On the Paradoxes and Ethical Implications of Time Travel

[May 2009]

Though current science possesses no method to physically transport a human being through the dimension of time, other than by a forward advancement of an insignificant fraction of a second from the exploitation of relativistic time dilation, it is worthwhile now to ponder the ethical implications on a society in which macroscopic time travel in either direction becomes possible. If such a thought experiment could shed light onto behavior of the future, we could start learning from it at the present, from pretending that such a time traveling device existed. Suppose in the future, humanity creates some standard of conduct, a time traveler’s law code. The code must be primarily designed to prevent logical paradoxes that could spontaneously appear especially from alteration of the past; this would cause the system to crash, much like a computer system that encounters a fatal error. Only after solving this dire problem can the code of ethics begin to worry about voyagers who stay within the system but travel due to immoral reasons.

The intention of the traveler does not necessarily determine whether a crash will occur or not. If I were to travel into the past to stop the Holocaust from happening, and successfully prevented it, then the current me would have no reason to go back in time to stop the Holocaust since it never existed. This creates a paradox as the back-in-time me would not have a place of origin in that timeline. On the other hand, if I had no intention of preventing the Holocaust, and accidentally did so, then a paradox would have occurred as well, also creating a contradictory situation that crashes the world. Luck, therefore, plays an unavoidable factor in the formation of a catastrophic situation.

Even if the code were designed such that all such human errors, including sporadic emotional impulses, are eliminated, it would still need to investigate the motives of a time traveler before letting them travel to the past. For example, if I apply to go back in time with the idea of assassinating Napoleon Bonaparte before he takes rule, I should definitely be stopped since such an event would drastically alter the course of history. Napoleon’s death could potentially result in the time machine never being invented: again, a paradox. The classic Grandfather Paradox also poses a problem; if a traveler goes back in time and kills his grandfather before the traveler’s father was born, then the traveler himself would never have been born. The paradoxes so far would have apocalyptic effects on space-time if the universe must travel in one timeline.

They can be resolved, however, through the hypothetical Many Worlds Interpretation (see Vaidman). Any quantum event with at least two possible outcomes creates multiple worlds in which each outcome is obtained, but we see only a certain outcome in our world, so there exist many other such parallel worlds around us with different outcomes. Perhaps if we travel back in time, we actually venture to a different world where none of our actions would affect the world we originally came from. Yet, if the Many Worlds Interpretation is wrong, for we currently cannot prove it, then the code must stop any disastrous paradoxes from happening.

Another type of paradox occurs from the transfer of possessions. You would not create any paradox in giving your past self the winning lottery numbers, because as that past progresses to the present, the original time travel remains possible, and you still have a motivation to perform the time travel (to give yourself the winning numbers and complete the cycle). However, if you were to change the transfer of lottery information to that of a technology which was developed between the two dates on the ends of the time travel, you will have created a highly interesting paradox, though it might not at first appear to be cataclysmic. To elucidate, if Steve Jobs, in 2009, sent an iPhone, released 2007, to himself in the year 1999, he would have a working model of an iPhone in 1999. He can then reverse-engineer, copy, and advertise this gadget as a marketable product in 2007. Later, in 2009, he goes back in time and hands his young self the same device, completing the cycle. Now comes the real kicker: Where did the original iPhone come from? If we trace its history, we find that it goes through an infinite loop of time, a remarkable paradox, as it belongs to its own timeline; even the atoms that comprise the iPhone are nonexistent before 1999 or after 2009.

This dilemma appears non-catastrophic at first, but the laws of physics begin to show that there hides an inherent impossibility. If the same iPhone enters the loop each time, then over an infinite number of years in the time loop, the phone should have corroded from contact with the air for an infinite duration, and the atoms should have undergone so many radioactive decays as to render the device nonfunctional. Thus, the original time travel transaction could not have happened, unless it occurred in a different timeline under the Many Worlds Interpretation. How can a law code stop such an event from happening? Only by strictly enforcing certain rules against time travelers can it prevent the world from crashing.

So far, in order to avoid a potentially catastrophic paradox, time travelers must obey the three fundamental laws:

  1. The traveler must not alter a historical event that originally provided the motivation to time travel in the first place, as doing so could remove that motivation.
  2. The traveler must not kill anyone, as the action would radically change history, perhaps to the point where the traveler cannot initiate the time travel in the first place, due to the nonexistence of the machine or the incorrect position of the traveler
  3. The traveler must not give anyone an object that can fall into an infinite time loop.

Even if travelers obey these laws and avert all such logical paradoxes, ethics would still remain an issue, in the form of giving oneself information that should not be known in the past.

