But here, upon this bank and shoal of time,
We'ld jump the life to come.
---Macbeth, Macbeth, Act I, Scene 7, William Shakespeare
Gosh that takes me back…or is it forward? That's the trouble with time travel, you never can tell.
---The Doctor, Doctor Who, "The Androids of Tara"
From the hapless Macbeth wanting to know how his killing of King Duncan will work out to Doctor Who, time travel has been a fascination of humanity. The possibility of time travel has been considered in literature, philosophy, and science, bringing to mind paradoxes which can or cannot be solved, mathematical formulas which allow for its possibility, and physical objections which claim that it is not possible. As a thought experiment, it has been reviewed and dissected, discounted and examined; as a plot device in science fiction, it has been used extensively, allowing its characters to encounter ethical and moral dilemmas, change or refuse to change history, glimpse the future, and travel back to the past to try to change a future which is undesirable. The ways in which time travel has been used in science fiction are as varied as the manner in which scientific and philosophical thought experiments have been envisioned. Some of the possibilities are examined here.
Recently, serious discussion about time travel has been undertaken by theoretical physicists. In part, the reason for such discussion lies in the problems associated with the effort to develop a unified theory of physics and the necessity for the unified theory to have a reasonable concept of the relationship between cause and effect. If time travel is a reasonable possibility, the nature of cause and effect is thrown into turmoil. Linear time might still exist but the possibility that people could travel back and forth in the timeline could cause chaos, as concepts of past and future would become muddled.
Prior to beginning this discussion, it is important to define time travel for the purpose of this article. Time travel occurs when a traveler undertakes a journey in a specific amount of time (perhaps fifteen minutes) but ends up at a time (forward or backward) which is greater than the fifteen minutes which the traveler has experienced. In general, when this article discusses time travel, it will mean that the traveler undertakes a physical journey with the assistance of a spaceship, wormhole, or other time machine which propels the traveler physically backward or forward. Although some science fiction utilizes time travel by other means (perhaps through a journey of the mind), in general, this article will be concerned with the physical time traveler, not the virtual one.
We are all travelers in time in the sense that we move in a linear fashion through our own lives from the present to the future. We experience time in the sense that it passes and that the time we experience is all the time there is in the universe. We cannot jump into next week or go back to 1945. But based on the persistent allusions in literature and philosophy, we wish we could.
The special theory of relativity allows for a certain kind of time travel, which it could be argued is really not time travel at all. But, it is the only kind of time travel which has been experimentally observed in the laboratory and so it will be discussed here. This kind of time travel is called time dilation. Einstein's theory of special relativity shows us that objects in one inertial frame of reference approaching the speed of light experience the passage of time more slowly than slower-moving objects in another frame of reference. Time slows down so that the faster-moving object experiences a shorter passage of time than the slower-moving object.
This effect is illustrated by the Twin Paradox, which really isn't a paradox at all. One of two twins leaves Earth on a spaceship traveling at the speed of light. The other twin remains behind. In the traveling twin's experience, she returns one year later and finds that the stationary twin is now ten years older. This is because of time dilation, an effect of special relativity which has been experimentally proven in the laboratory by observation of the decay of muons in particle accelerators.
Similarly to special relativity's effect on time because of speed, the theory of general relativity has an effect on the passage of time because of gravity. Einstein predicted that, like speed, gravity would slow time. Clocks in the attic run slower than clocks in the basement because the basement clocks are closer to the mass of the earth and therefore more affected by the earth's gravity. Extrapolating from here, it is possible to consider that a black hole, with its infinite gravity and infinite mass, would likewise slow down time. If it were possible to enter a black hole and emerge unscathed, one could use one's journey therein to travel in time.
Physicists have posited from these two ideas that it might be possible to travel into the future because of the effects of time dilation, but they have been less willing to accept the idea that one could travel backward in time, claiming that to do so would violate known physical properties of the universe.
One idea for a time machine which would allow travel backward and forward has actually come from this idea of the slowing of time in the vicinity of a black hole or a neutron star, which has extremely dense mass and extremely high gravity. If one were to construct a wormhole with one mouth of the wormhole close to the neutron star and the other in some part of the universe not close to the neutron star, time travel would become possible by transversal of the wormhole. In this way, you would be able to travel backward in time as well as forward since the different ends of the wormhole would experience time at a different rate. The two mouths would be separated not only by space but also by time. However, there is a restriction on this kind of time travel. The traveler transversing the wormhole could not return to a time before the wormhole was built.
