Editing General relativity (section)

=== Exotic solutions: time travel, warp drives ===
[[Kurt Gödel]] showed<ref>{{harvnb|Gödel|1949}}</ref> that solutions to Einstein's equations exist that contain [[closed timelike curve]]s (CTCs), which allow for loops in time. The solutions require extreme physical conditions unlikely ever to occur in practice, and it remains an open question whether further laws of physics will eliminate them completely. Since then, other—similarly impractical—GR solutions containing CTCs have been found, such as the [[Tipler cylinder]] and [[Wormhole#Traversable wormholes|traversable wormholes]]. [[Stephen Hawking]] introduced [[chronology protection conjecture]], which is an assumption beyond those of standard general relativity to prevent [[time travel]].

Some [[exact solutions in general relativity]] such as [[Alcubierre drive]] present examples of [[warp drive]] but these solutions requires exotic matter distribution, and generally suffers from semiclassical instability.
<ref>{{Cite journal
|last1=Finazzi|first1=Stefano
|last2= Liberati|first2=Stefano
|last3=Barceló|first3=Carlos
|date=15 June 2009
|title=Semiclassical instability of dynamical warp drives
|journal=Physical Review D
|language=en-US
|volume=79|issue=12|page=124017
|doi=10.1103/PhysRevD.79.124017
|arxiv=0904.0141
|bibcode=2009PhRvD..79l4017F
|s2cid=59575856
}}</ref>