Can a world where everyone has a time machine be consistent?

Question

In a universe everyone has a time machine and can travel both in the past and in the future.

At some point, no one knows when, an eternal war to control the time started.

The concept of history has lost meaning. There is no safe place, in any age and in any place of universe.

This universe is different by ours, so rules of physics can be different, but I want make it as more as realistic as possible.

Now the question. Is there is a way to make this universe consistent?

If we assume a single temporal line, will some paradoxes happen? For example the Grandfather paradox, would killing your grandfather kill you thereby preventing you from killing your grandfather.

If we assume a multiverse where everyone that goes in past creates a new temporal line, I expect that everyone goes back in past creates its own timeline, so if you go back before the invention of time machine then you are safe.

So thinking about a multiverse and a time machine that allows you to travel across any timeline, or will this generate other paradoxes?

Answer 1

Yes and no.

All time travel depends on how restrictive time travel is. There are several types of time travel!

Note that if everybody had a time machine, Hawkins should have had a booming party on 28th of June 2009, to which he sent out the invitations on the 29th of June. He gave it to remind of his 1992 conjecture that travel into the past is effectively impossible.

Flipflopping Time

Whenever you travel back and try to prevent something, you create a closed 2-state loop for yourself, and are therefor forever trapped in either of the states.

• Jack Traveller travels back 50 years and prevents his grandfather Albert Traveller meeting his grandmother Janine Jacobs by running him over with a car. Because his gramps never met Janine, Peter Traveller and his son Jack Traveller are never born. Because of this, Jack - who never existed and ceased to exist the moment he ran over Albert - never traveled back to even start the car to run over his grandfather Albert. Because Albert wasn't run over, Peter and then Jack get born. Jack travels back 50 years and runs over Albert...

Grandfather Paradox at its finest - Gramps Albert, Peter and Jack have become Schrödinger's Cat, the existence of the later 2 dependent on the state of the first, which is simultaneously dead and alive until we manage to collapse the quantum formula!

Closed timeline loops

Quantum mechanics tells us, we only need to look for Jack after the point of time-travel and we can collapse the whole thing: If Jack exists there, he never traveled and Gramps never died. If he doesn't, then Jack had never existed in the first place.

Don't change the past!

But you can certainly can do 12-Monkey style time travel with this: DON'T CHANGE THE EVENTS, but send notes to the future, where they will be received after the point of departure! This way causality is saved, Grandfather paradox avoided.

Self-enforcing-timeline

You can't change the past. Or as Dr. Who said in The Aztecs: "You can't rewrite history! Not one line!" It's a little up to interpretation how this can be seen, but the gist is:

If you attempt to change the timeline there's only two options: Your very attempt to change the timeline is either doomed to fail or by some thing or another your very action only did create the timeline you traveled from to begin with.

• Jack never knew his grandfather who was killed in a military accident. When he travels back 50 years to try to meet him, he runs over his gramps before he meets his grandmother. Later he meets a girl - Janine Jacobs - in a bar and (after kicking Private Paul Pascal's ass) things lead to one another and they dance the horizontal tango. 9 months later, Peter Traveller, Jack's dad is born and some 25 years later, Jack.

Jack had to make the timeline happen the way he had known it after he traveled back. And if he wouldn't have been the one to dance with his Grannie, it would have been Private Paul Pascal, who fathered Peter. In either case, Peter was never (even before the time travel happened) the son of the Albert Traveller to begin with like everybody thought!

Butterfly Effect

A variant of the general timetravel rule is explored by The Butterfly Effect. It imposes one extra limit: One can only travel backwards and only to his own past, taking control of his own body back in the event one travels to, and displacing the previous consciousness.

For an inside observer, the timeline fluctuates, but no other but the time-traveler does even know that anything changed. Only one state of the possibilities created by the time-travels exists: the one that was created by all the changes culminated. And since only the time-traveler can remember those 'alternate pasts' created by his time-travel, his mental health suffers.

There's no time-travel, it's travel to a replicate of the world.

