The question I have is about the details of cause and effect occurring out of standard temporal order. Distilling it down to a single scenario:

Consider two people, Alice and Bob, in a closed room. Bob has a device that can send him back in time 2 minutes. After 5 minutes in the room (at T=5), Bob uses the device. From Alice's perspective, at T=3, a second Bob, Bob2, appears in the room. He is the Bob 'from the future.' This is pretty standard in science fiction.

Now, for my confusion:

After this occurs, At T=4, Alice pulls a gun and kills Bob1, then takes and uses his device. Thus, Alice2 appears at T=2 from Bob1's perspective.

Now, at T=3, does:
1) Bob2 appear, because him appearing then was a part of the timeline when Alice used the device?
2) Bob2 fail to appear, because extrapolating Alice's unrealized timeline out past her loop, Bob1 could not go back to become Bob2?

ie, Can you change the past/present by changing the future?

EDIT: It was asked what would happen to Bob2 in Timeline 2 in my interpretation of time travel. My theory is that paradoxes resolve at the nexus of the timelines. For instance, in the above scenario Bob2 would vanish at T=5, because this is the temporal point where he should have been sent back from the other timeline. However, this point has been reached in the new timeline without said event occurring, so he ceases to be. (Also editing question with this)

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    $\begingroup$ You skipped over Question 0: At T=4, when Alice kills Bob1, does Bob2 vanish in a puff of smoke or fade out like an old newspaper clipping? $\endgroup$ Nov 8, 2016 at 22:41
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    $\begingroup$ @TwoBitOperation:  If you have a theory of how time travel works in your world/universe that explains how Bob2 can persist when Bob1 is no longer able to bring him into existence, you should explain that.  The more we know about your world, the more relevant answers we can give you.  Please do not respond in comments; edit your question to make it clearer and more complete. $\endgroup$ Nov 9, 2016 at 6:59
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    $\begingroup$ Relevant: en.wikipedia.org/wiki/Novikov_self-consistency_principle. $\endgroup$ Nov 9, 2016 at 7:42
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    $\begingroup$ I read this as “In a world with time travel, could one change the president by changing the future?”... $\endgroup$ Nov 9, 2016 at 10:35
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    $\begingroup$ isn't that effectively the same as the grandfathers paradox? you travel to the past, and then your past self gets killed (either directly or by killing your grandparents). it doesn't matter who does the killing, like in this example, bob2 could be the one killing bob1. the end result would be the same. $\endgroup$
    – eMBee
    Nov 9, 2016 at 11:23

8 Answers 8


There is no one answer to your question. It depends entirely on which time travel approach you take. There are many models of time travel in fiction, and since there is no evidence of time travel in science, fiction is all we have to go off of.

My personal favorite system for authors to explore is a causally consistent universe, where if something happened, you cannot prevent it from happening, but you can alter it. This approach is valuable because it can actually be ratified against modern scientific laws.

As an example, there was a proof a few years back done which proved that you could always be causally consistent in a simplified universe with 2 billiard balls and a wormhole. The basic setup is that you throw one ball such that it strikes the other and that other ball passes through the wormhole into the past. If you strike it just correctly, you can have the other ball pop out of the wormhole (in the past) and collide with your billiard ball before the collision occurred, knocking it off track. The argument would be that the collision never occurs, therefore the other ball never travels through the wormhole, thus inconsistency.

The paper showed that there was no configuration of billiard balls and the wormhole which could not be made consistent. The trick was that you would throw the ball forward, on a collision course, and the future-other ball would pop out of the wormhole to collide with yours, but at a slightly different angle than you had calculated in the inconsistent case. At this angle, the future-other ball grazes your billiard ball just right such that yours still collides with the present-other ball to enter the wormhole... exactly at the strange angle it came out from. It showed that, no matter the configurations, you could find such a consistent solution.

However, consistency is not the only option. There are plenty of science fiction systems with multiple timelines, or timelines with particular rules which are not obvious (I recommend the movie Primer for an example).

