Parallel Universes: Multiverse vs Many Worlds - Splits?

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Hi, sorry - forgive me for posting (I am not much of a science guy but I am still interested). I am just watching a documentary (I know not a great source for scientific information - but it got me thinking). In the documentary it talks about the many worlds theory (which I believe is different from the multiverse theory? It would seem that other copies of Earth makes more sense in multiverse theory as opposed to many worlds?) In the documentary it says that (with many worlds theory) in every decision we make we split the universe. I.e. If I choose a to order a latte over a mocco, then the universe will split into two parallel universes. I know this theory isn't fully accepted but the problem I see with it is that if everything is identical in the universes before hand (down to the smallest particles in our bodies) - then wouldn't the exact same decision be made every single time? - i.e. we have no free will - only fate due to the way certain atoms behave in our bodies? Meaning there could be no split and therefore no creation of a parallel universe? - I don't know much about science tho - just interested in others thoughts :). Or is it something to do with a particle being able to be in two places at the same time?
 
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Nugatory
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but the problem I see with it is that if everything is identical in the universes before hand (down to the smallest particles in our bodies) - then wouldn't the exact same decision be made every single time?
Not according to quantum mechanics. The randomness is inherent in the theory: if I send an electron with its spin oriented vertically into a measuring device (Stern-Gerlach apparatus, if you want the details) oriented to measure the spin along a horizontal axis, I will get spin-left or spin-right randomly and with 50/50 probability. It's tempting to think that the problem here is that we just don't know enough about the exact state of the electron, that if we just knew more about it we'd have enough information to calculate whether it would be spin-left or spin-right - but that's wrong. There are subtle statistical differences between "random when we measure the spin" and "gotta be spin-left or spin-right, but we don't know the state well enough to calculate which it will be", these differences can be tested for in experiments, and the results of these experiments confirm that the quantum-mechanical picture is right; for more you can google for "Bell's Theorem" and pay particular attention to the website maintained by our own @DrChinese.

This inherent randomness may be the most basic difference between classical and quantum mechanics; your line of thought works just fine in classical mechanics.
 
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Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?
 
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Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?
We have some rather good textbooks in the theorem proof style ie mathematically reasonably rigorous on many worlds these days:
https://www.amazon.com.au/Emergent-Multiverse-Quantum-According-Interpretation/dp/0198707541&tag=

There is a strong connection between Many Worlds and De-coherent Histories:

De-coherent Histories, without going into the details, was the formulation of QM Feynman was converted to a bit before he died.

The following paper by Gell-Mann and Hartle might also help
https://arxiv.org/abs/1106.0767

But since this is a B level thread the following commentary in ordinary language is likely more helpful:
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

If somebody were to ask me personally I would side with Gell-Mann - despite the mathematical Beauty of Many Worlds there is really no reason to introduce it - its nice features can be accommodated in a theory such as as de-coherent histories without the baggage of the idea of many worlds. However it must be emphasized as Gell-Mann does in the video what Everett thought of as a world is not necessarily what popularization's think of it as.

Just for completeness sake we come up against the real fundamental issue in QM. Everything is quantum. But then how do you rigorously define an observation? Some mentors/science advisers here think, like Einstein, it makes QM incomplete (Einstein towards the end, and after many discussions and disagreements with his good friend Bohr, the most famous one being the Einstein Box thought experiment, I will give a link to at the end, believed QM was correct - but to his dying day believed it incomplete - but that is another story best left to one of the many biographies such as Subtle Is The Lord). Others say it's an unsolved problem but think it can be eventually solved. Still others like me think that some key theorems are missing and once they are worked out whats going on will be a lot clearer. I think its more dotting your i's and crossing you t's sort of thing. But until its done, those that believe QM incomplete have a perfectly valid view. And who knows - solving it may lead to a revolution in understanding of QM - we just do not know yet.

Now for a more modern take on the Einstein Box Thought experiment after which Einstein never questioned the actual validity of QM - but always thought it incomplete:
https://arxiv.org/abs/1203.1139

Later of course Einstein was able to state clearly his real objection in the EPR paper. It was not resolved until the work of Bell. Its interesting the above author (who is a science adviser here) thinks the Einstein Box experiment is really an early version of EPR.

Thanks
Bill
 
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Is it something to do with a particle being in two places at the same time?
Yes, it is exactly this. Or three, or four....
Before the split the particle in question is smeared out across multiple locations. After the split each branch of reality sees the particle in a different position. and hence different things happen in different branches.
 
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DarMM
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De-coherent Histories, without going into the details, was the formulation of QM Feynman was converted to a bit before he died.
Thanks for the video (although the cards at the bottom by the video uploader are a bit odd!) Just since you seem to know this stuff....

I've finished reading Omnès's book. He says (as is typical of decoherent histories) that decoherent histories tells you when you have classical histories/sequences of porpositions and can assign a probability to them (very similar to Richard Healey's views). He says which event actually occurs in a measurement and what is occurring outside of decoherence is basically beyond science, representing a limit beyond which mathematics cannot model the world.