Instead of really traveling back to your past, you travel to an exact copy of the universe as it had been before your interference. By your interference that universe develops differently - but you don't really have effected your native timeline. Depending on the rules, he can travel back to his native timeline (and see nothing changed) or he travels to the point later in the new timeline. Or, by traveling back, you actually kill yourself.

• Jack Traveller travels back 50 years and (once again) runs over his gramps before he met his (former-future) grandmother Janine. In this new timeline, Janine becomes a nun and neither Peter Traveller nor Jack Traveller are born. But there is the Jack Traveller not native to this timeline.

Only in such a setting time-wars are actually somewhat feasible, but they end with all sides winning simultaneously, as each generates its own alternate world that never had enemies to begin with. And each dissident group branches off into its own, perfectly controlled dystopia of different means...

Answer 2

Worlds according to Novikov's self-consistency principle would remain consistent. This principle argues that, if time travel is viable, it is simply impossible to travel back in time to cause an inconsistency. The universe will intervene.

A famous challenge was the Polchinski Paradox, which would be a good place to start looking at the challenges of a consistent universe with time travel. In this paradox, a billiard ball is traveling towards a wormhole which will take it back in time. The wormhole's exit is configured such that, when the ball goes through the wormhole in the "future," it comes out in the "past" on a collision course which knocks the future ball off course so that it never goes through the wormhole. This would be a very straightforward version of the grandfather paradox using only physics.

It took a few years, but several Caltech students eventually proved that there is a solution. When the ball comes out of the past-wormhole, it doesn't come out on quite the same trajectory. It comes out on a perturbed trajectory which collides with the future ball exactly right to cause it to deviate in a way that causes it to emerge from the past-wormhole on the same perturbed trajectory, maintaining consistency. They proved that for all possible configurations of 1 billiard ball and two wormholes, there were an infinite number of self-consistent solutions possible.

They also postulated that time-travel systems more complex than a billiard ball and pair of wormholes would also provide a similar infinite set of solutions. Obviously proving so would be a nightmare, but given that they solved the math for the single ball/single wormhole case, some deference should be given.

The key to this is that there are an infinite number of possible solutions. Thus if your human actors act in a way to make it impossible for 99% of situations to occur, there are still an infinite number of valid possibilities.

Answer 3

What if you can only view the past?

With the amazing Chronovisor 9000
We can know who killed that hippie at the cross 2000 years ago.
We can get to witness the Ancient World Wonders!
Fancy looking how beautiful Cleopatra was?

It allows everyone to get a time machine and yet the TIMELINE is safe.

Brb, I have to snap pictures of early horses for my homework.

Answer 4

I'll fix this mess. - Universe

A really good source for this type of question I feel is the Orion's Arm Universe. That page goes over several of the more popular and scientifically backed theories around time travel.

I wanted to point out one theory that could be very interesting for your universe. To quote the above link:

Such a lack of continuity will invoke the Chronological Protection Conjecture: the universe will morph and mutate until it reaches a history in which time travel has never been invented, ever, at any time in the past or future. This is the only stable state for the universe, the one we are in now. The CPC has been proposed by many people, including Hawking, Asimov and Larry Niven.

In effect, your universe could take place during a time in which the universe hasn't yet become stable. Time travel is rampant, people are going back to the past and rewriting history until... time travel ceases to exist. The infinite war never happened (take a page from WarGames: the only way to win is not to play).

If you don't make time travel impossible (eventually), then the future (past?) is inevitable (again from Orion's Arm Universe):

The future sophonts would come back in an exponentiating wave to constantly change the present and the past, and whole galaxies of material particles will begin to exist in space time reference that did not have them before - some? many? most? matter and events may turn out to be acausal, going round and round in closed timelike loops and increasing the total mass of the universe, which may begin to collapse in the distant future, sending chronistic refugees in massive tardises back to our time thus accelerating the collapse; increasing the mass of the present day universe until it collapses. The collapse will get closer to the present day, until it eventually happened yesterday and we will cease to exist.

