Tipler cylinder 661465 222919269 2008-07-01T19:09:23Z DOI bot 6652755 Citation maintenance. Added: doi. Formatted: pages. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. {{expert-subject|Physics}} {{Refimprove|date=November 2007}} A '''Tipler cylinder''', also called a '''Tipler time machine''', is a [[hypothetical]] object (usually conceived of as being composed of [[neutron star]] matter){{Fact|date=March 2008}} [[Theory|theorized]] to be a potential mode of [[time travel]]&mdash;an approach that is conceivably functional within humanity's current understanding of [[physics]], specifically the theory of [[general relativity]], although later results have shown that a Tipler cylinder could only allow time travel if its length were infinite (see the discussion of Hawking's theorem below). [[Frank J. Tipler]] showed in [[1974]] that in a [[spacetime]] containing a massive, infinitely long [[cylinder (geometry)|cylinder]] which was spinning along its longitudinal [[Coordinate axis|axis]], the cylinder should create a [[frame-dragging]] effect. This [[frame-dragging]] effect warps spacetime in such a way that the [[light cone]]s of objects in the cylinder's proximity become tilted, so that part of the light cone then points backwards along the time axis on a space time diagram. Therefore a [[spacecraft]] accelerating sufficiently in the appropriate direction can travel backwards through time along a [[closed timelike curve]] or CTC.<ref name="rotating cylinders">{{cite journal | first=Frank | last=Tipler | authorlink = Frank J. Tipler | url=http://www.geocities.com/theophysics/tipler-rotating-cylinders.pdf | title=Rotating Cylinders and the Possibility of Global Causality Violation | journal = Physical Review D | volume = 9 | year=1974 | pages = 2203–2206 | doi=10.1103/PhysRevD.9.2203}} Available in [[Graphics Interchange Format|GIF]] format here: pages [http://www.vialattea.net/curvatura/articoli/t1.gif 1,] [http://www.vialattea.net/curvatura/articoli/t2.gif 2,] [http://www.vialattea.net/curvatura/articoli/t3.gif 3] and [http://www.vialattea.net/curvatura/articoli/t4.gif 4.] See also [http://adsabs.harvard.edu/abs/1974PhRvD...9.2203T here.]</ref> CTC's are associated, in [[pseudo-Riemannian manifold|Lorentzian manifold]]s which are interpreted physically as [[spacetime]]s, with the possibility of causal anomalies such as going back in time and [[Grandfather paradox|potentially shooting your own grandfather]], although paradoxes might be avoided using some constraint such as the [[Novikov self-consistency principle]]. They have an unnerving habit of appearing in some of the most important exact solutions in general relativity, including the [[Kerr metric|Kerr vacuum]] (which models a rotating black hole) and the [[van Stockum dust]] (which models a cylindrically symmetrical configuration of rotating pressureless fluid or [[dust solution|dust]]). Some [[physicist]]s argue that since Tipler cylinders allow closed timelike curves they violate [[Roger Penrose]]'s [[cosmic censorship hypothesis]], as [[naked singularity|naked singularities]] would be visible.{{Fact|date=February 2007}}<!-- closed timelike curves do not themselves imply naked singularities, is there a separate argument showing the tipler cylinder leads to naked singularities? Either way this sentence is confusing. --> Others argue that since [[causality (physics)|causality]] is not built into [[Einstein's field equation]], these regions may actually be able to exist (''see also'' [[Gödel metric|Godel's universe]]). A limitation of the Tipler cylinder is that it is only possible to travel to times (and places) in which the cylinder already exists{{Fact|date=November 2007}}. Thus, one could not travel backwards further than the date that the cylinder was activated{{Fact|date=November 2007}}. A more fundamental objection to the practicality of building a Tipler cylinder was discovered by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine in any finite region that satisfies the [[weak energy condition]], meaning that the region contains no [[exotic matter]] with negative energy. The Tipler cylinder, on the other hand, does not involve any negative energy. Tipler's original solution involved a cylinder of infinite length, which is easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,<ref name="Earman2">{{cite book | last = Earman | first = John | title = Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes | publisher = Oxford University Press |date= 1995 | pages = p. 169 | id = ISBN 0-19-509591-X}}</ref> he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."<ref name="futureofspacetime">{{cite book | last = Hawking | first = Stephen | title = The Future of Spacetime | publisher = W. W. Norton |date= 2002 | pages = p. 96 | id = ISBN 0-393-02022-3}}</ref> This result comes from Hawking's 1992 paper on the [[chronology protection conjecture]], where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a [[Cauchy horizon]] that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."<ref name="chronology protection">{{cite journal | first=Stephen | last=Hawking | authorlink = Stephen Hawking | url=http://link.aps.org/abstract/PRD/v46/p603 | title=Chronology protection conjecture | journal = Physical Review D | volume = 46 | year=1992 | pages = 603–611 | doi=10.1103/PhysRevD.46.603}}</ref> ==Tipler cylinders in fiction== *[[John DeChancie]]'s ''Starrigger'' series uses vertically-aligned Tipler cylinders (officially called [[Roy Kerr|Kerr]]-Tipler objects) to create spacetime gateways along an intergalactic highway. *The ''[[Continuum RPG|C°ntinuum]]'' [[role-playing game]] postulates the use of Tipler cylinders for "deep" time travel such as to the [[end of the universe]]. *[[Larry Niven]] wrote a [[short story]] (whose title, he admitted, he borrowed from Tipler's paper) in which the Tipler cylinder was believed to be a functional mode of time travel, but where a [[natural disaster]] overtook any species that tried to build one of their own or use one which had been constructed (and abandoned) by another species. *[[Poul Anderson]] in The Avatar. *The [[Astranagant]] in ''[[Super Robot Wars]]'' uses time-based weaponry inspired by the Tipler cylinder. * In [[Kristofer Straub]]'s webcomic ''Starslip Crisis'' the artist Volox created a full-scale model of a Tipler cylinder, but it was said he built the time machine "only '''artistically'''. Not scientifically." == References == {{reflist}} {{refbegin}} * Frank Jennings Tipler, [http://adsabs.harvard.edu/abs/1976PhDT........61T ''Causality Violation in General Relativity'',] Ph.D. thesis at the University of Maryland, College Park (1976). Source: Dissertation Abstracts International, Vol. 37-06, Section B, pg. 2923. Also available as Dissertation 76-29,018 from Xerox University Microfilms, Ann Arbor, MI. * [[Roger Penrose|Penrose, Roger]]. "The Question of Cosmic Censorship." ''Journal of Astrophysics and Astronomy'' Vol. 20 (September, [[1999]]): 233. * [[Robert Wald|Wald, Robert]] (ed). ''Black Holes and Relativistic Stars''. [[University of Chicago]] Press, [[1998]]. ISBN 0-226-87034-0 {{refend}} [[Category:Megastructures]] [[Category:Physics in fiction]] [[Category:Time travel devices]] [[fr:Machine de Tipler]]