Mr Young,

I was not very clear, and I had some of my ideas mixed up.

For every timeline, there is an infinite amount of timelines co-existing with it.  We could call one timeling, timeline 1, and a second, timeline 2.  Since timeline 1 and 2 co-exist, they are exactly the same- there is absolutely no difference between them.  Every event that occurs in one has happened exactly the same way in the other, and at the exact same time.  This is true of all the co-existing timelines.

Before I go any further, some people may disagree in their opinons of nature vs nurture.  I am assuming that if an exact genetic duplicate of a person is brought through the exact same circumstances, all occuring at the same time, those two people will turn out the exact same, as their nature will be the same, and they will have been nurtured in the exact same manner.  Thus, I really don't care whether nature of nurture is more important, or whether they are equal.  Now, to continue...

When a person (let us call him Bart) decides to travel through time (and let us say that Bart is from timeline 1), a second Bart, Bart2, also trys to travel through time, since timeline 2 is exactly the same as timeline 1.  However, when Bart "throws the switch," to begin traveling, he is moved to timeline 2 in place of Bart2, and the two timelines spilt apart.  The other timelines, of which there is an infinite number, split apart and either, form a third timeline, co-exist with timeline 1, mirroring its actions, or co-exist with timeline 2, mirroring its actions.  Timelines 1 and 2 no longer co-exist, and different actions can occur in each.  Timeline 1 goes on as if nothing had happened, only the world may marvel at the strange and sudden dissapearance of Bart.  "Hey," somebody may say, "have you seen Bart?  He wanted to build a timemachine.  One day I went to his house and he wasn't there.  I haven't seen him since.  Do you know what could have happened to him?"  Meanwhile, in timeline 2, Bart has taken the place of Bart2.  Since the two timelines were the exact same, all of Bart's and Bart2's memories are the exact same, they want the same things, have the same things, and have the same personality.  Thus, if Bart stayed there, he would never notice a difference.  However, Bart is not staying there, he is moving back through time.  Let us consider two different cases.  One where Bart changes the past drastically, and one where Bart never changes the past at all.  Also, keep in mind that Bart2's present self is destroyed.  In the past of timeline 2, a younger Bart2 is very much alive, and is the exact duplicate of Bart's previous self (since the same circumstances that helped shape Bart also happened to Bart2 at the same time and place).

               timeline 1
                  /---------------
                 /
timeline 1 /
/\/\/\/\/\/
\/\/\/\/\/\
timeline 2 \
                 \
                  \----------------
               timeline 2

The original   The timelines after
timelines,        they split.  Different actions
co-existing.    can now occur in each timeline.

Bart lives in a world ruled by his evil brother, Ken.  Bart decides that he wants to kill Ken, but Ken seems to be all powerful.  So, Bart decides to go back in time and kill their grandfather.  "Well," Bart thinks, "I will die too, but at least Ken and I will never have been born, so the world will be spared his evil."  So Bart builds a time-machine, and travels back to the past.  Whe he first starts to move back through time, before he has gone one second back in time, he is moved to timeline 2 and Bart2 is instantly destroyed.  In his place is Bart (with all of the same thoughts, desires, and memories).  Bart is now back in time, he finds his grandfather, kills him, then decides "Well, I will now return to my own time.  I will probably fade out when I get there, but I will get to see a world not ruled by Ken."  So Bart travels to the future.  But he is traveling to the future of timeline 2.  Since he is traveling to the future of a different timeline, he does not affect the circumstances of his birth back in timeline 1, so although all Bart2's siblings are never born, and Bart2's father is never born, Bart himself still exists, and can see what the world would be like if he had never been born.  He can make a place for himself in this new world, but no one will have previously known him.  He would be like Jimmy Stewart in "It's a Wonderful Life."

Now, consider the case of Bart (as he travels back through time), suddenly saying "No! I can't do it!"  So he hits the "reverse" button.  The machine never makes it to the past, but instead returns to the future.  The only thing is, Bart is still in timeline 2.  He will never notice a difference, and neither will anybody else.

