Date:  Wed, 30 Sep 1998 11:16:40 +0200
From:  Holger Thiemann

Hi,

thanks for your answer. I hope we can continue this discussion a little bit.  I don´t want to attack your theorie, but I would like to test it´s ´water proofness´ a bit further. First I have to say that the theories I presented are not my theories, but just a few ideas and thoughts about the two different approaches given in several movies and stories. I do not claim or even believe that one of this theories is reality and I´m not even convinced, that time travel is possible at all (but I think the work of Stephen Hawking and his black hole theories tends to lead in that direction). But Anyway the two theories I presented were the only ones which I felt satisfied with.  You added a third very interesting theorie and I would like to test it for self consistency.  But first I would like to comment your comments ;-)

With the immutable time theorie you seem to have a problem with the concept of free will. I think that this concept is based on a point of view. I have my free will if I can do what I want and if I can make my decisions without influence. In my opinion it doesen´t matter if future is predetermined or not. If it is predetermined that I will eat pizza tomorrow, this will be exactly what I will want tomorrow. It is my free will to eat pizza tomorrow and I will make this decision without influence. I do it because I want to do it and not because history wants me to do it to fulfill my fate. So my free will is only affected, if I got a way to know about the future (perhaps a time traveller tells me about that). In this I have a cassandra complex, because I know about the future without beeing able to change it. In this case my free will isn´t affected anyway because what the time traveller tells me is most likely what I would have done anyway. Of course it leaves a bad taste, to know that one could do nothing to change the future. But perhaps the time traveller is lying and I am able to do something different from what he has told me. Why should a time traveller give me bad information about the future which I can´t change anyway (except if he is a sadistic one). More likely he gives me the information I need to do what I (from his point of view) have already done. He gives me the key to save the earth for example. It´s appeasing to me to know that I will succeed (because he told me that I will succeed). The only thrill is that he possibly could have lied about the outcome. OK. So far for the free will in this case. On the other hand you should have problems problems with the free will in your theory, too. Let us say you will die in an accident tomorrow. A time traveller travels back and so time rewinds to today. He has a mission that doesn´t affect you. And so chances are big, that the accident will happen in this new C-D segment anyway. He perhaps knows about that and could perhaps even witness the accident. SO he knows that you will die tomorrow. You of course have your free will and would be able to prevent the accident if you would know about it. But he doesn´t tell you, so you do nothing to prevent it and so you will die tomorrow. From the travellers point of view your fate was predetermined and he knows exactly what you will do tomorrow by your free will. So there is no great difference in free will between the two theories as far as you don´t know about the future. You will make decisions tomorrow by your free will. But the decisions you make are predetermined in both scenarios. The only difference is that with your theory the events can be changed perhaps by a later time traveller. And what about your free will if the universe is caught in an infinity loop? We are forced to live the same two (slightly different) lives over and over again and will do the same every times without any chance to know about the loop or to break it. Is that free will?

The critic about the theory of parallel histories was the moral implications. Hm. Know you are the one who introduces the will of god. Or do you think the universe could have bad conscience. Not all possible histories have to exist. Perhaps there is one initially one, spawning others, when time travellers arrive in the past. And by the way, two histories could converge, if they lead to the same state. Because if you are not able do distinguish the two, then there is no way to discover the different histories. An atom has no history at all. Two ataoms with exactly the same state are even the same and could be replaced without changing anything. So do differentiate between two universes which are exactly the same is not possible, even for the universe. Think about the electrons in the same orbit of an atom. They must have different spins to exist. There could not exist two of them, which are the same in parallel. So I think to say that two different timelines could not converge is more a metaphysical point of view.

If I have to decide which theory I want to be reality, then I would take one of ´my´. If time is immutable it has the big advantage that my life, everything I have done and reached, the whole history of mankind is fixed and can´t be destroyed altered or wiped out by some stupid time tourist. Why should I rack to reach something if there is a great chance that it will all be undone by a time traveller eventually? In the multiple history scenario it is the same. All I have done is fixed in one of the histories. And no time traveller could undo it. He just could spawn a new history line (or change to another one) but my history line with my acquisitions is preserved.

