Let's say that we have a time machine. We take a trip to the past to.. Let's say, kill Hitler. We go back, kill Hitler while his was still a child and return to the present day. But now in this timeline no one knows who was Hitler, and that include us. So if we never knew who was Hitler then we wouldn't have time travelled to kill him in the first place. We created a time paradox. So my quiestion is: what happens now that we have created a time paradox? Will the universe be torn apart? Or will the universe bring things back to normal as they were before we time traveled?

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    $\begingroup$ For the downvoters, isnt it friendly to leave a comment as to why you think the question is bad? $\endgroup$
    – Demigan
    Aug 4, 2018 at 17:36
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    $\begingroup$ I expect the downvotes are due to the question being largely opinion-based and/or unanswerable. Real science can't say for sure how a paradox caused by time travel would work because (at least so far) no one has actually done it. As for science fiction, there are just... so many stories about time travel, most of which address what happens when a paradox occurs in some capacity. It might be better if the OP can at least establish some ground rules about how time travel works in their world. For example, are we allowing the multiverse theory, or can there only be one universe? $\endgroup$
    – Steve-O
    Aug 4, 2018 at 20:08
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    $\begingroup$ You do need to consider how time travel functions in your fictional world. What are its limits? Whether it is deterministic or non-deterministic? Most time paradoxes arise with classical causality. The universe can survive more than a few simple time paradoxes. It's only humans who get confused about them. $\endgroup$
    – a4android
    Aug 5, 2018 at 9:04
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    $\begingroup$ The answer to this question very much depends on how you define your time travel to work. There are a load of different time travel paradigms, some much trippier than others. $\endgroup$
    – Joe Bloggs
    Aug 5, 2018 at 9:55
  • $\begingroup$ Does your world allow for multiple timelines, and if so, does it allow for branching? $\endgroup$ Aug 6, 2018 at 4:29

2 Answers 2


This scenario seems fairly simple. 3 timelines will keep switching with each other.

Timeline 1, the "true" Timeline. Hitler lived, the time traveler decides to go back and enters Timeline 2.

Timeline 2: this one automatically exists the moment you time travel regardless of what you do. Because in Timeline 1 you never traveled back in time and never changed anything. Should you kill Hitler, then you won't make the decision to go back in time but you'll do something else instead.

Timeline 3: you do something else instead, Hitler lives and assuming you didn't do worse things with your "something else", you'll go back in time to kill Hitler, meaning you end up in a loop between Timeline 2 and Timeline 3 while Timeline 1 continuous on without you ever existing in it (if it isn't destroyed that is).

When you look at time travel, there's in effect a dozen different things that could happen (https://www.google.nl/amp/s/amp.space.com/21675-time-travel.html). The way you describe it I would assume a more classical sci-fi interpretation, but these often seem to miss critical parts in their reasoning to make sense.

  • $\begingroup$ This only works with multiple timelines. Much more challenging to consider what happens in the case of a single timeline universe. $\endgroup$
    – a4android
    Aug 5, 2018 at 8:59
  • $\begingroup$ @a4android I dont see your point. Either the Timeline starts to regress, and any time traveling cannot be undone. So you go back to kill Hitler, time progresses and you or someone else decides to go back in time, even if they kill the original you, you'll still pop into the Timeline to kill Hitler (although someone could wait to kill you as soon as you pop in). Eventually the Timeline becomes filled with time travelers until the universe is full. Alternatively you get the same situation as with the 3 timelines, but with a single Timeline that simply progresses from the point of timetravel. $\endgroup$
    – Demigan
    Aug 5, 2018 at 10:24
  • $\begingroup$ You are confused.Single timeline: A kills Hitler, returns to own time & finds they are an orphan -- unless A was born before Hitler was killed. Making A very old. Orphan A lives on in a now Hitler-free timeline. There will be nobody who wants to kill A because of a non-existent Hitler. The time police might intervene if killing Hitler was unlawful & they knew of his existence. This assumes a detection system that anticipates time paradoxes & prevents them happening. A wants to kill Hitler, but stopped by time police. No time paradox. $\endgroup$
    – a4android
    Aug 6, 2018 at 4:30
  • $\begingroup$ There is a James Hogan short story where time travellers (TTs) try to kill Hitler. The past fills with TTs trying to kill AH & others trying to prevent it. History goes into overload. Sorted out by time police or similar, history continues unchanged. $\endgroup$
    – a4android
    Aug 6, 2018 at 4:33
  • $\begingroup$ @a4android as far as I can tell there's 3 ways a single timeline can work. A regressive timeline or cloning timeline where you could go back 1 minute and a few meters away, then tell yourself to move a few meters away again and keep creating clones of yourself (and possibly the time machine) this way. If you killed yourself before you go, the previous time travel isn't undone. There's two progressive timelines. In one each time you go back the timeline will "reset" to that point, and if you convince yourself to go back to another point in time the you that told him will never have traveled. $\endgroup$
    – Demigan
    Aug 6, 2018 at 12:44

John Brunner answer this very elegantly in his short novel Times without Number. Each time a time traveler goes back and changes the past there is a risk of changing it so much that time travel is not subsequently discovered. Sooner or later that happens, and now you have an unchanging history.

The conclusion: If time travel can exist, we will still never remember it having been invented -- though we might remember people who claimed to be time travelers from the future visiting us...


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