How the prospect of visiting the past or future captivated the imagination of writers and scientists
The tradition of time travel and time travel paradox stories dates back centuries. In the 19th century, classics such as Rip van Winkle by Washington Irving, A Connecticut Yankee in King Arthur’s Court by Mark Twain, Looking Backward by Edward Bellamy, and so forth, delighted readers. They could be considered examples of novels involving time displacement, via either sleep or some kind of disruption.
However, it was Wells’ The Time Machine, published in 1895, that embodied the first description of a controlled voyage through time using technology. The protagonist in that novella built a device that enabled him to travel into the far future, witnessing the degradation of the human race into two new species, the Eloi and Morlocks.
Because it described time travel solely to the future and back, The Time Machine did not involve any paradoxes. The protagonist sets out into the future, stays for a brief interval, then returns to the present. Although his presence certainly alters the events of the world to come, such a change violates no laws of logic or physics since the order of cause and effect is not interfered with in the story.
A time travel paradox, involving an irresolvable disparity between two disparate versions of reality, would need to include backward time travel before the era of the time traveller’s origin. This could lead to twisted time lines and the possibility of changing history through the removal of a cause before its effect. Such a situation would prove paradoxical if it leads to an irreconcilable contradiction.
Starting in the pulp era of science fiction in the 1930s, speculations abound about time travel leading to an alternate reality. A classic story about tampering with history is Ray Bradbury’s ‘A Sound of Thunder’. Through a minor detour during a time traveling expedition, the historical chain of events is tampered with in a disastrous manner. The death of a butterfly, stepped on by a time traveller during a journey to the age of the dinosaurs, triggers a chain of events that grow over time, leading to a difference in current political events (a change in who wins a presidential election), along with an alteration in English language spelling.
Another pivotal novel in the alternate history genre, Bring the Jubilee by Ward Moore, considers how time travel could affect the outcome of the Civil War (Moore 1965). A time traveling scholar arrives back in the year 1865 and ultimately changes the course of the Battle of Gettysburg. This results in an alteration of the outcome of the Civil War and in a subsequent branching of history. The ironic twist to the story is that the scholar is from a world in which the Confederacy has been victorious over the Union. Through his inadvertent tampering, he helps trigger a Southern defeat and brings about the familiar narrative of history. He is stranded in the past because, paradoxically, he has destroyed the timeline for which time travel had been invented.
Time Travel in Physics
Discussions of the paradoxical aspects of backward time travel would remain a purely a literary device if it were not for mathematically rigorous examinations of the question in reputable physics journals beginning in the late 1980s. In 1988, motivated by a request from Carl Sagan to provide scientific justification for the interstellar transportation used in his novel Contact, Caltech physicist Kip Thorne assigned his then-PhD student Michael Morris the task of constructing solutions of Einstein’s field equations of general relativity that offer traversable wormholes linking otherwise disconnected regions of the universe (see Morris and Thorne 1988). In general relativity, mass and energy shape the fabric of the spacetime manifold. The greater the concentration of mass in a region, the more distorted space and time would become. While the technology to assemble enormous quantities of mass into wormholes is far beyond our present capabilities, and requires a hypothetical substance of negative mass, called exotic matter, the physics community took Morris and Thorne’s proposal seriously. There were a number of follow-up proposals, similarly published in scholarly journals.
In a subsequent paper, Morris and Thorne, along with Ulvi Yurtsever, demonstrated that by propelling one of the mouths (terminals) of the wormhole at a near-light-speed, a loop could be created that would allow astronauts to journey backward in time (Morris et al 1988). That is because time at the fast-moving mouth would move more sluggishly than at the other end and would thereby be, relatively speaking, back in the past. What would be created is a loop backward in time, called a closed timelike curve (CTC). It is unclear whether the laws of physics (as they stand now) allow the existence of CTC or not; for the purpose of this paper we will assume that if the evidence of CTC is found in a text then CTC can exist, at least, in the context of a literary piece. Friedman et al also argue that the existence of CTC would prevent the ‘free will’ to be exercised when a human being attempts to change the past.
Creating such a device is an extremely hypothetical proposition, given that it would require exotic matter, enormous quantities of ordinary matter, and a civilization advanced enough to fashion these substances into just the right geometry, and hurl one mouth through space at close to the speed of light.
Nevertheless, even the mere possibility that backward time travel might be possible someday raises important questions about causality. What would prevent someone from trying to tamper with history and create unsettling conundrums, such as in the famous grandfather paradox of an ancestor-destroying time traveller trapped between existence and non-existence? Could there be, as novelist Connie Willis suggests in her Oxford time travel series, an aspect of time that avoids paradoxes?