The other timelines, the ones that split into a third timeline?  In those timelines, Bart's machine never worked.

Since a third of infinity is still infinity, each of the three sets of timelines still is surrounded by an infinite number of timelines, mirroring its every action.

Here is a summing up.

If someone went back in time to stop a war, he would only be partially successful, since he would know that somewhere in the universe, there are two different sets of timelines where the war continues.  In the first set of timelines, he was successful, as far as traveling back into time goes, but the denizens of that first set of timelines will bever receive the benefits of his travel through time.  In the second set of timelines he was never able to travel back into the past, and so he never got a chance to try and fullfill his mission.  Thus, he failed.  However, there is nothing that he can do about those other two timeline, so he can either relax in the world he is in, forgetting all about those timelines, or he could just pass the whole thing off as bull, the whole time thinking that he was really in his original world.  And he would never be able to detect any difference.

Now, as for traveling between dimensions, without traveling through time or destroying your alternate self, so that you could meet someone exactly like yourself, that is all a completely different matter.

I wonder though, if all of this were actually true, and it was possible to move back and forth through time, then shouldn't any person who trully believes that they have traveled through time, no matter what changes for the better they say they made, shouldn't they be charged with first-degree, premeditated murder?  However, without being able to travel through dimensions (which I am not going to consider, and be traveling through dimensions, I mean physically leave one dimension, then enter into another dimension by a means totally unrelated to time-traveling), it would be very hard for the prosecutor to come up with evidence, and the person would probably just be "helped" with their mental problems.

Have a great day,

Bart
AKA Kelvin Rankeen

Bart--

Before I approach the details of your latest letter, please bear with me while I make a few general comments.

First, your notion (albeit borrowed) that one could travel back in time only to another dimension, and forward only to the same dimension, is an interesting one; but it isn't really time travel.  What you are arguing is, in essence, that travel to the past is impossible, but travel to the future is possible.  (It is an interesting aside that some would say the opposite, that one can go to the past because it already exists, but one could not move to the future because it is not yet fixed.  I disagree with that idea, also.)  Thus in one sense, we are arguing two different ideas--you are trying to say that one cannot go back into one's own past, while I am arguing that if one could there would be specific consequences.  It may be the case that time travel is impossible, or that it is impossible for creatures; but if we are going to discuss time travel as a possibility, let us discuss real time travel, but be mindful of the possibility that our efforts to travel in time might fail and result in a situation such as you have suggested.  It's not terribly useful to begin by postulating that time travel is not possible, but that we will be fooled into thinking that it is when we discover how to dimension-hop into the past of a parallel universe.  It is useful to be aware that our efforts at creating a time machine could result in such a device, and that, under the right circumstances, it might be some time before we discovered the error.

Second, whenever anyone mentions infinite anythings, I generally become very suspicious.  Nothing in our reality is truly infinite--the universe, as vast as it is, is not, nor anything it contains.  God is said to be infinite, and infinite in many ways; but even granted that some of us have met God and confronted His revelation of Himself, we haven't perceived the infinite--only a portion of it.  The Infinite could not reveal Himself either within our finite universe or to our finite minds.  Our number system is infinite, and infinite in multiple ways; but the names for our numbers are finite, and none of us grasp even the largest of the named integers or the number of points between any two.  Relativity postulates that an object which achieved the speed of light would have infinite mass and momentum; but if I understand it aright, no object can reach that velocity (and certainly no object is known to have done so), so it, too, is theoretical.  Nothing we know which exists in any material way (beyond the theoretical) within this universe is infinite, and none of us truly comprehend the concept.  Thus, the notion that there is an infinite number of anything always rings the warning bells for me.  People who use the word rarely know what they're saying.

