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.

My question is: Does the conclusion make sense?

Side note (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's 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.

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


  • $\begingroup$ One way to deal with it is called Novikov self-consistency principle $\endgroup$ – Mrkvička Jan 10 '17 at 19:52
  • 1
    $\begingroup$ Welcome to the site, Elric. If that's how you want to define the laws of time travel in your world, go ahead. I don't understand what you want from the community. Maybe you could edit to clarify? $\endgroup$ – Frostfyre Jan 10 '17 at 19:53
  • $\begingroup$ Thanks. I have read about this principle of self-consistency for some time now. Sorry my bad English, I will try to clarify. 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. If time travel to the past is possible and should happen, and if there is only one version of the past, paradoxes would be inevitable, and if so, the logic inherent in the Universe must unleash a kind of resistance in order to prevent any paradox. $\endgroup$ – Elric Jan 10 '17 at 21:01
  • $\begingroup$ I have once read a rather silly SF novel where the protagonist, who works as a guide for a time-travel agency, complains that two thirds of the onlookers at the crucifixion of Jeses were time tourists... $\endgroup$ – AlexP Jan 11 '17 at 13:06
  • $\begingroup$ "The objection that time travel entails the possibility of changing the past rests on a mistake. It is a mistake that is made time and again in the stories of science fiction writers, and it is simply this: to suppose that times, and the events which occur at those times, happen more than once. On the basis of this supposition it seems plausible to assume that the events of a certain time can turn out differently each 'time they occur. This mistake may be motivated by stories of time travel which..." $\endgroup$ – Elric Jan 14 '17 at 20:36

Backwards time travel is self limiting.

Imagine you invent a time machine. You go back to last week and re-watch a sports game you found interesting. No change, no problem. Next time you do something more interesting (or not next time but ten times from now) you give old you lottery ticket numbers. Bam he's rich. But now he has no reason to work hard in his workshop. The struggle leading to the invention of time travel never existed and time travel doesn't exist anymore. Time travel thereby erases itself from time. Now imagine something way more innocuous, you go back a few hundred years and visit Venice, but the mere fact of you being there causes ripples of effect. The world is a complex system with sensitive dependence on initial conditions. Tiny changes compound over time becoming literally unpredictable. A bus is five seconds late because of some insanely complicated chain of events caused by your presence hundreds of years ago. Your parents never meet, bam! time travel unravels itself again.

This will (must!) happen each time time travel is invented. All possible chances to change the past must be considered simultaneously. (because they all immediately have causal influence on the time stream) Every single possible excursion into the past is a chance to change the future. Every change has the possibility of wiping out the exact time and place that leads to the invention of time travel. The time stream will then cycle through all possible changes till it settles on one where time travel does not exist. (a time stream where time travel stops changing things and thereby stops destroying itself)

Time travel has been/will be invented many times. It destroys itself each time.

  • 1
    $\begingroup$ This smells like a story. Is there a story like this? $\endgroup$ – Willk Jan 10 '17 at 21:02
  • $\begingroup$ Interesting your line of reasoning, but what happens when I put the following condition: (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. $\endgroup$ – Elric Jan 10 '17 at 21:20
  • 1
    $\begingroup$ @Elric Watch that third step, it's a doozy! This would necessitate preventing information or matter transfer from the future to the past. This restriction would certainly protect causality and preserve the timeline. To truly separate it you'd have to show up in the past as a ghost. Cannot be seen or touch anything. Not that interesting (but a potential goldmine for historians) $\endgroup$ – jorfus Jan 11 '17 at 18:06
  • 1
    $\begingroup$ Or there could be some crazy unseen ghost in the machine where the time traveler can do anything he wants but somehow nothing ever manages to change anything. Like you go back and kill Hitler, but it turns out that Hitler was always killed by a time traveler and the kind gentle vegetarian painter gets replaced by his murderously insane younger brother. $\endgroup$ – jorfus Jan 11 '17 at 18:09
  • $\begingroup$ @Will there's no story like this, I invented it. I've been bothered by the causal paradox of time travel for a long time so I spend an inordinate amount of time thinking about it. $\endgroup$ – jorfus Jan 12 '17 at 20:36

Not the answer you're looking for? Browse other questions tagged or ask your own question.