Fiction involving time travel first became popular during the 19th Century. For example, in A Christmas Carol by Charles Dickens (1843), ghosts take Ebenezer Scrooge on return trips to his past and probable future.
A Connecticut Yankee In King Arthur’s Court was a fantasy novel by Mark Twain (1889) in which his protagonist makes an unintentional return trip to the distant past.
Then H.G. Wells (1895) wrote The Time Machine, a science fiction novel in which a time traveller makes a return trip to the future in a time machine he built using the premise that time is the fourth dimension.
Time travel at the movies
By 1960, several film adaptations of these novels had been released, which helped to bring the concepts surrounding time travel into the consciousness of the movie-going public.
Over the years, the popularity of this genre increased; the number of films released involving time travel or other temporal phenomena continued to grow until the end of the century.
All of these films use some implicit model of time travel: most have rules about how time works in the past and/or in the future, and they all involve a new ‘timeline’ that, for example, can replace the original timeline or exist in parallel to it.
These different models of time travel are used to convey to the audience how time is used in the plot. But they had not to my knowledge been formulated and recorded in any literature when I began my research.
Time travel is just one kind of temporal phenomenon; others include rewinding time, stopping time, fast forwarding time and splitting a timeline into two or more parallel versions.
For the purpose of brevity, from this point onwards, when I mention films involving time travel, I am referring to films that involve one or more of these types of temporal phenomena.
The physics of time travel
Theoretical physicists have proved that the fundamental laws of physics do not prohibit time travel. Nahin (1993, p. ix) writes, “Time travel was once solely the province of science-fiction writers [but] how times have changed! One now finds scholarly analyses of time travel in serious scientific journals written by eminent theoretical physicists.”
So, although time travel was once confined to fantasy and science fiction, it is “now simply an engineering problem” (Kaku, 2003).
The problem is that physicists do not all agree about what would happen if a person, an object, or some information were to travel back through time. Philosophers as well as physicists disagree on whether or not changes to the past would be possible.
Chaos theory is the area of scientific research that includes the ‘Butterfly Effect’, which “alludes to the idea that the flutter of a butterfly’s wings in Brazil may set off a chain of events that, over time, leads to a tornado in Texas” (Riley, 2006).
So, the smallest of changes to a past event could cause a completely different future to unfold (as in A Sound of Thunder, the famous short story by Ray Bradbury (1952), which became a film of the same title.[1])
The paradoxes of time travel
Many people do not agree that a timeline could be replaced, as this might cause temporal paradoxes. Philosopher David Lewis (1976, p. 149) in his landmark paper The Paradoxes of Time Travel concludes, “We might expect that when a time traveler visits the past there will be reversals of causation…. Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could.”
Time travel “continues to produce a fruitful cross-fertilization of ideas between scientists and philosophers as theorists in both fields struggle to resolve confounding paradoxes that emerge when time travel is pondered seriously” (Joel Hunter, 2004).
For example, the grandfather paradox occurs when a time traveller goes back and changes the past to the extent that they will never be born. One solution to this paradox is that if a time traveller’s presence caused such a level of interference with events, the timeline would split into two. An alternate timeline would then exist in a parallel universe.
Deutsch (quoted by Highfield, 2007) says that “Many sci-fi authors suggested time travel paradoxes would be solved by parallel universes, but in my work that conclusion is deduced from quantum theory itself.”
Nevertheless, not all physicists agree that our universe can have parallel worlds, as many prefer Novikov’s Principle of Self-consistency, which is a “conjecture on how nature may enable the laws of physics to accommodate time travel” (Nahin, 1993, p. ix).[1]
Causal loops
Causal loops can only occur in a universe where closed time-like curves (CTCs) exist, and these are consistent with the general theory of relativity.[2] In a causal loop, information that comes back from the future can cause changes that end up causing the predicted event to take place. This is known as a self-fulfilling prophecy, and a famous example is the ancient Greek legend of Oedipus.[3]
A predestination paradox can also take place within a causal loop. This is when changes that a time traveller makes to the past end up creating the present they came from.
