# Is this time travel scheme internally consistent?

I know there are similar questions, but not similar enough, and in fact, I don't remember seeing this kind of time travel in fiction.

Before I start, it doesn't matter how the time machine works in this world (could even be many different types), and time travel is allowed to violate matter/energy conservation laws (i.e. time traveler or time machine appears out of thin air).

Now the rules are as follows:

• Only time travel to the past is allowed.
• The timeline always changes due to the arrival of the time traveler.
• There's only one real timeline. The "old" timelines are destroyed forever.
• There's a first ever time traveler by the time of arrival.

That last part is important. Clearly, time travel events are countable. So if you go back in the real timeline, you eventually find the earliest arrival. It doesn't really matter when this time traveler departed in their original, destroyed timeline.

I believe this system is internally logically consistent and doesn't allow for any paradoxes. If you travel back and kill your grandfather, you will just have to live your life knowing your double will never be born.

Here's an illustration of multiple time travelers (or time travel events) in this scheme:

As you can see, time travelers are ordered by their arrival time and nothing else. They can bring records, devices and anything else from their own timeline and all of that will exist, even though the timeline itself is lost.

The first ever time traveler doesn't have to be the "original" one either. For example, it could have happened like this:

Somebody invented the time machine, tested it out, which created a new timeline, then somebody else got a hold of the same technology and traveled back far enough to become the first arrival.

So what I'm asking is this:

• is this system internally consistent?

• how to explain why somebody later who finds out the arrival time of the 1st (2nd, 3rd) time traveler can't travel father back?

I mean, that would make everything inconsistent again. Maybe they can and will simply become the first time traveler themselves? But where does it end? Now I'm confused again. Logically it has to end somewhere so we get a final timeline with a set and ordered list of time travelers up to some point.

Maybe Primer idea is not that bad (for those who haven't seen the movie SPOILERS ahead): we can only travel back to the point when the time machine was first turned on. But I don't like this restriction very much.

Some extra stuff that's not very important:

People often complain that maleable timeline stories don't have high enough stakes because the character can always go back and change something else, or if multiple realities coexist, then nothing even matters. But here's only one eventual reality and nobody will ever be able to recreate their original timeline, especially due to multiple time travel arrivals after their own.

I think there's a good narrative potential here. For example, there could be a whole society (or two competing ones) made of time travelers who are all by definition come from different timelines, quite possibly very very different. With different societies, technologies etc.

• Infinitely divergent, without any corrections able to be made because: multiverse. Opportunities for drama are up to the writer, it's not as if this hasn't been applied innumerable times to great effect. Dramatic stakes are more emotional and relational than to do with setting - but that's a writing issue. This seems to need narrowing to the one question (the first one), the second would be off topic. Please edit down to one question for comfort. Jun 17 at 21:37
• Isn't this the same scheme as Primer?
– rek
Jun 17 at 21:40
• Thanks for the edit, that helps clarify the issue at base, how to stop the infinite divergence of timelines with their inherent likelihood of making things more complicated. Ps. Marvel's solution was to cite the existence of a set of stones - the "time stone", which can be destroyed. I leave it to you. (But when do you need to destroy it......?) Jun 17 at 22:32
• You can go a different route than primer. Only travel as far back as your own time alive, or travel costs something, like it'll age you. That way, people cannot constantly go back and change things and it limits how far you can go, which reduces the amount of times someone going back further. Alternatively it is just a fixed given. You cannot go back further than the first traveller, because of time machine shenanigans (they can't cross over a time displacement point or something). Jun 18 at 4:58
• @walen "As explained, it is impossible for more than one time traveler to exist in the same timeline" Suppose I'm from a timeline where the first time traveler arrived on Jan. 1 1800. If I take a trip back to Jan 2 1800, won't I be able to meet the first time traveler with no inconsistency? I'm only "over-writing" the previous timeline from Jan 2 1800 on, everything before that should stay the same, including the arrival of the other traveler. On the other hand, if I went back to before Jan 1 1800 I'd probably negate the other's arrival, though this isn't actually spelled out in the OP. Jun 20 at 22:37

