In The Terminator you cannot travel through time with your clothes; the motivation for this rule is clearly aesthetic and cinematic rather than scientific.
When it comes to plots involving time travel, the general rule is a plain-stated one: do not ask questions. The writer expects, perhaps unfairly, that the audience approach any plot that meddles with chronology as ordinarily experienced by humans with a more-than-usually sturdy sense of the suspension of disbelief. Time travel can be encountered on its own terms, being the focus of a story, a novel, a film, or even a series; or, it can be encountered as an occasional feature in a science fiction series, something that creates a plot that is resolved within the ensuing forty-five minutes of television. Time travel is also a favourite theme for two-parter episodes, however, given the scope it can afford show writers; Star Trek two-parters that deal with time travel occur at least once per series excepting The Original Series (which had no two-parters): The Next Generation had ‘Time’s Arrow’ and even its final episode (‘All Good Things…’), Deep Space Nine had ‘Past Tense’, Voyager had ‘Future’s End’ and ‘Year of Hell’, and Enterprise had whole ‘seasons’ devoted to story arcs caused by a ridiculously conceived and poorly implemented ‘Temporal Cold War’ (don’t ask). This does not count the movies, of which 4 feature time travel as a primary plot device (ST:IV, Generations, First Contact, and JJ Abrams 2009 film), nor single-hour episodes utilizing this writer’s trope. All in all, time travel appears in fifty-two ST episodes and films!
Since our protagonists are already regularly in the habit of transiting the vastness of space by various improbable means, the writers tend not to be worried about abusing physics further; indeed, even when time travel is the focus of a feature, questions are not invited. Each incarnation of time travel applies new rules designed to provide precisely the level of flexibility and dramatic tension that the plot requires, or sometimes that the creators simply thought would be ‘cool’. The best fictional forays into this theme do so self-consciously, testing the rules that govern a universe in which time travel is possible; a common remark made is that if time travel were eventually to become possible, then we ought to be surprised by the absence of an inundation of time travellers in our present and in our history, especially at key events. At the very least, we ought to be surprised by these time travellers’ apparent commitment to secrecy whilst amongst us and their flawless competence in maintaining it. Most examples infer great difficulty and cost associated with time travel, or more commonly implement a system of state regulation where time travel is illegal or restricted. HG Wells surely picks the former, since the inventor in The Time Traveller does not encounter other temporal sojourners in any of the pasts or futures he visits, implying his relative singularity in his achievement. HG Wells also does not have to deal with any of the effects of relativity, and does not treat time as spacetime but as a separate entity, as evident by the way a traveller can ‘see’ the passage of the days (albeit as a blur) as he travels in an accelerated fashion without ever moving.
Before employing further example, it’s worth pointing out that it is difficult to discuss time travel problems and the merit or drawbacks of a particular plot without also spoiling it, so elect not to read if you are precious about having plots ruined.
Disclaimer thus given, I would immediately like to spoil the plot of Robert A. Heinstein’s clever short story “-All You Zombies-”, which takes advantage of the potential for paradox, predestination, and questions of determinism inherent in time travel. A young man, speaking to a bartender, has the nom de plume ‘Unmarried Mother’ and keeps a column in confession magazines; he reveals that he began his life a female, grew up in an orphanage, became impregnated by a lover who then abandoned her, and consequently discovered she had an intersex condition. Complications from birth resulted in her undertaking an operation that resulted in her becoming nominatively male, and his baby was abducted, never to be recovered. The bartender offers him the opportunity to travel back in time to when she was seduced and the emotional trauma of having this child began.
Unmarried Mother takes the opportunity, goes back to 1963, falls for a young woman and impregnates her; meanwhile the bartender leaves Unmarried Mother to travel forward 11 months, kidnaps the child and takes it back to an orphanage in 1945. The bartender returns to 1963 to collect the Unmarried Mother, saying, ‘Now you know who he is—and after you think it over you’ll know who you are . . . and if you think hard enough, you’ll figure out who the baby is . . . and who I am.’ This presents a thoroughly enjoyable exercise in causality. Considering the biology first of all, it is certainly possible to create a sexually produced clone from one set of biological material; the generation of gametes discards half of the host’s genetic material, so it is possible that a gamete could encounter another with precisely the genetic material that is missing from the first. I am assured by those more expert than I that this sort of likelihood is on the remotest end of a spectrum of probability, but of course the fact that the child must be born in order for it to travel back and experience the sexual encounter means that the impregnation is 100% likely to create a sexual clone whilst at the same time being a thoroughly improbable occurrence.
This is arguably time travel at its most thought-provoking, if also at its most outrageous. 2012’s film Looper posits a potentially fun and altogether more accessible use for time travel; it is explained within the first 5 minutes of film time that, in the future, time travel will exist, but will be illegal, and furthermore the disposal of bodies will become almost impossible thanks to advanced forensics, so organized crime syndicates will carry out hits by kidnapping targets and sending them to the past (a past which is nonetheless in the future relative to the audience) to be killed by a network of assassins known as ‘loopers’. Mostly fine, at this point, relative to credibility: if time travel were possible, this is entirely the kind of thing that would seem plausible once you had made that initial leap in accepting time travel’s reality. It also exhibits a rather chaotic notion of consequentiality: whereas inevitability is vital to the plot in Heinstein, it is made plain that in Looper the past is alterable, thus creating the dramatic tension necessary to drive the plot – inevitability in this film would perhaps render it a tragedy, in a completely technical sense, with the ultimate downfall of the characters being the result of their attempts to prevent that very downfall.
