# Time Traveller's Problem: Is This a Stable Loop? [closed]

I come from Europe, year 2017. One day, I fell through time to 1317. I have no idea how that happened. There must have been some kind of invisible one-way "rabbit hole" in my attic.

So here I am, in 1317 in the town I know very well from 2017, looking for any way to get back to my hometime. I don't like this era, really. If I find a way back, I am pretty sure I will never return to 1317 again, even if I knew how.

The problem is I don't know how this time travel works. Could I change something in the past? Could I affect the future I am used to and maybe risk my own future existence? Or is it a stable time loop, so there is nothing to worry about?

What should I do to find out? I need a safe way to find out if this is a stable time loop or not. I know when the king died from my history classes, so I could try murdering him, for example... If I were successful, it would be a proof this is not a stable loop. But... I find this idea exteremely bad:

1. I could get killed by his guards before or after my attempt.
2. If I am successful (which means this is not a stable loop), this event will surely change the future I know and I want to keep as intact as possible.
3. I don't like killing people, really.

I need a proof I can affect the future without affecting it too much. What can I, as a character in this strory, do instead?

• I'm not sure you can, really. Let's take the kingslayer example. You know when your history book said the king died, sure. But maybe the book was wrong? There is no way you can be sure wether the loop is closed until you get back to 2017. And by then your problem is solved. – Guran Aug 24 '17 at 7:46
• time capsule... but I seriously doubt time-loop would work this way :> – user6760 Aug 24 '17 at 7:49
• Note there's always the option of branching universe. You know the king dies of cancer in 1320, you murder him in 1317, you go back to 2017, check history books, king died of cancer in 1320 - the timeline of your travel was a branch; you returned to your original timeline, nothing you did in the past happened in it. – SF. Aug 24 '17 at 8:06
• If you don't like killing, try to save somebody that the books says died. – Stig Hemmer Aug 25 '17 at 7:23
• @JohnClifford He doesn't necessarily has to be the direct cause. He only needs to not do anything that would prevent himself from travelling back and time. On the other hand, if we take into account the butterfly effect, then every single thing he does in the past in a stable time loop (so he doesn't prevent the time travel), would be an indirect cause of the time travel. – Kaito Kid Aug 26 '17 at 14:20

Generally, there are couple of ways of interacting with the past that I know of:

1. "Passive" - That one says, that whatever you do in the past is what has actually happened. That means that from, let's say, "cosmic perspective" the things in 2017 are as you know them because you've time-traveled and affected them. When you come back, everything would be just as you left it. Pretty boring, but lets you leave all the sci-fi problems behind and focus on the action.

i.e. Kennedy is known to be dead in 2017 because you've traveled in time and killed him in 1963.

1. "Active" - According to that one, you are actively changing the future from the past, meaning that the future you come back to will be different from the one you left. There you have to be careful not to kill your grandpa and so on. More interesting, but very prone to logical paradoxes and writer's errors.

i.e.Killing the man who was about to murder your parents would actually bring them to life.

1. "Cloning" - Every major decision creates an alternate universe where something went differently. Depending on the needs the decisions vary in scale from killing Adolf Hitler to eating cereal on breakfast. That one is completely safe and lets your characters meet their counterparts from alternate future.

i.e.Adolf H. comes to the world where he was killed in 1939 and rebels the 2020 population.

The best way to check which of the realities you are in is to try to destroy an object you know from the local museum. It doesn't have to be anything valuable, just make sure what you knew in 2017 was not a copy.

If you want to put your character in the "passive" environment, he wouldn't be able to change anything. You will recognize this one by failing to break it several times.

If you succeed, you are for sure in option 2 or 3. Unless you know, how to travel between the worlds, you can just assume it's the "active one".