Now I have a few further questions to your theory. Perhaps I can point out a few new aspects.

You seem to be aware about chaos theorie and you write about the minor changes of history which are caused by the pure presence of a time traveller. How could a sawtooth snap ever end? How could a history with a time travel ever become self preserving? If I travel back, I will change history. I introduce knew atoms from the future. And when I travel back (at point D) again it will be a little bit different traveller this time (surely with a few atoms I introduced the first time). So I have a sawtooth snap and the chances that a time segment becomes self preserving is nearly zero, because eventually the same time traveller must travel back, without changing his next version by any atom. This is very unlikely to happen (it resembles a little bit to my converging of histories). So after the first time travel the universe would be trapped in an infinite sawtooth snap (perhaps periodic perhaps not). The example in your primer doesn´t work, because even if the traveller tries not to affect his younger self he will do so in a very subtle way and it will be at every point D (F,H,J) an other time traveller who travels back and changes history a little bit different.

I have an other problem with point D. If I travel back today let us say 30 years and kill my father (I´m 26), then time will continue till today and I can live 30 years in this history were I killed my father. And then suddenly the universe discovers that something went wrong and time snaps back again to rewrite the A-B segment. I could prevent it perhaps, if I take some DNS of my dead father and clone him, let him have a child with my mother and let the son travel back to C. He would know about his origin and he would this time kill my father and clone someone from my fathers DNS and sent him back. So the infinity loop could be broken. On the other hand, why does time not proceed a little bit farther after point D. Perhaps after point D I decide to clone my father and my mother, to have them get a child and so on. Then history is ´repaired´. Why should time snap back exactly at D? In the C-D segment there are no events which give D any special meaning?

And an other problem: I travel back and my pure presence alters history a little bit. But eventually I will be born, I will by a time machine model FX-4711 and I will travel back to C. When I travel back this time I travel back in a history where an other version of me has arrived from an overwritten history. Why should this other self be deleted or be replaced by my second self? My second self is´nt the same as my first self arriving at Point C. And in the history of my second self there is the first self of me that has arrived at C. So if I travel back to C I would not replace my first self, but I would create a new Point (E) at which my first self (just arrived from B) and my second self (just arrived from D) would meet.

I could continue that even further but I have to work now (hrmpf! :-( ) and I would like to see your comments first.

Hope you enjoy the discussion, too and waiting for the next round,

;-)

Holger

Holger--

You  would like to test my theory for self consistency.  Fine by me; I'm certainly open to someone trying to create a situation I can't explain.

I think that you have misunderstood my position on free will; it is not so different from yours.  I agree completely that my decision to eat pizza tomorrow, or do any other thing, is completely free, even if someone coming from the future knows that it will happen.  However, I think that if someone coming from the future were to tell me that I was going to eat pizza tomorrow, that would be a new factor in my decision.  At that point, I might decide that pizza really is what I want, and would then eat it; or I might decide that I don't like the idea that he thinks he knows everything I'm going to do before I do it, and so I'll order a hamburger instead.  If the factors leading up to my decision to eat pizza have changed, that decision should be able to change also.  So too, if the traveler from the future tells me the circumstances of my pending accident, I should be able to make choices which would avoid the accident.  However, I think there is an inherent danger in changing the future based on knowledge from the future, in that you destroy that knowledge--and at the same time, you've pointed out the possibility that the time traveler could be lying, which complicates the matter far more.