On the nature of choice, I've already discussed that extensively in several letters.  Is it heredity or environment?  Surely it is both, to some degree; much of my theory relies on the notion that the same person brought through the same events in the same way will have the same reactions and make the same choices at the same time, as long as it is the first time.  In short, we're agreed on that.

O.K., let's look at your ideas.  You suggest an infinite number of an infinite number of an infinite number of time lines, all exactly the same.  I'm suspicious that you're trying to bury the failings of a theory in numbers--I'm reminded of a James Garner piece about a scam in which you lose a little on each transaction, but make up for it in volume.  It doesn't work that way.  But let's assume that there is merit to the notion.  Each of these universes is exactly the same, because, unlike the typical multiverse theories, nothing is really random, everyone will do what he would do, and without any outside influence each universe with the same exact beginning would develop in the same exact fashion.  Although Holger Theimann suggested in an earlier letter that any two such universes which had returned to being exactly the same would merge into one, you and I agree that that is not so, that universes which were identical could exist independently.  So these universes cannot be distinguished from each other, unless something happens which changes them.

But something can happen to change them.  The universes are the same until the moment someone leaves universe 1--or wait, no, they are the same until someone arrives in universe 2.  Which is it?  Your explanations of the matter are inconsistent.  But you must mean that universe 2 changes at the moment Bart 1 arrives, and so diverges in history from universe 1.  It remains to be seen whether those changes are sufficient to prevent Bart 2 from making a similar trip to universe 3.

But I am now quite confused by the next point.  When Bart 1 arrives in universe 2, he replaces Bart 2--and I see no reason for this.  Let us suppose that Bart 1 was born in 1990, and in 2010 he becomes the pilot of the time ship.  He moves back to 1980, before his own birth, and looks around, but maintains a discrete distance from events, so that he will have as little effect as possible.  Since this is an experimental backwards trip, he has been provided with all that he needs to survive 30 years, at which time he is to report to the project to inform them of their success, the day after his launch.  But are you suggesting that because Bart 1 is living here in 1990, Bart 2 will not be born?  That seems entirely implausible; and if it is correct, Bart 1's mission is doomed--Bart 2 will never make the trip, so no one will expect Bart 1 to appear to announce the success.  No, the arrival of Bart 1 in universe 2 has no direct effect on the existence or life of Bart 2; he will be born, make his choices, and live his life, unless some action of Bart 1 specifically prevents it.  And if it's true for a Bart 1 who arrives before Bart 2's birth, it's true for one which arrives during Bart 2's life.  But perhaps that wasn't your point.

Your explanation becomes bogged down.  It seems that it is essential to you that Bart 2 must choose to make the same trip which had been made by Bart 1; but what if it were not so?  After all, Bart 1 has entered universe 2, and he could make serious changes in future history, intentionally or otherwise.  What if he changes Bart 2's life significantly, or even prevents Bart 2's birth?  What if he scuttles the time travel program?  Even very little changes could have drastic effects.  If Bart 2 is born but doesn't make the same trip, there will be 2 Barts in universe 2, and none in universe 1.  Perhaps you mean that Bart 2 is only destroyed if Bart 1 makes a return trip to the future; but there is no reason why this would be so, either--Bart 1 could go back to 1980, change some detail which would prevent time travel, and move to the future to find himself, Bart 2, in some other field.  There could, by your view, be two Barts at this moment, and there is no reason why either of them wouldn't be there.

You're also overlooking a critical point about the development of time travel.  The first working time machine which will take a person into the past will not be built by some tinkerer in his basement.  It will be created by government-funded corporations, university research facilities, or similar large entities.  Much like a NASA space launch, there will be many people involved in the development and operation of the project beyond the monkey in the can who is sent back.  These people are trying to reach the past, and expect to see results.  If their monkey vanishes without a trace, the project will be scrapped; they will quite rightly conclude that time travel is impossible--their pilot never reached the past.

