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Let's take the following scenario:

A person finds a time machine. He uses it to travel to the past, and kills his grandparents. Now because of this, his parents are never born, they do not meet, and he himself ceases to exist.

I have heard one predominant result of this murder:

Since he killed his grandparents, he doesn't exist. Therefore, he could not have gone into the past and committed the murders. Therefore, he exists. Therefore, he doesn't exist and so on, ad infinitum.

However, I think that instead of an endless loop, we would end up with an altered timeline:

Since the grandparents are dead, they no longer exist. Anything they might have done to the world in the normal timeline after their deaths doesn't happen. Time moves on, and we come to the present day. There is no boy, no parents, no grandparents. The loop doesn't happen, because the grandparents are already dead. It doesn't matter that the boy isn't there to kill them again.

So which would be a more accurate expectation? Or is there a third option that I'm missing completely?

I am looking for answers from a purely physics perspective.

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Why the downvote? – Raghav Sood Sep 29 '12 at 7:38
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The FAQ says that one should not ask "Questions about fictional physics". Now, as I understand it there is still some debate among the knowledgable theorists about whether closed timelike curves are admitted or not, and some debate about just how that would work, but I am not sure that either this question nor some of the answers are approaching the problem from the perspective of physics. Please take care to stay on topic here. – dmckee♦ Sep 29 '12 at 15:17
@dmckee I'll add that as an edit. Thanks. – Raghav Sood Sep 29 '12 at 15:28
In your second version, who kills the grandparents, or what do you mean they don't exist? – mtrencseni Sep 29 '12 at 17:58
@mtrencseni I mean that since they are already dead, they don't exist in that future. – Raghav Sood Sep 29 '12 at 17:58

closed as off topic by Qmechanic♦, Manishearth♦, Emilio Pisanty, Sklivvz♦ Dec 27 '12 at 16:37

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4 Answers

up vote 2 down vote accepted

which would be a more accurate expectation?

Since this is a Physics Q&A forum, we should approach this question from the perspective of Physics.

Until you have a hypothesis for the mechanism by which matter can be instantaneously moved from one point in time to a prior point in time, you have nothing on which to base predictions.

So far as I know, there is a maximum speed that we can move through spacetime. If I were to move backwards in time, at the very least I would expect to gradually get smaller and younger, to know less and have my memories slowly evaporate.

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up vote 1 down vote

causality is SAVED we can not move 'backwards' any time machine if working will be only producing closed timelike curves from the instant to the future but not to the past.

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up vote 1 down vote

The wikipedia article http://en.wikipedia.org/wiki/Quantum_mechanics_of_time_travel gives Lloyd's prescription with a consistent history, and Deutsch's prescription where going around a CTC can put you in a different parallel universe.

I'm more inclined to favor Lloyd's prescription as it's based upon path integrals.

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up vote 0 down vote

I guess essentially the question is unanswerable. Neither possibility is more "accurate" than the other because the entire scenario depends on travel back in time and this creates paradoxes that violate causality.

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