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Assume a world with no multiverse theory, just a single unbroken timeline. If you were to go back in time to the town your parents lived in on day of your conception, the air displacement due to your presence in the world is going to prevent your own birth. The reason for this is simple. Semen is a liquid. The slightest alteration in the father’s movements is going to make that liquid move differently.

The displaced air starts a chain reaction. The difference in air pressure alone, however slight, is going to make things act differently. All it takes is for the change due to your displaced air to interact with something, that interacts with something, and so on and so forth, until somewhere in that chain is your father.

The crux of this is the air displacement. How far away from your parents physically, or temporally, do you have to be before the air displacement from your time travel excursion no longer prevents your own birth? For the sake of making this more interesting, you are only in the past for a single second before being sent back to your own time. One second of air displacement.

Slight change in appearance is enough to make it a different me. If a different sperm reaches the egg, I die. As a hypothetical dramatic event that causes this plot point, imagine that somebody is trying to kill me by catapulting me back in time for the aforementioned single second. I can alter where and what time I "land", but not by much. How much do I need to alter it to be safe.

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    $\begingroup$ But if you're in a continuous timeline, doesn't that mean that whatever you do will already have happened and thus travelling back in time will henceforth have been the requirement for your birth? $\endgroup$ – dot_Sp0T Sep 27 '16 at 8:40
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Serban Tanasa Sep 30 '16 at 13:38

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Viewing from a point of single, unbroken timeline - it doesn't matter how far are you from your parents. As silly as it sounds - you could even pop up in the room they are having sex in.

Why?

The key point here is the very first sentence in your question. If we assume there is only one timeline, killing you by sending you back in time will just not work. You are as you are exactly BECAUSE you were sent back in time (or not, depends if given person decides to do it. It really doesn’t matter, it happened.). Without Multiverse this paradox is simply unavoidable. Furthermore, the person that sends you into the past can just as well be killing himself - not immediately, but as you mentioned in some comments - the butterfly effect might have just as well killed him. Or made him never meet you. Or, or, or. The possibilities are endless. If you assume that traveling into the past will change the future - you just cannot avoid multiple paradoxes at once.

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    $\begingroup$ Thinking about it, that does make sense. The only way the effects of chaos theory can have any real affect on the future is if the timeline works in such a way that those affects already happened. Making the paradox self correcting. I could handwave and say that it doesn't work that way, but it does work that way for the chaos theory to matter. Answer accepted. $\endgroup$ – Space Ostrich Sep 27 '16 at 9:02
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    $\begingroup$ So an universe with a single timeline is immune to time paradoxes... makes sense. In wich the time traveler had already gone and he is the cause of his own birth, just that detail was lost in the telling of his own history. $\endgroup$ – Mindwin Sep 27 '16 at 13:09
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    $\begingroup$ One could summarize this answer by saying that "all time travel is a causality loop" or "In this world, you may be able to time travel but you can never actually change what happened." $\endgroup$ – Shufflepants Sep 27 '16 at 14:32
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    $\begingroup$ @n00dles Free will would still exist, in as much as it does now. We can look back at things right now in a causal chain and say "X happened, which caused Y to happen, which lead to the current situation." Causal loops like the answer posits don't change that. You go back in time, behave however you want, which causes other things to happen, which leads to the situation where you went back in time. You can't view the entire loop at once, so you can still choose what to do at every point. $\endgroup$ – D.Spetz Sep 27 '16 at 20:05
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    $\begingroup$ @n00dles What you describe is a different paradox, which should be impossible with the causal loop that the answer describes. You also ignore the fact that memories can be wrong (it wasn't future-you that you saw die, just a clone of yourself) and that you are not always in control of what happens to you (you accidentally hide in a room that someone else uses to time travel, and they bring you with them). Wrong info and absurd coincidence are a hallmark of this kind of time travel in stories $\endgroup$ – D.Spetz Sep 28 '16 at 14:19
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Assume a world with no multiverse theory, just a single unbroken timeline. If you were to go back in time to the town your parents lived in on day of your conception, the air displacement due to your presence in the world is going to prevent your own birth. The reason for this is simple. Semen is a liquid. The slightest alteration in the fathers movements is going to make that liquid move differently.

The simple answer is: no you won't, because if you cause an air displacement and this produces the slightest alteration in anything, the net result will be that the sperm that became you will eventually fertilise your mother's ovum.

