John Richard Gott III (born February 8, 1947) is a professor of astrophysical sciences at Princeton University. He is known for his work on time travel and the Doomsday argument. Exotic matter time travel theories Paul Davies's bestseller How to Build a Time Machine credits Gott with the proposal of using cosmic strings to create a time machine. Gott's machine depends upon the antigravitational tension of the (hypothetical) strings to deform space without attracting nearby objects. The traveler would follow a precise path around rapidly separating strings, and find that he or she had moved backwards in time. Gott's solution does require that the strings are infinitely long, though: a theorem by Stephen Hawking proves that according to general relativity, closed timelike curves cannot be created in a finite region of space unless there is exotic matter present which violates certain energy conditions, while cosmic strings would not be expected to violate these conditions, so strings of finite length wouldn't work. Gott also proposed a "time mirror": a time travel device based on the principle of time delays. The device would be situated near a black hole some hundred or more light years from Earth. The device would act as a light collector and would power the light rays deformed and curved by the gravitational depression of the black hole. The collector would then reveal the past as detailed by the photons that had originated from Earth. Since Gott believes that time travel is not cosmologically excluded, he has presented the possibility that the universe was created out of itself (at a later time). This controversial suggestion was published with Li-Xin Lin, and it was described by Gott as "it would be like having one branch of a tree circle around and grow up to be the trunk. In that way, the universe could be its own mother." In his own book, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time, Gott argues that travel to the past is quite possible, although probably only after the construction of a working device (during its existence), and certainly not onto the time traveler's own past timeline (he argues that either the many worlds interpretation of quantum mechanics must be invoked to overcome the Grandfather paradox, or that all time travel remain self-consistent, i.e., one can visit the past but not change it, as in the Novikov self-consistency principle). Although