closed timelike curve
In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is "closed," returning to its starting point. This possibility was first raised by Kurt G?del in 1949, who discovered a solution to the equations of general relativity (GR) allowing CTCs known as the G?del metric; and since then other GR solutions containing CTCs have been found, such as the Tipler cylinder and traversable wormholes.