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This is part one of two related questions.

Let us suppose that we have an expert in a a particular period of time, a person who is one of the world's foremost experts on the period. This person also has the ability to speak the local language and blend in with the locals.

This person travels back in time to the period in question and does... nothing. They simply live as a local and observe events as they occur. This means that they must interact with the locals, may even form relationships with some, but do not reveal their origin, and do not attempt to change the course of events.

In theory, as soon as the time traveller appears in the past timeline, the effects of their presence will ripple out at the speed of light, affecting events at the quantum level, altering the outcome of random events. However, humans are macroscopic structures, and not quite as easily influenced. However, the vagaries of random chance will accumulate, and eventually have a noticeable effect.

How long is it likely to be before our expert notices divergences from their memory of the historical course of events, and how long before the world and its history is no longer recognisable?

While this question is applicable to time travel to any place or time, it is easier to have detailed knowledge of more recent times.

EDIT:

This is considered to be a "many worlds" scenario - the time traveller cannot invalidate their own existence or presence. They are also not trying not to have an effect on the course of major events, they are just not trying to have an effect.

We can presume that the observer is observing a major world event such as a war or diplomatic negotiations at close range. They are not "out of the way", just not getting involved.

As to the effect a time traveller might have, their mere presence will have all sorts of effects. At the very least, their presence may lead to the weather being different over the course of some weeks or months. Personal interactions can lead to all sorts of changes. Maybe the time traveller bumps into someone and stains their clothes, and as a result that person is not taken as seriously by their superiors, or a rider guides his horse around the time traveller, and as a result the horse doesn't step on a stray nail and become lamed...

While this question should be applicable to any time period, for the sake of example, let us assume that the time traveller appears in Europe at the outbreak of WWI.

Also, the question is not "what might change?", but "how long would it take before the changes became noticeable and how long before everything was completely different?"

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closed as too broad by sphennings, Anketam, L.Dutch, Mołot, Ash Nov 20 '17 at 12:38

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ This would be highly dependent upon the period in question. An expert of pre-agrarian humans is going to have a very different level of understanding than one who studies the history of early bulletin board systems. $\endgroup$ – sphennings Nov 20 '17 at 1:42
  • $\begingroup$ Time travel is forbidden under most theories of physics. As such this question is impossible to answer in a fact based manner. $\endgroup$ – bendl Nov 20 '17 at 1:53
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    $\begingroup$ The answer to this question greatly depends on when and where the time traveler goes. If this person goes to a small quiet town and no major events are happening then he likely will never notice any changes. As such I am voting to close as too broad until more details are given. $\endgroup$ – Anketam Nov 20 '17 at 2:51
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    $\begingroup$ @MontyWild What time period are you asking about? Without knowing the time period this question is too broad. $\endgroup$ – sphennings Nov 20 '17 at 3:26
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    $\begingroup$ The best answer I have for this is a story answer not a world building answer. In short you the author get to decide and it can be as short as 24 hours if you write an event that clearly messes things up. For example DS9 - Past Tense $\endgroup$ – Anketam Nov 20 '17 at 11:40
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The answer is really "it's up to you." As an author, you get to pick how your particular time travel system operates.

I can think of 3 different approaches, leading to 3 dramatically different scenarios.

The first is the chaotic argument. Your presence changes the weather. Within a month, it's raining over New York instead of sunny skies. Planes are routed differently, a couple never meets.. you know the drill. The chaotic argument is that chaotic systems are sensitive to initial conditions and that our planet is a highly chaotic system.

The counter argument is that the course of humanity tends to be an aggregate of large numbers of small events. We like to say that we could have predicted the fall of Rome using data 100 years before the fall. We like to believe that concepts like freedom and the idea that slavery is evil are simply the only possible way things could go. By this argument, we should have no problems with an observer breathing a few air molecules.

Finally, there's the Novikov Self Consistency Principle. Under this rule, if the traveler chooses to go back in time to a specific time and place, he had always gone back to that specific time and place, and all of the changes he will do have already been written into the past. He is incapable of engaging in any activity which is not self-consistent with the fact that his self in the future (future by global timeline, his past self by his own timeline) will need to make those decisions.

All of these lead to drastically different stories. Which one you want is really up to you.

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  • $\begingroup$ The Novikov principle does not hold in a many worlds scenario. $\endgroup$ – Monty Wild Nov 20 '17 at 22:55
  • $\begingroup$ True. In that case, you only have two approaches. Both of them are sufficiently correlated to real life evidence that you can pick and choose. $\endgroup$ – Cort Ammon Nov 21 '17 at 1:24
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There are two options here and the second is more likely than the first but for both I'm going to use Pratchett references.

The Quantum Weather Butterfly

This little butterfly is the one whose wing flap changes the weather on the other side of the world. You've all heard of it in the pop science explanation of chaos theory. The tiniest change can have ever amplifying effects. The mere presence of your time traveller affects the weather, the food supply, the time it takes people to get to their destinations because they have to go round him. History ends up utterly different because of his presence.

However it doesn't really work like that.

There are margins for error and corrections all over the place. The messenger was delayed by a couple of seconds, but he was still 20mins early for the meeting.

The air pressure changed as the traveller arrived but the coming storm completely overwhelmed that. The meal he ate at the inn came out of their stores but they always allowed for a few more people just in case. The extra second here and there made no difference to the lives of the people.

Narrativium or the Narrative Imperative

Events have momentum.

WWI was a war waiting to happen. It could have occurred any time after the unification of Germany, Queen Victoria is said to have spoken of its inevitability and her dislike of having to go to war against her own grandson. The alliances were already set, the whole of Europe was sitting there waiting for that war to kick off.

One uninvolved person isn't going to stop a war. A good war takes years of building alliances and diplomacy over who will and won't get involved. To the point where the scrap of paper that triggered Britain's involvement in the war predated German unification by over 30 years.

Maybe your traveller could have convinced the Archduke to wear his bullet proof vest that day, but something was going to start that war. It had to happen, sooner or later.

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    $\begingroup$ The time traveller doesnt do anything in particular, but, for example, perbaps some trivial change results in a bullet in WWI travelling in a slightly different direction at a slightly different time, and intersecting with the brain of a certain German would-be artist named Adolph Hitler... Tiny change, big difference - eventually. When would our time traveller notice? $\endgroup$ – Monty Wild Nov 20 '17 at 23:01
  • $\begingroup$ @MontyWild, my argument is that it's highly unlikely to happen just by wandering through life with his head down. His changes are tiny and the windows to make major changes are even smaller. Even if you kill that artist, is there too much social pressure behind the rise of fascism and the starting of the war? Would there just be a different man in charge and no other real changes? $\endgroup$ – Separatrix Nov 21 '17 at 9:03

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