The grandfather paradox is a paradox of time travel. The paradox is this: Suppose a man traveled back in time and killed his biological grandfather before the latter met the traveller’s grandmother. As a result, one of the traveller’s parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.
An equivalent paradox is known as autoinfanticide – that is, going back in time and killing oneself as a baby .
The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced.
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, you 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
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.
Parallel universes/Alternate Timelines
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.
Theories in science fiction
Parallel universes resolution
The idea of preventing paradoxes by supposing that the time traveler is taken to a parallel universe while his original history remains intact, which is discussed above in the context of science, is also common in science fiction.
Restricted action resolution
Another resolution, of which the Novikov self-consistency principle can be taken as an example, holds that if one were to travel back in time, the laws of nature would simply forbid the traveller from doing anything that could later result in their time travel not occurring. For example, a shot fired at the traveller’s grandfather will miss, or the gun will jam, or misfire, or the grandfather will be injured but not killed, or the person killed will turn out to be not the real grandfather, or some other event will occur to prevent the attempt from succeeding. No action the traveller takes to affect change will ever succeed, as there will always be some form of “bad luck” or coincidence preventing the outcome. In effect, the traveller will be unable to change history from the state they left it.
This theory might lead to concerns about the existence of free will (in this model, free will may be an illusion). This theory also assumes that causality must be constant: i.e. that nothing can occur in the absence of cause, whereas some theories hold that an event may remain constant even if its initial cause was subsequently eliminated.
It also may not be clear whether the time traveller altered the past or precipitated the future he remembers, such as a time traveller who goes back in time to persuade an artist — whose single surviving work is famous — to hide the rest of the works to protect them. If, on returning to his time, he finds that these works are now well-known, he knows he has changed the past. On the other hand, he may return to a future exactly as he remembers, except that a week after his return, the works are found. Were they actually destroyed, as he believed when he travelled in time, and has he preserved them? Or was their disappearance occasioned by the artist’s hiding them at his urging, and the skill with which they were hidden, and so the long time to find them, stemmed from his urgency?
Relative timelines resolution
It could be that the universe does not have an absolute timeline that is permanently written after events happen (or, in the deterministic view, at the start of time). Instead, each particle has its own timeline and therefore, humans have their own timeline. This might be considered similar to the theory of relativity, except that it deals with a particle’s history, rather than its velocity.
Physical forces affect physical particles. If your body’s physical particles go back in time, you will be able to kill your grandfather (no physical forces will mystically stop you), and nothing will physically happen to you as a result, because there are no physical forces that can “figure out” what happened and this new timeline develops, because the universe simply has no mechanism for unmaking it. Your younger self does not need to be born in order to fulfill a destiny of going back in time, because there is no written-in-stone absolute timeline that needs to be followed. If you were able to find and observe the younger versions of the particles that make you up, they too would follow physical laws and hence wouldn’t form into a younger version of you (because one of your parents wouldn’t be there to form you).
Some science fiction stories suggest that causing any paradox will cause the destruction of the universe, or at least the parts of space and time affected by the paradox. The plots of such stories tend to revolve around preventing paradoxes.
Some speculations suggest that, under no circumstances whatsoever you would be able to “kill your grandfather”. The only result of the time travel would be your knowledge that you’ve caused some event in the past. For example, if a protagonist kills his grandfather, it would turn out that the victim is not his grandfather at all.
Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible. For example:
Nobody has ever built a time machine that could take a person back to an earlier time. Nobody should be seriously trying to build one, either, because a good argument exists for why the machine can never be built. The argument goes like this. Suppose you did have a time machine right now, and you could step into it and travel back to some earlier time. Your actions in that time might then prevent your grandparents from ever having met one another. This would make you not born, and thus not step into the time machine. So, the claim that there could be a time machine is self-contradictory.
Consideration of the possibility of backwards time travel led to assert that time might itself be a sort of illusion. He seems to have been suggesting something along the lines of the block time view in which time does not really “flow” but is just another dimension like space, with all events at all times being fixed within this 4-dimensional “block”.