I would like to propose the following situation: (1) time travel to the past is possible, (2) there is only one version of the past, one timeline and (3) the past (or the future) cannot be changed. If so, then paradoxes must emerge (the grandfather paradox, for example). If this situation comes to be true my question is: Shouldn't a "logical constraint" arise to prevent the emergence of paradoxes in time? Maybe some kind of force field unknown to current science? This could explain why there are no tourists from the future appearing publicly. They simply can't because a logical (non) physical barrier prevents them. (However, they may be disguised among us in the present society.)

I will try another approach. There are four premises and one conclusion:

(1) time travel to the past is possible,

(2) there is only one version of the past, one timeline,

(3) the past (or the future) cannot be changed and

(4) there can be paradoxes (the grandfather paradox, for example)

Conclusion: Logic recommends that a kind of resistance or barrier should arise in order to avoid any paradox.

Trying to clarify:

If we take into account premises 2 and 3 and imagine a time travel to the past, we have to admit that a situation like that of the grandfather paradox would not be possible. For reason tells us that what has already happened cannot be changed, and since there is only one version of the past, a single timeline, it is not possible for the traveler to divert to another. Therefore the past must be considered fixed, unalterable. But even if the traveler tries to kill his grandfather, logic recommends that some kind of barrier should prevent the action of the traveler, being the past fixed. Whenever the traveler tries something against his grandfather that means changing history, a resistance must arise in order to prevent his actions. It's as if someone bumped into a wall. Only events and actions that do not imply changes in history would be possible. In other words, the traveler may be part of history but cannot change it. This would be a logical constraint in the structure of space-time that could not be broken.

My question is: Does the conclusion make sense?

I believe this text is a relevant complement: Heather Dyke (2005). The metaphysics and epistemology of time travel. Think, 3, pp 43­52


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    Hi, Welcome to the Philosophy SE, this question is not about philosophy per se and might be better suited for worldbuilding.stackexchange.com – Alexander S King Jan 10 '17 at 17:49
  • Ok, I'll see... – Elric Jan 10 '17 at 18:32
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    Given that the SEP has two articles which address this exact question I don't see why it isn't appropriate for the phil.se. The question seems to be to be asking "is a logical contradiction enough to disprove or prove time travel" and that exact question is handled in both of the articles – Not_Here Jan 10 '17 at 18:35
  • "Some kind of force field" is not a logical constraint. Colloquial meaning of "time travel" is very vague and confused, hence "time travel to the past is possible" is equally so. Paradoxes mean that this phrase simply makes no sense on straightforward interpretations, that is the "logical constraint". Careful authors either introduce some form of parallel universes (i.e. not really time travel) or assume hard determinism. In this last vein Hawking's chronology protection conjecture is probably closest to what you have in mind. – Conifold Jan 10 '17 at 19:19
  • What makes you think there aren't? One, most of us don't announce we're from the future. Two, those of us who do speak out end up getting labeled as crazy. That's why most of us don't speak out. – user4894 Jan 10 '17 at 19:36

There is no impetus to believe that there is some sort of non physical force field that prevents time travel; current scientific theory defines time travel and explicitly states what the potential barriers are to its actualization. These explanations are not based entirely on the philosophical argument that the physical world should avoid the grandfather paradox, however those arguments are often considered as extra evidence to believe time travel is impossible (if only as results of thought experiments). The current understanding of time rests in the foundations of our best physical theories and as such the answer to this question rests in them as well. To quote the conclusion of the SEP article on Time Travel and Modern Physics:

The only serious proof of the possibility of time travel would be a demonstration of its actuality.

The answer cannot be found within a logical argument alone.

Metaphysical stances on time

To quote the Internet Encyclopedia of Philosophy:

At the beginning of the 20th century, the appearance of the special and general theories of relativity and the theory of the Big Bang transformed the investigation of time from a primarily speculative and metaphysical investigation into one that occupied scientists in their professional journals.

Current understandings of time are no longer rooted solely in our phenomenology of temporal experiences. We now have empirical ways to test a theory of temporal (spatio-temporal) structure.

In regards to time travel, in the Stanford Encyclopedia of Philosophy's article on time in the section on time travel it states:

One question that is relevant here is whether time travel is permitted by the prevailing laws of nature. This is presumably a matter of empirical science (or perhaps the correct philosophical interpretation of our best theories from the empirical sciences). But a further question, and one that falls squarely under the heading of philosophy, is whether time travel is permitted by the laws of logic and metaphysics. For it has been argued that various absurdities follow from the supposition that time travel is (logically and metaphysically) possible.

The author then goes on to explain many paradoxes that arise with the belief in time travel, such as the grandfather paradox. However, then it is suggested:

Despite the existence of these and other arguments against the possibility of time travel, there may also be problems associated with the claim that time travel is not possible. For one thing, many scientists and philosophers believe that the actual laws of physics are in fact compatible with time travel. And for another thing, as I mentioned at the beginning of this section, we often think about time travel stories; but when we do so, those thoughts do not have the characteristic, glitchy feeling that is normally associated with considering an impossible story.

