The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime cause by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past. The first of these was proposed by Kurt Godel, a solution known as the Godel metric, but his (and many others) example requires the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.

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