Time travel is a fascinating fantasy idea that has a logical justification in addition to its obviously entertaining function. In particular, such travel is inextricably associated with the endless paradoxes generated whenever the traveler decides to move into the past or the future. This raises legitimate questions about whether the traveler can change the course of events, about the objective measure of time, and what justifies the discrepancy between the two temporal functions.
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These are all questions Lewis sought to answer in his reflections on “The Paradoxes of Time Travel.” More specifically, the author conducted an eclectic analysis of the phenomenon of time travel from the perspective of precise terminology, logical validity, and explanations of apparent paradoxes. The article is full of Lewis’s personal statements and illustrative cases to facilitate understanding such confusing material.
It is paramount to recognize that Lewis’s article is a summarizing and multifaceted work that adequately covers the basic philosophical and logical questions outlined as early as the first paragraphs of the material. Broadly speaking, this includes defining the permissibility of time travel, finding a terminological description of the basic elements, and explaining the geometry of time from the perspective of an outside observer and time traveler. Thus, the reader reading “The Paradoxes of Time Travel” may feel the initial confusion and logical incoherence of the text’s components, but through the scenarios and descriptions given by the author that accompany almost every page, the overall point is reached.
Consequently, Lewis began his work by attempting to explain the spatial geometry of time. Acknowledging the fiction of time travel — but, more importantly, not pointing out the impossibility of it — the researcher makes a cursory comparison of linear, planar, and four-dimensional time. Lewis originally adhered to the concept of a Cartesian model of time, in which the traveler is free to move between axes, whereas the average person lives by a linear function symmetrical about two axes.
However, this idea of two-dimensional time is not supported by logic from the point of view that the same event cannot be dissected into two axes. In other words, the nature of time is inseparable and represents one line rather without division into directions: straight or cyclic. Straight time aligns perfectly with what Lewis proposed, namely the four-dimensional model. Aligning the three spatial coordinates with the time axis allows the traveler to cascade along with one of the axes without splitting events into different versions at different times.
The difference between the stages of travel in time constitutes change. In attempting to formulate a definition of qualitative change, Lewis was particularly careful to refer to illustrations. In particular, a person’s growing up or hair growth determines the passage of time, and in this sense, the traveler can go back to a time when he was still a child without hair, for example, this forms the Cambridge change model (Lewis 146). However, some elements are constant at all times, such as numbers or the laws of physics. In this sense, even when traveling through time, the traveler will find that unchanging elements or events have not been subject to transformation.
A key idea, set at the beginning of the paper and found up to the last paragraph, was the division of the nature of time into personal and external. To better convey the meaning of this division, Lewis gives an example concerning the time it took to travel. For instance, if a traveler moves one thousand years into the past, the journey itself may take him about an hour, but the final destination will have a chronological difference from the point of departure of minus a thousand years.
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Thus, to respond to the seeming inconsistency of time — Lewis, as mentioned above, adheres to the hypothesis of the inseparability of time along the axes — the author introduces the terms “personal” and “external” time. The hour elapsed for the traveler determines his personal account, indicated on his wristwatch, while external time characterizes the difference that objectively exists between the beginning and the end of the journey.
Near the middle of his text, Lewis raises the most intriguing questions concerning time teleportations: the phenomenon of the traveler’s personal identity and the grandfather paradox. There is no doubt that the individual who has returned to the not-too-distant past is in the same reality system as the younger version of him. For example, a twenty-year-old man who has gone back ten years may be in contact with a ten-year-old himself. Lewis urges the reader not to make the mistake of differentiating between the two individuals and especially emphasizes that both the young and adult versions of the individual are the same person, bound together by a mental connection. For outsiders and even the young version of the traveler, all given events occur according to the course of external time, while the return to the past is the man’s personal experience.
On the other hand, such an effect raises the question of the existence of a causal link between the past and the present. In particular, in the case when an adult traveler tells a young one about the device of a time machine, this knowledge as a timeless phenomenon is transferred from the present to the past so that it then turns into the present. Simply put, a young explorer could not have created the time machine without encountering a more adult copy of himself. Lewis emphasizes that similar connections produce casual cycles and loops, which, however, do not explain the origin of the time machine’s knowledge.
Approaching the second half of the article, Lewis discusses the grandfather paradox, which best covers time travel’s causal mechanism. In particular, if a grandson from his own time returns to 1921 to kill his hated grandfather, this obviously raises a number of questions about the permissibility of such a situation (Lewis 148). On the one hand, the grandson’s personal reality does not exclude changes in the past, so there is no contradiction in killing his grandfather.
However, this version is untenable from the point of view that the events in time are interconnected. The murder of the grandfather, according to Lewis, would lead to the impossibility of the birth of the father and then of the grandchild proper in the future. In other words, the grandchild would not have to exist and could not go back in time to kill the grandfather. Consequently, the grandfather would not die, and the grandson would survive. Obviously, such thinking leads to untenable conclusions and logical fallacies, which is why this effect is called the time paradox.
One of the last thoughts mentioned in the paper is the hypothesis of the branching of time when events occur during travel. Although this idea has been mentioned cursorily, it is an important thought that allows considering the phenomenon of time in an alternative way (Lewis 152). Thus, time is not linear and does not represent an axis on a four-dimensional coordinate plane; instead, it can branch. The grandfather paradox perfectly describes this model: in the case of murder, the world continues to develop along one path, but the unkilled grandfather forms a parallel, alternate reality.
Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly, vol. 13, no. 2, 1976, pp. 145-152.