Closed timelike curve (abbreviated CTC) is physics jargon for a solution of general relativity that describes time travel. The 'curve' means a world line of somebody, 'timelike' means that the world line does not exceed the speed of light, so it is a 'valid' world line, 'closed' means that the world line is a loop, which means that it meets itself at an earlier time.

These things were invented by Kurt Gödel after meeting Albert Einstein in The Institute for Advanced Study. Mathematically these solutions are agreed to be valid, but their physical significance is unclear. It may be that general relativity is not valid in the conditions where CTC:s would occur.

One example of conditions, where general relativity makes CTC:s possible, is a _very_ dense, _very_ rapidly rotating, _very_ heavy, object. According to Einstein's field equations, such an object could curve the space-time around it so much that you could travel backwards in time by moving around in a suitable manner.

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