Seems internally consistent to me, but it seems like it might /lessen/ the stakes - anybody with time traveling tech can make an any amount of what happens completely moot at any time. In fact, the whole "meta timeline" seems like it would get so messy with temporal resets that it would have to eventually reach the only steady-state possible: a time traveler that goes back far enough and ensures that time travel becomes impossible for whatever reason. Or in other words, a world without time travel or time travelers ;)

• Which leaves the whole of the rest of the plot within a closed loop. "It happened, but it didn't" sort of scenario. Might someone then rediscover the tech, then...... (The sequel: "Just when you thought it was safe to trust causality..") Jun 17 at 21:43
• Thanks for the answer. But your conclusion is not possible within the proposed scheme. We already have 1st ever time traveler. Nobody else arrived before them. We also have 2nd time traveler, 3rd and so on. Their places in this list are set. You can't prevent time travel. There's only one timeline, the rest are just memories Jun 17 at 21:47
• @YuriyS Who's memories? Anyone from any timeline can re-invent travel and then go back to before the first event, unless you say they can't. If the creation of an infinitely divergent multiverse is possible at all, then it will result in uncontrolled interference as far back as time exists, unless there is some definite end-stop put in place, invention doesn't seem enough, it must be something which can be removed from existence completely, else infinite timelines and chaos. Jun 17 at 22:05
• They can't because they haven't. That's where the consistency comes from. Again, only one actual timeline exists and in that timeline has to be the first ever arrived times traveler. It's simple enough to place some restrictions to prevent stupid stuff like time travel all the way to Big Bang Jun 17 at 22:08
• They haven't when? @YuriyS (don't forget the @ function to poke people). Are you saying that time-travel can never (even after 100,000 years of technological and scientific evolution) happen? Jun 17 at 22:09

Larry Niven addressed this one--in a universe such as this there's only one stable state--the one in which a time machine is never invented.

Thus all that will be observed is no time machines.

This is interesting.

Usually it's either grandfather trope or multiverse trope:

1: Grandfather trope. The events of the current timeline determine it's past & future and you must preserve the grandfather rule

or

2: Multiverse trope. The events of the current timeline are unlinked and you can change whatever you want because you are from another dimension. However many times you jump there exist the necessary number of hypothetical dimensions to support your adventures.

But this seems a bit like something slightly different, which I might call

3: Linear meta-time The meta-timeline is linear & meta-causality cannot be violated. Because a nonexistent future can influence the past, then the events of past futures must be preserved - ie you cannot go back and change events which happened in a now destroyed timeline.

A weird artefact seems to arise from this:

Each jump requires the existence of n^2 + 1 timelines where n is the total number of jumps ever made. Each jump must create a series of new timelines timelines. I may have screwed that math up but you get the idea, it's more than one.

For example:

For the first jump, 1 new timeline/dimension was created and 1 was destroyed. You are now jumped into the past of another you. You make a second jump, where does the original you jump into on the original jump? The first timeline or the second? If the original jumps into the first timeline then who jumps into the second? You could reconnect the jump back to the current timeline but then what happened in the original jump target timeline? This would end up being a violation of meta-causality.

If people can jump from nonexistent futures, meta-causality must be preserved because the current timeline depends on meta-events in the meta-past, ie it's causality is not linked to it's own retrograde timeline. Therefore, the jumpers into the current timeline must have an origin other than the origin they had in the previous timelines ie, they must come from n parallel timelines. The multiverse trope deals with this differently by saying there are infinite dimensions anyway.

Another artefact of this meta-linearity is that time travel is ungetridofable as soon as the first jump is made*

You can't go back and prevent the first jump because the jump source is in another, nonexistent timeline. The only way to eliminate time travel is to create a dimension where every copy of the time machine is destroyed, which assuming the machine is carried on the jump will mean knowing every past jump and travelling to every jump point, including your own. You will generate n+1 new copies of the time machine for every n copies destroyed.

If the answer is "yes" then every other jump from a nonexistent future also still gets made and you will always need to reference the previous timelines to contextualise the current timeline.