The idea that history might be self-corrective was addressed by Fritz Leiber in his ‘Change War’ chronicles, including his novella The Big Time. In his stories, two rival, time-traveling factions, the ‘Snakes’ and ‘Spiders,’ constantly attempt to change history to the other group’s disadvantage. However, the fabric of actuality resists such changes through ‘The Law of Conservation of Reality,’ that minimizes the future impact of all shifts in timeline. For example, as one character mentions in The Big Time, when through historical tampering, the Roman Empire was prematurely defeated, a Germanic Empire took its place.
Leiber’s notion of time correcting itself was echoed by Willis in her Oxford series, albeit through a different mechanism. In Blackout and All Clear, whether or not a drop opens, allowing time travellers to return to the present-day, serves as a means used by time to sort itself out, eliminate contradictions, and restore reality to the way things ought to be. The main characters find themselves trapped for different intervals in the past, unable to get back, because time has found a way of preserving itself in that fashion.
The Principle of Self-Consistency
One way of ensuring that time travel never changes history is to posit that the past and present must always tell a self-consistent story. Such was the approach taken by Russian physicist Igor Novikov, along with a group of Caltech and Wisconsin researchers that included Kip Thorne, John Friedman, Michael Morris, and Ulvi Yurtsever, in postulating the principle of self-consistency. This is a way of permitting closed timelike curves but excluding the possibility of discrepancies between the realities before and after time travel. Even causality violations are allowed, as long as it leads to a logically consistent loop. The local framework can be time-reversed, assuming it provides a self-consistent global framework. As Novikov and the Caltech researchers wrote: ‘We shall embody this viewpoint in a principle of self consistency, which states that the only solutions to the laws of physics that can occur locally in the real universe are that which are globally self-consistent’
In a self-consistent CTC, a time traveller’s actions in the past can precipitate a chain of events in history as long as that leads to the world from which the voyager originated. In other words, the only changes to the past that are allowed are those that were meant to be.
As physicist Ian Redmount of St. Louis University, a former student of Thorne, noted: ‘The evolution of a physical system should be self consistent, even when you include influences from the future acting back in time. This means that if you travel back in time and attempt to shoot your parents before your birth, your gun misfires or you miss; the sequence of events already includes the effects of your attempt.’
Novikov and the Caltech/Wisconsin group offered as an example of a self-consistent CTC a game of pool in which a ball enters a pocket, say the right centre, travels back in time, and emerges from another pocket, say the left centre. The pockets would represent a wormhole used as a CTC. Suppose the re-emergent ball then hits the previous version of itself back into the right centre pocket. The entire process would obey all physical laws of momentum and energy conservation. Although the law of cause and effect would be reversed, because the ball from the future would affect its past, the global picture would be self-consistency.
Well before Novikov and his colleagues’ proposal, the essence of the principle of self-consistency has provided a means of avoiding paradox in a number of time travel stories. For example, in Robert Heinlein’s convoluted tale ‘All You Zombies,’ the protagonist, through several twists of fate and episodes of time travel, turns out to be his own mother and father. While in the story there are several different intersecting CTCs, each part of its narrative is entirely compatible with every other part.
Self-consistency does not eliminate the strange possibility that something could be created out of thin air. In another story, ‘Find the Sculptor,’ by Sam Mines, a scientist creates a time machine, travels five hundred years into the future, and finds a statue there of himself, erected in honor of the first time traveller. He then uproots the monument and takes it back to his own time as proof of his successful journey. Consequently, the statue is set up to commemorate his voyage. The question Mines asks at the end of the tale is, ‘When was the statue made?’ Clearly, though this story is self-consistent, it is troubling.
Time travel is a longstanding dream. While Einsteinian special relativity permits the possibility of travel into the future, travel into the past might engender disturbing paradoxes involving backward causality and other dilemmas. Consequently, it remains to be seen whether or not, even in the abstract, it could be scientifically viable. Nevertheless, in fiction, which lacks such constraints, it is fun to imagine such possibilities. After all, it would be the only way to revisit history and experience it personally. Who could turn down such a chance, even if just in one’s imagination?
Adopted from Out of the Darkness into the Darkness: Time Travel in Ernesto Sábato’s El túnel and Connie Willis’ Blackout and All Clear, by Victoria Carpenter and Paul Halpern