Time can be affected by speed. The faster you move, the more that time slows down. Time slows down due to gravity. A clock on earth runs slower than a clock on board a sattelite in outer space. If we were living in a rotating universe, this universe could create loops in time. Linear timelines would become looped. The special theory of relative allows you to go into the future, but the general theory of relativity allows you to go back into the past.
In this installment of Epiphany, Dr. Ronald Mallett explains how Einstein's theories of special and general relativity hold a key to understanding time travel.