There aren't any obvious logical difficulties with that system; it would be easy to implement those rules in an interactive simulated environment like a video game. There might be technical aspects of modern physics that pose a problem for this existing in a real universe that works like ours, but I assume you aren't trying to create an extremely hard science scenario, just to avoid blatant inconsistencies.

The existence of time travel that works in the way that you describe tends to imply that the world the story takes place in, the A-series model of time is correct: in other words, there is a meaningful difference between the past, present, and future that goes beyond just information about the relative order of events.

The idea that your model leads to an "unstable" timeline that is brought up in Loren Pechtel's, Gene's, and Mr Bullshit's posts only poses a logical problem in a world where the B-series model is correct. That's because the B-series model implies that the future "exists" as much as the past or present, and the future is where time travelers would come from.

But if you use a model of time where the future does not exist, you can say that regardless of whether it's true that a timeline like the one you describe is almost certain to eventually be reset sometime in the future, the reset hasn't happened yet at the time that you are narrating the story. In other words, there is no way a "jump from a nonexistent future" by a hypothetical time traveler from an alternative future branch can affect the present, because future branches do not exist, only from the present (which is constantly changing as time "passes").

Narratively, one potential issue with this type of time travel is that it effectively permanently destroys a universe's worth of people each time someone travels back in time. (While also re-creating the people of an earlier state of the universe.) So it can be argued to be morally equivalent to murder on a gigantic scale. If it isn't treated with this level of moral gravity in the story, that might cause dissonance for some people.

Also, I think some people might feel like this system does not feel as much like true "time travel" as other possible models. It can be thought of as a "universe reset button" for everybody but the time-traveler and whatever they take with them; using spatial metaphors, this is less like "traveling" from one location to another and more like exploding and then rebuilding everything in a particular location aside from what's in one particular protected spot.

In fact, while the illustration you gave is one way the information could be presented, the timeline of your world could also be presented as one non-branching line that includes the "destroyed" timelines up to the point where the time traveler resets the world state of the world to what it was at a previous point.

I think there's an inconsistency, or a risk of it.

See update below

You are saying 2 things that contradict.

First, as history now stands (and as it "always has been" except for perhaps some peoples memories") there is a first time traveller arrival. Call that time of arrival, T.

That implies that history can include a time traveller arrival.

Now, suppose I want to travel further back in the past than T. No reason, I just want to.

Your response to that is, it can't happen because it didn't happen.

If that's true for me wanting to travel back before T, then it was also true for whoever the traveller was,who arrived at T.

So how did that arrival (at T) itself, ever happen?

As Loren's answer says, the only stable solution is no time travellers, or do you have a different resolution?

Update

On reflection maybe this wouldnt be a contradiction? From that world's perspective, a person might say "perhaps there have been different ways things happened, but we can never know, and they don't exist now". That's a consistent statement.

One might also say that there is only one history, but it is not immutable. I think you're conflating the idea that within time there is only ever one history, "how it was, always is, always has been", and as a kind of transcendent external observer we here can see that there is only ever one way things happened during the life of the universe, but that history can be modified by human actions, to a new and also-consistent complete history of the universe, that those in it also perceive as the only way it's ever been.

In other words, one history does not necessarily mean one immutable history.

In other words I think your argument based on a "first time travel arrival" feels like Zeno's incorrect argument why you can never travel from A to B, by constructing events that appear superficially to contradict the notion.

• Yes, this is kind of the same thing. I don't have a reason for why no one can travel before the time traveler 1 arrival, I'm just using 2 axioms and logic, axiom 1 time travel is possible, axiom 2 there's only one linear timeline. Which clearly means that there has to be the first ever arrival of a time traveler. Logic doesn't have to be intuitive, but it's hard to argue. One can deny my axioms, but I'm free to choose them unless they contradict each other in a way that can't be resolved. Though clearly, I can invent a reason why you can't travel arbitrarily far back in time. Jun 20 at 20:51
• There has to be a first ever arrival, but that doesn't mean that I can't program a time machine to arrive earlier. If it succeeds, I will do so, the world will change, and there will still be just one earliest time arrival, and history will be consistent, and consistently record I was that first arriver. There will never have been another, or different, first time arrival. History will always have been as it is now. It just won't be the same first time arrival that some memories seem to suggest, because having one history doesn't mean having one immutable history. Jun 20 at 21:28

I think you can only have a consistent main timeline if you can never jump back earlier than the most recent arrival of another time traveler. This would allow you to get a diagram like your first one. But it would forbid your second diagram where a time travel path gets cut off.

