Tipler Cylinder
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A Tipler Cylinder is a hypothetical cosmological object theorized to be a potential mode of time travel—an approach that is conceivably functional within humanity's current understanding of physics, construction of the device notwithstanding.
Frank J. Tipler suggested in 1974 that if a sufficiently long cylinder with the mass of several neutron stars was induced to spin along its longitudinal axis, the cylinder should create a frame-dragging effect and warp spacetime in its locality as the spin approached the speed of light.
This frame-dragging effect causes the light cones of objects in the cylinders proximity to tilt, 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.
CTC's are associated, in Lorentzian manifolds which are interpreted physically as spacetimes, with causal anomalies such as going back in time and potentially shooting your own grandfather. 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. 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. Thus, one could not travel backwards further than the date that the cylinder was activated. One could hypothesize that this is why humans have not yet encountered a time traveler.
Though Tipler's paper originally suggested that the cylinder might have to be of infinite length, later calculations indicated that the model would function if it were "only" a few thousand kilometers long; however singularities appear at the end.
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 titled, he admitted, borrowed from Tipler's paper) in which the Tipler Cylinder was known to be a functional mode of time travel, but any race that tried to build one saw their work destroyed by a natural disaster before the Cylinder was complete.
- Poul Anderson in The Avatar.
- The Astranagant in Super Robot Wars uses time-based weaponry inspired by the Tipler Cylinder.
- In Terry Pratchett's Discworld series the History Monks use Procrastinators, rotating stone cylinders that are an obvious spoof of Tipler Cylinders, to move amounts of time around and fix temporal disturbances.
References
- Tipler, Frank J. "Rotating Cylinders and the Possibility of Global Causality Violation." Physical Review D 9 (1974): 2203-2206.
- Penrose, Roger. "The Question of Cosmic Censorship." Journal of Astrophysics and Astronomy Vol. 20 (September, 1999): 233.
- Robert, Wald (ed). Black Holes and Relativistic Stars. University of Chicago Press, 1998. ISBN 0226870340