Avoiding preventing your own birth in a time travel paradox?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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 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.

Answer 4

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.

Answer 5

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).

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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."

Answer 10

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.

Answer 11

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.

Answer 12

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.

Answer 13

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.

Answer 14

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.

Answer 15

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).

Answer 16

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.

Answer 17

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...