Time travel is the result of Einstein's Special Theory of Relativity (note: i am saying special not general. there is a difference). in time travel there is a dilation of time i.e if a clock is in an inertial frame of reference that is moving with a speed comparable to the speed of light then it is observed that the clock is moving slow. the needle takes long time to transfer from one digit to other.
time travel is not possible because you cannot move anything with the speed of light.
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still we don't know if this time dilation even happens because this is just a theory not a verified law.
Many ppl do not believe this theory 
scientists still working on it