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you can't change the past and if someone did go into the past wouldn't our world be constantly changing
Today, we know that time travel need not be confined to myths, science fiction, Hollywood movies, or even speculation by theoretical physicists. Time travel is possible. For example, an object traveling at high speeds ages more slowly than a stationary object. This means that if you were to travel into outer space and return, moving close to light speed, you could travel thousands of years into the Earth's future.
Complementary time travel
Since quantum mechanics is governed by probabilities, an unmeasured entity (in this case, your historical grandfather) has numerous probable states. When that entity is measured, the number of its probable states singularises, resulting in a single outcome (in this case, ultimately, you). Therefore, since the outcome of your grandfather is known, your killing your grandfather would be incompatible with that outcome. Thus, the outcome of one's trip backwards in time must be complementary with the state from which one left.
Novikov self-consistency principle
See the Novikov self-consistency principle and Kip S. Thorne for one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible timelines are those which are entirely self-consistent, so that anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from being made since this would represent an inconsistency. This is the view taken by the author of Doraemon.