Time travel: Difference between revisions

Revision as of 21:48, 28 January 2007

Revision as of 21:48, 28 January 2007

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'''Time travel''' is the concept of moving backwards or forwards to different points in [[time]], in a manner analogous to moving through [[space]]. Additionally, some interpretations of time travel suggest the possibility of travel between [[Multiverse (science)|parallel realities]] or [[universe]]s.<ref name="deutsch">{{cite journal|last=Deutsch| first=David| authorlink = David Deutsch|date=[[1991]]| title=Quantum mechanics near closed timelike curves| journal=Physical Review D| volume=44| pages=3197-3217}}</ref>

==Origins of the concept==

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There is no widespread agreement on what should qualify as the first time travel story, since a number of early stories feature elements suggestive of time travel but are nevertheless somewhat ambiguous. delayed talking to the narrator and so the other monks burst in thinking that some harm has come to him, and suddenly the narrator finds himself back under the tree in the present (August of [[1837]]), with his coach having just passed his spot on the road, leaving him stranded in Newcastle for another night.

[[Charles Dickens]]' [[1843]] book [[A Christmas Carol]] is considered by some<ref name=" seen as an inspiration for all later science fiction stories featuring time travel.

Since that time, both science and fiction (see [[Time travel in fiction]]) have expanded on the concept of time travel, but whether it could be possible in reality is still an open question.

==Time travel in theory==

Some theories, most notably [[special relativity|special]] and [[general relativity]], suggest that suitable geometries of [[spacetime]], or specific types of motion in [[space]], may allow time travel into the past and future if these geometries or motions are possible.<ref name="Thorne1">{{cite journal | first = Thorne| last = Kip S. | | title = [[Black Holes and Time Warps]] | year = }} p. 499 </ref> In technical papers physicists generally avoid the commonplace language of "moving" or "traveling" through time ('movement' normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of [[closed timelike curve]]s, which are [[worldline]]s that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, but the physical plausibility of these solutions is uncertain.

Physicists take for granted that if one were to move away from the Earth at [[special relativity|relativistic]] velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (although according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return). On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that issues of [[causality (physics)|causality]] be resolved. What if one were to go back in time and kill one's own grandfather (see [[grandfather paradox]])? Additionally, [[Stephen Hawking]] once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel&mdash;a variant of the [[Fermi paradox]], with time travelers instead of alien visitors. (Of course this would not show time travel is physically impossible, only that it is never in fact developed; and even if it is developed, Hawking notes elsewhere that time travel may only be possible in a region of spacetime that is warped in the right way, and that if we can't create such a region until the future, then time travelers would not be able to travel back before that date, so 'This picture would explain why we haven't been over run by tourists from the future.'<ref name="Hawking warp">{{cite web |url=http://www.hawking.org.uk/lectures/warps3.html |title=Space and Time Warps |accessdate=2006-11-20 |author=Hawking, Steven |format=html}}</ref>) However, the theory of [[general relativity]] does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from [[semiclassical gravity]] suggest that when [[quantum mechanics|quantum]] effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the [[chronology protection conjecture]], suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of [[quantum gravity]] to join quantum mechanics and general relativity into a completely unified theory.

==The "presentist" view==