In some models, a time machine only allows travel back to the point where it was first built e.g. http://en.wikipedia.org/wiki/Tipler_cylinder (Ignoring the "it does actually have to be infinite in length to work, you can't just assume an infinite length to simplify the maths and hope that a long enough approximation will work" detail....)
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branchandroot
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Date: 2010-04-09 09:50 pm (UTC)(Ignoring the "it does actually have to be infinite in length to work, you can't just assume an infinite length to simplify the maths and hope that a long enough approximation will work" detail....)