An intersection between science and science fiction occurred on the issue of this form of time travel when Carl Sagan wrote his novel Contact in 1985. The story deals with an alien civilization making contact with Earth and sending instructions on how to build a machine which would transport humans in spacetime to another world and another time. Sagan discussed the possibility of such travel with Kip Thorne at the California Institute of Technology. Sagan's original idea was to use a black hole as the means whereby the travel through spacetime would occur. Thorne suggested a wormhole instead because, unlike a black hole from which nothing but Hawking radiation can emerge, there is an entrance and an exit in the theoretical construct of a wormhole.
The theory of wormholes in science is a feature of the general theory of relativity. Gravity warps both time and space and, therefore, were there to be tunnels or holes in the fabric of spacetime, it might warp back on itself (like a piece of paper which was folded in half), so that one could enter the wormhole in one part of the universe and emerge in another part. Such a wormhole might also make it possible to enter at one time and emerge at another, particularly if the ends of the wormhole were positioned in parts of the universe where time passed at different rates due to difference in the effects of gravity in the vicinity of the end of the wormhole.
Thorne's idea of the wormhole is that it would be an object of substantial but not infinite gravity and that its tendency would be to remain closed in on itself by the effect of that gravity. In order for the wormhole to open and allow passage to the other side, an antigravity force would have to be generated to open up the wormhole and keep it open while objects passed through. The force which he theorizes is necessary to keep the wormhole open is negative energy, which is not forbidden by the laws of physics, and, therefore, makes the use of the wormhole as a time machine theoretically possible.
It should be noted that the effect of something which looks like negative energy has been produced in the laboratory. Quantum vacuum experiments showing the fluctuation in energy density allow for the possibility of negative energy (i.e., less than zero energy of the quantum vacuum). Theoretically, the idea of Hawking radiation, the production of positive energy from a black hole, also permits the possibility of negative energy. Negative energy is produced by the extreme spacetime curvature near the wormhole which, in turn, disturbs the vacuum fluctuations. Negative energy flows into the black hole to be consistent with the idea of the conservation of energy.
It should be noted parenthetically that Hawking has revised his previous position on black holes with respect to the idea that nothing (except Hawking radiation) can escape a black hole. He has abandoned this idea in a paper and series of lectures which he has given to various physics conferences in 2005 and 2006. The exact basis of this change in theory is apparently quite difficult, as a number of the physicists who have heard it cannot explain or understand Hawking's thinking on this issue; but, suffice it to say that he has changed his previous position and no doubt at some time in the future it will become clearer as to why.
Another thought experiment involving time travel is that which arises out of the writings of Kurt Gödel. In the 1940s, Gödel's solution to Einstein's field equations of general relativity led to a theory of backward time travel through the idea of closed timelike curves. Gödel theorized a universe which was spinning uniformly (not expanding as is posited by most theoretical physicists today). In such a universe, it is possible to go back in time by making a big circle in space and returning to the point at which you left in space and in time. Gödell wrote, "By making a round trip on a rocket ship in a sufficiently wide curve, it is possible in these worlds to travel into any region of the past, present and future, and back again, exactly as it is possible in other worlds to travel to distant parts of space."
There have been numerous objections to the idea of the Gödel universe time travel. One of the most notable is that of Stephen Hawking, who writes, "By traveling in a spaceship on one of these closed timelike curves one could travel into one's past. This would seem to give rise to all sorts of logical problems…what would happen if you killed your parents before you were born?" Hawking's answer to this is to theorize the existence of something he calls the chronology protection conjecture. According to Hawking, "The laws of physics prevent closed timelike curves from appearing."
Another objection to time travel has been suggested by physicists, including Mag Tegmark, who state that the absence of time travel may be due to the anthropic principle. They argue that a universe which allows for time travel and closed timelike curves is one in which cause and effect could never be discerned and, further, that intelligence could not evolve because it would be impossible to sort events into past and future or to make predictions. One of the ways in which philosophy and science fiction resolve the issue of the problem of backward time travel is to say that it is possible as long as the time traveler is prevented in some way from changing the past. You go back into the past to kill your grandfather but are prevented from doing so because you slip on a banana peel and miss your chance, or the gun doesn't go off. This particular solution is rejected by many philosophers because it requires the occurrence of too much coincidence to protect the timeline. They allow for time travel on the basis of the idea that what you may observe happening in the past has already happened, including the fact that you were there and could have taken an action but did not.