You will argue that they will have the pilot from another timeline who comes into their world.  But you have lost the sense of sequence, of causation, in all of this.  Consider this:  are all of the timelines of your infinite infinite infinite universes linked?  Or are they all independent?  If they will all reach 1990 at the same moment, then sending someone back in time is changing history, even if it isn't changing history in the same world.  If this is the case, you still have all of your anomalies.  As Bart 1 leaves universe 1 and changes universe 2, Bart 2 leaves universe 2 for universe 3; but if the arrival of Bart 1 in the history of universe 2 makes Bart 2's trip impossible, it undoes the history, and Bart 2 doesn't make the trip.  But Bart 2 has already made the trip to universe 3 in the original history, so he has already changed the history of universe 3 such that Bart 3 can't make his trip; but Bart 3 has already left for universe 4, and that history is likewise undone.  And either somewhen Bart N lands in universe 1, undoing the trip of Bart 1 and creating an infinity loop despite the dimension hopping, or no Bart ever reaches universe 1, and the time travel program there is branded a failure, and abandoned.

On the other hand, if the timelines are completely independent, there is an atemporal sense in which one of the universes will reach 1990 first.  In this case, in sending Bart 1 back to the past of another universe, they might be changing history which has not yet occurred.  Yet in this case, there is no universe which is temporally ahead of the first one, and so no Bart can be sent back in time to this first universe from any other.  It is exactly the same as if you had cascading timelines--universes which are staggered temporally.  In this case, moving sideways across the timelines would carry you into the history of your own world in that it would take you to a parallel world in which that is the present.  But there is no logic to a system which would allow you to move sideways into worlds which are temporally behind yours but which would not allow you to move sideways back to your own world, a world in which you did not change anything, because it was not the world in which you acted; and the notion that if you moved forward in time you would remain in the world which you changed would require a very different technology, one which removed you from time altogether and kept you suspended outside the effects (and benefits) of time while history passed beneath you, bringing you into the future when it became the present--much like Buck Rogers, who moved to the future by being in suspended animation for centuries.  (This theory is suggesting that travel to the past is actually travel to the present of another dimension.  You can thus travel to the future either by traveling to a dimension in which that which would be the future is already the present, or by stepping outside time and waiting for that future to arrive.  In the former case, the changes which you as traveler had made to the past would not have happened in the future, since you were not acting within the history of that dimension; in the latter case, travel to the future would be entirely different as a process from travel to the past.)  Still, in this case, no time traveler could come from the future to the present of whichever is the first universe, and unless the developers of the time machine think it valuable to visit worlds which are temporally staggered behind them, the program will be abandoned--and I'm assuming that the pilot will return to his own timeline when he moves to the future, for if he doesn't, time travel again will be abandoned as a failure.

And if the lead timeline stops sending people into the past, they'll stop arriving in the subsequent timelines, creating the mystery that "time travel use to work, but now it doesn't", and leading to the abandonment of time travel projects in other timelines.

You suggest that if Bart 1 fails to complete his trip to the past, but returns to the future without landing in the past, he will land in universe 2, but will not be able to distinguish it from universe 1.  You must be assuming that every Bart has done the same thing; and there is some logic to that.  But back in universe 1, no Bart ever returned; so if he does it again, no Bart will return to universe 2, because there was no Bart to leave universe 1.  Since what you are mistaking for time travel is really dimension hopping with a one-way ticket, it will eventually be abandoned as universes realize that what is happening isn't what they thought.

