Grandfather Paradox

Posted: February 1, 2011 in Philosophy
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            One of the most damaging rational arguments against the possibility of time travel, in particular time travel with backwards causation[1], is the “grandfather paradox.” To answer this, some have claimed that the paradox is just “telling inconsistent stories.” Herein, I shall briefly describe both the “grandfather paradox” and the “telling inconsistent stories” answer to the paradox. I will then give a critique of the “telling inconsistent stories” answer that claims that it restricts free will, reply to that criticism, and then give an assessment of the dispute.

            The “grandfather paradox” is a paradox concerning time travel where a time traveler, let us say myself, goes back in time, say to 1930, and I kill my grandfather before my mother is conceived. The paradox is this: if I kill my grandfather, then my mother never was born. If my mother was never born, then I would not exist. If I never exist, then I never go back in time to kill my grandfather. Therefore, my grandfather lives, and then I am born and can go back in time to kill my grandfather. This posits that in 1930 my grandfather both lives and dies.[2]

            An answer to this paradox is that it is simply telling inconsistent stories. What this means is that the paradox is attempting to invoke two sets of contradictory facts about 1930, namely one where my grandfather lives, and one where my I kill my grandfather. According to this view, a time-line, by definition, has a single history. It is necessarily self-consistent. In other words, there can only be one set of facts about 1930; the one where my grandfather lives and continues to live until the point where actually does/did die. Thus, to claim that my grandfather is both alive and dead in 1930 is pure nonsense, because in this time-line my grandfather did live through 1930 and long enough to conceive my mother.

            Those who advocate the grandfather paradox want to appeal to, intentionally or not, that there is one 1930 where my grandfather lives, and then there is a second 1930 when I go back in time to kill my grandfather. However, since time-lines must be self-consistent, when 1930 happened in the first and only instance, I was there and did not kill my grandfather. This says that I can affect the past, but not change it. This means that when I go to the past I cannot change anything, because my presence was always there. My being there, and any actions that I have done, has already been taken into account about the facts of 1930. This idea was brought up in the movie 12 Monkeys when Madeline Stowe’s character asks Bruce Willis’ why he does not try to stop the virus from being released, as opposed to just getting a sample of the non-mutated virus to create a vaccine from. She seemed nonplussed by his response of, “I can’t stop it. It’s already happened.”  

            The idea that I must always be in that version of 1930 is highlighted as well in, 12 Monkeys. In Bruce Willis’ character’s childhood memories at the airport on the day that the virus was released included the ado caused by his adult self trying to track down the virus. His memories included those of him as an adult. Again, the voicemail clues he followed to track down the virus were the very same clues he was leaving. This as well shows that one can affect the past, but not change it. 

            An argument often used to counter the “telling inconsistent stories” argument is that it constricts one’s range of actions and poses a limitation on free will.[3] Proponents of this view claim that if time travel is possible, then, if free will is to be maintained, then it must be possible that I can kill my grandfather. To say that I cannot kill my grandfather is to say that I do not actually have free will, as killing my grandfather is excluded from my possible actions, thereby saying that I cannot kill him, if not completely eliminating free will, severely limits it. It claims that accepting the premise of the “telling inconsistent stories” argument is a concession to, at the very least, a kind of limited determinism. If I cannot kill my grandfather, if Bruce Willis cannot stop the virus from being released, then we lack true independent agency to act as we wish. This argument has a certain intuitive appeal that seems to undermine the idea of free will.

            If we accept that a time-line must be self-consistent and necessarily has a self-similar history, then this argument is guilty of a variation on the fallacy of misplaced probability. The chance of me not killing my grandfather in 1930 is 1. It does not undermine free will that I cannot kill my grandfather in 1930, because when 1930 happened I was there and did not kill him. It is not so much that I cannot kill my grandfather, as it is that I did not kill my grandfather. I very well may have gone back to 1930 with every intention of killing my grandfather, but for whatever reasons, I chose not to, or was in some other way prevented from killing him. Foreknowledge of my actions, in this instance not killing my grandfather in 1930 from the perspective of someone in 2008, does not undermine free will. It just so happens to be the case that I did not kill my grandfather. It cannot happen any other way because it did not happen any other way; not that it did not happen any other way because it could not happen any other way.

            It is an underlying assumption, in both philosophy and science, that the universe, however conceived, is a rationally ordered thing; it follows certain rationally definable rules and laws. If this assumption is true, then, either instances of logical paradoxes are just impossible to produce in reality, or our underlying assumptions of the universe must be revised to reconcile with the paradox. One could reconcile our view of the universe with a “multiple worlds” postulation about the universe. This view however, side steps the question of backwards causation though, by claiming that the world/time-line affected was not the one in which I was born. That time-line remains unaffected, and another one where I kill my grandfather exists. However, if we would like to maintain the idea of backwards causation in time travel along a single time-line, then this consideration must be put aside.

            We now return back to our basic assumption of the world/universe being a rationally ordered thing. If this holds true, then it can be said that it is possible only to do that which is logically possible, and that logical paradoxes of this kind are just impossible to produce in reality. The “telling inconsistent stories” reply is consistent with only being able to do what is logically possible, and to a degree that is what its essential claim is. To attempt to do/say something that is a logical impossibility is just to attempt/do something that is just inconsistent with the way universe works. Without drastically redefining how the universe works, it reduces the paradox to nothing more than a fun, provocative thought experiment, and fodder for writers of science fiction.    

[1] Backwards causation states that effects can precede their causes.

[2] There is a secondary postulation here that my existence is not dependent upon who my biological parents are, that I exist independently from my particular biological parents.  However, this postulation goes beyond the scope of this essay and will not be addressed here.

[3] Many still hold this view even though it has been established that one can recreate a similar paradox independent of human action, such as the radio message paradox used in class.

  1. tricksmeme says:

    Though I think backward time travel very unlikely, it is still fun to think about. 🙂

    I would suggest that a timeline split would prevent the paradox. Time traveler travels back in time and kills grandfather. From the point of change (which I would suggest really is when the time traveler travels back, not when he kills his father), a split happens, and a new line is created. The time traveler does not disappear as he is from the first timeline when the grandfather does exist, which loops back to the point on the line of killing the grandfather. For the time traveler, killing the grandfather happens after the time traveler is born, etc. Now we have two playouts of time, one in which the grandfather lives, the other in which the grandfather dies, both interconnected by the single point of the split (think of a Y). Or, if we do not accept a split, it would be one line that changes at the point of change, in which case the time traveler would disappear when he kills the grandfather and the rest of the line is replaced with the new set of causality. Or, like 12 monkeys, the time traveler is already part of the causal chain, so he could not kill the grandfather. But if there was ever a ‘first’ line, that particular line could not have consisted of the time traveler as of yet, and that still presumes either a split or a replacement timeline happened at some point.

    Just food for thought.


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