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    $\begingroup$ @TwoBitOperation I highly recommend Sanderson's First Law of Magic. "An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic." Written for magic, it applies to extreme science fiction (like time travel) as well. Satisfy that law, and readers will appreciate it. Whatever time travel system you choose, if you follow Sanderson's Law, it will be accepted. $\endgroup$
    – Cort Ammon
    Nov 8, 2016 at 19:56
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    $\begingroup$ @CortAmmon: Can you provide a link to said paper and/or explain further? In particular, you say that for each configuration, there is a consistent solution; I'm wondering what information about the system is given to us in the "configuration", and what information remains to be solved. $\endgroup$ Nov 8, 2016 at 23:50
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    $\begingroup$ @MeniRosenfeld It looks like the wikipedia page on the Novikov self-consistency principle has a section dedicated to the different variants of this problem. $\endgroup$
    – Cort Ammon
    Nov 9, 2016 at 0:15
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    $\begingroup$ The paper in question, referred to above, is "Billiard balls in wormhole spacetimes with closed timelike curves: Classical theory (by) Fernando Echeverria, Gunnar Klinkhammer, and Kip S. Thorne Phys. Rev. D 44, 1077 – Published 15 August 1991" The causal consistency works because there are an infinite number of paths for the bowl ball to collide with itself, earlier. $\endgroup$
    – a4android
    Nov 9, 2016 at 0:45
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    $\begingroup$ I recommend just looking at a timeline of Primer and skipping the movie. Might be great in its consistency, but I loathe the writing. $\endgroup$
    – Nick T
    Nov 9, 2016 at 18:22

In a world where changing the past is possible, changing the future can potentially change the present or past, since time is non-causal. Whether changing the past is possible depends on what model of time travel you choose to use in your universe. But even if changing the past is impossible, supposing that changing the future is possible,

the present will be altered by even observation of the future

(Well, technically, the closest future to the present)

Consider reading Philip K. Dick's short story Meddler. In the story,

a government takes "time dips" (essentially just taking a snapshot) into the future, to see the result of a policy they had just passed. Everything was hunky-dory, but when they looked again to the same time, they found the world had problems that hadn't existed before. In fact, each time they checked, it got worse and worse until the human race had disappeared entirely.

(It's a really good read, btw)

Though the story doesn't explicitly explain why this happens, the implication is that the characters' knowledge of the future affected their present course of action, diverting the future timeline away from what they had seen on the previous observation.

But on the bright side, there's no chance of a time paradox.

  • $\begingroup$ but why would observation of the future necessarily change the past? Perhaps the observation happens to be of the sort that locks it all in place. After all, you should always see the same future unless you are looking into parallel dimensions. In that case, the future would cycle through many different versions in some non-temporal sense of time until it landed on a final future "set in stone". In that case, the past changed, but it also permanently fixed its future in stone. $\endgroup$
    – user64742
    Nov 9, 2016 at 1:58
  • $\begingroup$ That sounds like an interesting story, thanks! $\endgroup$ Nov 9, 2016 at 21:14
  • $\begingroup$ @TheGreatDuck I think it's more of a "changing the future can change the past relative to it" as in if you change the future and are aware of it, your course of actions in the present might change to those that led to that future in the first place, thus changing the future's past. $\endgroup$ Nov 10, 2016 at 12:39
  • $\begingroup$ @MiguelBartelsman "if you change the future and are aware of it" Stop right there. You cannot be aware. The instant you observe the future the future will immediately become whatever future occurs from you viewing it. So you cannot change the future from what you saw because it will instantaneously for all times change to a version that implies no change occurred. Your actions from then on out will simply fulfill the future you saw. $\endgroup$
    – user64742
    Nov 10, 2016 at 15:30
  • $\begingroup$ That's basically what I said... if you see the future, the future will change because you will do thing differently than you would have. $\endgroup$ Nov 11, 2016 at 12:53

Agree (and upvoted) Cort Amon's answer: It depends on how time travel works in your universe.

Basically you should decide if travelling back in time can affect the future or not. That is, if your time is linear or a tree.

If time is linear, everything you do happens in the same timeline, so you can affect the future you came from.

If time is a tree, everything you do now creates an alternative timeline (now), so if you travel back from future, in the exact moment you arrive you create an alternate timeline, so you can never travel back to THAT future you came from. Maybe to one quite similar, but not to the same. You came from a future in which you weren't in the present, and now you are in the present, so it's a different timeline.

If there are different timelines, Bob1 never weren't killed in the timeline he was when he travelled back, so he will appear. If there is only one timeline, Bob1 has no opportunity to travel back.

