"In relativity, time and space are intertwined in the fabric of space-time, so time can contract and stretch under the influence of gravity." Closed timelike curves (CTCs), sometimes referred to as time loops, in which the world line of an object returns to its origin, arise from some exact solutions to the Einstein field equation. However, the chronology protection conjecture of Stephen Hawking suggests that any such closed timelike curve would be destroyed before it could be used. Although CTCs do not appear to exist under normal conditions, extreme environments of spacetime, such as a traversable wormhole or the region near certain cosmic strings, may allow their brief formation, implying a theoretical possibility of retrocausality. The exotic matter or topological defects required for the creation of those environments have not been observed.