Although the theory of time travel has been questioned for several reasons, in this essay I will argue whether the case of the Grandfather Paradox is capable of successfully proving the impossibility of time travel. To better explain the meaning of this theory, an overview is given as to what time travel is in terms of personal and external time, as well as any other terms that may seem unclear. The essay will then focus on providing an understanding of the paradox itself, followed by a few critiques. Finally, the response of David Lewis to the theory will be discussed later. In the last segment, I will therefore argue and provide reasons as to why the Grandfather Paradox is unsuccessful in proving the impossibility of time travel.
In the ‘‘Paradoxes of Time Travel’’, Lewis believed that a possible world in which time travel occurred would have been the strangest world, different from the world we assume to be ours in fundamental ways. (Lewis, 1976:145). In the context of time travel, the difference between the two ‘possible worlds’ can be better understood in terms of personal and external time. However, to better understand these two terms, it is essential to define time travel first. For the sake of clarity, time travel is teleportation from the present to the future or the past; whereas external time is universal, it refers to time as we normally see it. (intext). Given that an individual within our ‘regular world’ was to turn 13 years old in 13 years, this would mean that the time marked on calendars, universal clocks, and on the personal time of individuals would match. (intext). Considering what is meant by personal time, according to Lewis, the concept of personal time, is what takes on a certain role in the sequence of events that constitutes the life of the time traveller. Essentially, personal time is an ‘inner body’ watch, in the sense that it belongs to an individual. Provided that an individual time travels to the future and back to the present after a fixed period, this would result in time dilation, which refers to the slowing down of time per the theory of relativity, resulting in the difference between personal and external time. This is because, when traveling in time, the theory of special relativity by Albert Einstein comes into effect. Assuming a time traveller travels at a fast pace, the amount of time spent in the future or past is lessor than that spent by other individuals in the present. Therefore, the personal time of an individual who time-travelled at the age of 25 would be younger than his or her peers who would have been affected by external time, resulting in them being much older or having died out. Thus, a time traveller travels solely through his or her own personal time, preventing the establishment of a new external time that may affect the lives of people and create a ‘new possible future’.
When referring to the ‘Paradox of Time Travel,’ it is important to understand that time travel is a possibility. This means that the paradox of time travel is an abnormality rather than an impossibility (Lewis, 1976:145). This then raises the question of whether certain actions may or may not be taken. In his ‘paper’, Lewis imagined Tim, a time traveller who hated his grandfather. Thus, Tim travelled back in time to the year 1921 to kill his grandfather. Despite his ability to kill his grandfather, he could not do so hence, Lewis explained that, for Tim to kill his grandfather would mean changing the past. (Lewis, 1976:149). However, Lewis writes that past moment events cannot be divided into smaller, or temporal parts and therefore these events cannot be changed. (Lewis, 1976:149). The Grandfather Paradox, therefore, entails that for Tim’s grandfather to be killed would mean killing him and not killing him in the year 1921. Hypothetically, if we were to talk about the ‘new’ and ‘original’ of 1921, that would mean that Tim’s grandfather was killed in the ‘original’ of 1921. However, it is logically impossible for any individual to be successful in making any changes to the past provided, there is only one timeline. As individuals, we can never re-write the past but instead, a new possible future through our actions. Given that Tim can travel back in time, means that he was never successful in killing his grandfather. Logically, this is because Tim would never have been born or been able to travel back in time to kill his grandfather. In turn, neither of them would exist. Therefore, it can be said that Tim was unsuccessful in changing the past as he merely influenced or affected a past event.
Following what Lewis believed to be true in the ‘’Paradox of Time Travel’’, the theory of time travel was challenged with criticism, but not all criticism was negative. For instance, Paul Horwich argued that time travel enables a person to have an impact on the past but not change it. (Horwich, 1975;435). Other philosophers, however, have raised concerns and have argued that the best way out of an irrational confusion that arises is to conclude that time travel is true and will be impossible for eternity. (Smith,2013). Such concerns can however be argued against in the sense that in the context of the Grandfather Paradox, Tim cannot kill his grandfather as this is something that has not been done hence, in light of eternalism, the existence of Tim’s grandfather is inevitable.
Relative to the term eternalism, this simply means that all events are fixed. Regardless of whether a time-traveller was to travel backward or forwards in time, this event was bound to happen.
By 1976, Lewis provided a solution suggesting that the paradox is not a paradox, but that what is thought to be a paradox is based on the equivocation of the word ‘can.’ (intext). What this means is that the assumption of the contradictions may be considered valid for the reasons given. They are compatible because the word ‘can’ be mistaken for something else, (Lewis, 1976:149), meaning that a person can do something relative to one set of facts, and yet cannot do something relative to another, more inclusive, collection.’ For example, Tim does not but can kill his grandfather versus Tim does not and cannot kill his grandfather ultimately imply that, Tim can fulfil his intentions because he has the capability of doing so and yet he cannot as it is impossible to change the past. The question then arises to ask, how does one have the capability of doing something, but cannot as it may be impossible? Well, to help answer this question, given that Tim does not execute his grandfather, provided there will be a few things in his life that remain true to reality and hinder Tim’s attempted murders. If Tim could kill his grandfather, that would contradict the past so we can therefore conclude that time travellers are constrained by invisible forces that threaten to help stop them from creating such paradoxes. (Horwich, 1975:435).