The Gödel universe is a hypothetical universe, derived from the equations of the general theory of relativity, that admits time travel into the past; it is infinite, static (not expanding), rotating, with non-zero cosmological constant. Kurt Gödel, best known for his incompleteness theorem and one of the first scientists to be intrigued by the possible physical basis of time travel, theorized the existence of such a universe in a brief paper written in 1949 for a Festschrift to honor his friend and Princeton neighbor Albert Einstein. Although largely ignored, Gödel's paper raised the question: if one can travel through time, how can time as we know it exist in these other universes, since the past is always present? Gödel added a philosophical argument that demonstrates, by what have become known as Gödel's lights, that as a consequence, time does not exist in our world either. Without committing himself to Gödel's philosophical interpretation of his discovery, Einstein acknowledged that his friend had made an important contribution to the theory of relativity – a contribution that he admitted raised new and disturbing questions about what remains of time in his own theory.
Physicists since Einstein have tried without success to find an error in Gödel's physics or a missing element in relativity itself that would rule out the applicability of Gödel's results. In the 1949 paper, Gödel introduced the now-famous grandfather paradox.