Science fiction has adopted a number of these ideas in its time travel stories and its various devices employed to protect the future by protecting the past. An example of this would be a corollary of Star Trek's Prime Directive, which prohibits Starfleet officers from interfering in any society, planet, universe, or time period in which they find themselves. Another example would be from Back to the Future II, when the Doc tells Marty that he cannot come into contact with his future self because this would create a huge temporal distortion.
Time travel is a possibility as a consequence of the implications of special relativity, general relativity, and quantum mechanics. But, there is no experimental evidence of time travel and our understanding of the current laws of physics seem to indicate that it is not possible.
Nevertheless, the dream of time travel persists and is examined extensively in theoretical physics, which discounts its possibility, and philosophy, which does not. Science fiction uses the arguments and thought experiments of both disciplines to give us stories revolving around the implications of being able to travel in time.
Time travel in the Star Trek universe is accomplished in a variety of ways.
One of the more confusing of Star Trek: The Next Generation's episodes is called "Time's Arrow," in which the crew travels back in time through the use of some sort of time teleporter, the mechanism of which is never actually explained. The show deals with an interesting variation of the grandfather complex. Events take place in the past with the deliberate idea that they will have an effect in the future. Data's head is found in an archaeological dig in San Francisco dated back to the 19th century. It is determined that somehow Data had traveled back in time to the 19th century, where he was killed. If this was true, then what was he doing alive in the 24th century? The paradox is resolved in an ingenious and somewhat confusing fashion. Picard also travels back in time to the 19th century, where he finds Data's head and taps binary program code into the head in order to reactivate his program so that his head (which will be discovered five hundred years later, as we learned in the beginning of the show) can be reattached and reactivated in the 24th century.
Many questions are presented by this scenario. For instance, at the beginning of the show when Data's head is discovered (in the 24th century), the actions of Picard in the 19th century have already taken place. But Picard doesn't know about them. Will the message for Data's programming be visible in the 24th century before Picard travels back to the 19th to tap in the code? A cause in the 19th century has an effect in the 24th century before the cause in the 24th century (traveling back in time by Picard) produces the effect in the 19th (Picard arriving at the place where Data's head is recovered). Confusing? Absolutely. This episode is, however, a prime example of the use of a fascinating thought experiment as a plot device. It is a murder mystery. Who killed Data? How did it happen? Can it be prevented? The device which moves the story along is the mechanism of one of the paradoxes of time travel.
Another Star Trek plot, in a Deep Space Nine episode, is similarly interesting in resolving the paradox of events occurring 200 years in the future changing events which already occurred 200 years in the past and, in the process, destroying the timeline which came after the events which occurred in the past.
The premise of this episode, called "Children of Time," is that the Defiant becomes caught in a temporal anomaly caused by its crashing into an energy barrier around a planet which it is about to explore. The crash causes quantum energy fluctuations which create, for an instant, two alternate universes (which is depicted by a momentary image of one of the characters looking as if she has been doubled). Upon landing on the surface of the planet, the Defiant's crew discover that they have moved into an alternate timeline as a result of the the quantum doubling effect-in other words, they have taken an alternate path, like the electron in Richard Feynman's two-slit experiment. (In this famous-at least in physics circles-experiment, Feynman shot two electrons through two slits at the same time. On the basis of the uncertainty principle, Feynman was able to show results which indicated that the electron went through BOTH slits at the same time. Of course, you could not be looking at the electron when it traveled through the slits because the act of looking would add a feature to the experiment and you would then see the electron go through only one slit. The experiment introduced the idea of quantum doubling-or one electron with two histories.) The crew find themselves standing among people who are the descendants of the survivors of the crash into the energy barrier which happened in the planet's new timeline 200 years before… even though, from the crew's perspective, it hasn't happened yet. (Another way to look at it, in the style of Gödel timelike curves, is that the Defiant's present remains constant but, in crossing the energy barrier, they have actually moved 200 years into the future--all time in all places in the universe being universal.)
