# Another temporal dilemma...

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Here's a seemingly impossible to answer question: To those unfamiliar with Tipler's rotating cylinder, it's a finitely long super-dense rotating cylinder, where if you circle it with a space-craft in the right direction, you can actually travel into the past; that is, until the time machine was first created. Now my question is, if your spacecraft is continually moving around the cylinder, until it reaches when the cylinder was first created, then what stops it from going before? Does a solid force stop it in its tracks? And if the answer is that you can go into the past before the crylinder was created, then how can you get back again (as the equations show that it's always possible to go back to your starting point in time), when the cylinder hasn't been created yet?

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I've heard about that theory on the Discovery Channel or something. I just don't understand what the cylinder has to do with it. I heard somewhere else that all you have to do to travel to the past is go past the speed of light.(Too bad I don't remember where I found these things.)

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Well, the actual cylinder is the time machine. Because it's super heavy, and therefore super-dense, then it actually drags space-time with it as it rotates. (Kind of like the ergosphere of a black hole...) Anyhow, if your spacecraft travels with this warping, it can carry you back to the past. (I hope this a good enough explanation. It's a bit unclear in the book I read it out of...)

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Welll, you had to create the cylinder at some point, right? So At some point there was a huge, non-rotating cylinder, with all its light cones just like they would be in normal space. Then, it started spinning, slowly and then more and more quickly. Soon, the light cones had tipped far enough to overlap and allow TT. What I think would happen if you went back to near the time of the creation of the cylinder is: the rate at which you were going back in time would decrease, until you go to the point at which the cones had just started to allow TT, and you would just stay at that point, sort of oscillating back and forth right at that point in time. You couldn't go any further back, because the cones weren't tilted enough before that. I don't think there'd be a wall, you'd just slow down to a stop.

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