From Wikipedia's article on "Kurt Godel."

"In the late 1940s, Gödel demonstrated the existence of paradoxical solutions to Albert Einstein's field equations in general relativity. These 'rotating universes' would allow time travel and caused Einstein to have doubts about his own theory."

More detailed information about this body of work can be found here:

http://infoshare1.princeton.edu/libraries/firestone/rbsc/finding_aids/mathoral/pmcxgod0.htm

Also, 72GET!!!