Does Gell-Mann have similar views do you know?
 
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Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?
I'm curious and doubtful about the Many Worlds theory ( first suggested by Hugh Everett, 1957 ) because I don't understand how an entire universe can split to create a second universe which claims to be identical to it's mother universe, but not violating the law of conservation of energy?
How do you get a second universe ( call it the baby universe ) from it's "mother" universe with exactly the same amount of matter/energy in BOTH universes as the original mother universe?? It would require energy to be created - a violation!
 
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How do you get a second universe ( call it the baby universe ) from it's "mother" universe with exactly the same amount of matter/energy in BOTH universes as the original mother universe?? It would require energy to be created - a violation!
It doesn't violate conservation of energy as they're just two different states of the same material as such. Another way to think about it is that both worlds were already there and then became different (this would be more in line with the modern view on Many-Worlds).
 
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I don't understand how an entire universe can split to create a second universe which claims to be identical to it's mother universe, but not violating the law of conservation of energy?
There is no "splitting" in the many worlds interpretation. The entire universe is in a pure state that evolves in time according to a unitary transformation. Unitary transformations preserve all conservation laws.

The "worlds" in the MWI are names that we give to particular parts of the pure state of the universe. They don't affect the above at all.
 
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But if they "became different" even ever so slightly, then there would be at least one molecule, atom, sub-atomic,string, SLIGHTLY different that would be part of describing the slight difference and so, necessarily an imbalance in matter and therefore, a loss or gain in matter/energy and thus a violation.
 
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DarMM
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But if they "became different" even ever so slightly, then there would be at least one molecule, atom, sub-atomic,string, SLIGHTLY different that would be part of describing the slight difference and so, necessarily an imbalance in matter and therefore, a loss or gain in matter/energy and thus a violation.
Systems move into different states that have the same total energy, so no. That is in one world the molecule does X and in another world it does Y, but being in X and Y are associated with the same energy.

A flaw this blatant would have stopped Many Worlds in its tracks years ago if it were true.
 
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PeterDonis
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But if they "became different" even ever so slightly
If what "became different"? Go read my post again, carefully.
 
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in one world the molecule does X and in another world it does Y, but being in X and Y are associated with the same energy.
This does not have to be true for subsystems; they can go into superpositions of states with different energies. The conservation law only has to hold for the entire system, i.e., universe as a whole.
 
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This does not have to be true for subsystems; they can go into superpositions of states with different energies. The conservation law only has to hold for the entire system, i.e., universe as a whole.
True, I was just assuming a world where the only thing that changed was an isolated molecule like @PEGELLA 's example, or at least how I read it. Of course if not isolated one has to replace "molecule" with "universe" or whatever is the smallest isolated system containing the molecule.
 
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Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.
 
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Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.
I don't think most are, some are. Or at least I'm not aware of anything showing most are.

What level of quantum mechanics are you familiar with, e.g. texts you have read?
 
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DarMM
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That survey was at a very small conference, other surveys give completely different results.
 
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Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.
It is certainly true that many worlds has steadily grown in popularity. In the 1960s of was it ignored, in the 1970s it attracted the attention of flakes and nutcases, alongside DeWitt and Zeh's more sober assessments. In the 1980s it was treated seriously with the emergence of decoherence into the mainstream. Since then it has steady risen in profile. A mainstream physicist can now declare him/herself a fan without it ending their career. Progress indeed.
 
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You can see my response on quora in the link. In brief, the article claims MWI is incomprehensible, but is itself incomprehensible about why. In MWI reality forms a branching tree and our consciousness branches along with reality. There is nothing in incomprehensible about that, although it takes some adjusting too. But then so does relativity - we long ago got over the idea that the universe was intuitive.
 
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I think the author of this article identifies the most severe problem of MWI:
I would have said the most severe problem of MWI is the derivation of probabilities, of which there is no noncircular justification.
 
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You can see my response on quora in the link.
You mean in comments section? I don't know how to identify it.
In brief, the article claims MWI is incomprehensible, but is itself incomprehensible about why. In MWI reality forms a branching tree and our consciousness branches along with reality. There is nothing in incomprehensible about that, although it takes some adjusting too. But then so does relativity - we long ago got over the idea that the universe was intuitive.
Well, if you take the "split" literally, these objections might seem incomprehensible. So we can say that there are two versions of MWI. One with literal "split" and the other one with "separation" of two superposition components within the same global wavefunction.
So we can say that arguments in the article apply to MWI version with the "separation" concept.
 
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I would have said the most severe problem of MWI is the derivation of probabilities, of which there is no noncircular justification.
Justification is not a strict requirement for scientific theory. It's enough that predictions are unequivocally derivable.
But if a theory is built on philosophical basis that is inconsistent with philosophical basis of scientific approach, there is nothing much you can do about it.
 
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