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How the Many-Worlds Theory of Hugh Everett Split the Universe

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Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#41

Post by ncallaway »

lisper wrote: post #35
ncallaway wrote: post #32

Earlier quoted context omitted.

The label "Many Worlds Interpretation" is a popular label for the theory, but it's not a particularly accurate one. The original paper by Hugh Everett was titled '"Relative State" Formulation of Quantum Mechanics'. The underlying principle of Many Worlds isn't actually that there are many worlds. Is that there is a singular wave function, which is the true reality. The things we refer to as "worlds" are massively-com…

Everything you say is true. Nonetheless, if you read papers written by self-identified advocates of Everett, they invariably refer to it as "many-worlds" and they talk about "the branching structure of the multiverse" and "copies" or "versions" of yourself. So regardless of what you or I think Everett meant, the present-day advocates of his theory are clearly trying to advance the idea that there are, in point of met…

Sure, but I would think every one of those advocates would also agree with what I wrote.

That the "world" described by many worlds is a convenient short-hand for a single position in a massively entangled system of particles in a complex-superposition.

It's the "Many Worlds" is common moniker, and the rhetoric does use the concept of worlds all the time, but whenever you get down to the specifics of the theory there really aren't many worlds.

I think it's unfair to hold the shorthand rhetoric against people when they attempt to get to that more precise level. It would be like using the plain english definition of "observation" against someone trying to provide a more rigorous definition of what an "observation" is under the Copenhagen interpretation.

Ultimatley, this discussion all stems from the original statement: "MWI insists that experiments have multiple outcomes in point of metaphysical fact." It's a fairly moot point at this point since that statement was removed.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#42

Post by tim333 »

lisper wrote: post #13
doubleunplussed wrote: post #11

Earlier quoted context omitted.

It should be noted though that few interpretations have an explanation for the born rule.

Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.

Though Everett in his disertation doesn't call it an axiom he kind of assumes it:

>It is convenient at this point to introduce some notational conventions...

>For a complete orthonormal set we define... [the Born rule]

>In the probabilistic interpretation this distribution represents the probability distribution over the results of a measurement...

https://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation... page 34

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#43

Post by aklein »

drcode wrote: post #30

Sean Carroll's new book on quantum physics that came out yesterday is the clearest laymen's exposition of quantum physics I have ever read: https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook...

Another book I'm enjoying is "Einstein's Unfinished Revolution" [1] by Lee Smolin, which also has a deep dive into foundations of QM.

[1] https://www.amazon.com/Einsteins-Unfinished-Revolution-Searc...

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#44

Post by tim333 »

platz wrote: post #14
lisper wrote: post #9

Earlier quoted context omitted.

The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...

> The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenge…

"self-location uncertainty" seems to still assume what is basically the Born rule:

>MWI probability postulate: The probability of self-location in a world with a given set of outcomes is the absolute square of that world’s amplitude. (http://philsci-archive.pitt.edu/14389/1/defence-location-unc...)

It just applies the absolute square of the amplitudes to the chance of being in a given world rather than the chance of an event. The thing that needs explaining is why either probability is given by the square of the amplitudes.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#45

Post by inscionent »

fallingfrog wrote: post #8

You can do a really simple thought experiment that provides a persuasive argument in favor of the many worlds theory. Or at least, it shows that wave function collapse is relative to an observer, not universal everywhere. Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the cat is found to be either alive, or dead, a…

If a tree falls in the woods, and no one is around to hear it, does it make a sound?

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#46

Post by drcode »

aklein wrote: post #43
drcode wrote: post #30

Sean Carroll's new book on quantum physics that came out yesterday is the clearest laymen's exposition of quantum physics I have ever read: https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook...

Another book I'm enjoying is "Einstein's Unfinished Revolution" [1] by Lee Smolin, which also has a deep dive into foundations of QM. [1] https://www.amazon.com/Einsteins-Unfinished-Revolution-Searc...

I'm not familiar with that book, but I can say that Dr. Carroll's book is far better than any previous book on the subject I've read, in terms of getting rid of the confusion inherent in QM.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#47

Post by lmm »

lisper wrote: post #34
jbay808 wrote: post #31

Earlier quoted context omitted.

> Um, what do you think that the "MW" in "MWI" stands for? Ok yes, it is named that, but what the theory postulates is really just that "when Schrodinger opens the box to check on his cat, he himself is in a superposition just like the cat was -- and what it feels like to be in a superposition is that you don't notice you're in a superposition, because your superposed states each only observe one cat."

> your superposed states each only observe one cat Again, I have to ask: what do you think "many worlds" means? Because what you've just described is exactly what the advocates of the MWI describe when they talk about "many worlds", which they absolutely do. The MWI literature is festooned with references to "copies" or "versions" of yourself, and "the branching structure of the multiverse".

