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Asked by anon-254838 on 18 May 2020.
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Susan Cartwright answered on 18 May 2020:
We don’t know. Almost by definition, we can’t know: universes are closed systems, so information does no tpass from one to another. Evidence for parallel universes would most likely come indirectly – if a theory makes lots of predictions which are testable and pass the tests, and it also predicts parallel universes, then you would be likely to accept that this is indirect evidence for parallel universes (because it is direct evidence for the correctness of a theory that predicts parallel universes).
There are a couple of theories that would predict parallel universes. One is the Many Worlds Interpretation of quantum mechanics. In QM, you cannot preduct the actual result of a measurement, only the probability of obtaining a given result. The MWI states that all possible results of a measurement “exist” in some multi-dimensional space, but you are only aware of one. The other results effectively generate parallel universes (hence the “many worlds” of the title). However, there are other solutions to what’s called the Quantum Measurement Problem (i.e. how is the result you actually get generated from the wavefunction, which gives multipke results wuth defined probabilities), and I do not know of any way in which you could prove that the MWI is correct. (But then, I’m not a quantum theorist.)
Another theory which leads to parallel universes is the cosmological theory of inflation, which states that the very early universe underwent a period of exceptionally rapid (exponential) expansion. In some versions of inflation, the end of inflation, when the universe settles back into slower expansion, creates a “bubble” of non-inflation in an infinite sea of continuing inflation, and there will be infinite other bubbles, each representing another universe. Again, I don’t see how one would prove this, though I think that different versions of inflation do predict different properties of primordal gravitational waves, so detecting those (probably by their impact on the cosmic microwave background – they are too faint for the likes of LIGO) would be a good start.
So, in summary, the answer is, “We don’t know, but some theories give us reason to think so>”
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Richard Fielder answered on 18 May 2020:
As Susan says, the problem is that it’s very difficult to tell. If parallel universes interacted with our own in a big enough way for us to detect, then we should have already detected them – it’s difficult to come up with a theory that allows enough interaction for us to find, but small enough that we’ve somehow missed it so far. That means most realistic ideas that include parallel universes end up looking exactly the same even if those universes don’t actually exist. The Many Worlds interpretation of quantum mechanics that Susan mentioned is one explanation for why the maths in the theory of quantum mechanics works the way it does, but the actual theory is identical no matter which interpretation you prefer – whether those other universes actually exist is more of a philosophical question than a scientific one.
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John Davies answered on 18 May 2020:
Sorry Alfie.
This not my subject. I do solar system astronomy, mostly comets and asteroids. I like to tell people if that if I cannot land on it, and it cannot land on me, I do not really care. I’m joking of course but I’m not very clever when it comes to Big Bangs, curved space and parallel universes. 😉 -
Gabriel Gallardo answered on 20 May 2020:
Maybe! There’s a school of thought in quantum mechanics that every decision or every measurement we make creates a new universe. For example, if you decide to have an apple for lunch rather than a banana, there’d be a universe where you have an apple and a universe where you’d have the banana! Though there’s no way to know for sure if these universes actually exist.🤔
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