It seems to me like that future is inevitable in a universe in which time travel is possible. People will try to escape the pending Death of the Universe and the only way to do that will be to go back in time. As more and more people travel back to a "safer" time, they draw the end of the universe closer and closer until the universe corrects this mistake and just... dies.

Answer 5

Time travel is bad enough when one person has it. Give it to everyone, and you're aiming for a world of confusion.

However, there is a way of getting around this, and the answer lies in what it means when you "create a new timeline". The fact that going back in time always creates a new timeline means that people in already existing timelines can never be in a "target" timeline for a backwards-going time traveler.

Or you could look at it from a many-worlds perspective. In an infinite number of universes with infinite possibilities, the possibility always exists for a backwards-going time traveler to spontaneously manifest from nothing. Go back in time, and you automatically "target" the one universe among innumerable universes that happened to create you at that moment in time. However, the chances of being in a universe that spontaneously creates a time traveler from nothing is so infinitesimally small that it might as well be zero. (This is why we never get time travelers from the future, no matter how many parties we make.)

That means that another time traveler coming from the future will never appear. You can travel forward without changing the universe you're in, but travel backwards and you're in a completely new universe, and completely safe from anyone you met in the future. If they go back in time to follow you, they will appear in a different timeline and may find another one of the infinite "yous", but you will never encounter them. (Or rather, the possibility of you encountering them is one out of infinity, which is basically zero.) (Math pedants, don't comment on this, it's wrong but it's close enough.)

Of course, this messes up your story's premise of a universe where no time is safe, so I'd add one caveat: When someone travels back in time, they create a "trail" that can be tracked with a time machine, allowing others from the future to follow the original traveler into the same universe they "created".

You will never meet an unrelated backwards time traveler, but you can meet a traveler who is pursuing you in particular (or anyone who is following them, or anyone who is following the person you are chasing). This allows "time chases", but you can't just go back to the time they went to and expect to find them, you have to "lock-on" to their personal timeline by tracing the point they left and follow them directly.

This is also the only way that a time traveler can return from the past - by traveling back (forward?) along their own "trail". Otherwise, traveling forward will keep them within the new timeline they created/found, and the universe they left behind will never see them again. If they do return, any changes they made in the past will have no impact on the future they return to, since they happened in a different universe.

There can also be "time tunnels" that are permanently "open" using the same principle, and people can walk "backwards" and "forwards" through them whenever they want, although in this case it's less about moving through time and more a portal between two universes, where one happens to resemble the way the other one looked in the past (or at least it used to before all the time travelers showed up.) Together, these time tunnels can make up a "portal network" of parallel worlds, initially identical but gradually diverging until each of them becomes its own unique locale over time.

Answer 6

Just because you intend to change history, doesn't mean you can.

As a non-time-travel example - just because you decide to win the lottery this week, doesn't mean you will. Just because you decide to get that perfect job, doesn't mean that the company will choose you from all the possible candidates. Anyone who thinks that they can change history is being astonishingly arrogant, egotistical, and overconfident.

The past has already happened. While you certainly can try to change history, you will fail. Circumstances beyond your control and/or knowledge will have already prevented you from managing this.

Even when you think you succeed, you actually haven't: go back in time and arrange for Adolf Hitler to be killed during the first World War, and it will turn out that someone stole a dead man's identity to get themselves a clean slate. History doesn't change, but our understanding of what actually happened might. Truths uncovered, conspiracies unravelled, misunderstandings cleared up. That group who went back to see the aliens land at Roswell in 1947 should have just looked in the mirror.

On the other hand, going back to ensure events do happen is practically guaranteed to be a success - and if it doesn't, then you didn't need to intervene anyway.

Your factions are all fighting each other in the background, but it's all ultimately pointless - nothing that they do can actually change the past, it just decides who claims bragging rights. Of course, once time-travel is actually invented, you can plan for reïnforcements from the future - but if you lose the battle, you can't send more than were already there.

Answer 7

The grandfather paradox works in single timeline only if you decide it works there. Otherwise the attempt on your grandfather life was always there. It was just unsuccessful.

