What happens when the century's greatest logician meets the century's greatest physicist? In the case of Kurt Gödel and Albert Einstein, the result is Gödel's revolutionary new world models for relativity theoryAlthough most famous for his Incompleteness Theorems in mathematical logic, Gödel was a philosopher in his own right, with a special interest in the philosophical problem of Time.Most people are unaware that Gödel and Einstein were close friends for many years at Princeton's Institute for Advanced Study. Gödel extended Einstein's General Theory of Relativity with cosmological models—now known as "Gödel Universes"—with extraordinary properties, including the possibility of closed, timelike curves that allow the philosophical fantasy of time travel to become a scientific reality.For Gödel, however, the reality of time travel signals the unreality of time. If he's right, the real meaning of the Einstein revolution remained a secret for half a century and is only being revealed now.