Types of Time Travel

Answers About Time Travel

While science fiction is often less “science” than “fiction”, one puzzle that has baffled authors, philosophers and physicists is that of time travel. Is it possible? If so, can you change the past? If either of those questions leads to a “no”, why not? And either way, what are the implications and how could we know?

While I haven’t spent much—read “any”—time working at an advanced particle accelerator, and have yet to publish my very own science fiction, I have studied time travel extensively from a philosophical point of view. And while I don’t claim to have a definitive answer, I can illuminate on what those answers should address. Through the course of this paper I will show what considerations need to be solved in order to answer these questions, what possible and likely answers would be, and what implications could be inferred from those likely answers.

Considerations and Assumptions

The first thing that needs to be considered is time travel itself: what is it? This is probably the easiest to answer. When we want to know if time travel is possible, we want to know if we can “get” to the past. Einstein’s Theory of Relativity has been shown to allow “time travel” into the future, but this isn’t interesting unless we can get back to the present. And we also want to actually travel there; we don’t just want to see it or wake up and find out it was a dream. So by convention, “time travel” will refer to “actual backwards time travel”.

So, what is “time”? There are many different theories about time that fall (roughly) under two categories: the “A Theory” and the “B theory”. These have evolved from an attempt by J.M.E. McTaggart to prove that time does not exist. While many philosophers aren’t convinced by his reasoning, they have used his way of describing time to develop theories about what time is and how it flows. The “A theory” is roughly that events are ordered by their properties. As time flows, an event’s property changes from “after” to “now” to “before”. The curious result of this is that everything exists in relation to the present; in fact only the present really exists. Taken one way, objects are what they seem; three dimensional entities that change as time passes.

On the other hand, the “B theory” states that events are ordered by their relations to each other, referring to each event’s relation as a coordinate, such as A happens at t=1 and B at t=2. This means that all times exist “simultaneously”. In this way, “objects” are really slices of four dimensional objects that have different temporal parts. This view of time seems to coincide with the model that modern physics uses, where time is a dimension just like the three special dimensions, and the four together make the “space-time continuum”.  Unless I explicitly state otherwise, I will be assuming that the B Theory is correct.

The next consideration is what physics says about time travel. Both special and general relativity allow for time travel in certain circumstances: by traveling faster than the speed of light or through a region of sufficiently warped space-time, time would run backwards. Unfortunately, both of these methods would require an infinite amount of energy, or negative energy, which hasn’t yet been confirmed to exist.

Quantum mechanics offers alternative opportunities. While the collapsing of a wave-function may or may not be reversible, some physicists believe that wave-functions can have multiple states simultaneously, allowing for time travel scenarios to be consistent. As well, physicists are unsure whether or not the phenomenon of quantum entanglement may allow for faster than light communication, or even demonstrate reverse causality.

Considerations in metaphysics are less optimistic. Almost by definition, “causality” means that a cause will occur before its effect, seeming to deny any chance of time travel. The “Novikov self-consistency principle”, developed by physicist Igor Dmitriyevich Novikov, offers a way out of this. It says that time travel might cause “closed causal loops” where each event in a chain of events has a preceding cause, but the whole chain might loop back to itself due to the changing reference frames of the system. This however might prevent free will, as a time traveler would be “forced” (somehow) to prevent a paradox by fulfilling each step in the chain. This, however, isn’t a problem if we assume the universe is deterministic, which I choose not to assume.

This leads us to the final consideration: what is a paradox? The most common paradox associated with time travel is the “grandfather paradox”. If you could go back in time, you could kill your grandfather before your father was born, which in turn means you would never be born, which means you never killed your grandfather, which means your father was born, which means you can kill your grandfather, which means—well, you get the idea. This paradox occurs whenever a time traveler would change the truth-value of an event upon which depends the causal chain that allowed that change to happen in the first place.

The other main paradox of time travel is called the “ontological paradox”. An example would be to travel to the past with a copy of Handel’s Messiah, and then give it to Handel for him to copy and present as his “own” work. The question arises, where did the music come from? Handel only wrote it because we gave it to him, and we only had it because he wrote it. Here the causal-chain has no origin, even though each event does. This is the result of the Novikov self-consistency principle, and while it isn’t necessarily a paradox it involves scenarios such as saving your grandfather before your father was born, allowing you to be born to save him. This has earned it the nickname of the “reverse grandfather paradox”, as it would cause the grandfather paradox if it didn’t play out right. The ontological paradox is closely related to the “predestination paradox” where you see your future self getting crushed by a boulder, so you run to hide, only to put yourself in the path of the boulder. Because both paradoxes rely on self-fulfilling causal chains, I will assume they are the same thing.