• I think the question is not how the various paradoxes could work, but how the person in the story could doublecheck which one is applied – Martijn Aug 24 '17 at 9:57
• When I fail destroying a museum object, I know nothing. I may be in a stable loop, but it also might have been bad luck... – vojta Aug 24 '17 at 10:10
• @Martijn Exactly. – vojta Aug 24 '17 at 10:10
• You could make a timecapsule on a place only you know. And recover it in the present. Just make sure it has no impactful information when some random archeologist accidentally finds your timecapsule. – Totumus Maximus Aug 24 '17 at 11:40
• @Martijn After rereading the question, I've added second paragraph. Several failures prove it is not an incident - dropping the vase just for it to be caught by a bypasser is normal once, but if you fail to destroy it time and time again, it gives you the idea of what's going on. Plus, that's a bit meta, it allows for explaining things in narratively pleasable way. – Jakub Tustanowski Aug 24 '17 at 12:02

You could redirect your test to something less protected and with a lesser impact on the future, for example carving something on a rock that still exist in the future.

You know this rock where blank in the future, so if you can carve a mark on this rock this mean you are in an unstable loop, because there is no way this mark could disappear between 1317 and 2017, and without any important impact on the future.

The only issue is to recognize a rock as being a part of your town in 2017, without being too important otherwise the rock could be replaced. The best solution would be caving on a place you use to go and know perfectly.

• Agreed, knowledge of the town is your best weapon. Failing a visible landmark, you can try burying a time capsule full cheap historical artifacts in an area that was only developed in your lifetime. No contemporary will want to steal it, but it would be likely to make the papers in your own time. – Random Aug 24 '17 at 8:02
• Excellent idea. Grab a rock and knock a chip out of the cornerstone of a bridge or someday-to-be-famous landmark that is just recently built. Or at night, carve your initials in it. or a few crossed lines. You would know if some difficult to replace cornerstone had actually been replaced, it would have been talked about while you were growing up. As would mysterious intentional hash marks or even the initials of your own name; you would undoubtedly remember that: So if you get back to 1317 without remembering it, and make it happen, and nobody cares or replaces it: You have changed the past. – Amadeus-Reinstate-Monica Aug 24 '17 at 11:35
• Assuming memories are stable to the time traveler. What if changing the future through the past changes the way you remember how things were? Hence, if you carve your name somewhere you will remember from your time that the carving was there. You could double check by writing down your memory beforehand? – Three Diag Aug 24 '17 at 11:37
• @ThreeDiag But then what you wrote down would change... You could carve a description of what you remember being on there onto the stone (i.e. nothing -> the word nothing -> the phrase "the word nothing") so that when that turned out to have been a quine you would know that either your memories were changed to what happened or there was a stable time loop after all - both of which are pretty much identical as far as the traveller is concerned. – wizzwizz4 Aug 24 '17 at 12:07

Try to find out if there is another rabbit hole that will take back into the future of, hopefully, 2017. If time travel only happens by passing through a rabbit-hole, then changing or attempting to change history may not result in sending you back to your hometime.

Your question assumes that changing causality, the sequence of events in the past, will somehow return you to your era. If this assumption is wrong, then killing the king may only make you a murderer. If time is strictly deterministic, the past will remain unchanged because you and your actions were always part of the past. So anything you do will only make history happen the way it did happen.

You have correctly identified your main problem: how to prove if your actions will change the future. The trouble is, you can't. Do something in the present, which is also your past, and you have no way of knowing if it does cause changes in the future or the magnitude of those changes.

For example you know from history the king was killed. But you remember the date correctly? If you kill the king, this could happen earlier than in the history you were taught. This could set in motion a series of changes to history that by 2017 will have completely changed the course of European history. But this in and of itself may do nothing to return you to the future. Consider, what if it did. Now you are returned to a completely different Europe of 2017. A Europe you may have no part in, so you might not have even been born there and where. Now you are a Stateless person in time.

In summary, you prove what the consequences of actions are going to be in future. Everything you do will be risky. There may be no way back.

• No, I don't think changing causality would bring me back. Getting back is not the most important part of my question, actually. I just want to know how to check which "time-travel option" applies in my case (by "me" I mean the person in the story). – vojta Aug 24 '17 at 10:15
• @vojta Problems with time travel are all about causality. If doing anything in 1317 changes its future that will harm your future self, which would actually be your past self, assuming you never get back to 2017 or anywhen in the future of 1317, that is changing causality. You are either in a deterministic universe where nothing you do will change history so your future self is safe or you're in an alternative or parallel timeline where, yes, your actions might harm you in the future. But this won't be you it will be an alternative you. But it's not guaranteed. – a4android Aug 24 '17 at 12:05

As another alternative, which I could do in this situation with knowledge I actually have. In the village of Lyminge in Kent is a church. It was already old in 1317, being originally a Saxon abbey. Its construction incorporates Roman tiles from the ruins of Roman buildings on the site when the Saxons built it. If I smash those tiles, they can't be replaced like-for-like, because the Roman ruins from which they came have been erased by the 400 years that have passed. So I will have made a trivial change to the future.

