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I just discovered Primer: it is nothing short of amazing IMHO. I've read various sites and graphics explaining how their machine work and I've got a question which, I think, would help me better understand how it works.

What if I'm the original and I plan to use the machine to go six hours back but with a twist...

First to simplify things nobody is ever going to enter the machine besides one time at 2013-oct-15 at 6pm.

  1. I turn the machine on at 11:45am 2013-15-10 with a 15 minutes delay and then leave the area to not have an encounter with my double when it pops up at 12:01pm or so.

  2. I then stay in the area (but somewhere where he can't see me : for example because I have binoculars) and wait for my future-self to come back in the past

  3. My future-self comes back and from my point of view it is just that: my future-self that knows what the lottery number are going to be that day etc.

  4. Because I have my binoculars, I can see my future-self coming out of the facility where the time-travel machine is.

  5. I'm glad that I saw my future-self with my binoculars, I dodge him, I go check the lottery numbers (say the lottery happens at 5pm) and then at 6pm I enter the machine.

And that's it.

How many timelines are now created? An infinity because there's a loop? As I understand it I go in the machine at 2013-oct-15 6pm and come back at 2013-oct-15 12:01pm with "lottery numbers knowledge", I'm seen by past-self, my past-self learns the lottery and enters the machine, he comes out in the past to be seen by his past-self and rinse and repeat? Is this correct?

(basically I'm trying to understand what's going on if I try to use my binoculars to see the one that already has the "lottery knowledge" instead being the one that has the "lottery knowledge" that tries to see the one that hasn't)

Now what if there's an additional twist: what if in addition to "nobody is ever going to enter the machine besides one time at 2013-oct-15 at 6pm" I decided that "I'll enter the machine at 2013-oct-15 at 6pm no matter what, except if I did see my future-self (the one with lottery knowledge) come out at 2013-oct-15 a little bit after 12:01pm, in which case and in which case only I won't enter the machine" ?

Then say I'd be before the first ever use of the machine and I'd check with with my binoculars and not see my future-self popping back in my timeline, so I'd enter the time machine at 6pm. But that means I'd have been back in the past so I should have seen my future self and hence decided not to go into the machine.

I don't know if I've been clear enough but how would that work in the way time travel works in Primer?

Is this later scenario "always enter the timemachine but not if I see my future-self" a paradox? (I don't care about there being two me in one timeline... I just don't see how things would work out with this simple rule : "Always enter on 2013-oct-15 at 6pm unless I've seen my future-self come out".

My question is similar to something posted by someone nicknamed 'Indigo' on http://qntm.org/primer who wrote:

I kept asking myself, how could symmetry really be broken? Wouldn't I always see the end result? In other words, if I wanted to go back a few hours to make a change.. to say hello to myself for instance; wouldn't I have seen myself greet me a few hours ago in the first place? And so, you can get trapped in this circular logic if you don't accept certain rules like the possibility of multiple timelines.

But I do accept the Primer rules and I do accept the possibility of multiple timelines. What I'd like to know is how things would work out if before ever inventing my first time-travel machine I'd fix to myself the very simple rule above: "Always enter on 2013-oct-15 at 6pm and on 2013-oct-15 at 6pm only, unless I've seen my future-self come out".

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    My brain exploded about halfway through this question. – Johnny Bones Mar 27 '15 at 15:10
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Q: What I'd like to know is how things would work out if before ever inventing my first time-travel machine I'd fix to myself the very simple rule above: "Always enter on 2013-oct-15 at 6pm and on 2013-oct-15 at 6pm only, unless I've seen my future-self come out".

A: There would be two versions of you in existence. One version -- "You A" -- would be 12 hours older than "You B" because "You A" lived through the original 12pm to 6pm as well as the subsequent 6 hours in the box. "You A" would be the only person in the universe who experienced those particular 12 hours.