Types of Time Travel

With that in mind, can we travel to the past and change it? What “type” of time travel is allowed? There are many different answers to this; I will address each in turn and see how each measure up to our assumptions. I will notate each type of time travel by how it is related to the other types and which considerations it weighs more heavily.

  1. Time travel is not possible. Stephen Hawking gives the greatest evidence for this when he brings up the fact that since we cannot see any time travelers, they must not exist. This could be for a variety of reasons, each of which has interesting implications.
    1. The simplest reason would be that, according to one interpretation of the A theory, there is no past to travel to. The present is all that exists. This would pose serious questions to physics, such as “How can we account for such consistency and predictability in nature if there is no past?” However, due to the success of the theories of relativity, it is likely that this type isn’t the case.
    2. It could be that unknown laws of physics prevent time travel. Stephen Hawking predicts that this would be a form of temporal feedback (analogous to audio feedback), that would disrupt any attempt to time travel. This would likely only be testable if we could access negative energy and build an otherwise workable time machine.
      1. An alternative to this is that time travel is not possible now. General relativity predicts the existence of wormholes, which are portals that can connect any two points in space-time. However, you can only go back as far as the wormhole-based time machine was first created. This would likely be confirmed as soon as a time machine was built: a time traveler would likely emerge right after. If a time travel does turn out to be possible, this type would change to another type of time travel.
    3. Time travel is physically possible, but is impossible to execute due to unreal recourses. In order to travel faster than light it would take infinite amount of energy, which is not attainable by humanity. If resources such as negative energy are found to not exist under circumstances that they should, then this type of “time travel” would be confirmed.
    4. All timelines “evolve” so that time travel never becomes possible. Another way of describing this is that time travel is entirely possible, but all scenarios where time travel would happen are prevented by chance. While this is not really satisfying, it could be that because of all the possibilities that can happen due to time travel, the one that will always be a possibility is that time travel is prevented from ever being invented. This type of time travel is identical in many ways to the types 1.3 and 2.1.2 (see below).
  2. Time travel is possible, but history is fixed. This is the generally most agreed upon type of time travel among physicists and philosophers. While we could time travel, we couldn’t change the past to anything other than what it was. As to what prevents the changes and what it implies about the fundamental nature of our universe is up for debate.
    1. “Changes” to the past would have already been accounted for according to the Novikov self consistency principle. Here the potential time traveler would be the cause for things that already led to the present. This is an example of a closed causal loop where the entire scenario is self-consistent. This brings up questions of free will and determinism, as the time traveler cannot choose to do anything other than what is already the case; time travel just happens to shuffle the outside ordering of causality. If the universe is deterministic, then there would be no problems with this type of time travel as the time traveler would be impelled to act in a way that is already the case. However, if free will does exist, a time traveler would have to (voluntarily) forfeit their free will at the start of the journey.
    2. Alternatively, it may be that unknown laws of physics prevent the past from being changed. A time traveler would be able to “keep” their free will, but every attempt they make to change the past is frustrated by natural means. While invoking “unknown laws” means this type of time travel is unlikely, examples of natural means preventing the past from changing could be:
      1. Random chance– So long as there is a statistical probability of the paradox-forming action failing, it will fail. Maybe birds fly by and distract you from shooting your grandfather, or your time machine randomly breaks and can’t be fixed until you give Handel his music, etc. Absurd occurrences may happen, but as long as they are not impossible, the causal chains will remain unbroken and the past (relatively) unchanged.
      2. Ejection– Any changes to the past are prevented by removing the time traveler from the past. A time traveler is “thrown” back to the moment that they entered the past (or back to the present) any time they would create a paradox.
      3. Intent-prevention– If time is a figment of the mind, a time machine might always “land” off course so that an individual can never achieve their goal. Here the time traveler’s own sub-conscious might prevent him or her from reaching the intended destination.
      4. Incorporeality– Any time a time traveler would try to change (or even effect) the past, they are rendered incorporeal, as Mr. Scrooge was in A Christmas Carol. This however strains the “actual” part of the definition of time travel.
    3. Every time time-travel is invented, events unfold so that time travel isn’t invented. Because time travel will always incur a paradox, all paradoxes are naturally fixed by un-doing the time travel. For instance, assume you wanted to build a time machine to see a dinosaur. You complete your machine and see the dinosaur, but because you have seen a dinosaur you have no reason to build a time machine in the first place. A “looser” version than type 0.4, but “more restrictive” than type 2.1.2, though all prevent time travel.
  3. Time travel is possible, and the past can be changed. This is usually what people mean when they talk about time travel. However, it is considered the least likely by scholars for various reasons. Problems with causality, memories and paradoxes need to be considered.
    1. Time travel may cause paradoxes to occur. This is the “loosest” type of time travel. It would imply that we don’t live in a consistent universe and that effects could exist without causes and vice-versa. While paradoxes may be devastating to reality, they would just be the sad (or interesting?) truth of our world.
      1. The “Butterfly Effect”– An alternative is that the past is extremely sensitive to change, and small differences in one causal chain would change big things. Often, the very presence of a time traveler may have drastic effects on the present.
      2. The “Time Dilution Effect”– In this universe time travelers would always or often sever the causal chains that allowed for time travel, so that time travel never happens. Again, this has a similar effect as types 0.4 and 1.3 (all of which time travel is prevented by time travel).
    2. The past is resistant to change in proportion to the importance of the event. The questions then arise, how would the past be resistant, and how would it know the importance of the event?
      1. Minor changes are allowed, but major ones (changes that would create a paradox) are not. This type is unlikely, as “wearing a different colored shirt” would entail a paradox in some sense. This just seems like a slightly looser version of type 1.1 where a time traveler loses their free will.
      2. The more minds involved with an event, the harder it would be to change that event. Again, if time is affected by the mind, then it could be that the quantity (and quality) of memories involved with a particular event could make changes to it more difficult. However, what would make the change more “difficult” to perform would take us back to type 1.2 ideas.
    3. The past is elastic and will “fix” changes by the time the present comes around. This would again invoke an unknown natural law, and would imply that time itself has forces that govern it. If this turns out to be the case, then it would require an entirely new model of physics or would have to be incorporated into the Standard Model (a “Time Boson” would need to be discovered). This however could explain the one way nature of time and the asymmetric nature of many physical processes.
  4. Time travel is possible by means of entering alternate timelines. While this doesn’t necessarily satisfy our definition of time travel, it is still traveling through time and space.
    1. According to the “Quantum Many Worlds” interpretation of Quantum Mechanics, there are an infinite number of parallel worlds that are the actualization of all the possible outcomes of all choices. However, traveling to one of these would cause problems with causality, as a cause would seemingly enter a universe from nowhere. Our universe is, by definition, a closed system.
    2. Traveling back in time could create a new timeline, one separate from the original. Similar to type 3.1, but this new timeline is created at the moment of time travel; it didn’t “exist” all along. This is an easy way out, but essentially causes a second universe to be created, the physics of which we don’t now understand.