The simplest form of a single unbroken timeline is a deterministic one. Nothing anyone can do by travelling into the past will change the past because if you were there you were always were there, in the past, so nothing changes.

The standard deterministic model of time travel works best for single unbroken timeline scenarios. The alternative is Novikov self-consistency, but that is essentially a physicist's version of determinism.

Basically you don't have to avoid doing anything to prevent changing the past or preventing your own conception, because you can't. In fact, you may be responsible for your own conception.

Trying to burst in on your copulating parents and shooting them isn't advised. Something will happen to prevent you from doing so. Most likely whatever it is, it will destroy you. Don't try this at home.

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    $\begingroup$ "because if you were there were always were there, in the past"... hehe. Reminds me of the Douglas Adams quote: "One of the major problems encountered in time travel is not that of becoming your own father or mother... The major problem is simply one of grammar." (full quote here) $\endgroup$ – Simba Sep 27 '16 at 13:35
  • $\begingroup$ @Simba You can say that again, but choose carefully if you say in either the past or the future. $\endgroup$ – a4android Sep 27 '16 at 13:44
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    $\begingroup$ By far my favourite interpretation of temporal mechanics, and I wish way more TV shows made use of it. I find self-fulfulling prophesies to be supremely narratively satisfying for some reason. SG1 came pretty close but then closed the episode with Sam pointing out that Heisenberg blows pre-determination out of the water... which is kind of annoying. :) $\endgroup$ – Lightness Races in Orbit Sep 27 '16 at 16:48
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This sounds chaotic

What you're referring to is chaos theory, popularly summarized by the essay title Predictability: Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?, subsequently known as the Butterfly Effect

A chaotic system is defined mathematically in a complicated manner, but the basic idea of it is that a tiny change in the initial conditions of a system has a vastly different change in the trajectory of the system. It's also important that the potential trajectories all cross each other at some point (otherwise, predictable actions like exponential growth would count as chaotic).

A pendulum on the end of a pendulum

A pendulum on the end of a pendulum is a chaotic system. Moving the pendulum ever so slightly either direction results in a vastly different path that the end of the second pendulum will follow.

Atmospheric conditions are considered chaotic, and this is the reason that meteorologists generally give weather forecasts one week or less in advance. Given that, it's likely true that, as you say, any change in atmospheric conditions could result in a different offspring, or no offspring.

How far away do I have to go to avoid threatening my own future?

If you change something in the past, you likely can't, on the basis of distance alone. You might be able to make the change far enough away that your change will not affect your conception (maybe), but this does not put you "in the clear," so to speak. The chaotic system that is reality will have changed and its future trajectory might or might not include your time traveling antics. Maybe you cause a rainstorm in a different country that causes the automotive death of the brilliant scientist who would otherwise have designed the time machine.

Maybe your atmospheric changes affect economics and someone murders you out of desperation before you can ever time travel (though your son might become Batman, so that would be a plus).

Your only certain chance of protecting the future of your past self in order to avoid creating a time paradox would be to find a completely isolated system which has remained isolated since the time period you intend to travel to, and travel to the inside of it. By "completely isolated" I mean airtight and not resting on the ground (it would need to be in free fall, or in outer space). Since it has been isolated the entire time, it doesn't matter what changes you make to it (unless you use some tricky momentum tricks with a well-timed travel to the past or present to cause it to become unisolated), because should it ever become unisolated, that will be in the future of the moment you left in the present.

But all this depends on a vital assumption: that you are changing the past. So...

Are you actually changing the past via time travel?

I'm gonna paraphrase myself here:

The Novikov self-consistency principle states that any time travel is mathematically required to be self-consistent. That is to say, the traditional time paradox isn't possible.

Consider drawing your situation to its ultimate conclusion. You travel back in time, disturbing the air, which prevents your birth. Traditionally, this is a time paradox. If we were never born, we could have never time traveled and prevented our own birth. There are a number of suggested "resolutions" to the time paradox, almost none of them good for us.

Novikov's solution is different. Novikov suggests that none of this could have happened in the first place. For some reason, regardless of whether it the reason is known for any particular instance, it is simply not possible to create a time paradox, much in the same way it is not possible to create any other kind of paradox.