When "You A" emerges from the box at 12pm, events would unfold exactly as they had before -- except for any differences caused by "You A." So if "You A" prevents "You B" from traveling back in time at 6pm, that simply means "You B" does not travel back in time at 6pm. The trip made by "You A" still happened, so now both "You A" and "You B" are in existence, even after 6pm.

Overall, the universe in Primer does not appear to be subject to the Novikov consistency principle, which asserts that if an event exists that would give rise to a paradox or to any change to the past whatsoever, then the probability of that event is zero. Because clearly the characters in Primer change the past (like when they use tranquilizers to incapacitate previous versions of themselves). In fact, I believe a key point of Primer is that, if time travel were truly possible, paradoxes would be unavoidable.

  • This requires pictures! But great explanation nonetheless – Tablemaker Oct 16 '13 at 13:10
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    @Shane Finneran: +1, I see, makes sense! Would that be considered a "paradox" that there would be two me in existence? – Cedric Martin Oct 16 '13 at 17:11
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    @Cedric: Technically, I don't think the two copies of you would be a paradox in itself. The part where You A prevents You B from entering the box would be a paradox. But I'm not sure there's much difference between the two, as time travel is fiction. :) – Shiz Z. Oct 16 '13 at 20:01
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My future-self comes back and from my point of view it is just that: my future-self that knows what the lottery number are going to be that day etc.

Not necessarily! You might see no one emerge from the building. If you decide to use the binoculars, from your point of view, it is possible you will see yourself, and it is possible you will not. Note that in the film, Abe shows his past self (meaning the self he remembers being) to Aaron outside the storage facility, not his future self (the self he does not remember being). That way Abe is certain to see himself.

Now what if there's an additional twist: what if in addition to "nobody is ever going to enter the machine besides one time at 2013-oct-15 at 6pm" I decided that "I'll enter the machine at 2013-oct-15 at 6pm no matter what, except if I did see my future-self (the one with lottery knowledge) come out at 2013-oct-15 a little bit after 12:01pm, in which case and in which case only I won't enter the machine" ?

The singular result of this decision will be that on Oct 16, two of you exist simultaneously. If we start on Oct 14 and follow the flow of time, it is obvious that you do not see yourself emerge from the building the next morning. Therefore you watch the lottery and get in the box. Therefore you do see yourself emerge from the building; one of you gets rich at the lotto, the other does not get in the box. There are two of you from that point on. The probability that there are not two of you on Oct 16 is zero.

Of course, this explanation contains a blatant apparent self-contradiction: you both see and do not see yourself emerge from the building at that point in time. The solution is simply that there is more than one timeline. Really this is a treatment of spacetime as essentially subjective. Under this model, it is academic what happens to the universe after you get in the box. Because you can never get back there; that time stream is lost to you forever and vice versa.

This model resolves the Grandfather Paradox basically by saying, "so what?" If you go back in time and kill Hitler, then Hitler is dead, and the world will be quite different in the future, even though you remember it quite differently. So different, in fact, you probably won't even be born. Enjoy a Hitler-free 1936. Maybe you'll meet a nice dame, settle down, have some kids. Enjoy the world you're in now, because you can't go back to the world you remember, ever. You can't even be sure it still truly exists.

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Think of it as An infinite stream with a boat in it on a rope. You put the boat in with a hundred miles of rope in it and walk alongside at the same speed as it casually goes down stream. You then get in the boat and row back to where you started at the same speed and get out meeting past you. The boat travels down stream again and you walk alongside it again this time with "past you". "Past you" gets in the boat after a hundred miles and rows back down stream as you carry on walking. You never see him again because he's two hundred miles downstream and only going the same pace as you. He never catches up. It doesn't matter how many duplications of this happen because they will never meet. This could go on "forever" in an infinite loop but it makes no difference at all from your perspective. You never meet.

Obviously if past you doesn't get in the boat then he'll stay in your timeline and there will be two of you strolling downstream unless one of you gets in the boat. This is what happens in the film to create duplicates. If you prevent yourself getting in the box, they stay in your timeline.

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