According to the way we understand our universe now, the most likely type of time travel would be type 0.2.1. Several solutions to General Relativity show that this is the case, and this also gives a satisfying response to Hawking’s lack of evidence: time travelers can’t come back in time this far. However, if no new discoveries are made (particularly if negative energy doesn’t exist), then the correct type of time travel is type 0.3, where it is theoretically and physically possible, but we will never have the means of developing a time machine. If time travel isn’t possible, we would continue to live in a universe that is “safe” from time travel. We would never have to worry about encountering paradoxes or violating free will (or worse: finding out determinism is correct).

On the other hand, if a method of time travel is discovered, likely time travelers from the future would emerge from the newly made time machine, creating the first closed causal loops. These time travelers would also be obligated to enter the time machine at the right future moment to make their appearance in the “present” possible. They would likely also be constrained by what they would be allowed to do in our “present”, so as to not incur paradoxes in their “present”. This would be a type 1.1 universe, which would in turn imply that we don’t really have free will. It would also likely confirm the B Theory of time, as the four-dimensional timelines could loop back on themselves.

Only if these four-dimensional timelines are flexible and we actually have free-will could we change the past. The most likely candidate would type 2.3, where the grandfather paradox is solved by the ontological paradox, which in turn can be explained by “random” chance arising from statistical thermodynamics and Quantum Mechanics. However, a traveler might still lose some free will, as some of these random happenstances could affect the time traveler.

In the end, while time travel is theoretically possible, it is highly improbable. And even if it was discovered, no meaningful changes to the past, or present, could be made.




Arntzenius, Frank, Arntzenius,. “Time Travel and Modern Physics.” Stanford University. Stanford University, 17 Feb. 2000. Web. 11 Dec. 2013.

Grey, William. “Trouble with Time Travel.” Philosophy 74.01 (1999): 55-70. Cambridge Journals. Web. 11 Dec. 2013. <http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=71265&gt;.

J.J., Smith, Nicholas. “Time Travel.” Stanford University. Stanford University, 14 Nov. 2013. Web. 11 Dec. 2013.

Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly 13.2 (1976): 145-52. Web.

Markosian, Ned, Markosian,. “Time.” Stanford University. Stanford University, 25 Nov. 2002. Web. 11 Dec. 2013.

Toomey, David M. The New Time Travelers: A Journey to the Frontiers of Physics. New York: W.W. Norton, 2007. Print.

(I would also like to mention the use of Wikipedia for inspiring me on a means of organizing the different types of time travel. While there is little to cite there, I didn’t come up with the method on my own.)


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