If I work out a mathematical proof that shows that $1=2$, then there is something wrong with my proof, because we already know $1 \ne 2$. What is wrong exactly? Well I dunno, it depends on the "proof." Being unable to determine the reason does not mean that the paradox is allowed to exist.

In our case, the only way you could travel to the past into the same room with your parents when you were conceived would be if you had traveled to the past into the same room with your parents when you were conceived.

There is a pretty detailed example given at the above link involving a billiard ball traveling into a wormhole with the precise trajectory that will cause it to strike itself and knock itself off course, preventing it from entering the wormhole. Novikov refuted this example by redoing the mathematics himself and finding a number of self-consistent solutions.

Think of it from the perspective of a non-traveling observer. Your parents are working on making you, when suddenly out of nowhere, the adult you steps out of a wormhole. Your parents apparently don't react by stopping (they must be into this sort of thing) but you've shaken things up enough to cause a different sperm to join to an egg than it would have. But this is ok, because as our observer finds over the next decades, the resulting child is you, the person who stepped out of the wormhole that day. In this solution--rather than how we normally think of time travel as messing up the opportunity for time travel, therefore being inconsistent--your time traveling has instead complemented the opportunity for time travel, enforcing its own self-consistency.

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  • $\begingroup$ The only issue I have with that theory is that it implies that if I were to shoot my parents something would happen to prevent that. Which doesn't really make any sense. For starters, you could send something back that is physically impossible to prevent. For example, a cubic metre of star. Nothing is going to stop that cubic metre of star from obliterating everything in that room, but the theory indicates that something would. $\endgroup$ – Space Ostrich Sep 30 '16 at 19:27
  • $\begingroup$ @SpaceOstrich Being unable to determine the reason does not mean that the paradox is allowed to exist. $\endgroup$ – Devsman Oct 5 '16 at 18:54
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The flapping of the wings of the butterfly you disturbed on the other side of the world caused such a storm that your parents spent the night hiding under the kitchen table instead of making babies.

If one is going to consider, as you are, the butterfly effect in time travel, there is no such thing as a safe distance within the light cone of the event in question.

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    $\begingroup$ I can give an example, though I suppose I should clarify that I'm looking for a minimum here. If I was to appear one hour before conception on the other side of the planet, there is no feasible way the butterfly effect is going to reach the parents in time. $\endgroup$ – Space Ostrich Sep 27 '16 at 7:35
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    $\begingroup$ I agree with the application of the light cone concept as a way of mapping out all POSSIBLE event chains that would stem from your appearance in the past. As long as you fall within the light cone, your appearance has a chance to affect your conception. $\endgroup$ – Xenocacia Sep 27 '16 at 7:45
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    $\begingroup$ @SpaceOstrich, someone who knew your parents saw you in Singapore and thought you were your dad, who he knew should have been in Copenhagen, called your mum and rather spoiled the moment. $\endgroup$ – Separatrix Sep 27 '16 at 7:49
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    $\begingroup$ @Xxy Ok I see the problem. I didn't mean that each butterfly literally causes it's own unique storm, I meant that each storm is literally dependent on every minor perturbation in the past, if you're counting far enough back, including each single flap of a butterfly's wing. So we can predict the weather with fair accuracy a few days ahead, but the further ahead you try to project, the more smaller and smaller effects become able to affect the big picture, all the way down to the actions of a single molecule. $\endgroup$ – Stephen Lujan Sep 27 '16 at 14:47
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    $\begingroup$ So we'll never be able to predict the weather a year ahead of time, even with sensitive instruments all over the world and supercomputers more powerful than any in service running. We can't simulate every molecule on earth, and even if we could we would never get an accurate measurement of the velocity and temperature of every molecule, and even if we could, the earth is still not a closed system. There are variations in the radiation we receive from the sun. $\endgroup$ – Stephen Lujan Sep 27 '16 at 14:51
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Sorry

It is called a "paradoxon" because no amount of thinking or arguing will make it go away.

But even if we assume the naive version of time travel, where you somehow pop into being, displace all stuff, and carry on blithely, it's hard to give a good answer.

Applying the "butterfly theory" in your scenario seems unlikely to me. Sure, some air molecules will move around, maybe. It depends on whether you displace the air, creating a rather strong wave (like an ultrasonic boom), or if the air volume just disappears to make exactly the space you need, or if you can pop in in a way that kind of expands slowly, to remove the "instantaneous" effect (making it less loud).

