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It's something that's been used several times as a plot device on Star Trek, notably in the TOS episode "The Tholian Web", and the TNG episodes "The Next Phase" and "The Pegasus". I'm certain it's appeared elsewhere as well, but here's my question: Is it theoretically possible, as we currently understand physics, for physical objects to pass from one reality or dimension or phase into another, and back again? It could be through some natural but heretofore undocumented phenomena, or via a technological device. Either is fine. :)

The reason I'm asking this is that for a story I'm working on, I'm hoping to use it as a variation of "The Flying Dutchman" myth, but if the science isn't there it suddenly gets harder to justify. Thanks in advance.

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    $\begingroup$ You can imagine that there are more than one universe. You can imagine that there are infinite realities. Nothing has any consequences for anything but sci fi writers. The answer is clearly no because what you are asking doesn't have a physical meaning behind it. Here is my tip for you: just do it. If you don't know physics, don't shoehorn a faulty explanation into your story. This has been done a zillion times, you named examples, your audience will accept it more without a bs explanation $\endgroup$
    – Raditz_35
    Commented Mar 29, 2020 at 13:05
  • $\begingroup$ Is the important part that they 'leave our reality' or that they 'enter a different reality'. If the former, you may be able to take the approach that their component particles no longer interact with the rest of the universe in regards to one or more of the fundamental forces. $\endgroup$
    – Gary Myers
    Commented Mar 29, 2020 at 22:04

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Is it theoretically possible, as we currently understand physics, for physical objects to pass from one reality or dimension or phase into another, and back again?

No, based on our current understanding of physics. Because, based on our current understanding of physics, there is only one universe and one reality.

Theories of multiverses, many worlds and so on, lacking experimental verification, cannot qualify as science and/or physics.

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    $\begingroup$ Straight from the Dutchman themselves. $\endgroup$
    – Black
    Commented Mar 29, 2020 at 22:07
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    $\begingroup$ Second paragraph is too strong in my opinion, there are plenty of papers on the multiverse published in physics journals. $\endgroup$
    – Allure
    Commented Mar 30, 2020 at 3:20
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    $\begingroup$ @Allure, publication on physics journals doesn't count as experimental verification. See string theory. $\endgroup$
    – L.Dutch
    Commented Mar 30, 2020 at 5:27
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    $\begingroup$ @L.Dutch-ReinstateMonica yes - but the answer is asserting that they are not science and/or physics, which is different from saying they are not experimentally verified. $\endgroup$
    – Allure
    Commented Mar 30, 2020 at 5:54
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    $\begingroup$ Granted, but maybe the question can generously be understood as "are there models of physics in which such a thing would be theoretically possible". But honestly, I am guessing that even for models that do have a multiverse the answer is no. $\endgroup$
    – kutschkem
    Commented Mar 30, 2020 at 7:24
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The only more or less scientific version of multiverses is the Many Worlds Interpretation of quantum mechanics, which basically states that any possible quantum state is realized in some universe.

Unfortunately, the mathematics behind this include the assertion that any contact between those universes is impossible, which is why it isn't regarded as a theory but just an interpretation - it can't be proven, and not even be falsified.

So, no, it is not theoreoretically possible to shift between realities in current physics.

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Yes, it's theoretically possible. It's actually already been done in hard science fiction - see the sequel to H.G. Well's The Time Machine, The Time Ships. The book espouses the Many-worlds interpretation of quantum mechanics. I don't have the book on hand anymore, but the gist is that the time traveler goes back into the future where he discovers he's entered a different future than the one he originally went to. He meets a Morlock named Nebogipfel, who explains that because he publicized his original experience, he had changed the future. The original future is still there, but it's now in a "different universe".

The question now is whether the time traveler in this universe can communicate with the time traveler in the original universe. Nebogipfel asks the time traveler if he thought it's possible, and the time traveler said "no". Nebogipfel then says that this is true only if quantum mechanics is linear. If there are small non-linearities, then communication becomes possible. If information can be passed from one universe to the next, then presumably physical objects can also be passed. In fact in the finale of the book, a super-species known as the "Constructors" send the time traveler back to the original future by turning back time to the Big Bang (see the synopsis on Wikipedia).

Mind you, I tried asking for the math behind this on the Physics SE, but nobody seems to have seen it. Still, it's a hard science fiction book which is supposed to be scientifically accurate. Therefore the math presumably must exist. Caveat: as far as I know, there is no experimental evidence that quantum mechanics is nonlinear. However, you can always postulate nonlinearity at a level below the experimental threshold.

tl, dr: it's possible if you 1) accept the Many-worlds interpretation of quantum mechanics and 2) assume quantum mechanics is slightly non-linear.

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    $\begingroup$ Anything is possible if you make enough assumptions, and the assumptions here seem to come down to "it's possible if we assume it's possible". I was under the impression that the question is based on the real world ("as we currently understand physics"), so any "hard science" based on assumptions that don't hold, or haven't been shown to hold, in the real world would presumably not be valid. Scientific accuracy in Sci-Fi typically falls into one of: (1) blatantly not how the real world works, (2) assuming things we have no evidence to believe is true or (3) made up material. $\endgroup$
    – NotThatGuy
    Commented Mar 30, 2020 at 15:59
  • $\begingroup$ @NotThatGuy apologies for late response. Although you're not wrong that science fiction stories are often scientifically wrong, this is a hard science fiction story - and in that genre, scientific accuracy is highly valued (en.wikipedia.org/wiki/Hard_science_fiction). To quote, One requirement for hard SF is ... a story should try to be accurate, logical, credible and rigorous in its use of current scientific and technical knowledge about which technology, phenomena, scenarios and situations that are practically or theoretically possible. So it's possible. Not proven, but possible. $\endgroup$
    – Allure
    Commented Jan 17 at 3:47

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