After acing a test, a student could turn back time and reveal to the past test taker the answers; that cheater would then ace the test, continuing the deviant cycle. Would this be considered an moral practice? (More strangely, where did the answers originally come from?) Most likely, the law code must prohibit such immoral actions. However, some emotionally charged events could feel quite painful to potential time travelers due to restrictions from performing certain tasks. If I knew I had the option to go back in time to save a dead relative from a car accident, would I restrain myself from doing so? That would alter the course of history, though, and would violate the first law. If I went back in time and killed John Wilkes Booth before he assassinated President Abraham Lincoln, I would clearly violate the first and second laws and therefore cause a catastrophic paradox.

The third law, however, is a much more tricky proposition. Although Steve cannot send his actual iPhone back to 1999, he could, however, go back to 1999 and email himself the schematics of the iPhone. A paper blueprint would not work, as it comprises a real object, and would disintegrate over an infinite span of time. The email works because it represents a virtual image of an object and furthermore, he does not have to send the same email every time; he could even delete the email after receiving it, and his time travel would remain logically valid. However, a certain logic trick slightly complicates matters: if Steve becomes overambitious and releases the iPhone in 1999, then a violation of the first law would have occurred, and the system would crash. However, if he waits until 2007, it seems that time flows normally.

From these laws, it appears that as long as the time traveler makes no physical contact with the world of the past, no paradoxes will arise. However, the moment the time travel machine finishes the journey, the traveler will inherently touch the air or some part of the surroundings. Parts of his skin, and the air he breaths, would scatter across the winds, never to return again to the time travel machine. The only way, then, is to create a completely enclosed room that the time traveler may observe from, perhaps with a one-way mirror, so that the traveler may see the people of the past, but not vice versa. This avoids the situation that the traveler could potentially communicate to the past society how to mitigate some war or misfortune, a violation of the first law. The enclosure must exist at the time of the more previous era; therefore, if we build such a structure right now, future time travelers could observe our world only starting from the moment the structure finishes. Anything before that is history.

Ethics itself, however, could change if we gain the ability to directly observe our own history, even if we cannot alter it. Knowing how certain events unfolded would be a crucial benefit in the area of crime investigation. Past crime scenes could be reconstructed almost perfectly with multiple video recordings and three dimensional imaging techniques, and would greatly improve the quality of court evidence, and perhaps even deter crime. On the other hand, felons and suspects could potentially escape to an earlier period of time, and if they blended in with, for example, a medieval society, it would create a paradox to send police back in time to arrest him; the act of seeing modern humans with modern technology could unnerve any medieval observer. Thus, suspects and criminals must not be allowed to time travel, as they can also cause a logical paradox. While backwards time travel is not possible today, the thought experiment could definitely lead us to think about how to set up a law code for when it does become a reality.

On the other hand, we already know how to travel into the future as a consequence of Einstein’s relativity theories, and such travel would not cause logical paradoxes, as long as the traveler moves only forward in time. Due to a very slight relativistic effect, we can make our astronauts just slightly younger, by a fraction of a second, than people on the earth. Supposing it is possible in the future to travel at speeds approaching the speed of light, travel into the far future should also be possible. There will hardly seem to be any practical applications of forward time travel, other than to go into the future to hopefully cure a disease currently incurable, or to simply try to experience a higher standard of living. The difficulty arises that no information can possibly be sent backwards; the traveler cannot ever contact his original society. So we would never know if there actually is a cure for the disease until we go forward, leaving everyone else behind; they would miss us, and if we go so far into the future as to cross generation gaps, our permanent absence will be not unlike a death to the current community.

Forward travel, however, must also be limited by some rule. We cannot allow free access to everyone, as if every person goes into the future, no one will be present now. Say everyone chooses to jump forward one million years: nearly all remnants of human civilization would be disintegrated by the time we are supposed to arrive, and we would either have to rebuild or die. Who is allowed to time travel? And for what purposes, i.e. would people be allowed to escape to the future if an epidemic occurs? Speaking of which, our current selves would be pitifully unadapted to diseases of the far future; perhaps the human race itself has evolved or progressed on. Thus, should there also be a limit to how far forward a traveler may go? The law code must be able to deal with such issues.

The art of travel into the past looks in total less exciting than it once did. Under artificial rules and constraints, the time traveler is reduced to a mere observer, without the ability to impact anything in the past for fear of causing a massive paradox. Ethics appears more harsh than before, in the mitigation of paradoxes and control of the pursuit of self-interests. Forward travel, too, seems to lack the daringness and adventure of what it once seemed to promise. The only hope for free time travel, then, is for the Many Worlds Interpretation to be proven correct by the time we have invented a time machine; otherwise, the universe could suddenly terminate.


Vaidman, Lev. “Many Worlds Interpretation of Quantum Mechanics.” Stanford Encyclopedia of Philosophy. 2002. Stanford University. 12 May 2009 < http://plato.stanford.edu/entries/qm-manyworlds >.