Suppose the time traveler knows that tomorrow I will eat pizza, and tells me so.  At this point, if I choose to eat pizza, everything will proceed quite normally; but should I choose to eat the hamburger, I have now altered the future such that the traveler now knows that I chose to eat the hamburger.  We must re-play history, because he cannot honestly tell me that I ate pizza.  Since he cannot do so, he didn't, and that bit of information was no longer part of my decision to eat the hamburger.  Without that piece of information, we have three possibilities--either he will tell me nothing about what I ate, and I will choose to eat the pizza, creating the infinity loop (since I've restored the information which had been erased), or he will tell me I ate the hamburger, which will involve a new set of choices (will I choose to eat the burger, confirming the altered timeline, or rebel against his foreknowledge by eating pizza, restoring something closer to the original timeline, or pick yet a third food, continuing into a convoluted sawtooth snap?), or he will tell me that I ate pizza (for reasons which could range from his knowledge that I chose to eat the burger because he mentioned the pizza to the changes in history are so extreme that he now lies to me instead of telling the truth), which will confirm the altered timeline.  But let us suppose instead that I originally ate the burger, and for some reason--perhaps to test whether time is immutable through a misguided notion of changing something small and insignificant--he told me that I ate pizza.  He perhaps wants to know whether his prediction that I will do something different from what I actually did will change my choice.  Now if I eat the burger, I have done the same thing, albeit for a different reason, and the timeline may stabilize (once I do the same thing for the same reason).  But if I choose to eat the pizza, his reason for telling me I ate pizza has been erased, and unless he has a different reason for doing so, he will not tell me this, and I will eat the burger.  We could now set up an infinity loop in which we alternate between him causing me to eat the pizza by telling me I ate the burger, and causing me to eat the burger by telling me I ate the pizza (or by telling me nothing at all).

My "free will problem" thus focuses on whether someone from the future can change the past at all.  I say he can--at great peril not merely to himself nor to human history, but to the dimension of time itself.  Star Trek Next Generation did an episode in which they discovered that warp drive was creating rifts in sub-space, that the fabric of the space-time dimension was being damaged in a spatially localized area; I'm suggesting that the fabric of the space-time dimension could be damaged in a temporally localized area, and in such a way that it became non-continuous throughout the universe--that time itself could become looped back like a mobius strip, repeating events which are freely chosen but which ultimately lead to their own repeat.

"The critic about the theory of parallel histories was the moral implications. Hm. Know you are the one who introduces the will of god.  Or do you think the universe could have bad conscience. Not all possible histories have to exist. Perhaps there is one initially one, spawning others, when time travellers arrive in the past. And by the way, two histories could converge, if they lead to the same state. Because if you are not able do distinguish the two, then there is no way to discover the different histories. An atom has no history at all. Two ataoms with exactly the same state are even the same and could be replaced without changing anything. So do differentiate between two universes which are exactly the same is not possible, even for the universe. Think about the electrons in the same orbit of an atom. They must have different spins to exist. There could not exist two of them, which are the same in parallel. So I think to say that two different timelines could not converge is more a metaphysical point of view."

You say that two universes could converge if they moved into identical present configurations.  Obviously, I disagree.  You fail to distinguish here between what is true and what is observable.  I may be entirely unable to distinguish two manufactured rubber balls, two cars, even a pair of identical twins; but they are not therefore the same.  The history of an object is one of its properties in more than a metaphysical/theoretical sense, if time is perpetually existent.  Look at it this way:  if we assume that two universes have converged such that in the present we cannot distinguish between them, but we choose to go back in time (and are able to do so) from what in your mind is a single converged universe, into which history will we travel?  The entire concept of time travel assumes that the past and the future exist in a real form at another point along a dimension which is distinct from the three spatial dimensions, but not entirely unlike it.  Thus every particle of matter in your convergent universes would have two distinct histories, existing in two historical locations.  No, "Things which are not the same are different", as Young's Theorem states (postulated in the 1950's by C. B. Young, Jr., then of Western Union's engineering department).  If the two seemingly identical universes do not have the same history, they are different, even if I cannot detect those differences by any means possible to me.

As to my "moral objection" to the multiple universe theory, I used it to make a point.  We are told that all possible universes exist; but I think that the many universes in which we are all serial killers, or in which the majority of us are serial killers, or serial rapists, or sadistic torturers, cannot exist in that sense.  Arguably, there could be a universe in which all humans were like that; but apart from the short life expectancy of the race in such a circumstance, it would not be possible for us to be there--our parents would not have met and married and produced us as their offspring, because the social changes in such a world would be too drastic to support the identical population.  Thus, it cannot be the case that there is a world in which all humans are serial killers, and yet all of us are alive with the same families.  But the reason this is so goes very deep--to our own free will and destiny problem.  You have already pointed out that if tomorrow I would have eaten pizza, and nothing interferes with that choice, tomorrow I will freely choose to eat pizza.  It is not merely my moral choices which come from deep within my nature; it is every choice I make--how hard do I work, what do I do to relax, where do I shop, what do I buy.