OR
The attempt was successful. And it's called "we need to kill Adolf Hitler in the cradle". Which result in newborn Adi being killed but being replaced by another baby by the babysitter.
This is the idea behind Doctor Who "Fixed points in time". So no matter what you do the history will change itself to get to that fixed point anyway.

Which can be used by you for the story. Everything that is happening is due to time-travellers trying to change the time. Breaking a broken thing fixes it. So one time traveller tries to break Gavrilo Princip's gun. Unbeknownst to him a second TT did the same two days ago. TT fix the gun while trying to break it.

Another thing you can do is to make so many timelines they overlap themselves creating one "proper" one that cannot be changed. Think about stereoscopic image that create one when looking at two slightly moved apart.

Answer 8

While your question focused on time travel in relation to different models of time. Time travel in different models of time will have different paradoxes or not (depending on the nature of time in each model).

For example, a multiverse where time travellers either go to timelines where no matter what they do can create a causality violating event or they simply create new timelines, then paradoxes like the Grandfather cannot occur.

This answer will suggest that consistent world with universal time travel is possible, but it depends more on what sort of time machine everyone is using.

If a time machine is a portal. This could be a wormhole, to be scientific, or simply a scientifically unexplained time-gate. Assume a portal with a fixed duration displacement in time. Now time travel will be restricted to which portals someone goes through and when they emerge. if time travellers have to use access cards to travel in time, irrespective of the past or the future. Then time travellers will be able to track their trips and be able to avoid creating paradoxes. Also, control systems could be installed in all portals which monitor all time travel and can thereby prevent paradoxes from occuring.

For example: "I'd better not go a fortnight into the past there, because I will meet myself or change my own history." or "Your journey is forbidden. It will violate causality. This is not allowed under the statutes of the Chronological Protection Act. Please select another portal for a legal temporal excursion."

Assuming everybody carries small portable time machines. Basically cell phones (or mobile phones, depending on your country of habitation) with time travel capacity. A traveller keys in the time of their destination, presses activate and goes when (I could have said "there", but we're talking time travel, so it's when instead). This world has monitor towers tracking every time machine and every time traveller.

if a traveller arrives in a time where they either meet themselves or cause a causality violating events, the monitoring towers will intervene and either block the traveller from going when or might activate their time machine to move them to somewhen where they cannot violate the flow of causal events.

This answer has described two types of time machines: fixed interval time machines and flexible interval time machines. Both types of time machines are accompanied by overarching control and monitoring systems designed to prevent paradoxes and causality violation from happening. This will produce a consistent world with universal time travel (i.e., where everyone has a time machine or access thereof).

On the other hand, it is possible to postulate models of time where causality violations and time paradoxes are prevented from happening or those like the multiverse where causality cannot be violated, despite appearing to have happened (or unhappened, as the case maybe). However, what is often forgotten is that new dangerous technologies are developed, steps will be taken to try and make them safe. With time travel that can easily include mechanisms, built in to the very time machines themselves to prevent paradoxes. Strangely enough, this is called commonsense.

Answer 9

Ah, time travel. Be warned that it is very difficult to write a time travel story that is both consistent and interesting. But if done right, the result can be amazing!

Trish's answer lists a number of different possibilities. I would like to add one more: Time travel in the Multiple World multiverse.

IndigoFenix's answer also talks about Multiple Worlds, but I think they got one important aspect wrong. They say that there is a very close to zero probability of a time traveler appearing because that would be the same as a quantum fluctuation where the person just spontaneously appears from nothing. Quantum mechanically possible, but very improbable.

In the Multiple World multiverse, one Present leads to many Futures. Suppose 10% of those Futures leads to a traveler going back to this Present.

I believe there is then a 10% chance of a traveler appearing. Not 0%. Note that those 10% of the Futures are not necessarily the same 10% as those sending a traveler. There will be Futures where a traveler goes back and disappears. There will be other Futures where a traveler appears but nobody ever goes back to become them. It only adds up if you consider all the Futures together.

So, the Grandparent Paradox is typical example of this, here there is no overlap between travelers leaving and arriving.