• Yes, churches are the easiest thing to survive 700 years. Go carve something in a wall, at two meters of height and it won't be erased in any restoration because people would think it's already historical. – Alberto Yagos Aug 24 '17 at 11:09
• @AlbertoYagos It won't be erased in a 20th or 21st century restoration. But it might be erased in 1340. – Mike Scott Aug 24 '17 at 11:10
• If you make it small and put it at 2 meters, they won't. In the church of my city there are graffittis made from medieval students in 1390 like that. – Alberto Yagos Aug 24 '17 at 12:34

Find a tree that you knew to be over 700 years old in 2017, and chop it down. A building might be repaired, an artifact might be replaced, repaired or re-made, but none of these can happen with a tree. If you can chop it down, then you can change the past.

• That is a good idea... However, how can I know it is the exact tree? The tree looks totally different 700 years ago. I would have to chop all plants and trees in that location and hope the tree I know from 2017 is really 700 years old (which I cannot know for sure). – vojta Aug 24 '17 at 10:21
• @vojta In the UK, I'd suggest a yew tree in a churchyard. You should be able to identify it exactly, as there will only be one such tree near the church. And there are lots of old churches with old yew trees. – Mike Scott Aug 24 '17 at 10:23
• Or just take a tree you know still stands and has nothing cut in its bark. Take a knife and cut in your initals. There is however a chance that your memory is affected by this too and you start remembering these very initals as always being there. This might lead you to believe your actions don't change anything while they in fact do... Depends on your rules really. :) – Morfium Aug 24 '17 at 10:33
• @Morfium Do you remember any trees of that age well enough that you can be sure the initials weren't already there? I don't. – Mike Scott Aug 24 '17 at 10:35
• Well, I don't know what the character does all day in the present. Maybe he has a favorite tree that's quite old. I just thought cutting down a tree in front of a church might be a little problematic to do. So writing something in it might be a little less hard to do. – Morfium Aug 24 '17 at 10:40

The best course of action, in this case for you, is to invent something. Throughout history, there has been many very complicated inventions, like computers, cellphones, cars, etc. Those inventions, even if you know about them, you wouldn't be able to create them from scratch if you never studied them in the present.

On the other hand, some inventions are literally just a very good idea. If you know it's possible, it might take you a few hours or a few years, but you can make it. For example, if you traveled to the VERY distant past, you'd be able to "invent" fire, wheels, and even more advanced objects like boats and bicycles.

In 1317, honestly the biggest thing that comes to mind that wasn't invented yet but is still simple enough that you could figure it out, is the printing press.

It is a fairly complicated machine, but the "how it works" part is simple enough that your character definitely understands it without having done specific studies. So he could probably make it work within months, or years if many events slow him down.

The goal is basically to invent something that would leave a HUGE impact in the future (inventing the printing press one century earlier would definitely change the world). So you find something that would change history a lot, so there is absolutely no way you can be mistaken about "did history change?", but is still simple enough that it is believable a random guy from 2017 would be able to make it from scratch without plans.

Then, if you are in a stable time-loop type of world, something will prevent you inventing it or making it public. Maybe events out of your control keep stopping you from building it (natural disasters, vandalism, etc). Maybe you get killed before completing it and nobody knows what that half-machine in your workshop was supposed to be. Maybe you get kidnapped by a guy who sees the potential of your machine, steals it, and tries to force you to give its secret. Then plot twist, that guy was Gutenberg's grandfather and it took them two generations to figure out how to complete this half-machine they stole from you. The possibilities are endless.

If you succeed at making your printing press, and making it public enough that no reasonable event could wipe out all traces and memories of its existence within a hundred years, you know you are either in a "Alternate timeline" or "you can change the past" situation.

The question is now: How do you know all those things that stopped your invention weren't just bad luck?

