Editing Time travel (section)

==== Wormholes ====
{{main|Wormhole}}
Wormholes are a hypothetical warped spacetime permitted by the [[Einstein field equations]] of general relativity.<ref name="Visser1996">{{cite book|last=Visser|first=Matt|author-link=Matt Visser|title=Lorentzian Wormholes|publisher=Springer-Verlag|year=1996|isbn=978-1-56396-653-8}}</ref>{{rp|100}} A proposed time-travel machine using a [[Wormhole#Traversable wormholes|traversable wormhole]] would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced [[Vehicle propulsion|propulsion system]], and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both these methods, [[time dilation]] causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently ''through'' the wormhole than ''outside'' it, so that [[Synchronization|synchronized]] clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.<ref name="Thorne1994" />{{rp|502}} This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;<ref name="Thorne1994" />{{rp|503}} in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with [[negative energy]], often referred to as "[[exotic matter]]". More technically, the wormhole spacetime requires a distribution of energy that violates various [[energy condition]]s, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition,<ref name="Visser1996" />{{rp|101}} and many physicists believe that the required negative energy may actually be possible due to the [[Casimir effect]] in quantum physics.<ref name="casimir">{{cite web|url=http://www.npl.washington.edu/av/altvw69.html|title=NASA Goes FTL Part 1: Wormhole Physics|work=Analog Science Fiction & Fact Magazine|year=1994|access-date=December 2, 2006|last1=Cramer|first1=John G.|author-link=John G. Cramer|archive-url=https://web.archive.org/web/20060627211046/http://www.npl.washington.edu/av/altvw69.html <!-- Bot retrieved archive -->|archive-date=June 27, 2006}}</ref> Although early calculations suggested that a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.<ref name="negative energy">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|author2=Sayan Kar|author3=Naresh Dadhich|title=Traversable wormholes with arbitrarily small energy condition violations|journal=[[Physical Review Letters]]|volume=90|year=2003|issue=20|pages=201102.1–201102.4|doi=10.1103/PhysRevLett.90.201102|arxiv=gr-qc/0301003|bibcode=2003PhRvL..90t1102V|pmid=12785880|s2cid=8813962}}</ref>

In 1993, [[Matt Visser]] argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.<ref name="visser_1">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture|journal=Physical Review D|volume=47|year=1993|issue=2|pages=554–565|doi=10.1103/PhysRevD.47.554|pmid=10015609|arxiv=hep-th/9202090|bibcode=1993PhRvD..47..554V|s2cid=16830951}}</ref> Because of this, the two mouths could not be brought close enough for [[Causality (physics)|causality]] violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "[[Roman ring]]" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.<ref name="visser_2">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=Traversable wormholes: the Roman ring|journal=Physical Review D|volume=55|year=1997|issue=8|pages=5212–5214|doi=10.1103/PhysRevD.55.5212|arxiv=gr-qc/9702043|bibcode=1997PhRvD..55.5212V|s2cid=2869291}}</ref>