There is also the Anti-Grandparent Paradox. In this version, the only reason the traveler exists at all is because they travel back and save their grandparent from some fatal accident. And the only reason they travel back at all is because their grandparent have asked them to. This is consistent, but hard to get off the ground in the first place.

There has to be Futures where a traveler appears, but fails to rescue their grandparent. Nobody is perfect, after all. To balance this there must also be Futures where the grandparent survives on their own, somehow, and their grandchild still goes back to rescue them. But why? Perhaps they want to rescue them from being crippled or maybe there is just an interesting sports event that day.

All this is fascinating, but is hard to turn into a working story. The basic multiverse story is "Everything possible happens in some universe. the End." Not very interesting. I hope you can write something better than that.

Answer 10

Aside from the variants already provided in other answers, there is another model of time travel effects that can be played with.

In this case, it's the "propagational change" type of time travel. Think of what happens to Marty McFly in Back To The Future Part 1 - some of his actions interfere with his own parents getting together, and it gradually propagates through the time loop.

This can be justified by considering "space-time" propagation of information - it happens as "the speed of light" at its fastest, when operating in space. It is reasonable enough to suppose that it has a "speed of propagation" in time, too, when we're talking about time travel.

To use the grandfather paradox case, if you travelled back in time to kill your own grandfather before he and your grandmother had produced the fertilised egg that would become your father, then you might have a few weeks, say, of time to do something to prevent your own destruction, because the change takes those few weeks to propagate through. If you travelled back to yesterday and killed yourself, you might have a few minutes to think about what you've done (and maybe travel back again to stop yourself from doing it).

And because it "propagates" in this way, your actual action in killing your grandfather (or yourself) still happened, in the same way that, if you suddenly caused the sun to cease to exist, it would still take about 8 minutes for this ceasing to exist to impact the earth. This is just the time version of the same thing.

One of the nice aspects of this is that it eliminates the need for universal agency, while also preventing the obvious paradoxes.

In the context of a "time war", it makes for some fascinating possible battles, where one side does something, and this leads to the other side having a time limit on possible counteractions to prevent their own demise. Perhaps each side have placed "units" at various time points, who have the ability to report forward in time (thereby skipping ahead of the propagation), so if someone went back and did something to Da Vinci, it might be someone who is currently living in the time of Galileo who detects it, and reports it forward, giving the "Prime Time" a chance to send an operative back and deal with it.

If a change propagates through a time that a time traveller (who has travelled back in time) is currently at, they would perceive the change pass through, but not be directly impacted by it, as their own "timeline" hasn't been hit, yet. The very act of travelling in time causes a "temporal delay" that is larger, the further from the "Prime Time" they are.

Interestingly, this would operate in a reverse fashion in the future - any changes would reach the time traveller before reaching the time that they're living in... assuming the change occurred in the past relative to the "Prime Time".

Answer 11

I believe what you describe is possible in simulation theory. Imagine that the time machine created an enormous black hole. Time is inscribed on the event horizon of this black hole, much like a video recording. The original particles are destroyed in this process, so there's no question that this is the only reality in this patch of spacetime.

Causality does not exist on this recording. Things only seem to possess causality because that is how they were initially recorded.

Inside, through some unknown physics the sensation of existing plays out.

The second part of this time machine is some sort of technology that allows people to edit the recording on the event horizon. This allows faster than light travel, and also time travel. No energy is produced or destroyed by editing the record.

After a lot of use, the historical record is starting to look pretty sketchy. Did the prototypical human Lucy really fight in the American Civil War with a light laser brigade?

I imagine there will be some political group of time travelers wanting to restore history. They'd be contrasted by political groups wanting few restrictions on time travel. And, hopefully, they'd come to some sort of agreement.

Reconstruction may be like the 2014 movie Interstellar where advanced humans can't seem to figure out their own history.

This is not a closed loop. New time is being created constantly as new material from outside falls into the event horizon. It may even be possible for advanced technology to exist allowing ingress and egress (like Fredrick Pohl's Heechee), and thus some sort of interaction between the poor causal worlds outside and the wealthier acausasal worlds inside.