You seem to want to have the first version. In this case, just pop in right on your parents and you will surely ruin their private moment. If you pop in next door, they will hear a loud noise and likely - unless they really are in the heat of the moment, stop to look who entered their living room. And so on. You will find lots of ways to avoid your birth without any need to look at the movement of air molecules, and, frankly, I don't see how the movement of air will change anything at the places "interesting" to your conception.

Outlook

In my version of a time travel story, I would very likely avoid such cheesy effects. Science that is advanced enough to solve the unsolvable problem of time travel will surely figure out a way to handle the pesky little problem of air displacement. In fact, I could imagine that everything would be easier if the volume at the destination is warped to the same volumne at the source and vice versa, hence removing this particular problem altogether.

If displaying the air really were the deciding plot device in your story, you would likely have lost me at this point, that's so Jules-Verne-ish. You are thinking about the little mouse (air effects) sitting next to the huge pink elefant in the room (time travel). Give me a story about the elefant, I don't care about the mouse. :)

Outlook²

EDIT: I add this third part try to reinforce the second part of the answer. In some comment, you write

That said, I'm not actually writing a story, this is a thought exercise in time travel, as I've often found that people don't tend to understand the full extent of chaos theory.

Again: considering the hardness of time travel in itself, the problem of how much change you need to "break" something is really quite uninteresting (if you are interested in a hard SciFi type of story). Single-timeline time travel in a hollywood format is so obviously impossible especially because of the Butterfly Effect that if you do a story with that, and then go ahead to do a scientific display by counting air molecules, it would just be offputting to hard-science folks, as well as to those that prefer soft-scifi.

Don't get me wrong, there are many excellent stories involving time travel.

For the "hard SciFi" stories you need a spin on time travel that just might work, i.e. which allows for and maybe explores the Butterfly Effect, but not in a single-timeline universe. For the "soft" ones, you need to just decide on some mechanism for time travel, don't go into how it works at all, and then explore all the non-scientific aspects (i.e., emotional drama from almost meeting yourself/your grandpa as a child, or meeting a girlfriend while time traveling and continuing traveling with them, maybe exploring the different ages by visiting ancient places, and so on).

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Yet another perspective to add here, arguably a more realistic take on the "paradox armor":

Let's say there's a 1 in 1 million chance that the correct sperm fertilizes the egg to make you, since I don't feel like looking up sperm counts on a work computer.

In a traditional, no-time-travel situation, the die has already been rolled, and cannot be changed. Your arrival, however, stirs things up again, and let's say that a different sperm gets through. In this case, you're never born, and thus you never come back, so nothing gets changed, so you're born, so you come back and change things.

But who says the second time you come back, you change things the same way? This cycle can repeat as many times as necessary for the 1-in-a-million chance of you coming back and still being you to occur. From the perspective of the observers - even the time traveler - it essentially looks like "luck favors a stable timeline" - but only because it had a billion chances to get it right.

Of course, there's a reasonable chance that the "stable timeline" version of you that's eventually reached won't, in fact, match the original version - but the original version never existed, and you'd have no memories of, for instance, not having your luxurious blonde hair.

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The answer to the question of how much you'd have to change or how far you'd have to be away, there is an answer that means you could be (relatively) close and not cause any issues. The answer is roughly 53km (or 29 miles) plus 9.81 meters, straight up.

The Earth’s stratosphere begins around 29 miles high, above this, you aren't going to affect the air pressure. Assuming you are only in the past for 1 second and that gravity accelerates you at 9.81 meters per second per second, if you materialized 29 miles plus 9.81 meters directly above the earth surface, you wouldn't cause any difference in the air pressure because you'd never touch the air.

Provided you can hold your breath for one second and not be ripped apart by the forces (or lack up) or survive the cold up there, you'd be fine. Even in a vacuum I believe an average human can survive for a few seconds.

I do accept that this is a bizarre answer but then the question isn't particularly straight forward either.