If you don't do this, your current main timeline is perpetually at risk of becoming irrelevant non-history. All it would take is somebody going back in time before the first known time traveler in your current timeline.

Here's an ASCII art diagram of that happening, with A indicating the arrival of a time traveler, while D shows where they came from in some other branch of the timeline.

      --------------------------
/                          \
|                           /
|                      --->D--->
|                     /
|                --->A--->D--->
|               /    ^     \
|              /      \    /
|             /        ----
|        --->A--->D--->
|       /    ^     \
|      /      \    /
v     /        ----
--->A--->A--->D--->
\   ^     \
\   \    /
\   ----
--->


What used to be the main timeline (with the three 'A's on the upward sloping timeline) wound up getting cut off by the fourth time traveler who went back before any of them, causing a new main timeline to branch off going downwards.

If you only allow the smaller loops and forbid the big one that cuts off the others, you can have a single coherent timeline that is "true", at the cost of not being able to go back too far into the past.

• This restriction is too strict I think. Your hypothesis is that someone always could jump arbitrarily far to the past. But it's very easy to set a realistic limit. For example, when Earth was covered by magma, only very advanced technologies could allow the time traveler to even survive there, much less change something in a meaningful way. Even when dinosaurs walked the Earth, not much could've been done to meaningfully change history. And remember, jumping to the future is impossible, so why would anyone want to waste their life in the distant past... Jun 20 at 21:04
• Though I suppose some form of "time travel repulsion law" could be set which doesn't allow arbitrarily close time travel events (like arriving during the same day). That would make the plot more coherent, as it would lead to only a few time travelers in a year or even a decade Jun 20 at 21:08
• Well, I'm only saying this is necessary to have a single coherent timeline when a departed timeline ceases to exist. If you don't care about coherence, then anything can go. If you don't need timelines to cease to exist, then you don't need as strict rules for what could have happened in some alternative past. The problem when you have both rules is that if anyone does go back to primordial lava earth, they've killed off all other humans by destroying all other timelines. If you relax your rules about bypassed timelines ceasing to exist, then you can have things be more sane. Jun 20 at 21:34

## You will never invent, build, or use a time machine.

Every timeline in your scheme is destroyed if that happens, which is to say, it's not real. Time travellers may arrive all the time, but they certainly never leave. If you sit down with your soldering iron and a gleam in your eye, rest assured ... it's not going to work. Because if it worked, you're not here, and you're not building it.

## Time travellers come from complex futures with multiple unseen timelines.

Your second diagram illustrates that there could be multiple revisions in the development of those futures.

## Time travellers can't arrive "now" in the past.

If they arrive in the past, your timeline isn't real and you aren't here. If they arrived in the past, that's your past and it's already fixed.

## Time travellers can't not arrive right now.

The last time traveller changed the future when he arrived. Someday someone will invent a time machine and come back and change that future and so someone else will come back and change it again and so on. If there's a day between now and when the next time traveller comes back, then that changed the timeline and things will go round and round until he arrives before now, in which case you're not here seeing it, or just after now, in which case we go around with the changes again. So one must be arriving right now. This goes even if the world gets destroyed in the potential future because then aliens have invented the time machine somewhere and they are coming back ... right now.

## Relativity hurts my brain

The colony on Alpha Centauri has some of those time travellers arriving. Does that mean that the instant they arrive there, our timeline is destroyed? Or is it only when the light arrives and the changes take hold? I suppose if the timeline is "destroyed" but replaced with one with no observable changes until light reaches us, that's not very destroyed. But what if a time traveller arrives here and on Alpha Centauri at the same time, in 2020, in some "rest frame", and then the two jump on the Interstellar Express flights when they start up, and kill each other's former self in 2120 or so, before either of them has a chance to build a time machine and jump back? Which one is in the fake timeline? Okay, now I've had it.