Answer 12

And answer that often comes is the following: if you go back in time you make a new timeline.

Ok why not but then what about the "present" timeline? The one you went out from.

I believe the present time line will be emptied because every one will want to travel back in time to create it's own timeline in hope of improving their life condition. Then the base timeline will be almost completely emptied, this phenomena will probably start as soon as the time machine is created.

I guess one interesting point can be the dilemna facing those staying, will you stay in this timeline or go backward and make your own at some point in history, hoping to find a better moment in history ?

The more you wait and the less information about the past you'll find so the benefit of going back is reducing (you probably will have more trouble getting rich with the knowledge you have about the past etc...)

At some point internet shuts down, we are living the classical apocalypse : power plants explode, their are big fires and no one to handle them (you can imagine whatever you want starting from there), water get polluted, crops gets harder and harder to grow...

Plus the time machine will probably not be that easy to use, so probably without the original scientists and the first billion people already gone it might be pretty hard to find correct guidance with the machine.

Every moment you wait your world is getting more and more empty and the distance between you and the past is getting bigger.

So my point is probably that instead of solving the complexity of managing different timelines, how do you handle the main one? What happened 100year after the time machine is invented? What's left of the world ?

Answer 13

This is something that the comic book Universal War One covers, the author Denis Bajram had a lot of work to keep it coherent with only one timeline.

In this universe (our near future), at the end a whole civilisation grows with teleportation through time and space at will. The idea is that at any point in time & space, the events are the sum of all past and future events, so you cannot really change what you are experiencing, but are just contributing to the events. Therefore there is no paradox, the actions you do in the past are really shaping the future as you have known it, to make stable loops.

This is really an interesting read for anyone willing to read a hard SF story with time travel. More on https://tvtropes.org/pmwiki/pmwiki.php/ComicBook/UniversalWarOne

Answer 14

To keep things simple I would say: 1. If you go back to the past you can go only as a ghost, i.e. you cannot touch anything, you cannot talk to anybody, and no one sees you. You can go through walls and be wherever you want. When you come back you can keep what you saw. 2. When travelling to the future, then you have to forget what you saw and what happened.

In general some level of constraints are needed, and you need to decide how strict you want to be.

Answer 15

Sure, you can travel in time, but it's not for the weak of heart. If you go back in time, you can create a new timeline, but if you knowingly or unknowingly create a paradox you're stuck in @Trish's limbo and objectively just disappear. If you really know what you're doing, you have maybe a 50-50 chance of avoiding this if you go back a day, but the butterfly effect would make it exponentially more difficult as you go back further.

Even if you avoid oblivion, you have no idea what your current timeline will be like. Return to your time, and your friends have all died in a war you inadvertently started. Even worse, the environment (local or global) may be toxic and kill you, or you're a hunted fugitive.

Going forward in time...which timeline to follow blindly? Normally, moving through time at the regular speed, we adjust one decision at a time, but we skip a lot of choices when taking the back way. Do you land in a time where you're already dead? Game over. Even minor errors in astronavigation could land you in the middle of the ocean, or in space. These dangers also increase exponentially with the length of the trip.

So everyone has access, as we do today with skydiving, but it's dangerous with the right conditions, knowledge, limitations, and expertise, and almost certainly fatal without them.

Something to think about...do you move within your own life, or create another copy of you at the target time? Or create a copy only if outside your own life?

Answer 16

If it is possible for anyone to go back and change the past, the timelines will eventually 'converge' or rather, time will simply become a spatial dimension.