The time traveller must go back in time in order for the self-consistency of his timeline to be maintained.
A causal loop implies that all past and future events are fixed in time and that although the future is unknown, it is pre-determined.
However, not everyone is entirely convinced about causal loops: Mellor (1998, p. 131) argues against the possibility of time travel by arguing against the possibility of causal loops; whereas Hanley (2004, p. 123) disagrees by arguing that “they are neither logically nor physically impossible.” Lewis (1976, p. 74) is uncommitted, “Perhaps there must be loops if there is reversal; I am not sure.”
There is also the possibility of mental time travel. In the World Encyclopedia of Time, Anderson (2010) asks if it is possible “that the human mind could master time? Or is it possible that the human mind itself is a time machine?”
In a review of recent findings, Grondin (2010, pp. 573-574) states, “Psychological time is a very elusive object of study [and] the past 25 years have seen a multiplication of theoretical propositions, including those embedded within a neuroscience perspective, to account for the ability to process temporal information.”
The nature of time
There is an ongoing debate in philosophy about the nature of time, which came to the fore when McTaggart (1908) proposed that all events could be ordered in time using either one of his two ‘series of time’. The debate has continued ever since, and most (but not all) commentators now side with McTaggart’s B-series of time, but dispute his conclusion that time must be unreal.[4]
There is also an ongoing debate in philosophy about time travel, and Richmond (2003, p. 297) notes:
For some years, the topic of time travel has prompted interesting philosophical discussions. However, despite decades’ worth of defences and rebuttals, there isn’t yet consensus as to whether or not time travel is possible, or, if possible, of what kind of possibility.
Some say that logic forbids time travel, while others maintain that metaphysical necessities or physical contingencies prevent it. Yet others hold that, for all we know, time travel might not only be possible but actually attained.
Zeh (2001, pp. 4-5) notes, “classes of phenomena characterizing a direction in time have been called arrows of time.” The most important arrows of time, he says, are: radiation, thermodynamics, evolution, quantum mechanical measurement, exponential decay, and gravity.
The directions of time
Scientists working in different fields have to use different models of time from each other. For example, in the area of relativistic physics, at the fundamental level, there is timelessness because time is wrapped up in the space-time continuum. In the Newtonian world and even the quantum world, time is a physical parameter that is used to track change.
The equations used to describe these worlds work equally well in reverse; there is nothing about them that says they can only work in one direction. However, when we apply these equations to our macroscopic world, time has only one direction – we live in a world of increasing entropy.[5] [6]
Physicists and philosophers have long postulated reasons for why time appears to be asymmetric, and therefore only able to have one direction when “the laws of science do not distinguish between the forward and backward directions of time” (Hawking, 2008, pp. 392-393). The problem is explained by Zeh (2001, p. 1) in his book, The Physical Basis of the Direction of Time.
If physics is to justify the hypothesis that its laws control everything that happens in nature, it should be able to explain (or consistently describe) this fundamental asymmetry, which defines what may be called a direction in time or even …a direction of time.
Surprisingly, the very laws of nature are in pronounced contrast to this fundamental asymmetry: they are essentially symmetric under time reversal. It is this discrepancy that defines the enigma of the direction of time.
Philosopher Price (1996, p. 16) argues that if we had an atemporal viewpoint taken from outside of time, it would reveal the true symmetrical nature of time as implied by the block universe.[7] In other words, there only appears to be an arrow of time to observers because they exist in the dimension of time.
Misconceptions about time
There is a difference between the alternate conceptions mentioned thus far and misconceptions in science, which for the public, further complicate the picture. For example, there are still those that refuse to accept that forwards time travel is possible even though Einstein’s 1905 theory of special relativity shows that the faster a vehicle moves, the more slowly it ages.[8]
From around the start of the last century, scientific discussions about forwards time travel with rockets were able to take place. Many scientists of the day were shocked that their cherished Newtonian time had been proven to be inaccurate, and it took a long time to convince them that this was so. Even though this theory has since been proven experimentally, there are still those who find it hard to believe, as Nahin (1993, p. 26) points out:
Some modern philosophers … are still not quite sure about this particular way of traveling into the future and have confused it with simply being asleep or being frozen…. The Victorian shock over the relativity of time has obviously not yet totally disappeared…. For example, one philosopher – Mellor (1981) goes on to declare … All in all, real forward time travel is … really only an overly grand description of processes slowing down or stopping.
