Time travel is a concept that has fascinated humankind for thousands of years. Ever since the invention of the concept of "time," humans have pondered the ramifications and requirements to actually traverse time, as if it were a river, or an ocean, in order to arrive at different time periods. Even today, in the era of science and technology, the concept of time travel remains an elusive one, and, perhaps, even impossible. Scientists have performed a great deal of research into the field of time travel, and come up with a few interesting theories, but as of yet, time travel remains a work of science fiction. The question, then, is will it always be science fiction? In order to determine this, it is necessary to examine some of the most prominent scientists in the field of time travel, their theories on time travel and its ramifications, and hopefully, discover the viability of time travel both now and in the future.
One of the most credible scientists in this field is, somewhat unexpectedly, Albert Einstein himself. Einstein thought of the concept of time travel as a sort of fourth dimension, which some people referred to as duration, as part of Einstein's concept of special relativity (Gott 42). This theory of special relativity is based on a simple concept. "First, the effects of the laws of physics should look the same to every observer in uniform motion… And second, the velocity of light through empty space should be the same as witnessed by every observer in uniform motion" (Gott 41). This view of time travel is interesting because it puts it into a new light, no pun intended, because it relegates it to the personal level. Einstein assumes that the laws of physics affect everyone in an equal way and forms his numerous hypotheses on time travel based on this assumption, which helps to keep his theories grounded. Einstein also bases these theories of time travel on pre-existing theories, mostly created by him, in order to have a solid foundation of science with which to work with. He doesn't account for the uncertainty principle within physics. Einstein applied this same simple concept to electromagnetism as well; a concept that, in Einstein's theories, is crucial to the utility of time travel. For example, one of Isaac Newton's theories stated that a state of rest, of non-movement, existed for everything but that it was impossible to tell if some objects, such as the solar system, were truly at rest (Gott 42). Einstein, however, disagreed with this assessment. He argued that if it was impossible to measure if an object (if the solar system could indeed be considered an object) cannot be measured to be at rest or not, then that state of rest does not exist (Gott 42). This means that observers in uniform motion could all make the claim that they themselves are at rest; a concept that Einstein extrapolated to electromagnetism as well (Gott 42-43). Thus, things like light beams "...could appear to travel always at the same velocity as it passed observers traveling at different speeds relative to each other only if their clocks and rulers differed" (Gott 43). This simple postulation helped to lay the groundwork for future theories on time travel. This theory also marked the creation of the famous concept, coined by Einstein, that time is relative.
This concept turned out to be key to understanding the inner workings of time travel, much of which involved traveling at speeds greater than the speed of light, which is considered quite difficult, for obvious reasons. The concept of relativity in time presents a number of exciting possibilities for the concept of time travel, not because they are brand new ideas to work with, but because they present the concept of time travel as personal and thus able to be molded to one's own perception. To put this into perspective, consider this thought experiment created by Einstein: a man, floating in space at rest (to his definition) is passed by a rocket, manned by an astronaut and moving at approximately the speed of light, and as the rocket passes, the man, rather quickly, shines a beam of light at mirrors on the sides of the rocket (Gott 49).
I see both light beams arriving back to the astronaut at the same time, just as he does. But he perceives the light beams hitting the front and back mirrors simultaneously because he is in the middle of his rocket and perceives himself as at rest. He and I disagree about whether the arrival of the light beams at the front and back mirrors are simultaneous events. It's not that one of us is right and the other one is wrong; each is right in his own frame of reference. (Gott 49)
This concept posits that time travel is merely relative, and that, even within a normal context, two individuals might not experience time the same way, just as the astronaut and the man floating in space experienced the movement of light differently simply based upon who, in their own perceptions, was at rest.