One dramatic change in the past like say someone blowing Hitler's brains off with a beam gun in plain daylight can be immensely impactful, but then it is followed by 10, 100, 1000, 10000000000000 time travelers interfering with the event. Since the length of time between where a time traveler kills Hitler and the present is irrelevant to how many time travels can have been made, either there will be a point where any new time travels to that point in time will have little-to-no impact, or no more time travels will be made to a given point in time. Consider these scenarios:

1. Date X becomes a time tourist mecca which eventually spawns its own regulatory mechanics, where the time traveler pileup means any newcomers have nothing to do but simply observe. If they somehow act in a way that might impact the stabilized timeline, it will already have been accounted for because there will be trillions upon trillions of time travel effects that check each other.
2. Only a limited number of people ever travel to Date X and make any changes, which means the impact on the timeline is also finite before the timeline up until Date X is considered static.
3. Some event happens where Date X becomes the latest time a time travel originates from. It can be an extinction event, loss of the technology by accident or on purpose, or simply the 'hobby' dying away being so saturated by past-comers. If someone stashed away a time machine after this date, no one is left to use it or no one ever finds it etc.

You can also see it as the timelines being subject to Darwinian natural selection: if a timeline is not stable against time travelers it will die and since a practically infinite amount of time travels will have occurred at any given present time, the surviving timeline will be practically 100% time traveler proof.

Answer 17

The TV show "Gargoyles" time travel was always a stable time loop (AKA a Bootstrap paradox) as a universe rule and was consistent about it. Episodes featuring stable time travel include "Vows", "Avalon Part II", "MIA" and "Future Tense" (a character who should know better suggests they try and change the past, which tips Goliath off that there's an imposter afoot.).

Essentially the concept in the Bootstrap Paradox is that history cannot be changed, but time travel is possible. The Bootstrap Paradox would take the Grandfather Paradox (you kill your grandfather in the past before he concieved your parent with your grandmother, but you don't blip out of existence. Turns out that Grandma was having an affair and the other man impregnated her and she lied and said it was the child of the person you thought was your grandfather. If you try and go back to kill the "Other Man", something else will prevent you from doing so... (maybe in your desperation, you cross the street and get hit by a car and die in the past... maybe the machine breaks)). It's important to note that in the Bootstrap paradox time travel, the preservation of the time line is not by some force that changes history... but rather that the preservation was always history as we Knew it before the time travel... it's just no one counted on the time traveler to be a player in the event. Essentially, all of history has always played out exactly as the events were recorded, including the appearence of the time traveler, who might not know he or she would create the very history they set out to change when they went to the past.

((For a real event idea, lets say we went back in time to save JFK from his assassination. Moments before the fateful shot, we actually find a secret service agent on the grassy knoll and go to warn him that the president is about to be shot. The secret service agent mistakes our warning as a threat and proceeds to tackle us. In doing so, he takes his eyes off the book depository building and misses Lee Havery Oswald taking aim as the motorcade enters Dealy plaza. Had we never time traveled to Dallas, November 22, 1963, the agent would have had eyes on Oswald and may have saved Kennedy... but since you time traveled, history is preserved.))

In my own work, I use the mantra "The past is Stone, the future is clay, the present is a kiln" to work around the implication of lack of free will that the Bootstrap paradox creates. Essentially how this works is that people who do not know anything about the future events may change the course of history, but any knowledge of future events prior to them happening will cause them to become certain as soon as the event is known. this means that it is possible for time travelers to change history, so long as they don't directly affect the change... essentially they have to play a very subtle manipulation game to get people in the past to make the choice for themselves that will alter the past (In our Kennedy scenario, we shouldn't warn the secret service agent about the president's imminent assasination, but if we stand with our back to the book depository and talk to him about the wonderful weather, he might have enough of our distracting yammering to turn his head and look at us with intent to tell us to buzz off... at the exact moment Oswald is taking aim... allowing him to give a "get down" alert that allows Kennedy to react just in time to save his life).

Answer 18

You are on Timeline 1 and travel back (or forwards that doesnt matter!). This lands you on parallel timeline 2, where everything is the same except that you are there now and can kill your own grandfather without repercussions.

A nice extra "rule" is that you cant go back to timeline 1. If you time travel again you end up in timeline 3. This is important, since any time travel from timeline 1 to timeline 2 will instantly have happened. If you are the first time traveler in 2020 and go to the year 2000 and the second time traveler only does so in 2030 but goes back to 1 minute to before you arrive, then time traveler 2 will already be there and changing the timeline you end up in as you dont end up in seperate timelines.