David Lewis argues that even though time travel is possible, you cannot go back in time and kill your grandfather before his original date of death. Lewis gives an example about a boy who is called Tim. In Lewis’ article “The Paradoxes of Time Travel”, Tim goes back in time with his time machine and he wants to kill his grandfather. In 1921, all conditions are available to kill grandfather. However, Lewis argues that Tim cannot kill his grandfather because we know that grandfather lived and got children and grandchild, he earned money which paid Tim’s time machine expenses later, then grandfather died in 1957. So to speak, grandfather lived in 1921. According to Lewis, if we argue that Tim can kill grandfather in “new 1921” which is in Tim’s extended personal time, then in the external time there would be one “original 1921” which is same with “new 1921”, and Tim both kills and not kill grandfather in 1921. Since this is impossible to do, Tim cannot kill grandfather and change the past. David Lewis uses the terms external and personal time, while he is claiming his arguments in the article. We can say that for everyday people there is an external time which is compatible with their personal time, on the other hand for time traveller his/her personal time is not compatible with external time. Even though, for Tim it seems possible to kill his grandfather in his personal time when he comes back in 1921, this situation is not compatible with external time which has one and only 1921 that grandfather lived.[1]
Based on Tim’s example, David Lewis concludes that we cannot change the past and also the future. Since the facts in the past include that “ Tim did not kill his grandfather in 1921”, they do not include this fact at the same time: “ Tim went back and killed his grandfather in 1921”. I think, Lewis’ arguments are indeed convincing in his model in view of external and personal time, otherwise they would be logically contradictory when we claim Tim can kill the grandfather who is the father of Tim’s parent. I would want to add an example to this claim of unchangeable past and future. Let’s suppose, I (call B) and my friend (call C) have a time machine, hence we can go back and forth through time. First I decide to go to 27 May 2021 which is my birthday and I am pretty sure that future B won’t be at home and she will celebrate her 30th birthday outside. I assume that future B still lives in the same home as I live now and she will hopefully have more stuff in the house compared to my current furnitures. I travel to 27 May 2021 at 10pm and as I presupposed 30th years old B is not in the home and before she comes back from her birthday party, I collect all money and sell furnitures, electronic devices, then I return back to now with a lot of money. Then my friend C travels to 27 May 2021 at 8pm and C finds future B’s home fully furnished. C collects all money in the house and sells furnitures, electronic devices, then C returns back to now with a lot of money. For the sake of the part I want to focus, I skip the possibly problematic but irrelevant parts for now such as being able to sell all stuff in two hours, to carry money by time travel, to be identical or mentally in continuity with me and future B. Still, this scenario seems to have a problem, though. How can we sell the same stuff twice? In my view, even this is not as problematic as Lewis’ grandfather paradox, the logic could be the same. Like as Tim cannot kill his grandfather in 1921 because grandfather died in 1957, I cannot sell the stuff in the house when I will be in 27 May 2021 at 10pm because in our scenario C sells the stuff in 27 May 2021 at 8pm and I would have found an empty house. If there is any need for further explanation, I can say that in keeping with my imaginary 27 May 2021 day scenario, this is how things are going on: 27 May 2021 future B leaves home before 8pm, at 8pm C comes and acts in accordance with scenario explained above, at 10pm B comes and finds an empty house, 28 May 2021 after midnight future B comes and finds an empty house unfortunately. This probably would not be the best gift for future B from present B in her birthday, anyways on the basis of Lewis’ argumentation I can claim that in future at 10pm I have to find the house as it has been already robbed by C. This is why I am convinced by David Lewis’ argument and I agree with him about his claim: “ Not that past moments are special; no more can anyone change the present or the future.”[2]
As I mentioned before, Lewis’ argument works for his model, but we can have other time models which can contradict with Lewis’ argumentation. Branching time model could support the idea of time travel but still it is debatable whether travelling through branches and changing events in different branches can be counted as changing past or future? I cannot say I totally object the claim that creating a new branch in a history is not a change, unlike David Lewis. On the other hand, I am definitely sure that presentism claims that there cannot be anything as time travel, since there is no time except present time. Hence, it is impossible to travel through time because there is no past or future.[3]
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[1]Lewis (1976, p. 149, 150).
[2]Ibid, p. 150.
[3]Markosian (2004, p. 49).
REFERENCES
Lewis, D. (1976). The Paradoxes of Time Travel. North American Philosophical Publications.
Markosian, N. (2004). A Defense of Presentism. Oxford: Oxford University Press.