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  • $\begingroup$ That's actually a pretty good idea. It'd probably hurt, but people can survive vacuum for a lot longer than a single second, and if I'm far enough up I won't hit anything that would cause issues on the planet. It'd make for an interesting experiment when i got back, how much will have changed from me just grazing the atmosphere a few decades ago. $\endgroup$ – Space Ostrich Sep 27 '16 at 14:47
  • $\begingroup$ But what if you are spotted on a satelite, or if you cause a minor interference to a gps(or other satelite signals), or your gravitational influence changes the environment, or your shoe falls off, or the energy from your arrival causes charged particles to make the aurora very slightly brighter or... It may not affect your parent's love-making, but it could have changed you in some way. $\endgroup$ – n00dles Sep 28 '16 at 4:37
  • $\begingroup$ There would be no chance of being spotted on satalite or interfering with the GPS signal when I was born. I think that a show falling off could be an issue but by the time it arrives at the surface of the planet, the love making will be over. Finally, when Felix Baumgartner did his Red Bull jump, I don't think it caused any brightness in the stratosphere. $\endgroup$ – Steve Matthews Sep 28 '16 at 9:33
  • $\begingroup$ You won't be ripped apart, ignore what movie authors dreamed up. Your body tissues are a lot stronger than that. At worst some capillaries will burst. Oh, and don't exhale, no matter how much you want to. $\endgroup$ – doug65536 Sep 28 '16 at 18:02
  • $\begingroup$ Shoe falling off would be impossible if I did it right. If I'm in a vacuum everything is falling at the same rate, meaning without some form of propulsion the shoe would fall at the exact same rate as me. I could be spotted on a satellite, but it's highly unlikely. As far as I know, no satellites were taking pictures of the edge of space with a level of accuracy or frequency that they'd spot a single second of person. $\endgroup$ – Space Ostrich Sep 30 '16 at 19:31
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I don't think you're right in your reasoning there. If your parents copulate within a few days of your actual conception, you'd still be here to type about it.

You might be a different sex, there might be a slight change in your appearance, but you will still be you.

For the sake of the story universe you have here, I'd say that you just need to stay out of the room.

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  • $\begingroup$ Slight change in appearance is enough to make it a different me. If a different sperm reaches the egg, I die. As a hypothetical dramatic event that causes this plot point, imagine that somebody is trying to kill me by catapulting me back in time for the aformentioned single second. I can alter where and what time I "land", but not by much. How much do I need to alter it to be safe. $\endgroup$ – Space Ostrich Sep 27 '16 at 7:37
  • $\begingroup$ Ok, I see the point there. You're probably safe if you don't go inside the house and don't spy on your parents. Why do you need to risk this jump back in time anyway? Is there a task you need to fulfil? $\endgroup$ – Snow Sep 27 '16 at 7:44
  • $\begingroup$ Currently, I'm going with a "person trying to kill me by sending me back in time" plot. That said, I'm not actually writing a story, this is a thought exercise in time travel, as I've often found that people don't tend to understand the full extent of chaos theory. They'll say things like "so long as you don't step on any butterflies", without realising that the air pressure change has already altered the lives of insects, and propagates through the food chain. $\endgroup$ – Space Ostrich Sep 27 '16 at 7:45
  • $\begingroup$ @SpaceOstrich Are you sure you die? It is the same egg. You are an egg cell that self-replicated. The different sperm cell simply changed the other half of the DNA. $\endgroup$ – doug65536 Sep 28 '16 at 17:47
  • $\begingroup$ If half my DNA suddenly changed, I don't think that's still me. It'd be my brother or sister. $\endgroup$ – Space Ostrich Sep 29 '16 at 8:44
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I will agree that in a single-timeline situation, the distance from your parents is irrelevant. I'd also suggest that the paradox is irrelevant. Upon arriving in the past, you would find that the future from whence you came no longer will exist, as all the so-infinitely-complex-as-to-appear-random interactions between every particle in existence need to be re-calculated.

The future you came from doesn't exist. You haven't been born, and you may not be born, but it has no bearing on you now. The universe would likely incorporate you into the present as simply another input into the vastly complicated calculations determining position and vector of everything else in the universe.

Travelling back to the future (ha.) would certainly be possible... but I suspect you'd have to take the long way 'round, since all facets of the future depend on their precedent actions, and the processor running this whole mess likely has a limit as well.

So... the very fact that you arrived in the past, regardless of the amount of time, or the distance from involved actors completely invalidates your previous present, but the universe replies with an existence-shaking "meh."

