# Mathematical backing for the many-worlds theory?

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I've been reading about the subject and stumbled upon the quantum many-worlds theory. Is there any observable or mathematical backing for a 'new universe' sprouting from every possibility?

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I only know "many-worlds theory" from the Internet. My understanding of it is somewhat the following: Usually, we assume that an ideal dice after rolling shows up with one of its sides on top, where each side shows up with a probability of 1/6. Now comes ... *hocus*, *pocus*, ...

When you roll a dice, it creates six parallel universes. In each of those, a different side shows up on top. A copy of you exists in all of these universes, each of them seeing another side being on top. By some fancy trick [or just by definition], one of those universes is distinguished from the other. Each of the parallel universes has a chance of 1/6 to be the distinguished one.

I see no way to put a mathematical backup into this. More importantly (and unasked for), I see no point in it. "Observable" (effects) would probably depend on how the fancy trick works. You can probably rule out some super-fancy tricks by demanding that you reproduce the boring old single-world physics, at least in the realm where it's been observed and conceived as single world by us. One caveat in my satire above is that some effects in quantum mechanics are a bit more alien to everyday experience than throwing dice. In this case, the detour over sci-fi may in fact be a gain (which we hopefully agree it is not for understanding the probability of throwing a dice). However, no one has yet bothered to explain them to me.

Bottom line: I know of a "many-world interpretation", not the "many-world theory". To my understanding (which is from internet forums, and not from any serious source at all), it is roughly as observable or math-backupable as the addition "god made" to the sentence "all objects in vacuum fall at the same rate"(*). I may admittedly miss the point of the whole issue, in which case I'd really be interested in a qualified explanation. Until then, I'm going to make fun of it

(*): I probably have to state that explicitly on a science forum with a "religion" subforum: I have no problem with people believing this or finding this to make more sense to them. I'd not particularly like to have this sold as the "godmade theory", the newest development in theoretical physics, though.

EDIT: As a matter of fact, there is (a) a bit more to the topic than I though, and (b) an explicit section of potential experimental tests. I particularly like

It has been claimed that there is a thought experiment that would clearly differentiate between the many-worlds interpretation and other interpretations of quantum mechanics. It involves a quantum suicide machine and an experimenter willing to risk death. However, at best, this would only decide the issue for the experimenter; bystanders would learn nothing.
(this particular example is not exactly stopping me from making fun of the "many-worlds theory", though) Edited by timo
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I only know "many-worlds theory" from the Internet. My understanding of it is somewhat the following: Usually, we assume that an ideal dice after rolling shows up with one of its sides on top, where each side shows up with a probability of 1/6. Now comes ... *hocus*, *pocus*, ...

I see no way to put a mathematical backup into this. More importantly (and unasked for), I see no point in it. "Observable" (effects) would probably depend on how the fancy trick works. You can probably rule out some super-fancy tricks by demanding that you reproduce the boring old single-world physics, at least in the realm where it's been observed and conceived as single world by us. One caveat in my satire above is that some effects in quantum mechanics are a bit more alien to everyday experience than throwing dice. In this case, the detour over sci-fi may in fact be a gain (which we hopefully agree it is not for understanding the probability of throwing a dice). However, no one has yet bothered to explain them to me.

Bottom line: I know of a "many-world interpretation", not the "many-world theory". To my understanding (which is from internet forums, and not from any serious source at all), it is roughly as observable or math-backupable as the addition "god made" to the sentence "all objects in vacuum fall at the same rate"(*). I may admittedly miss the point of the whole issue, in which case I'd really be interested in a qualified explanation. Until then, I'm going to make fun of it

(*): I probably have to state that explicitly on a science forum with a "religion" subforum: I have no problem with people believing this or finding this to make more sense to them. I'd not particularly like to have this sold as the "godmade theory", the newest development in theoretical physics, though.

