In a world with time travel, could one change the present by changing the future?

Question

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?
or
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)

Answer 1

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).

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

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.

Bob

• 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!"

Alice

• 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.

Answer 7

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.

Answer 8

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.