Time travel is a wide-reaching topic in science fiction, but one recurring area of fascination is the predestination paradox - the idea that an event may only occur because of efforts to prevent it, or as a result of its own consequences. As with most forms of time travel, this confronts one of the strictest restrictions in relativistic physics, the law of causality.

Note: Unfortunately it’s not really possible to talk about paradoxes without giving away endings, so I have to issue spoiler alerts for many of the texts below.

Bootstrapped 

An alternate term for this form of self-fulfilling prophecy, a bootstrap paradox, draws its name originally from the classic fantasy Adventures of Baron Munchausen (who lifted himself by pulling on his own bootstraps) and more immediately from a short story published by science fiction author Robert Heinlein in 1941 - “By His Bootstraps” (as by Anson MacDonald, in Astounding Science Fiction magazine).

[Image: Cover of Astounding magazine for October 1941 (source)]

In this narrative, a young student Bob Wilson is interrupted by two apparent strangers, one of whom forces him into the distant future. There an individual called Diktor uses alien technology sends him back to recruit himself, appearing as the first stranger. Realising Diktor is deceiving him, he then returns again to try to stop the recruitment (becoming the second stranger), and finally travels back to gather cultural and factual material Diktor had asked for. He decides to return with the material to a point ten years earlier in order to usurp the man, himself learning how to use the abandoned time travel device left by another civilisation. As might seem inevitable given this context, he eventually realises that he has himself aged into the role of Diktat and is responsible for setting the entire chain of events into motion and placing himself in a position of authority. Unsurprisingly, Bob himself is confused by this sequence of events:

The physical process he had all straightened out in his mind, but the intellectual process it represented was completely circular. His older self had taught his younger self a language which the older self knew because the younger self, after being taught, grew up to be the older self and was, therefore, capable of teaching. But where had it started?

Unfortunately, but unsurprisingly, the story offers no real solution to this problem:

He felt the intellectual desperation of any honest philosopher. He knew that he had about as much chance of understanding such problems as a collie has of understanding how dog food gets into cans.

Heinlein was evidently fascinated in such paradoxes as they feature in a number of his other fictions (e.g. "All you Zombies" (1958), The Door Into Summer (1956) and in the lengthy Lazarus Long sequence (1941-1987)). Indeed cosmologist and science popularist Carl Sagan cited Heinlein as an influence: his works “​​force the reader into contemplations of the nature of causality and the arrow of time. These are all works you ponder over as the water is running out of the bathtub or as you walk through the woods in an early winter snowfall.” (Sagan, New York Times, 28 May 1978)

Of course Heinlein wasn’t the only writer to explore these ideas, and a good example of a light-hearted take on the potentialities of the subject can be found in Murray Leinster’s 1955 story “Sam, this is you” [1]. In this tale a telephone linesman working on a broken line is disconcerted to receive a call from himself ten days in the future. But not as disconcerted as his fiancee is to learn that Sam has spoken to someone who knew their intimate secrets. The fact that this person is a future incarnation of Sam himself appears less important to her than the image of another person on the end of a telephone line, distinct from the version of Sam she is talking to… with consequences for her willingness to engage in further intimacy!

Interestingly, the causal loop here does not involve the time travel of individuals, but rather of a voice message - or seen in the abstract - of the information contained in electrical currents in a wire.

[Image: An illustration from the Galaxy magazine first appearance of “Sam, this is you” (source: Gutenberg.org)]

This is intriguing as it makes a (possibly unintentional) nod towards the roots of causal paradoxes in physics. Under the relativistic paradigm first developed by Einstein, nothing in the Universe can travel faster than light. Hence for one event to affect another, enough time must pass for light to travel between them. In other words, if you see something begin to fall, you cannot react and begin to reach out to catch it until the photons of light carrying that information reach your eyes. This dependence on information transfer is known as causality - an event which can receive light from another is said to be causally linked to it, and from a mathematical viewpoint, the main objection to time travel is that it violates this causal relationship. 

However one of the theoretical areas which is still researched regarding relativistic theory is whether these causality limits apply to the information being communicated, or merely to the photons carrying that information. Information occupies a privileged position in modern quantum relativity theories. The phenomenon of quantum teleportation suggests that under certain circumstances, pure information itself can travel faster than light [2]. Hence while Sam himself, or the vibrations of his voice, could not violate causality, the slim possibility remains that the information which encoded the electrical signals storing those vibrations might do so. 

Hope for Tomorrow?

While neither of the examples above are without their complications, the effects of time travel in both are largely beneficial for the protagonists. A far less positive view of the impact of causal loops can be found in two feature films: Twelve Monkeys (1995, dir. Gilliam) and The Terminator (1984, dir. Cameron).

In both stories, the future is a bleak post-apocalyptic wasteland, after the ravages of a pandemic and AI-controlled conflict respectively. In both cases, the actions of a time travelling individual attempting to avert an apocalyptic future are instrumental in setting protagonists (in the case of Twelve Monkeys, the time traveller himself) on their life’s journey… and thus in causing the scenario which led to their time travel. As in the two more light hearted examples above, the characters cannot escape the events in which they find themselves embroiled: their fate appears to be predestined. However here, the causal loops are very much not to the characters’ advantage and are used instead to emphasise the remorseless march of Fate, and the indifference of the Universe as a whole to the fate of individuals. In all four cases, no matter how much the characters wish to break the cycle, they find themselves unable to do so, raising the philosophical questions regarding the existence or otherwise of free will. These are questions which have always sat alongside the questions of fundamental physics when time travel is discussed.