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    $\begingroup$ OP rejected multi-universe/multi-timeline mechanics $\endgroup$ – Lightness Races in Orbit Sep 27 '16 at 16:50
  • $\begingroup$ @LightnessRacesinOrbit Yes the OP did, but this answer isn't about a multiverse or multiple timelines. It's more of an infinitely complex, paradox-free, computationally self-adjusting universe. An interesting variant on how the universe copes with paradoxes. $\endgroup$ – a4android Sep 28 '16 at 10:59
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    $\begingroup$ "The future you came from doesn't exist." Sorry but that, no matter which words you use to describe it, is expressly prohibited by the question's constraints as far as I can see :) Whether the one universe "adjusts itself" or multiple universes exist in parallel is, for all intents and purposes, a meaningless distinction. $\endgroup$ – Lightness Races in Orbit Sep 28 '16 at 11:04
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There are many different kinds of time travel; as you've eliminated multiple-universe or abandoned-timeline, you're really only left with only two options:

1) Fate. Everything happens the way it happens because that's how it was scripted; you don't prevent your birth by disturbing the air, you cause your birth by disturbing the air. Time is lock-step, and there is no way to mess things up. Or fix things, for that matter. No paradoxes are formed, because it's simply impossible. See "All You Zombies" by Robert A. Heinlein.

2) Paradox armor. Maybe timelines are fluid after all, but temporal paradoxes effectively destroy the universe; you go back in time and prevent your own birth, which means you're not around to prevent your own birth, which means... To keep from getting stuck in a loop, paradoxes resolve as a force; the closer you are to breaking the universe, the harder it is to do. You might have meant to appear in your parents bedroom, but a fluke causes you to appear 15 minutes too late, or a few feet too far away. The bigger the disruption and/or the closer you get to disrupting time, the more flukes show up, making actual disruption of a timeline completely impossible.

Now, there is one other type of time travel available: Hollywood time-travel.

3) In some instances of Hollywood time-travel, changes propagate slowly, "catching up" to "current time" like in Back to the Future, or changes skip causality all together, like in "Looper", happening immediately, but without actually changing any events between. The second case works out the best for you - you'll still be you, with your same memories, just in a different body (how different depends on how much the timeline was changed). However, in each of these cases, paradoxes become merely plot-holes. The science is lazily hand-waved away.

Good news! It's impossible to send you back to stop your own birth. In the case of #1, you cause, not prevent, your birth. In the case of #2, you simply cannot change the past; your enemy will send you back to the wrong time and/or place, no matter what. In the case of #3, you will succeed or fail, depending on what the plot requires of you, regardless of your actions or those of your enemy.

Time travel aside, air is very lightly packed; a human will react to sights or sounds long before air has enough force to move the human body enough to make a difference. Close to the moment, don't land where your parents can see or hear you, and you'll be fine. Anything more would be so difficult to calculate that your enemy wouldn't bother - just send something else back, like a blaring air horn.

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  • $\begingroup$ The third option is actually called simulated time, and is actually how it would be theoretically implemented in a programmed universe. Of course, it doesn't really exist now. The only time travel in things at the moment is groundhog day rewinding and simple scripted events meant to give the illusion of time travel. I wouldn't blow off 3 lightly. $\endgroup$ – The Great Duck Sep 28 '16 at 3:43
  • $\begingroup$ The trouble with #3 is that it is simultaneously the easiest "true time travel" to implement, and the worst at ignoring physics, causality, etc. It's where we get the phrase, "Meanwhile, 30,000 years in the past..." and other such nonsense. If this is a closed-loop system, then any changes in the past would propagate forward through time at a rate of 1 second per second; all time between the event and the present would be affected, which means you would be affected before you ever time-traveled. TL;DR: time travel is hard. $\endgroup$ – ArmanX Sep 28 '16 at 14:53
  • $\begingroup$ no they would propagate forward at one second per second of whatever deity is watchings clock. Meanwhile the future continue forward. Otherwise the time travel would repeatedly triggering itself and the universe would hang stuck at whatever moment you went back. $\endgroup$ – The Great Duck Sep 28 '16 at 21:43
  • $\begingroup$ Perhaps I wasn't clear: once a change occurs, it propagates through time as fast as time happens. Time traveling may skip some amount of time, but at no point would you be able to reach a time where your change didn't happen - when you arrive, the change has already happened. As soon as a change occurs, all future events, including the time traveler, immediately and instantly register that change. $\endgroup$ – ArmanX Sep 28 '16 at 22:37
  • $\begingroup$ You cannot do that in any sense though. It's not a difficulty thing. It's truly impossible. The changes would echo into the future right? They'd also echo into the future of the time travelled object meaning that the echoing goes back the past and continues echoing forward again. This cycle is literally an infinite loop and it will never end. As a result, I conclude that the future never comes into being. If the time traveler went back at moment x. Moment x + (infinitesimally small number) will never exist, period. By your model, time travel literally bring about the end of the world. $\endgroup$ – The Great Duck Sep 29 '16 at 2:21
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I'd say the only way to be certain not to have affected your 'present self' is to appear outside of the light cone of the event where you went back in time. i.e (your age + ~9 months) light years. I'm bad at math/s.

