref name="Hawking">S. W. Hawking, ''Introductory note to 1949 and 1952'' in Kurt Gödel, ''Collected works'', Volume II (S. Feferman et al., eds).</ref>