I'm confused about the timelines in which Bart's machine never worked.  Did they have different physical laws?  Or did the people in them make different choices, against that to which we agreed earlier (that the same people in every universe would make the same choices unless affected by an outside influence)?  You're starting to fall into the fallacy of the multiverse theory:  that things which happen in one universe can also not happen in another, just because everything that happens can also not happen.  You already stated that it doesn't work that way.  If what you say is true, then if I step off a cliff and fall to my death in this universe, there must be another universe in which I stepped off the cliff and floated in the air.  But a universe with different physical laws could not be exactly the same, and therefore flies in the face of your basic theory.  If the time machine works, it works in every universe in which it is created.  Your notion of the universe in which the time machine didn't work is intrinsically faulty--if it's the same time machine built the same way by the same people operated in the same way under the same conditions by the same crew at the same instant in a precisely identical universe, there is no reason why it wouldn't work.  Although my example of floating in the air seems absurd, it is not less absurd than your notion that identical time machines built in identical worlds would work in one but not in the other.  (I know it's appealing, and it's part of the theory of the Multiverser game, that a universe exists in which the time travel attempt failed; but it doesn't wash under your conception of identical people in identical universes.)

So, given your theory, if we sent someone back to prevent a war, we would never do it again, because as far as we know, we killed him and gained nothing for it; unless we're in universe 2, in which case we've been fooled into thinking that what we did helped--it was actually those in universe 1 who helped us, our own actions having no benefit to us but helping those in universe 3.  But wait--if those in universe 1 sent someone back who successfully prevented the war, why would we in universe 2 send anyone back at all?  We had no war, and so have no reason to prevent one.  We're back to your history problem.  Did the war happen in universe 2, only to be undone by the traveler from universe 1?  Or did the traveler from universe 1 prevent the war before it happened?  If the war happened but was undone, you still have temporal anomalies which must be explained (N-jumps, at least); if it never happened, then no one ever left universe 2 to prevent it, and when your traveler returns to the future of the world in which he prevented the war, no one knows what he's talking about, and his dimensional duplicate is there working on something else.

Let me make this clear.  Your system creates a different kind of temporal anomaly, resulting in temporal duplicates.  Under my system, if a person decides to travel back to change history, he will either fail, creating an N-jump, or destroy time with an infinity loop.  However, under your system, the traveler trying to change time could fail--and how this would work depends on the unanswered questions raised above regarding the relationship of the timelines--or he could succeed.  His success means that the event he intended to prevent/change has been corrected.  If it was corrected after it went wrong, that is, if traveler 2 leaves universe 2 to change the problem before traveler 1 arrives in the past to fix it, you've changed nothing--you still have the impossibility you are trying to avoid.  But if that history is changed before the moment from which traveler 2 would have left to fix it, traveler 2 doesn't leave; traveler 1, "returning", as he supposes, to the future, would find himself in a world in which the events he intended to prevent never happened, and his divergent self, traveler 2, never left to change them.  The family to whom he returns has never lost him; the job to which he returns is already filled.  I don't have a name for this type of anomaly--but since they aren't possible under my theory, I don't really need one.  But it's the mistake consistently made by the (enjoyable but completely incredible) series Seven Days:  When Frank Parker goes back seven days into the past and calls the base to report his mission, where is the Frank Parker who was waiting at the base seven days before?  That Frank Parker has not made the trip yet, and probably never will (unless the team was smart enough to realize that the other Frank Parker has to be sent back at the right moment to do what this one did).  Thus with each mission, there is one more Frank Parker in the world.  Oops.

Why should someone who traveled through time be accused of murder?  You must be assuming that foolish idea that the arrival of Bart 1 in universe 2 destroys Bart 2; but we've dealt with that.  Or is it the case that you're assuming that anyone who steps into the past must change it in ways which would intrinsically alter who lives and who dies?  While that is not necessarily so, even if it happened, the legal concept of proximate cause would prevent them from being charged.  That is, I may have altered history such that Mrs. Jones was at the convenience store at the time it was robbed, so that she was shot in this timeline but not in the history I remembered; but the guilt falls not on me, but on the man who pulled the trigger.  One might as well say that anyone who conceives a child is guilty of murder, since they all die eventually, and would not do so were they never born.

Thanks again for your thoughts.  I have again expanded and polished mine before adding them to the web site, although it took me several weeks.  Time is like that.

--Mark