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In the episode before the last one, why does Suzuha disappear when travelling back to the "present"?

From what I've read, going back in a time machine always changes the world line, and in the world line where Okabe saves Kris the time machine no longer exists therefore time travelling Suzuha doesn't exist.

However I don't see this explanation working without her disappearing as soon as they appear back in the past. It is not their returning to the future that changes the outcome (and subsequently the timeline), it's their actions in the past.

How can this be explained with consistency with the rest of the rules of time travel we've encountered in this show? If any assumption I've made is incorrect I'd love to hear it.

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    I have rewrote the answer in order for it to be more accurate, perhaps you can understand it better now. – Astral Sea Apr 22 '16 at 12:56
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I would love to just write this off as "the writers wanted a good goodbye scene" since quantum teleportation in the temporal dimension(time travel) is really not my forte when it comes to quantum mechanics. However, by the nature of this site, I will try to answer your question. This is going to be long, bear with me. I have included several questions you might have after reading it, if I missed out anything, feel free to clarify.

Before I re-answer, let me tell you a short story. After a few hours of discussion with my colleagues, we could not arrive at any conclusion despite bringing in several plausible theories. Then, out of the blue, our professor (who is like nearly 60 but still an avid anime watcher) just walks over and proceeded to blow our minds with one sentence "It's a cross-time temporal loop." Then we proceeded to face palm ourselves for our stupidity in over-complicating the situation when it was really so simple. Side note: my theories were not wrong, but the factor I inputted into them were wrong.

Firstly, you need to understand how their time machine works, cause we underestimated its capabilities before. Their machine is not a linear time travelling device, it is a trans-spatial time machine. When they travel through time, they possibly travel to another universe as well. Believe it or not, it is actually theoretically easier to make this type than a linear time machine cause time flows relatively, thus theoretically, in a 'space pocket' between universes, time would pass differently for separate universes.

But I digress, thus, Suzuha did not actually travel from the future, but instead from a potential future of the Steins Gate World Line. Now, Okarin never actually needed to go to the Steins Gate World Line, he left it as he saw Kurisu dead. But in reality, he was there in the beginning, he then proceeded to send himself all over the time lines with his microwave oven when he sent that mail to Daru.

What is the loop? The time loop created works like this:

  1. Okabe sees Kurisu dead, send mail to Daru, enters other timelines
  2. Okabe spends 3 weeks in alternate timelines, gets back to Steins Gate
  3. Time machine shows up with Suzuha saying they have to stop WW3
  4. Okabe goes back to past, fakes Kurisu's death, burns thesis, stops WW3
  5. Past Okabe sees Kurisu dead, goes back to step 1 and repeat

Remember the Attractor Field-World Line theory employed by the show, now imagine 2 lines intertwined but splits like a split hair at a certain point. To both these splits, their past is the same, but some event split this time line. This event was Okarin gaining the knowledge that he saved Kurisu.

Despite the aim of going back to the past was stopping WW3, we can never really say that WW3 was fought over Makise Kurisu's Time Travel thesis completely cause we have too little details. A more likely reason would be Okarin built the time machine based on her thesis using the knowledge he gained from the other World Lines and this information was obtained by world powers and they started the war to obtain this thesis to built their own time machine as Okarin was already dead in 2025.

The motives for WW3 was effectively instigated by Okarin. Okarin built the time machine to save Kurisu and thus as long as he does not know he saved her, the time machine would still be built, WW3 would still occur. That is essentially why Suzuha cannot disappear until the Okarin that knows he has saved Kurisu appears in the present again, or Okarin would go on to create the time machine to save Kurisu and start WW3.

But, to save Kurisu, the time machine is necessary, if Okarin did not have the motive to make it, where did it come from?

This is where the cross-time part comes in. Remember that I said Suzuha came from a possible future of the Steins Gate timeline, a split in the World Lines. This is where quantum causality comes in, as long as there is a chance, WW3 can still occur despite the thesis being burnt as Okarin can still make the time machine. The core reason for this as I mentioned is because he thinks Kurisu is dead. Remember that the first time he went back to the past, he stabbed Kurisu himself, no World Line change occurred, that means that they were still connected to a timeline that leads to WW3. However, when he saved Kurisu and burnt the thesis, still nothing happened.