Pickover (1999b) adds, “For centuries, these questions have intrigued mystics, philosophers, and scientists, and the subject of time has been central to the world’s many diverse religions and cultures.” In his book (1999a, p. xvii), he writes, “Is time real? Does it flow in one direction only? Does it have a beginning or an end? What is eternity? None of these questions can be answered to scientists’ satisfaction.”
Models of time
As shown, neither physicists, philosophers, nor psychologists can agree on the nature of time. These unresolved issues are then magnified by the introduction of the topic of time travel, where there is much uncertainty about whether time travel is possible and what would happen if it were.
With no universally accepted understanding of what constitutes time, various models of time currently exist, reflected in people’s personal perceptions and their philosophical position. It may not be possible to discover which of these models is a true representation of time even if the technology of time travel has been developed, or until we have a full theory of quantum gravity.[9]
Therefore, if scientists and philosophers cannot agree on a universal model of time, then how can filmmakers decide which models of time to use in their films? And if the filmmakers do not know which models of time are accurate, then what chance do the movie-going public have of separating fact, speculation and fiction?
Time travel in the public eye
The topic of time travel has been placed in the public domain by the high number of television shows, plays, novels, computer games, and particularly films in the two decades following the mid-80s when The Terminator (1984) and Back to the Future (1985) were released.[10]
However, the unresolved issues and controversies in science are not in the public domain, so what the public accesses is perceived as science fiction, loosely based on these issues.
Films involving time travel are also important because new science can come from science fiction as explained by Nahin (1993, p. ix).
Smart physicists seek insights everywhere, including from clever science fiction writers, who long ago began probing seriously the logical consequences that would ensue if the laws of physics permitted time travel. For example, Igor Novikov’s Principle of Self Consistency has its roots in science fiction.
When Carl Sagan was writing his novel Contact, he asked his friend, physicist Kip Thorne, how he could transport a person to a distant star and back with virtually no time having passed on Earth. By considering this question, “Thorne revitalized the whole modern field of the study of wormholes” (Palmer, 1997).
In the medium of film, the answers to many of the unanswered questions about time travel frame the underlying hypothesis around which the narrative is built. An analysis of the narrative therefore provides an opportunity for deducing the model of time travel employed in each film, which then enables a comparison of the models with conventional scientific ideas and with the public perception of time travel.
[1] These theories and others are discussed in more detail in the section, ‘The Paradoxes of Backwards Time Travel’ which begins on page 62 of Chapter Three.
[2] Most (but not all) cosmological models are globally hyperbolic and cannot accommodate CTCs.
[3] Oedipus unwittingly fulfils a prophecy that foretells he will kill his father and marry his mother.
[4] The A and B theories of time are discussed more fully on page 42 of Chapter Three.
[5] Entropy is the quantitative measure of disorder in a system. This hugely controversial topic is discussed more fully in the section ‘The arrow of time’ on page 50 of Chapter Three.
[6] There is also the case of the exception of CP symmetry violation, and therefore by the CPT theorem, time reversal asymmetry in the weak interaction, (where C is charge, P is parity and T is time).
[7] The block universe is discussed more fully in the section, ‘The block universe theory of time’ on page 44 of Chapter Three.
[8] This is known as ‘time dilation’ and how this may be interpreted as forwards time travel is discussed more fully in the section, ‘Time dilation and relativity’ on page 53 of Chapter Three.
[9] Quantum gravity is discussed more fully in the section ‘A universe without time’ on page 51 of Chapter Three.
[10] The rising trend in the number of time travel films released is discussed in the ‘Results and Analysis’ section on page 91 of Chapter Four.