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  • $\begingroup$ That is a nice thought! $\endgroup$ – The Great Duck Sep 29 '16 at 14:17
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Not necessarily relevant, but I did simulate a digital timeline per se (it failed because computers simply don't have enough memory to handle such a system, period) that supports time travel. In general, we can assume that changes to time ripple down the timeline. So one could under that system say that there is a period of initial time for the character to somehow stop the villain by somehow forcefully resolving the situation into a bootstrap paradox. They would have roughly their current age to stop the villain. Heck, the system may never catch up with them and reset them. It just means the past will unravel behind them into the new reality.

To put it simply the system requires two things:

  • Time travel alter the timeline

  • For the future to continue on

This means the future and changes occur concurrently at some proportional rate. Therefore, there will always be a period where the future "shouldn't" be the way it is do to changes, yet it marches on unchanged. This also means that those who come from that future will also be unchanged. So, in a way, the character could have years to fix this issue. Heck, they might even be able to put in a series of small temporal changes resulting in the villain never sending them back, while said changes cause themselves. It's perfectly reasonable.

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If we say you die because a other sperm reaches the egg, you would not live in the future so you wound not get sent back so you would not change anything so you would exist and then would get sent back and kill your self and then you would not exist and could not kill your self and so on.

So a world with no multiverse theory does not work in my opinion, and i think you have to be really close to them that the breeze really matter.
I think if you spawn in front of the wall of the room of your parents the breeze would not matter because it would not even reach them strong enough to change anything.

1.Edit (comments) An empty basement would maybe be a good place to spawn.

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  • $\begingroup$ If you appear on the other side of the wall, the pressure wave from the displaced air will propagate into the room, resulting in slight changes to the movement of the occupants, resulting in death. This will cause a paradox, sure, but that's the point. We want to prevent that paradox. $\endgroup$ – Space Ostrich Sep 27 '16 at 8:10
  • $\begingroup$ @SpaceOstrich To prevent that paradox you have to survive and dont get effected by anything, so i would say in front of the building is far enaught, your breeze would not effect something, e.g. a butterfly want to land on a stick now you spawn some meters away but not in danger of the butterfly, the butterfly would feel the breeze but i think it would still not effect it, it would still land on the stick because a butterfly does not change such small things because of the breeze, a bug maybe in front of you gets scared and this may cause something huge, so in front of the building should work. $\endgroup$ – Xxy Sep 27 '16 at 8:18
  • $\begingroup$ The butterfly would land in a slightly different place or orientation. Which would be enough to kick off the chain reaction. $\endgroup$ – Space Ostrich Sep 27 '16 at 8:26
  • $\begingroup$ @SpaceOstrich why sould it land on a different place and even if would a such super small thing effect something relevant? The butterfly land on the stick because the want to e.g. want to smell that stick, then he will have a special place to land, the most attractive place in his opinion, he would not change that place because of a breeze, he will always get effected by a breeze but it does not matter because he gets driven by other things also and they matter more then a breeze e.g. hunger, fear, reproduction. $\endgroup$ – Xxy Sep 27 '16 at 8:33
  • $\begingroup$ Think about all the slight, almost imperceptible movements you make every minute. They're all caused by something. Air currents, insect life, temperature. That's the crux of the whole thing, the tiniest change in movement will cause the semen to move differently, potentially changing the outcome of your birth. All it takes is that one butterfly landing differently. From this altered angle, perhaps it flies to a different location next, or lands differently again at it's next spot. Once the spread of changes reaches the father, it's no longer safe. $\endgroup$ – Space Ostrich Sep 27 '16 at 8:38
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If you were somehow able to go back in time and displace some air molecules, the displacement alone may be explosive if it were to happen instantaneously (and the air molecules stay in the timeline and are just pushed out of the way as you seem to imply). But, putting that aside (pun intended), just the greater density of your body would be enough to significantly change the timeline.