What happens, then, if, when visiting the past, you effect your younger self’s death? This is an example of the ‘Grandfather Paradox’, a phrase that ought to induce groans of pronounced lethargy from audiences these days, so often is it used to explain ‘how’ time travel works and yet with very little explanation of how exactly it produces a genuine paradox. In Looper physical injury to the younger self impacts the elder time traveller, but apparently does not interfere with memory (odd, if memory and consciousness derive from physical tissue, chemicals, and energy as we assume they do), since a time traveller is surprised when his fingers disappear. More unlikely still is what happens when a time traveller kills a younger self (be quiet, I warned you to stop reading if you were worried about plots), id est, he simply vanishes, yet his impact on the world seems to remain. The medium of film and the diachronic (although helped at times by anachronic ‘flashforwards’, which are flashbacks relative to the traveller) nature of the narrative means that all that the protagonist does did happen, and yet he simultaneously, by the end of the film, did not do it. Like I said, rule number one is not to ask questions.
I was gifted the audiobook of John Scalzi’s sci-fi spoof Redshirts last year, in which the infinite flexibility of time travel is lampooned. Having travelled back in time, the protagonists only have a certain number of days before they cease to exist due to the fact that their atoms are forced to ‘choose’ where to exist; the protagonists draw attention to the bizarre and nonsensical nature of this rule about time travel by asking why this should be so, remarking that all their knowledge of physics gave them no reason to think that this would be the case, with the obvious conclusion that the number of days they had was precisely the amount of time the plot needed to resolve itself in the way the writer wants and add the dramatic tension of their objectives being time-sensitive. The book itself, I felt, suffered in some stylistic ways that seemed more painful for being read to me aloud (the repetition of the word ‘said’ became as the throbbing of a distant headache, and the sarcastic tone came off as affected purely because of overuse – like an Aaron Sorkin script, the jokes and ironic remarks were too practised); yet nonetheless it was a pleasure to encounter parodies of the non-science and convenient demands placed upon the characters by these plots. The observations made were clearly originating in fondness for a genre, and likewise here, but there is an accompanying element of lament that more sophisticated things cannot be done.
Time travel risks cliché more than perhaps any trope in sci-fi. Few people recall the series Quantum Leap (probably a good thing), in which each episode involves visiting an historically or culturally significant period, place, or event in the past. This series demonstrated of one of the main reasons sci-fi gets re-used so much: perspective. Just like travel writing, its true value was not to be found in factuality but in polemic. Montesquieu’s Lettres persanes afforded the writer the disclaimer that he was merely reporting how people from other lands saw Europe, all the while levelling a cultural and political criticism at an establishment prepared to mete out severe responses to criticism. Likewise, time travel episodes of series that deal with humanity in a time period before the show’s enlightened future involve almost without exception a collection of remarks about the barbarous nature of the time period, and this provides much opportunity for comic effect, with eccentric fashion and music often making an appearance. The importance of money in our present constantly bemuses future travellers; by contrast, in an episode of Star Trek: Deep Space Nine that playfully suggests some characters from that series were the culprits of the rumours of aliens crash-landing at Roswell, the financially motivated Ferengi fall in love with the consumer culture of 1950s America.
Time travel is clearly highly adaptable as a plot element. When HG Wells used it, it represented the sense of wonder and trepidation towards the future that technological advancement created, and continues to create; in Star Trek it was often little more than convenient for a 45 or 90 minute plot. In the Terminator series it was simply ‘cool’, and the degree to which it engaged with a warning about nuclear holocaust and the removal of human decision in war was foiled significantly by the protagonists’ indulgence in high performance weaponry. In Dr Who it is treated with reverence and enough wanton playfulness that we hold it to no accountability. In all cases that I’ve considered it is difficult to obtain genuine gravity, genuine peril; the elegance of time travel as a plot device is also its essential drawback: if the past can be changed, then really very little is of lasting consequence. Most incarnations tend to struggle to produce genuine do-or-die situations, since it is perennially transparent that the constraints put upon our heroes are not constraints demanded by a sense of realism or even aesthetic constraints, but are demands required by the plot.
In this way, time travel risks being pornographic in its implementation when it is at its laziest. Because of this, I enjoyed Looper for its relative freshness, its change of setting compared to many other examples of time travel, such that I was able to respect this despite uncompelling character motives and shallow romances that mar the finished product. For a truly class act I cannot help but feel that we have to return to the written word, however, since few filmed scripts have matched the imagination of Heinstein and Philip K. Dick (whose A Little Something for Us Tempunauts (1975) deals directly with time travellers, and whose Martian Time-Slip reflects on the relationship between consciousness, thought, mental health, and the progress of time in a more philosophical way). Written word has an ability to accommodate time travel’s lack of intuitive sense that the rigidity of a film reel struggles to recreate because of its linear presentation; for that very reason, I welcome attempts that challenge this notion of mine and hoped to be proved wrong.