> The MWI literature is festooned with references to "copies" or "versions" of yourself, and "the branching structure of the multiverse".

Many MWI advocates will describe that physical wavefunction in those terms, but that's a metaphor/model/aid to understanding, not a "point of metaphysical fact". The only thing that MWI demands we take as metaphysically real is the wavefunction. Given a wavefunction like 1/sqrt2(|experimenter who observed a live cat> + |experimenter who observed a dead cat>) many people find it convenient to think of this as "two Everett branches with ...", in the same way that people who hear a particular pattern of sound might find it convenient to think of it as a chord of two or three notes. But if you prefer to think of it as simply a unitary (in the everyday sense) wavefunction that's fine, you'll make exactly the same experimental predictions.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#48

Post by platz »

tim333 wrote: post #44
platz wrote: post #14

Earlier quoted context omitted.

> The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenge…

"self-location uncertainty" seems to still assume what is basically the Born rule: >MWI probability postulate: The probability of self-location in a world with a given set of outcomes is the absolute square of that world’s amplitude. ( http://philsci-archive.pitt.edu/14389/1/defence-location-unc... ) It just applies the absolute square of the amplitudes to the chance of being in a given world rather than the chance o…

Fair enough - I'm not a physicist or read papers on physics. I'm just trying to re-tell the best version of the theories as I've understood them by steel-manning arguments laid out (by scientists that have some media exposure), rather than try to straw-man each theory with prima-facie criticisms that have already been answered.

I know MWI has significant gaps, this appears to be one of the biggest.

thank you for citing that paper directly.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#49

Post by lisper »

lmm wrote: post #47
lisper wrote: post #34

Earlier quoted context omitted.

> your superposed states each only observe one cat Again, I have to ask: what do you think "many worlds" means? Because what you've just described is exactly what the advocates of the MWI describe when they talk about "many worlds", which they absolutely do. The MWI literature is festooned with references to "copies" or "versions" of yourself, and "the branching structure of the multiverse".

> The MWI literature is festooned with references to "copies" or "versions" of yourself, and "the branching structure of the multiverse". Many MWI advocates will describe that physical wavefunction in those terms, but that's a metaphor/model/aid to understanding, not a "point of metaphysical fact". The only thing that MWI demands we take as metaphysically real is the wavefunction. Given a wavefunction like 1/sqrt2(|e…

Here's a quote from David Deutsch's "The Fabric of Reality":

"When I introduced tangible and shadow photons I apparently distinguished them by saying that we can see the former, but not the latter. But who are ‘we’? While I was writing that, hosts of shadow Davids were writing it too. They too drew a distinction between tangible and shadow photons; but the photons they called ‘shadow’ include the ones I called ‘tangible’, and the photons they called ‘tangible’ are among those I called ‘shadow’.

Not only do none of the copies of an object have any privileged position in the explanation of shadows that I have just outlined, neither do they have a privileged position in the full mathematical explanation provided by quantum theory. I may feel subjectively that I am distinguished among the copies as the ‘tangible’ one, because I can directly perceive myself and not the others, but I must come to terms with the fact that all the others feel the same about themselves.

Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."

And his summary:

"the whole of physical reality, the multiverse, contains vast numbers of parallel universes"

So no, Deutsch, who is arguably the leading advocate of MWI, does not consider parallel universes to be a "metaphor/model/aid to understanding". He insists that they are real. He even defends this position at extraordinary length in chapter 10 of TFOR.

Re: How the Many-Worlds Theory of Hugh Everett Split the Universe

#50

Post by lisper »

ncallaway wrote: post #41
lisper wrote: post #35

Earlier quoted context omitted.

Everything you say is true. Nonetheless, if you read papers written by self-identified advocates of Everett, they invariably refer to it as "many-worlds" and they talk about "the branching structure of the multiverse" and "copies" or "versions" of yourself. So regardless of what you or I think Everett meant, the present-day advocates of his theory are clearly trying to advance the idea that there are, in point of met…

Sure, but I would think every one of those advocates would also agree with what I wrote. That the "world" described by many worlds is a convenient short-hand for a single position in a massively entangled system of particles in a complex-superposition. It's the "Many Worlds" is common moniker, and the rhetoric does use the concept of worlds all the time, but whenever you get down to the specifics of the theory there…

> I would think every one of those advocates would also agree with what I wrote.

No, they wouldn't. They don't. See my response to lmm here: https://news.ycombinator.com/item?id=20941803

> "MWI insists that experiments have multiple outcomes in point of metaphysical fact." It's a fairly moot point at this point since that statement was removed.

I'll stand by that with one minor modification: contemporary advocates of MWI insist that experiments have multiple outcomes in point of metaphysical fact.

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