The answer is: you don't. You have to rely on math for this.

Imagine a random number generator, which gives you a random number between 0 and 9 every time you ask. The first time it gives you 4. Sounds pretty fair. Then you get a 4 again. Having twice the same number in a row had a 10% chance of happening, this is Lucky. Then you get another 4. Now it was 1/100, what a coincidence! Then another. Then another. Then another. At some point, it sounds like the machine isn't really random. There is still a very small chance it was only luck, but after getting 20 4s in a row, which had a chance of 1/10000000000000000000, you can pretty much say that it's not random. You can never be sure, but you can be pretty confident.

The same goes for your attempts. If a random group of kids break in your workshop and destroy the machine, that's possible. If the village gets flooded and you lose all your work, that's possible. If your house burns down and you have to start over, that's possible. But every time you see a somewhat-reasonable event that happens to ruin or seriously set back your invention, it could be complete luck, but at some point you have to decide "yeah, having all those events in a row is pretty unlikely. I guess it's not just bad luck after all". You won't have 100% certainty, but I think 99.99999% would be enough for you.

This idea also had a great advantage. If you don't succeed at ever going back to your 2017, you will most likely become rich and famous in the 14th century, and be in most history books later on. That's pretty significant.

• i'm not quite sure i understand the last paragraph, do you mean in case he succeeds inventing something, or in case he doesn't? because in case he doesn't, i really doubt current knowledge will make it easy for him to become rich in the past. – Brian H. Aug 24 '17 at 12:33
• 3 problems with your idea. First one is the fact, that it takes awful lot of time to establish a printing house even without the disasters. You would need at least three attempts to be "sure", which would take years. And in the case you actually can change the future, you would have caused the havoc by that time. Second, your goal is not to change the future significantly. And inventing the print will surely do that. Finally, even if you manage to establish a small time printing house in a year, you can't be sure you've changed anything. Gutenberg is just the first one we know about. – Jakub Tustanowski Aug 24 '17 at 12:36
• @BrianH. That's if he succeeds. If he fails to go back to his time period in a stable time loop situation, he can just keep his stuff on the down low to avoid getting screwed over by destiny, and he'll still get a decent amount of money by advertising his extremely quick hand-made copy shop. – Kaito Kid Aug 24 '17 at 12:40
• @JakubTustanowski He doesn't need anything that big honestly. Just make the machine, mass produce bibles for your village and spread the word. Bibles got so much attention throughout history that having a hundred of them completely identical all originating from one village would definitely get you "caught" later on, and people would figure it out. You don't need to go global for it to work, which is why the printing press is a good invention to choose. The invention itself is so big that your effort can be minimal. – Kaito Kid Aug 24 '17 at 12:42
• @JakubTustanowski Furthermore, if you are in a "you can change the past" situation, you're screwed either way. You better hope you're somehow paradox-proof, because if you stay in 1317 for a single day and kill and animal to feed yourself, this will have a huge impact 700 years later on basically everything (that pig never mates, within a few generations the entire pig ecosystem is slightly different, and everyone who ate them is slightly different, etc etc). – Kaito Kid Aug 24 '17 at 12:47

# If your goal is to not affect the future, you can't

Think of it like this, the only way to be sure whether you are capable of affecting the future or not is by succeeding, and if you wish to keep the future as it is, you must at all cost avoid even trying to figure it out.

Another thing is that a "stable loop" might not be as stable as you think it is, and in case you are writing a full story about this concept it may be quite a difficult task to convey this.

So let's for a second assume you don't care about having any effect on the future, you give some ruler crucial information for them to win a battle you remember them losing. A few things could happen:

• Your character's memories are changed, so he's always known that this ruler would win that battle, even if you (the writer) know events have changed, and might think this loop is unstable, it actually is a stable loop, a person's identity are his memories, change them and you change the individual, now your character is the "guy who made history work" instead of the "guy that changed history".

• Your character's memories stay the same, but guess what, it's just that he was terrible at history and got it all wrong, the ruler that won the battle was meant to win, just that your character thought he should lose. this one can be interpreted in both ways, this could be a stable loop and in fact he was really bad at history, or it is an unstable loop and future is changed.