You travel back to the past and kill your grandfather. So now, your father won’t be born and neither will you.
So if you are not even born, who killed your grandfather?
You travel back to the past and kill Hitler. So history changes and now, there is no Hitler.
Now, if Hitler doesn’t exist, why would you travel back in the first place to kill him?
Infinite loop paradox
A man travel backs to the past and marries a woman. After that, he returns to the present. But the woman gets pregnant and has a son. Many years later, that son becomes the time traveller who goes to the past and marries the woman.So who is the son and who is the father?
A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being, or existence. George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin. A bootstrap paradox involving a person could be, say, a 20 year old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20 year old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies”.
These ontological paradoxes imply that the future, present and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backwards time travel as being theoretically possible under certain conditions.