What do I buy--there's an excellent example.  When I walk into a grocery store, there are some things for which I am specifically looking, and other things for which I am generally looking.  I will buy a particular brand of dish detergent whenever I need it; I also buy the same soap every time, because I'm allergic to so many additives in so many products that discovering one which does not irritate my skin or my nose ends my search.  On the other hand, I will buy the cheapest tissue from my list of acceptable brands.  I will look at the meat, comparing prices and qualities to a mental list I've made of what is reasonable-chicken always if it's below $0.70, never if it's above $0.89; beef and pork, never above $3.00 (except on rare special occasions), preferably below $2.00.  Once I see what's available, I will select the meat I think will make the best most economical dinners for the family within my notion of providing variety in their diet.  However, someone who knew my mental list and my immediate resources and needs could probably predict with fair accuracy what would be in my shopping cart when I hit the checkout line.  And the more he knew about the nuances of my thinking and the details of the store and the track of my perception, the more accurately he could predict it--because although I have used my absolute free will to make the selections, that free will has directed me to very specific choices which I would make the same way under precisely the same conditions every time that it was the first time.

That's why the infinity loop works:  for everyone in it, it is always the first time.  They will always freely choose to do exactly what they freely chose to do the last time it was the first time; for those for whom any factors have changed due to the time traveler's interference, they will always freely choose to do that which they would so choose with his interference when he interferes, and always freely choose to do that which they would choose without his interference when it is absent.  For them, this time is always the first time.

Your preferred theory is based on an emotional choice, not a rational one.  As I said to another inquiry, why should it be possible for someone tampering with nuclear fission to kill me?  The fact is, we have the ability to affect each other, intentionally or unwittingly.  As John Wenman says in his excellent book, The Goodness of God, it is one of the good things about the world--we have the ability to benefit each other; it corresponds to that that we have the ability to harm each other.  Why should Hitler have been able to destroy the lives of so many people, Jewish and otherwise?  Why should the United States have had the ability to kill and maim so many people in Hiroshima and Nagasaki?  Why should the Soviet Union have been able to destroy the lives of formerly free people in countries beyond its borders?  If I blow up a building, and you're in it, you will suffer the consequences.  If I blow up a bridge on the road you are traveling, your progress will be impeded.  If I destroy history, your life will change.

As to my preference, I would prefer to discover that man will never travel back in time; it is too dangerous.  However, I know two things about this.

First, I know that many times in the past people have thought that God would not allow something as terrible as something which eventually happened; similarly, many things which were once thought scientifically impossible are now common realities.  To suggest that something will never happen, either because it seems too horrendous to us or because we cannot see any way it could be done given our current understanding of reality, is to overstep what is permitted to us--we cannot know what will be possible in the future, or where, if anywhere, the limits of our evil might be set.

Second--that is, the other thing which I know--is that our perception of what is too terrible for God to allow is conditioned by our reality.  In every century, the "problem of evil" has been thrown in the face of God, and always in the present tense--how could God allow the black death? how could a loving Father permit the atrocities of the holocaust, or the destruction of the atomic bomb? how could He allow children around the world to starve?  Without making many of the obvious arguments (such as, isn't it we who allow the children to starve), I would observe that the thing that is too horrible for God to allow is the greatest evil we have ever known; we cannot postulate concerning why God would allow that which we have never known, nor can we make a reasonable argument based on that which is relatively insignificant.  But it is the relative nature of these evils which become the basis of our measure.  I am convinced that if God had so designed the universe such that the greatest evil, the most severe pain, which would have been suffered by anyone amounted to a hangnail, philosophers would be crying out that a good God could not allow sentient beings to suffer the pain of a hangnail.  We cannot know whether God has limited the evil in this world, and we cannot know what evil He may have precluded.

In short, although I would like to believe that no one will ever have the power to travel back in time and destroy the time line, I cannot know that.

