If we travel time and went to the past then we would create a parallel universe where we go. Think it this way, you went to past and killed your grandfather this is called the grand father paradox. If you killed your grandfather then how were you even supposed to be born and if you were not born then who killed your grandfather.
This paradox can be explained by the parallel universe theory. If you make a time machine then you will create a parallel universe where you would have your own time frame. In that universe you can kill your grandfather and you won't born there.
This way we may be a living in some one else parallel universe. Some of your friend might be those time travelers.
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