Perhaps inevitably, there are also writers who rebel against such impotence and champion the idea that loops can indeed be broken - regardless of the resultant paradoxes. In doing so, it is usually human free will and the hope that comes with it that proves crucial.

An interesting example can be found in the 2015 Disney film Tomorrowland: A World Beyond (dir. Bird). This film (itself inspired by a futurism-themed "land" at the Disneyworld theme park) calls on a curious mix of science fiction tropes in its worldbuilding. It envisages that a group of nineteenth century scientists and creatives including Verne, Tesla and Edison created advanced technologies (a common Steampunk conceit), that these were used to construct a technocratic scientific utopia in a parallel dimension, and that this secret community used the 1964 New York World’s Fair (also seen as a setting in Iron Man 2 (2010, dir. Favreau) and Men in Black (1997, dir. Sonnenfeld) amongst other science fiction) as a recruitment fair. Amongst their inventions are ray guns and advanced artificial intelligences. It is gradually revealed that the scientists had created a tachyon beam monitor for viewing the future and revealed an imminent apocalypse. The paradox appears towards the end when it is revealed that the scientists attempts to broadcast this future in the hope of inspiring action, instead cause apathy and hopelessness and so lead to the disaster.

The clear implications of the film is that it is the actions of the residents of Tomorrowland that may both cause and prevent the expected catastrophe, and that it is the human traits of hope, creativity and perseverance that will be decisive. When looked at in detail, the film actually communicates a curiously mixed message. It is far from clear how the necessary change can be enacted in the time remaining, or indeed what fraction of the coming disasters can be averted as opposed to being natural phenomena. The main female protagonist is both creative and technologically gifted, but the majority of other characters imply that creativity and scientific ability are not normally found together. The film also appears to urge societal change while simultaneously suggesting that societies as a whole show apathy, and only exceptional individuals can make a difference. Nonetheless, the film is notable both for its commentary on the way in which the 1960s optimistic vision of the future has been corrupted, and for its more-than-commonly complex explanation of causality violation.

Here the information about the future is encoded in tachyons, theoretical particles which not only travel faster than light but can never be slowed down to travel more slowly (in relativity it is passing through light speed that presents the insurmountable barrier, travelling well above it is theoretically as straightforward as the low speeds of our familiar environment). In theoretical discourses, tachyons could potentially be causality violating. On the other hand, it is far from clear how they could be interacted with, and it is likely that any attempt to “read” them would result in the causality condition being restored. Thus, while this is a pleasing attempt to appeal to plausible sounding physics, it is still very much in the realm of science fiction.

A Rose By Any Other Name

Setting aside for the moment the question of whether time travel is actually possible, another question arises which is not addressed directly by any of the above, but is confronted by other science fiction: what are the implications for the creative process?

The television series Doctor Who gives an example of the Bootstrap paradox [3] in a rare fourth-wall-breaking monologue to camera: A time traveller sets out to find Ludwig van Beethoven, but finds that the composer appears not to exist in his own time and place. He chooses to copy out and publish Beethoven’s music himself to ensure it is not lost:

“He becomes Beethoven, and history continues, with barely a feather ruffled…. My question is this: who put those notes and phrases together? Who really composed Beethoven’s Fifth?” (The Doctor, “Before the Flood”, 2015).

[Image: The Twelfth Doctor explains the Bootstrap paradox in "Before the Flood"]

Doctor Who phrases this question as neutral, focussing on the nature of the paradox, even while posing the creative question at its heart. Author Douglas Adams confronted this issue more directly in his Hitchhikers’ Guide to the Galaxy series novel Life, The Universe and Everything (1982). Here we encounter the Campaign for Real Time, which was founded after a number of instances of time travel causing cultural loss. In particular we hear of the Cathedral of Chalesm that was rewritten out of existence when redevelopers of its site started shifting their demolition to earlier dates in order to avoid late completion penalties. Another example is provided of a pastoral poet Lallafa from centuries before, who was visited by time travellers who decided his use of correcting fluid would be an advertising coup. Enriched and frequently commuting to the future for interviews, he became so busy and successful that he never actually wrote his own melancholy poems, but instead copied them from a much-later printed copy:

Many people now say that the poems are suddenly worthless. Others argue that they are exactly the same as they always were, so what’s changed? The first people say that that isn’t the point. They aren’t quite certain what the point is, but they are quite sure that that isn’t it. They set up the Campaign for Real Time to try to stop this sort of thing going on. (pg 84, Pan edition, 1982)

In other words, the dilemma is whether the value of art is in its manifestation (in words, visuals etc), in the experience of the audience when encountering it (regardless of background information), or in the emotions, experience and culture it encodes. This is, of course, a very real question in modern society, in which the value of revered art works of the past are being questioned in the light of the culture they represent, and in which conceptual art has often jettisoned the former entirely in favour of the latter. The science fictional bootstrap paradox examples obviously represent an extreme case here, but are interesting in the context of how SF can reflect and give an opportunity to experiment with such societal issues.

Despite ongoing work on the fundamental physics, theory and philosophy of time travel, it is fair to say that it remains well beyond any plausible near-future technological application. Nonetheless, science fiction as always provides a sandpit in which scenarios, examples and their implications can be considered, helping to spur that work and to widen our view of the Universe and its possibilities.