But a more specific and direct answer to your preferred theory is that it makes no real sense ultimately.  If you are in a timeline in which your brother leaves the present and goes into the past, you would like to believe that he can return to the present to again find you.  But if he changes the past, then any future to which he is able to return will be one based on that altered past--otherwise, in what sense has he traveled in time?  But if he can only return to that future which is based on his altered past, then either the past must be altered for you also, or he cannot return to when you are.  Thus if we are suggesting that any trip to the past with a return trip to the future places the traveler in another reality, another timeline, then we have sliding--we don't have real time travel at all.

I had thought of the problem of the constantly changing sawtooth snap, and you are correct that the probability of a timeline stabilizing is extremely low.  My best hope is that it would be possible for the changes to approach zero on each repetition (in much the same way as the genetic change increases related to the man who replaces his grandfather) until they reach zero.  There may be a mathematical way to test this, but it is well beyond my meager skills in that area.  Anyway, if the changes will never reach zero, then you are correct--the N-jump will never occur, the N-jump termination of the Sawtooth Snap will never occur, and if ever anyone travels backward in time, they will immediately destroy the future.  (However, it is clear that as the differences in timelines increase, the likelihood of an infinity loop resolution increases.)

There is a sense to which you are right in saying there seems no particular significance to point D; however, there is a sense in which that moment is important in the chain of causality--it is where the break in the chain becomes significant.  That is, the C-D segment is caused by your arrival in the past; that arrival in the past is caused immediately by your departure from point B.  When the moment of point B arrives at point D, the immediate cause of point C fails, and thus point C fails.  The magic of point D lies in its place in the chain of causation; a break there will prevent the events of point C, which are a necessary cause of the entire altered timeline.  If you've ever played Mousetrap, you know that the Rube Goldberg contraption which is built during the game requires several sections to work in sequence.  If any piece of that chain is absent, the mouse is not caught.  But it's very clear in that context which piece of the chain is missing.  Each part of the machine works until the process reaches the missing link in causation, and then it fails.  So too with our time line, the causative link at point B is necessary, and if at point D we fail to cause point C, then the entire C-D timeline fails.  That's why D is so critical.

But, as you say, it is possible for me within the C-D time segment to find another way to assure its perpetuity.  In addition to those you suggest, you could find someone not descended from your father, and pass information on to him which suggests that he must carry out this task, and must assure that it is performed in the future.  Thus your stranger will take your place in the E-F segment, and if he tells himself to do it in that segment, the G-H segment will begin to stabilize.

But why shouldn't it be possible for the C-D time segment to stretch beyond point D, as long as it is corrected eventually?  I find the answer to that in the problem of infinity.  In theory, someone could go back to cause the C-D segment the next day, or the next year, or the next century, or the next millennium, or the next eon--or possibly never.  But once we reach never, causality is disrupted, and the time line is destroyed.  But if time has no end, "never" can't occur, and an uncaused timeline may continue perpetually.  But I cannot accept an uncaused event; therefore, there must be a moment in time at which the universe recognizes that causality has failed, that its current timeline is impossible, and that time snaps back to correct its history to reflect the change.  The most logical time for this to occur is at the instant that the immediate cause fails--viz., point D.

You ask why, when you leave point D to return to point C, you don't find yourself already there, recently arrived from point B.  This has an inherent appeal to it, but it fails to understand how the timelines are forming.  The reason an infinity loop occurs at point D is because traveler didn't leave from D to return to the past, and so he wasn't there.  Each time line is a repetition of the same hours, but a continuation of events from the last segment.  Let me see if I can clarify this.

Time moves from A to B, and at B, Traveler goes back to A; but his presence causes that to be C.  Because he has changed history, time cannot proceed beyond point B, because the history of that timeline has been erased.

Time now moves from point C to point D.  At point D, time can only continue if the history of the universe from C to D is confirmed by Traveler moving from D back to C and repeating his same actions in the same way--because the Traveler who left point B has destroyed his own history, and no longer exists as such, but only exists as the Traveler who reaches point D.  If that Traveler does not go back in time, no traveler does.

It is because of this that the B Traveler is not already back there when the D Traveler arrives.  Does that make sense?

And indeed, I look forward to continuing the discussion--it keeps me on my toes.