This question makes several assumptions:

1) The "Many Worlds" Interpretation will have been proven through future observation

2) CTC's exist within the rotating event horizon of a Kerr black hole

3) Quantum fluctuations would influence the outcome of travel along a CTC

4) At some point in the future the first stable rotating black hole will be created in a lab

I'm envisioning a fictional scenario where a rotating black hole (albeit a "small" one that is not too massive-although I'm not sure what's considered small or large enough to make the CTC possible in the first place) has been created for the purpose of harnessing its rotational energy via the Penrose Process. Some mishap/plot device ensues, and a person passes through the rotating singularity, traveling along a CTC to find herself transported unwillingly several hours into the past, or in this case, a parallel world line.

So, to restate the question:

Would the creation of the CTC and subsequent travel to a parallel universe mean the effective waveform collapse of the previously inhabited universe via quantum probability? Or, put another way, assuming alternate timelines only exist as potential probabilities in the quantum foam, would the previously inhabited (present) reality exist independently of the new branch of spacetime or cease to exist?

(Please excuse any gaps in my understanding, not being a physicist this is all still very murky in my mind despite all the research. That's why I'm here.)

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    $\begingroup$ Esse est percipi. Something exists from the point of view of an observer if the observer can perceive it, that is, if they can interact. If two parallel universes can interact, no matter how exotic the means of interaction, then they are not two parallel universes but rather two aspects or regions of the same universe. (Hint: that's why we call them parallel.) So, if one could arrive somewhere by travelling along whatever kind of path, it means that that somewhere is part of the same universe as the point of departure. $\endgroup$
    – AlexP
    Sep 10, 2018 at 0:56
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    $\begingroup$ I want to make a small point. As Cort Ammon says, the Many Worlds interpretation alone doesn't allow for travelling to "alternate timelines", as it's just an interpretation of ordinary quantum mechanics. Instead, what is usually proposed in such scenarios is a modification of quantum mechanics in which the states can evolve from pure to mixed (which can in principle be falsified). $\endgroup$
    – pregunton
    Sep 10, 2018 at 6:23

2 Answers 2



Even in the Copenhagen interpretation of quantum mechanics (CIQM) once the probabilistic wave function has collapsed and whatever was measured becomes real its wave function doesn't collapse again. Effectively once something is real it stays real.

Therefore, passing into a parallel universe, by any means, won't cause a collapse of a wave function. The previous universe, or as we can call it the "original universe" can't collapse because it's full of measuring devices and observers keeping it real.

Passing through a Kerr black hole to a parallel universe may make it impossible for the time travellers to return to their own original universe. All they will be able to do is through through their Kerr black hole into parallel universe after parallel universe without returning to their starting point universe.

Please note: many of those parallel universes will be effectively identical or indistinguishable from their original starting point universe, so this may not matter. Not unless other parallel versions of the time travellers turns up claiming they've returned home.

  • $\begingroup$ To your point about not being able to return to the starting universe, which I had anticipated, it would seem like this could lead to a causal loop of sorts. This would be one that avoids paradox, but due to the effectively identical states (or at least similar ones) of the parallel universes, the traveler may be doomed to repeat a series of actions or at least be confronted with similar obstacles/conditions that would lead to a repetition of events that would be difficult to extricate themselves from. $\endgroup$
    – Brian
    Sep 12, 2018 at 16:21
  • $\begingroup$ @science-conscientiouswriter Glad you anticipated that possibility. A multiplicity of parallel universes can avoid actual causality paradoxes, but may create a whole series of apparent paradoxes which can vary according to the model of the universe you are working with. Lots of confusion can happen. $\endgroup$
    – a4android
    Sep 13, 2018 at 2:50


First off, The Many Worlds Interpretation cannot be proven. What can be proven is that it is indistinguishable in every observable way from all other interpretations of QM. A future observation which proves MWI would actually have to be a substantial rewrite of QM based on something we have not yet even thought of observing. It would make the effort to prove String Theory right to look as mild as a butterfly fish compared to a great white shark. It is proven that you cannot prove MWI with the measurements we believe to be physically possible to make, so such a proof of MWI would be quite astonishing.

As such, if MWI is "proven," we really can't say anything about your world. It's so different from the known physics that you're really in a position to make up physics as you go.

Even if MWI is "proven," or merely just used, you wont see a waveform collapse in the vicinity of the CTC because MWI does not have the concept of waveform collapse. It is the Copenhagen Interpretation that needs the waveform collapse to make sense. MWI actually states that there are no truly objective observations. One can only talk of an observation with respect to the subject doing the observing. It's actually just a straight forward interpretation of the superposition principle we use to calculate probabilities in QM. The idea is that, because the equations for QM are linear, you can simply treat the different parts of a superposition of states as separate universes.

Finally, it's worth noting that even if you constructed such a strange situation, we don't really know what would happen at all. While time travel is theoretically possible around some CTC, it remains a mathematical oddity until someone actually does it. We know not whether it is something real and meaningful, or if it is something abstract as a complex number which does not really exist.

All that being said, if you're writing a book and need this to work, go for it! As an author, you have the privilege of rewriting physics as you see fit. What you describe is so far beyond our scientific models, that nobody is going to complain if you go out and invent new ones.

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    $\begingroup$ The question postulates science marching on & proving MWI QM. The resulting science, as you say, would be spectacularly different from current theories, which is to be expected. For a science-fiction story it only has to sound plausible enough. $\endgroup$
    – a4android
    Sep 10, 2018 at 2:23
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    $\begingroup$ @a4android That's why I pointed out that, as an author, the OP is welcome to write their own physics as they see fit. But if you're inventing a way to prove MWI, and trying to use waveform collapse in MWI, at some point I'd probably just ask that we not call it MWI at all. It's a new fictional theory based off MWI in the literary sense. $\endgroup$
    – Cort Ammon
    Sep 10, 2018 at 2:31

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