  • $\begingroup$ Totally agree. In a Multiverse ("tree") system, bob disappears from the room and Alice never sees him again. In an alternative universe he may appear and be shot by a startled Alice2, but then he's dead and Bob2 is going WTF? Alice2 picks up the device and goes back in time, but vanishes from Bob2's universe and appears in Box3. Hilarity ensues. $\endgroup$
    – Thucydides
    Nov 9, 2016 at 0:16
  • $\begingroup$ David Gerrold's novel The Man Who Folded Himself is a good example if time being a tree. $\endgroup$
    – lsd
    Nov 9, 2016 at 18:20
  • $\begingroup$ My universe is definitely in the tree version of things, and I agree with what you are saying. I think the wrinkle in my issue is that you have two competing timelines that both have influence over the 3rd, but are somewhat incompatible with each other. @lsd that book has been on my to-read list for far too long, and I really should pick it up soon. $\endgroup$ Nov 9, 2016 at 21:18
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    $\begingroup$ @TwoBitOperation if it is really in the Tree system, the point is "you can not affect futures of other timelines, and you can not affect your past at all". With this clear in mind, there is no wrinkle. In Timeline 1 nothing happens until Bob uses machine at T=5. Bob arrives at T=3 creating Timeline 2. At this point IT DOESN'T MATTER ANYMORE if Bob travels back in time at T=5. It was "Bob1 from Timeline1" the one who arrived and is no named as "Bob2 in Timeline2", so now we are in Timeline2 having Bob1 and Bob2. Whatever happens at T=4, Bob2 is already here. $\endgroup$
    – Envite
    Nov 9, 2016 at 21:50
  • $\begingroup$ @Envite I think I get what you're saying here, and if I'm correct it leans towards choosing outcome #1. Since Timeline 3 splits from Timeline 2, (which had Bob2 appearing at T=2) Bob2 must appear at T=2 in Timeline 3 as well (or is he Bob3 now? A question for another day...). There is no paradox because from a narrative POV we do not stay in Timeline 2. $\endgroup$ Nov 16, 2016 at 21:03

Here is another interpretation that might help your story. In this interpretation, whole universe is done in an instant. Any time travel, if exists, will not cause any changes as the result of that time travel is already factored in. Imagine the universe as a hyper-sphere with space-time combined. Now when it is created, time is also created with it. Now in this interpretation, time travel is a way to weave the universe as its results are necessarily tied to the current timeline. This means either Alice will not be able to kill the Bob due to a reason, or Bob will not be able to run the time travel machine, it will malfunction or something else will happen. At the end of the day, whatever would affect the past from the future, has already have that effect on the present. There are no multiple timelines and there will be no alterations.

  • $\begingroup$ The "whole universe is done in an instant" is usually called the block universe. This is the deterministic model of causality and time travel. The past even with time travellers, is what it was and always will be. $\endgroup$
    – a4android
    Nov 10, 2016 at 11:15
  • $\begingroup$ Thank you for naming it, I come up with it one day and didn't know the name. I just assumed someone else already published it. $\endgroup$ Nov 10, 2016 at 11:23
  • $\begingroup$ Quite right. The topic has been written up by philosophers interpreting Minkowski spacetime. Physicists have published extensively too. That's the trouble with good ideas, usually someone else has already thought of them first. $\endgroup$
    – a4android
    Nov 10, 2016 at 11:34

i believe this situation is effectively the same as the grandfathers paradox.

bob2 travels to the past, and then his past self, bob1, gets killed (either directly or by traveling further back and killing his grandparents).

it doesn't matter who does the killing, like in this example, bob2 could be the one killing bob1. the end result would be the same.

your question is about "whether an event at T=4 can affect an event at T=3".

although the grandfathers paradox generally focuses on the question of what happens at T=5, it is still the same situation, because T=3 is the moment the grandson arrives in the past, while T=4 is the moment the grandfather gets killed.

you can read more about the grandfathers paradox here: https://en.wikipedia.org/wiki/Grandfather_paradox


Most theories on time travel depend on their being a fixed timeline through the universe. A leads to B, which leads to C and so on... In your scenario because B didn't happen then C, D, E, and F unravel. The paradoxes start when a previous action which didn't happen promoted this unraveling.

However, consider for a moment that the universe doesn't have timeline - people do.

You have two people in your scenario, Alice and Bob. Instead of worrying about what the universe sees, let's look at what Alice and Bob perceive.


  • At T = 0 Bob has a time travel device, he doesn't use it. Instead he stares vacantly around the room. Maybe he checks out the slightly psychotic woman he's with?
  • At T = 3 Bob watches himself appear in the room "Oh, I obviously used the device!" he thinks. To his horror he watches Alice turn the shoot him. "Bother..." he thinks "Maybe that's not such a good idea!"