EDIT: As a matter of fact, there is (a) a bit more to the topic than I though, and (b) an explicit section of potential experimental tests. I particularly like

(this particular example is not exactly stopping me from making fun of the "many-worlds theory", though)

If I needed you to give a conceptual explanation and your opinions I would have asked for them, but I didn't(=p). Also, I apologize for using theory; as you said, 'many-worlds interpretation' is the correct term.

Edited by Sato
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Heh, that was a decent attempt, timo. I'll have a go as well.

You'll get many different interpretations of QM if you ask many different people. A common interpretation of the phrase many-worlds is basically what timo described, privileging consciousness as some kind of magic wand which splits universes. This is similar in many ways to how the Copenhagen interpretation is/was often presented (with the experimenter/consciousness as some kind of magic wavefunction collapsing wand).

The more coherent versions of each are as follows:

The Copenhagen interpretation is a bit ambiguous to the objective reality of the wavefunction. It deals with it as a model or abstraction of reality, which may or may not be true. Measurement is a process which is very difficult to define (and as far as I am aware, left somewhat undefined except insofar as what the result will be), but easy to point to (it's the thing that guy in the lab did with those results). It collapses wavefunctions down into single possibilities with measurable results. The main objection to it is that the idea of measurement and quantum collapse is a poorly understood, complicated process that needs to be inserted into the theory, making it more complicated.

There is room for some variations of interpretation within the Copenhagen interpretation, from completely denying the wavefunction as a way in which reality behaves and instead considering it a model with predictive power over large numbers of trials of an unknown but complicated process to ones where wavefunction collapse is an objective phenomenon.

The many worlds interpretation comes from simply taking the wavefunction at face value. We observe that particles take on superpositions when interacting with other particles in superpositions. You insert some (often vague) argument about macroscopic objects being one of the distinguishable states of this superposition. Measurement and 'universe splitting' is simply the process of becoming entangled with a specific state of another superposition. There is no privileged system which can do the measuring.

The main objection to this is, although it makes the theory simpler, it entails an absolutely preposterous number of universes (a split for every interaction that's ever resulted in a distinguishable difference).

Re. the dice example, depending on how far back you go (maybe after it's thrown), that may just be a deterministic phenomenon insofar as there was nothing in superposition large enough to change the result of the roll

Ie. there'd be universes where a photon bounced of the die in one direction, and universes where it bounced in another direction, but it might be showing 3 in all of them (if you go back to where some small change could alter the outcome, you would have splits with different rolls).

Re. The quantum suicide, the experimenter would be just as dead in (n-1)/n universes as if they had committed suicide by any other means, so strikes me as an absurd way of going about testing MW.

TL;DR There are lots of interpretations of QM, most sane variants of Copenhagen and MW have adherents within serious scientific circles (or at least people who will state that they use x interpretation when pressed), but Copenhagen is a lot more common. Most physicists just take a shut up and calculate approach as the maths works extremely well and doesn't care how you interpret it. Anyone who takes one or the other too seriously can probably be safely dismissed, esp. if they insert consciousness as a magic measuring-wand.

Edited by Schrödinger's hat
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Then, I assume, the answer is that there is no literal proof for this interpretation.

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Then, I assume, the answer is that there is no literal proof for this interpretation.

Correct they are largely a matter of ontology and metaphysics. Any interpretation which makes different, testable predictions would be considered a theory, and work would be done to check it. This is not to say people aren't trying hard to find testable predictions for these two (and other) interpretations, just that no definitive ones have been found to my knowledge.

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http://en.wikipedia....ugh_Everett_III

I remember this guy's son talking about him on the radio one day. His son had also said that the math to describe this theory is pages long, which is probably why no one posted it on the internet. You'd need to have a high degree to see it, know how to use computers, have enough time to care about putting it up on the internet, then finally type it all on the internet.

Edited by questionposter
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I've been reading about the subject and stumbled upon the quantum many-worlds theory. Is there any observable or mathematical backing for a 'new universe' sprouting from every possibility?

As mathematical physicists Neumaier has pointed many-worlds

is a smokescreen without a consistent mathematics behind.

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