Tipler Cylinder
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A Tipler Cylinder is a theoretical method of time travel that could conceivably work within current understanding of physics, construction of the device notwithstanding.
Frank J. Tipler suggested in 1974 that a sufficiently-long cylinder with the mass of several neutron stars could be constructed, and then induced to spin along its longitudinal axis. As the spin approaches the speed of light, the cylinder should create a frame-dragging effect and warp space and time in its locality. A spaceship traveling in the direction of the spin or against it in a certain path will travel through time along a Closed Timelike Curve, similar to effects theoretically caused by Kerr-Newman objects.
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 brought "online", nor forward past a date that it was destroyed (assuming such destruction is even possible). This could explain why we've not seen any time travelers yet, but can possibly expect some traffic once a cylinder is running.
Though Tipler's paper originally suggested that the cylinder might have to be of infinite length, recent calculations show that it would still work even if it were "only" a few thousand miles long.
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.
References
- "Rotating Cylinders and the Possibility of Global Causality Violation", Frank J. Tipler. Physical Review D9, 2203-2206.
- "The Question of Cosmic Censorship", Roger Penrose. Black Holes and Relativistic Stars, Robert Wald (ed).