  • At T = 0 Alice is in a room with Bob, maybe she likes him - maybe she doesn't. Either way she has much more interest in the time travel device he's holding... but does it work?
  • At T = 3 a second Bob appears "Oh, so it does work!" she thinks. Determined to possess the device for herself she shoots the first Bob and hopes some temporal paradox will save her
  • At T = 3 and a bit Bob2 looks at her and shouts "Oi - you just killed me", he's very upset but he doesn't disappear in a puff of smoke because he does exist in this timeline.

For bonus points

Bob 2

  • At T = 0 Bob has a time travel device, he doesn't use it. Instead he stares vacantly around the room. Maybe he checks out the slightly psychotic woman he's with?
  • At T = 5 Bob gives in and presses the button, he vanishes and reappears in a room with himself and mad Alice
  • At T = 3 and a bit Bob2 watches in horror as his younger self is shot... he's very upset by this
  • At T = 3 and two bits he sits down with Alice (who now also has a time travel device) and they talk about how upset they both are.

The reason I believe this is a much more plausible way of thinking about time travel is that there are no paradoxes and no one fades in and out of existence. Timelines and continuity depend on the viewers perspective and not from that of some omnipotent being.

  • $\begingroup$ But clearly you are describing different timelines. This doesn't help explain which actions are possible in the OP. $\endgroup$
    – JDługosz
    Nov 10, 2016 at 12:05

I like how the game Achron answers this question. Achron is a real time strategy game in which you can send units back or forward in time, and doing so costs more resources the farther in time you want to go.

The fascinating part is how the game's internal time engine resolves paradoxes. It can actually resolve the grandfather paradox, and it does so in a quite ingenious manner.

Let's say I build a barracks, from which I can train soldier units. The barracks finishes building at T=5.

Next, I simultaneously build a time machine and a soldier. The soldier finishes at T=10 and the time machine finishes at T=20. Now, I use some resources to send the soldier back in time to T=0.

The soldier waits there until T=5 when the barracks is built. I then instruct him to destroy the barracks which takes him 4 seconds, completing at T=9, one second before it created the soldier. I have now initiated the grandfather paradox.

What happens now is called a time wave. At T=10, the barracks no longer exists and thus it could not have trained the soldier that now stands in front of its ruins. This creates a time wave which takes 10 seconds to propogate. It's like an echo of time where what actually happened catches up to what should have happened. However, during the ten seconds the soldier is still standing there. Why? Because it wasn't until T=20 that the soldier went back in time and altered the course of events. The first moment where something different happened was T=10, ten seconds before T=20, where the time wave began. So for those ten seconds, nothing happens.

Now, at T=20, the time wave catches up with us. The soldier vanishes because he destroyed the barracks that created him.

But wait! Here's the interesting bit. We are now in what I would call phase two of a time wave oscillation. Waves (sine and cosine for example) oscillate continuously between two distinct states. What has happened now is that the soldier that destroyed the barracks at T=9 no longer exists, and therefore could not have destroyed the barracks in this branch of time. It doesn't even appear at T=0, where it came from the future.

So ten seconds after T=0 when the soldier no longer appears, the time wave propagates again, and now the barracks reappears at T=5 when it was originally created, and there is no soldier around to destroy it. At T=10, the barracks still exists and finishes training a soldier just as it did before the time wave hit, and we are back at phase one.

So we continue to oscillate between two conflicting series of events until some other event (like the opponent winning the game, or becoming bored and quitting) occurs to jar us out of the time wave.

Having recently watched Doctor Strange, this also reminds me of

the ending where Doctor Strange creates a time loop and defeats the being that rules the dark universe.

In this model of time, the answer is yes: you can alter the present by altering the future. My example was of sending a unit into the past, but you could just as easily send a unit into the future and create a time wave or paradox that way.


An omnipresent, omniscient algorithm God-like being, could view the future state, then the present state, (and all intermediate automata states in the sequence). Then, when everything looked ok, taking fine care that one state in the sequence truly and properly implied the next, just run the t(ime)-dependent algorithm.

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    $\begingroup$ A literal deus ex machina? $\endgroup$ Nov 9, 2016 at 6:52
  • $\begingroup$ I don't speak Latin. What is that? $\endgroup$ Nov 9, 2016 at 7:00
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    $\begingroup$ From Wikipedia: “literally meaning "god from the machine", the term has evolved to mean a plot device whereby a seemingly unsolvable problem is suddenly and abruptly resolved by the inspired and unexpected intervention of some new event, character, ability, or object.” $\endgroup$ Nov 9, 2016 at 7:04

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