You see, every particle with mass has a gravitational effect on all other particles with mass, limited only by the speed of gravity (about the speed of light). Yes, that means your toenail has a gravitational pull on the rings of Saturn and your elbow has a gravitational pull on the water in Lake Tahoe, and so on.

Thus, there doesn't need to be any complicated "butterfly effect" with some implausible-sounding chain of events in order to have enough of an effect to change the flow of some "liquid". All that needs to happen is for your body mass to suddenly appear anywhere in our solar system with enough time for your particles to have sufficient tug on your father's body fluids. And, of course, you wouldn't just affect your father, you would affect all of the body fluids of all the people of Earth and all the other particles too. The pull may be tiny, but it would be everywhere. There is no particle that gets to be exempt from the oncoming gravitation. Even if there isn't enough time to alter things enough to prevent your own conception, I believe all of the other changes to the paths of the particles of the world would certainly be enough to make your life different.

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Assume a world with no multiverse theory, just a single unbroken timeline.

That logically rules out time travel, which necessarily introduces deviations from the "single unbroken timeline".

What you're asking is contradictory, like "I would like to have a backwards goto in a computer program which consists of nothing but a single unbroken sequence of statements with no loops."

There is no way to have a world with time travel in which events unfold in a single, unbroken timeline, without resorting to ridiculous deus ex machina interventions and ad hoc explanations and plot twists (if it's in the context of telling a story).

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  • $\begingroup$ Unless everything in the timeline cannot be changed. In that sense, the character never changed anything. Fate decrees it so. $\endgroup$ – The Great Duck Sep 28 '16 at 3:39
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    $\begingroup$ @TheGreatDuck You mean that the loops have some stable solution in which everything Just Is consistent, and that's how it plays out. $\endgroup$ – Kaz Sep 28 '16 at 4:11
  • $\begingroup$ No I mean that the entire timeline past present and future were all created simultaneously be some deity-like being that simply made time travel result in no changes. $\endgroup$ – The Great Duck Sep 28 '16 at 21:39
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Basically, any travel to the past would cause change. What change is a matter of chance and action. A time paradox is impossible, as I will show you. As to eliminating yourself: NOT POSSIBLE! You could prevent the birth of a past you, but NOT YOU. You would continue existence. The proof is quite simple.

Passing through time is relative. Some of us go faster into the future than others. We have proved this with clocks and space satellites. So, if you go into the future faster than me, do I disappear? No. This proves that the past and future co-exist.

I would be in the past waiting to catch up with you, and that may never happen. If travel to the past is possible, then if you would go into the past would I disappear? No. Why? Because this would be an actual reversal of time which, even if theoretically possible, would be in actuality, impossible.

So, traveling to the past still leaves me in your future. When you go to the past, you will naturally travel into the future at a normal rate. I, in the meantime will be going into the future at a normal rate. NOTHING you do in the past will catch up to me! And nothing you do in the past will alter your birth, or your life in any way.

A little side note: If a man would leave his wife to go on a trip to a foreign land, on a speedy jet, he would likely pass through time at a different rate than his wife. When he got back home, his wife would not be the same wife that he left! The original 2 are now in different time zones with different versions of themselves.

There are a couple things to keep in mind. Going faster into the future still leaves a "Person" existing in all time periods he passes through. So, no one disappears from a time line. Also, there are at least 2 variations of time travel that may be possible. This depends on whether time travel is analog or digital.

Personally, I think time is like light. It acts both as an analog wave, and as a particle.

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Open a portal into the past but don't send anything through it so no air displacement occurs. Just let a few photons travel forward to your age. Now move your viewing portal to that special intimate moment of your conception and get a detailed view of the particular egg and single victorious sperm which lead to your creation.

Using that detailed information and some synthetic DNA technology which you've previously stolen from the future, create a perfect replica of the fertilized egg which became you.

Now go back and kidnap your pregnant mother...

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