Tipler cylinder

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A Tipler cylinder, also called a Tipler time machine, is a hypothetical object (usually conceived of as being composed of neutron star matter)^{[citation needed]} theorized to be a potential mode of time travel—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 which was spinning along its longitudinal axis, the cylinder should create a frame-dragging effect. This frame-dragging effect warps spacetime in such a way that the light cones 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.^[1]

CTC's are associated, in Lorentzian manifolds which are interpreted physically as spacetimes, with the possibility of causal anomalies such as going back in time and 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 vacuum (which models a rotating black hole) and the van Stockum dust (which models a cylindrically symmetrical configuration of rotating pressureless fluid or dust).

Some physicists argue that since Tipler cylinders allow closed timelike curves they violate Roger Penrose's cosmic censorship hypothesis, as naked singularities would be visible.^{[citation needed]} Others argue that since causality is not built into Einstein's field equation, these regions may actually be able to exist (see also 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^{[citation needed]}. Thus, one could not travel backwards further than the date that the cylinder was activated^{[citation needed]}.

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,^[2] 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."^[3] 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."^[4]

[edit] Tipler cylinders in fiction

• John DeChancie's Starrigger series uses vertically-aligned Tipler cylinders (officially called Kerr-Tipler objects) to create spacetime gateways along an intergalactic highway.
• The 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."

[edit] References