The Paradoxes of Time Travel

In 'The Paradoxes of Time Travel" David Lewis defends the possibility of time travel. Lewis addresses three different paradoxes with time travel and provides solutions for each. First, he discusses the discrepancy between time and time. Second, he proposes the possibility of closed causal loops. Finally, he defines the term ?could' and explains two different meanings of the word to discredit a possible paradox. Lewis's thesis states, 'The paradoxes of time travel are oddities, not impossibilities". I disagree with Lewis's second point regarding closed causal loops. At the end of this paper, I propose a rebuttal to Lewis's claims on this paradox.

At the beginning of his paper, Lewis sets the record straight and explains that he will not explore the ideas that time is circular or that there are particles that move faster than light. Lewis also clarifies early on that he will be analyzing time in one-dimension. The argument Lewis is fundamentally arguing is as follows. Preface 1, time travel gives rise to paradoxes. Conclusion 1, if time travel were possible, then paradoxes would be possible. Preface 2, paradoxes are not possible. Conclusion 2, time travel is not possible. Lewis is arguing that Preface 1 is false. He suggests that time travel does not augment contradictions. He proceeds by contemplating each of the following paradoxes and explains why each is not truly paradoxical.

Lewis first considers the paradoxical discrepancy between time and time. This is referring to the idea that if Tim leaves 2018 and arrives 30 years into the future, or past, but his journey only took him 10 minutes to complete. Lewis's argument is as follows. If time travel is possible, then the time separating Tim's departure from Tim's arrival is 10 minutes. However, if time travel is possible, then the time separating Tim's departure from Tim's arrival is 30 years. Although, the time separating Tim's departure from his arrival cannot be both 10 minutes and 30 years. Therefore, time travel cannot be possible.

Lewis responds to this paradox by differentiating between external time and personal time. If Tim were to carry a wristwatch, said watch would reflect Tim's personal time. External time is real time. The external time in Tim's journey is 30 years. This is the real word time that separates Tim's departure from Tim's arrival. All the while, Tim's personal time registered on his watch is 10 minutes. The difference between external time and personal time is similar to the difference between the distance between two towns by roads and the distance between the two towns as the crow flies. The distance between the two towns by road is 35 miles. However, the distance between the two towns as the crow flies is only 15 miles. This explanation Lewis suggests shows that there is no contradiction between the two. There just needs to be a distinction between the distance along the roads from the distance as the crow flies. This is similar to the case of time travel and the distinction between personal from external time.

This being the case, it can be possible that within your journey along the road it is possible to go both under and over the same overpass. The road can simply loop back around to cross over the same path. This is similar to the possibility that a Tim can be in the year 1973 twice. This could even provide the possibility of Tim talking to himself in 1973. Lewis offers the example of Tim(A) teaching his younger self, Tim(B), how to build the time machine. Tim(A) learned this information from his older time-traveling self, Tim(C), who visited Tim(A) and taught him when he was younger. This information is in a loop and is not available any other way. However, this raises the paradox of where the information originally came from.

The second paradoxical argument is as follows. If time travel is to be possible, then there could be causal loops. In other words, uncaused information, for instance how to build a time machine. However, there could not be causal loops. Therefore, time travel is not possible. In this paradox, Lewis denies the idea that there cannot be causal loops. Lewis suggests that there could be closed causal loops. Lewis explains that all of us have accepted some kind of uncaused thing. Whether it be the recognition of the Big Bang or the existence of God. Lewis concludes that there are unexplained and uncaused causes that we have accepted in our lives. Lewis adds these closed causal loops to the list of uncaused and unexplained oddities in our lives.

Lewis continues by providing a situation where Tim goes back in time to 1921 to kill his own grandfather. It seems that Tim ?could' kill his grandfather if he was in 1921 and had the intention with the proper supplies. However, Tim ?couldn't' kill his grandfather because if he does/had then Tim's father wouldn't have been born and therefore Tim wouldn't have been born. So, Tim couldn't have traveled back in time to kill his grandfather because Tim was born. This is a contradiction because it is not possible for Tim's grandfather to both be killed and not killed.

This argument is formatted as such. If time travel were to be possible, then Tim could kill his own grandfather. However, if time travel were to be possible, then Tim could not kill his own grandfather. Therefore, time travel is not possible. To solve this paradox, Lewis recommends that we take a closer look at the term ?could'. When it is said that a person ?could' do something it is meant that they have the capacity while holding certain contexts fixed.

Lewis reasons that he, unlike chimpanzees, can speak a language. Let us say Finnish for example. So, Lewis can speak Finnish. However, if you were to take Lewis to Helsinki he would not be able to translate the language because Lewis can't speak Finnish. Lewis never learned the language. When it is said that 'Lewis can speak Finnish", I am referring to Lewis's capacity and ability, as a human, to know how to speak a language. When it is said that 'Lewis can't speak Finnish" I am hold holding fixed the condition that his brain is capable of knowing the language he just hasn't learned it. Similarly, when holding fixed the future of Tim existing, Tim can't kill his grandfather. On the contrary, holding fixed only the past in which Tim wasn't born yet, Tim can kill his grandfather. Therefore, there is no paradox. The argument is simply to clarify the different meanings of ?could' proving the paradox invalid.

I fundamentally disagree with Lewis's proposal that there could be closed causal loops. A portion of Lewis's argument he makes he explains that people accept uncaused causes in their life as is and this would just be one more. However, I do not accept something can happen without cause. Therefore, I don't agree with Lewis's point. I believe that our world and universe functions on cause and effect. Now, if the Big Bang is an uncaused cause, I believe that it has set in motion a series of cause and effect and binds the universe to that truth. The idea that there can be more than one uncaused cause is something irrational to me. If that were more than just one uncaused cause in the physical world there would surely be more evidence of them existing. Therefore, there can not be a list of uncaused cause to answer questions we don't yet know.

In 'The Paradoxes of Time Travel" David Lewis defends the possibility of time travel. Lewis addresses three different paradoxes with time travel and provides solutions for each. First, he discusses the discrepancy between time and time. He explains by differentiating the two different forms of time, personal and external. Second, he proposes the possibility of closed causal loops. He justifies this by drawing a parallel to the Big Bang and other uncaused and unexplained metaphysical oddities. Finally, he defines the term ?could' and explains two different meanings of the word to discredit a possible paradox. He states that ?could' is in reference to having the capacity versus the practical ability. Lewis's states, 'The paradoxes of time travel are oddities, not impossibilities".

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