It is then explained to the crew which has just crashed that, in their old timeline, in three days from now they will attempt to leave the planet, crash into the barrier again, and crash land on the planet, from which they will not be able to escape, thus preserving the new timeline. If they try to find a way to avoid the crash into the barrier when they leave the planet, they will destroy the new timeline and their descendants' timeline will collapse and disappear.
This variation of the grandfather paradox presents a situation wherein the crew of the Defiant must go into the future to perform the same act which they have already performed in the past in order to preserve the timeline in the present.
The thought experiment here is used as a plot device to advance the love story which is being developed between Odo and Kira. When the ship crashes into the energy barrier, Kira is injured, potentially fatally, and can only be cured by 24th century medicine if they can get her back to DS9. Now that they know about the accident which will occur three days hence, the crew can take action to avoid it and go home, where Kira can get medical treatment and survive. If they do go home, they will cause the collapse of the current timeline and all of their descendants will disappear. Odo has waited 200 years to tell Kira of his love and he knows that, if they recreate the accident, he will watch Kira die again and lose his chance with her in the other, original timeline. He decides he cannot let this happen; he changes the flight plan of the Defiant so that it does not hit the energy barrier. As a result, the timeline in which they had landed is destroyed and all their descendants disappear.
In a sense, Odo by his action in the future is creating a third timeline, one in which the crash has both occurred and not occurred and in which the persons who participated in both events are aware of their occurrence. This is another resolution of the paradoxes of time travel. It is a variation on getting a second chance to take an action which can set things on a different course in the present. The action creates an entirely new timeline, a new history.
This is certainly an outcome which is envisioned by the thought experiments discussed above.
Another example of the resolution of the paradoxes of time travel is found in the Back to the Future movies. The hero, Marty, travels back and forth in time in these movies to have a variety of effects on events in his present life. In the first movie, he goes back to 1955 where he meets his own father, a timid nerd who is bullied by the local obnoxious, high school football hero, Biff. Marty encourages his father (George) to grow some backbone and stand up for himself and the girl who will become Marty's mother and, when George finally does so by punching Biff in the nose at a high school dance, he changes the course of everyone's life in the present. Similarly, in Back to the Future II, Marty travels forward in time to take care of a problem with his son. He disobeys the directive of Doc, his friend and the scientist who built the time machine, not to bring anything back from the future, and picks up an almanac of sports events which he brings back to the present. It is stolen by the unscrupulous Biff, who steals the time machine and goes back to 1955 where he gives the almanac to his younger self, who uses it to make a fortune betting on sporting events. The present is now changed again as a result of this, and the rest of the movie is devoted to Marty and Doc trying to bring things back to the way they were at the end of Back to the Future I.
These movies also describe a method by which the time travel is accomplished. The time machine which is used in the movie is an old Delorean which is powered by nuclear fusion. The graphic representation of the time travel sequences show the car speeding down a road and gaining speed and suddenly vanishing from view, presumably from achieving light speed travel velocity. Similarly, the Delorean appears in the new time by a flash of light (slowing from light speed?) and coming to a halt. One of the devices kept in the car is a clock which presumably will keep time in the frame of reference of the car. The car itself travels 30 years in either direction in a matter of seconds as delineated by the clock inside the car. The physical method of travel seems to combine the ideas of special relativity and Gödel's timelike closed curves, without explicitly explaining to the audience how all this travel is being accomplished. The philosophical thought experiment's use as a plot device is developed well while the science is not.
These paradoxes of time travel are generally resolved by scientists by their claim that a sensible universe could not allow such conditions to exist. Unfortunately for this position, the equations of general relativity do not forbid these possibilities. They make them possible.
These examples of time travel in science fiction are concerned more with the consequences of time travel rather than their method. This is probably due to the fact that, while it is possible to speculate about what would occur if humanity could travel in time, the technology to build a time machine or to perform experiments to test its possibility have yet to be performed.
Even so, science and science fiction continue to dream of the possibility and look to the future, inspiring us to do the same.
This article is an excerpt from a University of Chicago Master of Liberal Arts thesis written in 2005. The author has an abiding, one might say obsessive, interest in cosmology, physics, and time travel as well as Star Trek and particularly Deep Space Nine.