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Time travel is in many movies, books and video games. But there are differences on how it works and its consequences.

A thing I always think is: OK, they go back to the past to change the future, but isn't this future already a consequence of them going back to the past to try to change it?

Another thing: if some people can come to visit us from the future, so it means we are not the first ones since they are us but in the future. And does it means they have no future yet when ours is already written?

And if we could go in the future, would it mean we are only the past since a future exists? And could we go in the future until the very end or only until a time that is something like the true present and has no future yet?

My question is: is there a theory about how it would happen if we could travel in time?

Thanks.

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closed as unclear what you're asking by Hohmannfan, a CVn, cobaltduck, MozerShmozer, TrEs-2b Aug 19 '16 at 19:22

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Your question's title and body don't match. The title asks about consequences of time travel, the question body asks how time travel could happen. Please edit to pick either. $\endgroup$ – a CVn Aug 19 '16 at 12:25
  • $\begingroup$ Aren't causes and consequences indistinct in time travel? $\endgroup$ – Destal Aug 19 '16 at 12:32
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    $\begingroup$ "How it would happen", at least to me, means "how can we accomplish this?". "What are the consequences?" is just that. The two are different because they ask for completely different answers, regardless of whether you feel time travel turns cause and effect on its head or not. $\endgroup$ – a CVn Aug 19 '16 at 12:35
  • $\begingroup$ I suspect the OP means "what would happen if we could travel in time", referring to its consequences, but used the wrong form of words. Getting the form of the words right is important because each gives a different answer. $\endgroup$ – a4android Aug 19 '16 at 12:39
  • $\begingroup$ You are right that my question, which was about "untold consequences of time travel in a fiction, to keep it coherent at first glance, that makes it incoherent if you think a bit about it and the what ifs" has become more about "how it would work". $\endgroup$ – Destal Aug 19 '16 at 12:52
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There's a few, try Many-worlds interpretation, Quantum physics, Presentism vs. eternalism, The grandfather paradox, Ontological paradox, Theory of compossibility. All of which are mentioned on the wikipedia article for time travel.

Edit: Added Descriptions from wikipedia

Many-worlds interpretation

Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation (MWI) of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the MWI is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one. The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. The quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

Quantum physics

Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. The fact that these quantum phenomena apparently do not allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments.

Presentism vs. eternalism

Many philosophers have argued that relativity implies eternalism, the idea that the past and present exist in a real sense, not only as changes that occurred or will occur to the present. Philosopher of science Dean Rickles disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism." Some philosophers view that time is a dimension equal to the spacial dimensions, that future events are "already there" in the same sense different places exist, and that there is no objective flow of time; however, this view is disputed.

The bar and ring paradox is an example of the relativity of simultaneity. Both ends of the bar pass through the ring simultaneously in the rest frame of the ring (left), but the ends of the bar pass one after the other in the rest frame of the bar (right). Presentism is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own. In this view, time travel is impossible because there is no future or past to travel to.[71] Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present;[74] these views are contested by some authors.

Presentism in classical spacetime deems that only the present exists; this is not reconcilable with special relativity, shown in the following example: Alice and Bob are simultaneous observers of event O. For Alice, some event E is simultaneous with O, but for Bob, event E is in the past or future. Therefore Alice and Bob disagree about what exists in the present, which contradicts classical presentism. "Here-now presentism" attempts to reconcile this by only acknowledging the time and space of a single point; this is unsatisfactory because objects coming and going from the "here-now" alternate between real and unreal, in addition to the lack of a privileged "here-now" that would be the "real" present. "Relativized presentism" acknowledges that there are infinite frames of reference, each of them has a different set of simultaneous events, which makes it impossible to distinguish a single "real" present, and hence either all events in time are real—blurring the difference between presentism and eternalism—or each frame of reference exists in its own reality. Options for presentism in special relativity appear to be exhausted, but Gödel and others suspect presentism may be valid for some forms of general relativity.

The grandfather paradox

One subject often brought up in philosophical discussion of time is the idea that, if one were able to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his paternal grandfather at a time before his grandfather met his grandmother. If he did so, then his father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather. The paradox is sometimes posed with autoinfanticide, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant.

This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way,[77] an idea similar to the proposed Novikov self-consistency principle in physics.

Ontological paradox

The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that any actions, taken by a time traveler or by an object that travels back in time, were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox, ontological paradox,[79] or bootstrap paradox. The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses Somewhere in Time as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states:

The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be absolutely identical to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will wear in the normal manner. Its entropy will increase. The watch carried back by Reeve will be more worn than the watch that would have been acquired by Seymour.