Which clearly shows that there is a contrasting view as to whether the logical contradictions can be used to show if time travel is possible or not. This statement is echoed in two other articles on the SEP: Time Travel and Time Travel and Modern Physics. In the second article the author states:

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent.

Scientific Definition of Time Travel

The scientific description of time travel is a phenomenon known as closed timelike curves. Closed timelike curves are paths that a particle can (more like might be able to but probably will not be able to in practice) take throughout spacetime that allow it to arrive back at its previous position in both time and space. The main reason that, under non extremal gravitational conditions, time travel is believed to be impossible is because in order for a particle to travel on a closed timelike curve it needs to travel faster than the speed of light in some reference frame. This goes against the theory of relativity (for more understanding of why the speed of light cannot be exceeded an understanding of Lorentz transformations is required). Now, there are certain solutions of Einstein's field equations that permit closed timelike curves. The most famous example is the one given by Kurt Gödel. However, it is generally agreed that the universe Gödel's solution describes is not the correct formulation of our actual universe. This paper by Al Momin explains why we should believe that and includes arguments on the philosophical reasons we should reject the model, not just the scientific (which might be closer to what you're asking for).

Kip Thorne gives us a good explanation as to why this problem is so hard to solve for science:

Much of the forefront of theoretical physics deals with situations so extreme that there is no hope to probe them experimentally. Such, largely, was the case nearly a century ago for Einstein’s formulation of general relativity, and such is the case today for the attempt to quantize gravity. In these situations, thought experiments can be helpful.

At the present time the only thing that allows us to believe the nonexistence of closed timelike curves are the immensely successful experimental verifications of the laws of relativity, including the speed of light being a universal speed limit, as well as the cosmological data that rejects the idea that our universe conforms to the one described by Gödel and other similar metrics. The philosophical weight of such timetravel paradoxes as the grandfather paradox add extra impetus for our belief that time travel is not possible. However, of course, that is not a formal proof that they do not exist.

Additionally there is another credibly theory of time travel that deserves attention. This is of course the existence of Einstein-Rosen bridges (wormholes). There is a lot of treatment of wormholes in the current scientific literature and it has been well understood since Einstein and Rosen's original 1935 paper that wormholes that arise under "normal" conditions are non-traversable. This means that a particle cannot go entirely through the wormhole and come out on the other side. To understand why this is the case you should read more about Penrose diagrams of a Schwarzschild blackhole. It is true that wormholes may be made traversable but the conditions for them to be are very unlikely to exist naturally in the universe and many physicists believe it is not possible for us to create them given our current understanding of what exists in the universe.


Given our current metaphysical stances on time, as outlined above, the answer to time travel lies in empirical experiment. There is enough room in the solely logical aspects of the question to allow for both answers (as demonstrated in the IEP and SEP articles linked above) and the metaphysical interpretations point us towards empirical experiments to find the answer. The current understanding in our best physical theories is that time travel is allowed by general relativity, however it is unlikely that our universe contains the structure to support closed timelike curves and traversable wormholes.

  • Thank you for your answer. My reasoning is that time travel to the past would be possible, but that a logical (non) physical barrier should arise against the paradoxes in time specially. Also, I know there is the limit of the speed of light for a moving object, but if I am not mistaken, for an object falling into a black hole this limit of light would not apply, gravity would take it further beyond this limit into the singularity. But I may be wrong... – Elric Jan 10 '17 at 19:26
  • It is generally agreed that paradoxes do not disallow time travel by themselves and that time travel is mathematically compatible with general relativity. The reason why most physicists and philosophers believe it is not possible is because all of the cosmological evidence we have seems to point towards our universe not being structured to allow for closed timelike curves and traversable wormholes. You should really check out the papers I linked for more explanation, especially the one by Kip Thorne and the SEP articles. – Not_Here Jan 10 '17 at 19:35
  • @Elric The limit of the speed of light is on all energy, including matter, and applies even in black holes. Gravity slows time, so stuff moves slower in black holes than outside them. – user9166 Jan 10 '17 at 19:35
  • As for the speed of light, as @jobermark just said it applies in black holes as well. There is a difference between a particle traveling faster than the speed of light and something like spacetime expanding fast enough that it looks like a particle is traveling faster than the speed of light. en.wikipedia.org/wiki/Metric_expansion_of_space See this for more explanation, but the point is that this is not the same as something like an electron having enough energy as to cause it to exceed the speed of light. – Not_Here Jan 10 '17 at 19:38
  • Not_Here Thank you. I'll save your text for future reference. – Elric Jan 10 '17 at 23:01

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