That is because at this point, the two timelines still share a common past, and Okarin still wants to save Kurisu as the him that saved Kurisu is still in the past, future cannot affect them and they cannot affect the future yet as they are in a state of temporal uncertainty by being in the past. Same as the first answer but this time the factor is not the burning of the thesis but Okarin returning to the present with the knowledge that he saved Kurisu and does not want to create the time machine anymore, thus completely erasing any minuscule possibility for WW3 to occur. This leads to Suzuha disappearing as the two futures no longer have a common present. This returning of Okarin to the present is the diverging point of this two timelines. Thus, Suzuha can no longer stay in Steins gate as this is not part of their shared past.

Then what was the point of faking Kurisu's death?

Simple, it was to keep the loop going, if he broke the loop, the him that is trying to save Kurisu would definitely disappear for he never spent even a single moment with Kurisu. No matter how far quantum causality goes, it most likely does not extend into separate timelines for his knowledge is not from the universe he is in. Thus, the loop has to occur, he has to keep the past somewhat constant until his return to the present with the knowledge he saved Kurisu, only then will the timelines diverge properly.

What would have happened if they stayed in the past?

This depends on which travel they stayed in the past.

In the case scenario where they failed to save Kurisu, Okarin would disappear at the divergence point. Since this Okarin never returned to the divergence point with the knowledge that he saved Kurisu, it would go the WW3 route, the past Okarin thinks Kurisu dies and went on to create the time machine, starting WW3. This allows Suzuha to exist. Since Okarin traveled to the past from the Steins Gate timeline, he disappears since that is no longer his past now.

In the case scenario where they managed to save Kurisu and stayed in the past, Suzuha would disappear at the divergence point. Despite Okarin never returning to the divergence point with the knowledge that he saved Kurisu, they would still arrive at the divergence point where they will witness themselves going back to the past and thus arrive at the same case where even without a time machine returning them to the present except that the moment they witness Okarin going back, Suzuha disappears.

  • I have a problem with the first explanation: it seems to me that in the show time is not linear, so future past and present all exist simultaneously therefore passing the point in time where the paper burns seems irrelevant to suzuha's disappearance(as the future of that timeline is already predetermined, including the paper burning). About the second explanation, I wont pretend to understand quantum causality at all, but again I cant seem to see why he needs to return to the future and finish the loop for the timeline to change,leaving him in the past will have the same effect on the timeline – Arthur.V Apr 19 '16 at 17:09
  • The first explanation has to do with the nature of the interaction between the quantum of the time travelers, since they are in the past, they will interact with the time of the past and the future will not have any effect on them until they arrive at that point in time so it can be said they exist in a state of temporal uncertainty, like schrodinger's cat paradox, even if the time line is not linear, that does not mean the future can affect the past even if they were from the future. Perhaps if they stayed there, they would have just disappeared when the the thesis was burnt. – Astral Sea Apr 20 '16 at 11:57
  • The second explanation depends on what were deemed necessary effects by the "world" as some things happen and some things do not. My guess is that the erasure we see of them is not really them being erased, but their existence being overwritten into the newly established timeline, where they take up the place of their new selves. – Astral Sea Apr 20 '16 at 11:57
  • That is due to the No-cloning theorem, where when they went to the past, they effectively erased their future selves from the timeline, so the world “decided” they had to go back to their original place in time in order to restore this anomaly or else when their past selves arrives at this point in time, they most likely would disappear as the remaining information about their “future” is not with them but exists as a separate entity. Thus still achieving the same result, just that it took longer then using the time machine. – Astral Sea Apr 20 '16 at 11:57
  • Of course, like I said, this is not my forte, my forte is quantum mechanics, thus I have convinced my colleagues, who are more well versed than me in temporal mechanics to watch the show and come up with better theories. I will update the answer as soon as they give me their theories. – Astral Sea Apr 20 '16 at 12:02
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The story after Mayuri is saved, and before Makise Kurisu is saved, happens in the beta world line. When they go back to the past, they were still in the beta world line. Hence Suzuha doesn't disappear. And by saving her, then returning to the future, they have successfully moved to the Steins Gate world line.

Suzuha is not supposed to have existed in the Steins Gate world line until 7 years after. Because it will cause a paradox to have her exist, she is simply erased from existence. This is the law of Steins Gate - paradoxes simply cannot happen even if you try to cause one.

Neither the show nor the visual novel has explained it scientifically enough so I'm afraid I don't have a quantum mechanics-related answer.

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