On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.[26]:23

Theory of compossibility

David Lewis's analysis of compossibility and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradoxes. Consider Lewis’ example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised: can Tim kill his grandfather?

For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim's trigger finger. Can Tim shoot his grandfather? Considering these facts, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to.

Consider now the fact that in Tim's universe his grandfather actually died in 1993 and not in 1955. This new fact about Tim's situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesn’t but can, because he has what it takes", and, "Tim doesn’t, and can’t, because it is logically impossible to change the past", are not contradictions; they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts.

So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.

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  • $\begingroup$ Thank you I'll check that. It's fun that we can imagine how a lightsaber works, we still can say "maybe they found a way to stop light spreading or something", or how a spaceship works, or how we can regenerate cells to make someone live again, but with time travel it's just impossible to only imagine something that would make it coherent because of the paradoxes. $\endgroup$ – Destal Aug 19 '16 at 12:30
  • $\begingroup$ The biggest problem with 'many worlds' from a story perspective is that, if the protagonist believes this is how time travel works, it can undermine the motivation for going back to change in the past in the first place, since all it means is that you choose a 'different' world to go back to, but your 'original' world line still exists. It's inevitability in a more multi-faceted sense. $\endgroup$ – Dan Bryant Aug 19 '16 at 13:18
  • $\begingroup$ Still, there are interesting stories that can be told with a many worlds type of slant. One of my favorites is "The Man Who Folded Himself" by David Gerrold. It has a slightly different take, in that it's more many selves than many worlds, though many worlds get folded into the 'one timeline' over the course of the story's evolution. $\endgroup$ – Dan Bryant Aug 19 '16 at 13:22
  • $\begingroup$ @DanBryant nonetheless, although you doom the world you leave to be the same but without you, you create a new world with your desired characteristic. That sounds like about as good an outcome as possible in the circumstance! $\endgroup$ – A Simmons Aug 19 '16 at 17:39
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There is no such thing as a "true" present, past or future. These are relative terms. The future is everything ahead of that point in time where you are (called the present), the past is everything behind it.

There are various possible ways in which one could travel through time though all are still (and may forever be) unachievable to us.

You could use wormholes. Or abuse the reality-rewriting properties of quantum entanglement. Or perhaps a 4-dimensional being (where the 4th dimension is time) can move you somehow.

Regardless, there is an entire class of problems called the time-travelling paradoxes. For example, what happens if you kill your grandfather? without grandfather you cannot be born but without you your grandfather cannot be killed.

The answer to these questions are simply unknown. We do not have enough information about how time travel works in the first place. There are many different ways it could work, any of them as likely as another. Another answer already mentioned a few.

If you want to build a story involving time travel you'll first have to choose how it works. Then, based on that you can derive the answers to your questions and perhaps come back if you can't figure it out on your own. If you asked for curiosity's sake, the answer is that we don't know.

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  • $\begingroup$ I asked for curiosity sake, because everytime I watch a movie with time travel, I find paradoxes and incoherences. So as you said, I suppose they have to choose how it works for their movie and not give too much details. I remember in Futurama when Fry goes back in time and kills his grandfather, then makes love with his (young) grandmother, and doing so he becomes his own grandfather. Was he already his own grandfather before going back in time? Or did he change who is his grandfather by becoming his own grandfather? $\endgroup$ – Destal Aug 19 '16 at 12:24
  • $\begingroup$ That's up to the director to choose. If we go with a universe that follows the novikov self-consistency principle then he already was his own grandfather but just didn't know it. If we go with the many-worlds theory his "real" grandfather was his grandfather and by going back in time to kill him he created a new world in which he was his own grandfather, but the man who made love to his grandmother still was the grandchild of his "real" grandfather. Note that I haven't seen futurama so there may be other factors that disprove one of the theories I mentioned. $\endgroup$ – Annonymus Aug 19 '16 at 12:28
  • $\begingroup$ The Novikov self-consistency principle can't exist in that universe, for example, if prior to travelling back in time he had his genes tested to confirm he was related to his "real" grandfather (and had a positive result). The many-worlds theory solves pretty much all paradoxes one can encounter in a work of fiction. A slightly altered version of it could even explain inconsistencies between the past and the present before it was modified. $\endgroup$ – Annonymus Aug 19 '16 at 12:33

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