This thread was inspired by a post by slipster
in another thread. I searched for Tipler Cylinders, but didn't find much that discussed them directly, so I thought I'd start a thread about them.
Originally posted by slipster
In or around 1974 an American physicist named Frank Tipler wrote that, based on what is understood about the relationship between time and the spacial dimensions, there was a method--in theory at least--for constructing a time machine.
Tipler said that if one constructed a cylinder out of the "stuff" of which black holes are constructed, and spun it very fast, the material would not collapse in on itself and form a singularity. Some physicists dispute this. Tipler also thought that one would not be crushed by entering the space in the center of the cylinder; I guess this would work on the same principal as the idea that a person at the center of the earth should be weightless as the mass--and, therefore, the gravitational pull--acting on him or her would be equal on all sides.
In any case, Tipler said that if such a whirling cylinder was constructed and the device spun at sufficient speed, it would contort or "tip" the four dimensional geomerty of the space which it occupied. (I am probably not explaining this very well, but then, I'm not a physicist.)
Suppose you take an ordinary cube and draw arrows along its sides, marking some as "height", others as "length", and others as "width". If you then tip the cube on its side, two of the dimensions "switch places"; for instance, the arrow which is being "height" may now be pointing in the direction in which the "length" arrow was pointing a moment before.
In a somewhat analogous fashion, in Tipler's model, time would "switch places" with one of the three spacial dimensions. This would mean that if one moved forward or backward inside the cylinder, it would mean you were moving through time. Once the cylinder stopped spinning and the four dimensions were restored to their normal orientation, the person who had moved about inside the cylinder would find he or she had moved forward or backward in time.
Which direction in the cylinder would be the future, and which would be the past? In H.G. Well's novel The Time Machine the anonymous inventer says that he can make the parts of his machine rotate in either of two directions, but he won't know which way pushes the machine forward in time and which pushes it backwards until he actually tries it out. It would seem to be the same for Tipler's hypothetical time machine.
I have read that one limitation on the Tipler design is that it could not put a person farther into the past than when the cylinder was first built. This has to do with the idea that the cylinder itself does not travel in time except in the ordinary sense that everything and everybody "go" from their past existence to their future through the eternal present.
First off, I think slipster
may have gotten a few specifics wrong. If I'm not mistaken, I believe the black hole "stuff" of which the cylinder is made is
in fact a singularity -- albeit linear instead of point-like. I suppose this would make it a cosmic string, but presumably it doesn't have
to be quite so dense -- neutronium would probably work. And the reason that it doesn't collapse linearly to form a point-like black hole is that the theoretical cylinder is infinitely long (which makes it a bit of a practical difficulty). The cylinder is
spinning, but the traveller doesn't enter the center, but instead orbits around it and along its length in a spiral. The rotating gravitational field twists spacetime in such a way as to "tip" time into a spatial dimension, as you suggest, and demonstrate with your apt analogy. You travel forward or backwards in time according to whether you spiral with the rotation or against it, and it should be readily calculable which is which (I don't possess the ability to do it, but I suspect that orbiting with the rotation leads to travel in the forward direction). Upon reaching one's destination time, the cylinder does not stop rotating; instead the craft just leaves orbit. Backwards time travel is not, I believe, limited to the date of the creation of the machine, as it is in joined wormholes -- time travel is possible into the distant past, although I may be wrong about this. The more I think about it, the more the fact that the cylinder didn't exist in the past that you're traveling to seems like a hairy issue.
So that's my beginners understanding of Tipler Cylinders. And the GQ is (are), to what extent do I have my facts straight? I searched for "tipler cylinders" on google, and only got 64 hits, which seemed appallingly low. Also, a substantial proportion of them were bad sci-fi stories, pseudoscience, conspiracy theorists, and other assorted less-than-credible sources. I thought this was a well-respected and accepted theory -- where are the legitimate pages? Is Tipler a respected cosmologist? I've read a few of his books ("The Anthropic Cosmological Principle" and "The Physics of Immortality"), and he seems to be a borderline nutjob in some of his beliefs, but his science and reputation doesn't seem terrible. So what's the scoop on these babies, and what other cool info can you tell me about them? And what's the straight poop on Frank Tipler -- serious physicist or borderline nutjob?