Of course, as with any concept that is theoretical yet deeply complex, there are bound to be a number of paradoxes in the field of time travel that put a sobering reality on the concepts posited by Einstein and others. Perhaps the first and most important step to truly understanding time travel, one author believes, is to disregard the mainstream notion of time travel, such as the one presented in Orson Wells' "The Time Machine" (Lewis 1). This author makes a point out of defining "personal time" and segregating it from more objective measurements of time, oftentimes measured by clocks, wristwatches, sun cycles, and things of that nature (Lewis 2). "We may liken intervals of external time to distances as the crow flies, and intervals of personal time to distances along a winding path" (Lewis 2). This means that, while time is indeed relative to the individual, as Einstein postulated, there is a second, objective measurement of time, that is always constant, as the crow flies, as the author puts it. This puts a wrench in Einstein's theories, which are focused purely on the individual, and largely disregard the concept of a singular, objective and external measurement of time. However, according to the author, this does not discount the possibility of what could be considered "time travel" altogether, it simply means that it must be examined in a different light (Lewis 2-3). To that end, the author uses the example of an average Joe, who travels through time with respect only to what is known as external time, or objective time, such as clocks, and what he refers to as a "time traveler," who is connected to time only through his or her own personal time (Lewis 3). Segregating the concept of perception of time into what is essentially two opposites (the subjective and the objective) creates a few obvious contradictions. For example, one commonly-held belief considering time travel is the concept of, as the author puts it, punching someone in the face before a time traveler leaves, causing his eye to blacken centuries ago (Lewis 3). Yet this concept, according to the author, disagrees with his perception of time travel. "…The orders of personal and external time disagree at some point, and there we have causation that runs from later to earlier stages in the order of external time (Lewis 5). Essentially, the main argument the author makes here is that time travel, as it is currently understood, is impossible, because while time travel might be possible on a personal level, as postulated by Einstein, there will always be an objective and external measurement of time that is immovable and constant, a sort of vigilant scorekeeper (Lewis 5).
Another author takes a somewhat more abstract approach to time travel, accepting that it is a complicated concept, yet also acknowledging that there are a number of different models for time travel, some more plausible than others. To this author, general relativity is a sound concept for explaining the possibility of time travel. To illustrate some of his ideas concerning time travel, the author breaks it down into simpler "toy" models of time travel. One of these models treats time travel as a simple two-dimensional plane; "…We can represent it initially as the Euclidean plane, and the dynamics are completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an ‘X’ in space-time" (Arntzenius and Maudlin). This sets an interesting standard for time travel because it demonstrates how oftentimes the simplest interpretation of it can be the most effective at explaining it. The author also examines what he refers to as more realistic models of time travel, which deal with the concept of wormholes that have the possibility of being able to traverse space-time itself. This is more of a thought experiment than anything else but helps to illustrate how the concept of time travel can be applied to the "real world" even today. The thought experiment posits that a ball in space heading towards a wormhole, "…assuming that the ball does not undergo a collision prior to entering mouth 1 of the wormhole, it will exit mouth 2 so as to collide with its earlier self prior to its entry into mouth 1 in such a way as to prevent its earlier self from entering mouth 1" (Arntzenius and Maudlin). While this might seem confusing, it does nevertheless show a remote possibility for time travel existing in the universe even at this very moment, via astronomical anomalies that are not fully understood by scientists even today.
While time travel is a convoluted and complex concept, breaking it down into multiple models, then assessing the viability of each of those models individually, as was demonstrated by many of these authors, helps to put time travel into perspective. In order for scientists to begin making even the first tentative steps toward controlling the concept of time, they must first agree on a universal method of classifying time itself, such as if it is merely personal, as Einstein postulated, or external and objective. Once this is done, forward progress may be made in the field of time travel, which brings with it a number of obvious and exciting possibilities for humanity. Until then, however, time travel must remain an interesting, although distant, science fiction concept.
Arntzenius, Frank, and Tim Maudlin. "Time Travel and Modern Physics." Royal Institute of Philosophy Supplement, vol. 50, (2002), pp. 169-200.
Gott, J. Richard. Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time. Houghton Mifflin Harcourt, 2002.
Lewis, David. "The Paradoxes of Time Travel." American Philosophical Quarterly, (1976), pp. 145-152.