Well, there is a theory (which I agree with) known as the "many-worlds theory", which means that if you go back in time, you actually shift into an alternate universe.
The "Grandfather paradox" is when you go back in time and kill your Grandfather. But how could you do that? If you kill your Grandfather before you were born, then how could you be born in the first place?....Hence, you would have never existed to go back in time to kill your Grandfather lol.
but if you shift into an alternate universe when you go back in time, then all the people you recognize are NOT the people you think they are. They may have the exact same DNA, fingerprints, etc. but they are not the same people you left behind in your original universe.
There could be "an ensemble of parallel universes" such that when the traveller kills the grandfather, the act took place in (or resulted in the creation of) a parallel universe in which the traveller's counterpart will never be conceived as a result. However, his prior existence in the original universe is unaltered. Succinctly, this explanation states that: if time travel is possible, then multiple versions of the future exist in parallel universes. Alternate timelines would also apply if a person went back in time to shoot himself because in the past, he would be dead as in the future he would be alive and well.
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