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...
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 challenges against MWI that needs to be answered.
Carroll offers "self-location uncertainty" as a way to recover the probabilities predicted by the Born Rule despite the determinism of the Schrodinger Equation
> But even if both people know the wave function of the universe, there is now something they don’t know: which branch of the wave function they are on. There will inevitably be a period of time after branching occurs but before the observers find out what outcome was obtained on their branch. They don’t know where they are in the wave function. That’s self-locating uncertainty, as first emphasized in the quantum context by the physicist Lev Vaidman. (https://news.ycombinator.com/item?id=20927410)
* edit - OP's post was edited but i'm leaving this post as-is.
> MWI insists that experiments have multiple outcomes in point of metaphysical fact.
That doesn't seem right. under MWI there is only one Wafefunction and one big quantum state. There are not additional 'worlds'
For the record, the original just said (as quoted in the above response) "The main difference is that the MWI has trouble explaining the Born rule." Upon reflection I decided that wasn't really the main difference, that the main difference is the difference in ontology. But the fact that the MWI has to recover the Born rule from the linear dynamics is a major consequence of this difference in ontology.
The jury is still out about whether or not self-location uncertainty actually allows the Born rule to be recovered without begging the question. If it works, it would be a major breakthrough. Personally, I'll give long odds against.
[UPDATE]:
> There are not additional 'worlds'
Um, what do you think that the "MW" in "MWI" stands for?
Sure, I'm not sure how much Science has to say about ontology but I'm interested in the outcome.
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."
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".
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.
"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.
Either the other versions of reality are tangible, or they're not. If they're just abstractions, you have't solved the problem - you've taken a long way around a circular argument that starts with the wavefunction and ends with it doing something that results in a measurement, maybe.
Unfortunately explaining how that happens is the problem you're trying to solve. It's not an answer to the problem.
If they are real, there are all kinds of other issues, the Born Rule being the most obvious.
The wavefunction is physically real. Whatever you interpret it as is your business. We've solved a real problem: we can explain the evolution of the wavefunction and the results of experiments without needing some weird collapse/decoherence concept that breaks all the usual rules. The Born Rule is still something that remains to be explained, agreed, but it's a much smaller something than the full Copenhagen style collapse.
But you still have to peel back the words to think about what kind of mechanism they are describing, which Deutsch is trying to communicate. You can't just complain that he says "reality contains parallel universes" without accepting that he's taken the time to map "parallel universes" to a well-defined concept.
“To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor’s algorithm works. I do not merely mean predict that it will work, which is merely a matter of solving a few uncontroversial equations. I mean provide an explanation. When Shor’s algorithm has factorized a number, using 10^500 or so times the computational resources than can be seen to be present, where was the number factorized? There are only about 10^80 atoms in the entire visible universe, an utterly minuscule number compared with 10^500. So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed?”
[1] https://www.newyorker.com/magazine/2011/05/02/dream-machine/...
- Peter Shor (https://www.scottaaronson.com/blog/?p=218#comment-5753)
An analogous question: when three objects are orbiting each other in space, where is the computation being performed that provides the solution to the three-body problem that represents their motions?
Well, obviously, the computation in the latter case is being "performed" by the three bodies themselves, and the Shor computation is being performed inside a quantum computer that exists in our universe. It's just that the stuff that is performing the computation isn't atoms, it's entangled qbits. There's no need to bring entire parallel universes into it.
You seem annoyed by the terminology of words like "worlds" or "multiverse", but are you also thinking about what a MWI advocate is referring to by the usage of those words? They are shorthand for "the observable wave function" and "the global wave function".
There being "copies" of Schrodinger observing his cat is the most intuitive and expressive way to summarize Schrodinger being in a linear superposition of two states in a (cat alive, cat dead) basis. It's not any different from how a Copenhagenist would treat a photon passing through a polarizating beamsplitter as being in a superposition of having passed through and having been reflected; if the photon then goes and interacts with other things, they would keep track of what would have happened in each "branch" of the superposition.
If you have an issue with those, why don't you have an issue with the idea of superposition itself? It's exactly the same concept.
(UPDATE)
In particular, when MWI people say "multiverse", it's not a sleight of hand where we are being deceptive when we actually mean "observable and unobservable components of the wave function". The term "multiverse" had no prior meaning in physics; MWI is defining it to mean the global wave function, using that word to draw attention to the fact that it has unobservable components.
These words are being given rigorous definitions that align with their intuitive meaning. You seem to be arguing against that practice?
Should I complain that when a Copenhagenist says "collapse", nothing is actually falling down and so it's misleading?
You may consider this a distinction without a difference, but the fact of the matter is that people do argue about this. A lot.
Also "metaphysical fact" is a questionable term.
So detecting the first, definitely 'collapsed the wave function'. This is not an unknown result in physics.
As for collapse, Copenhagenists can't seem to explain why an interaction between a photon and electrons in a polarizing filter does not collapse the wave function, but an interaction between a photon and electrons in my retina does. They're made of the same stuff and should be following the same quantum rules; why does one of them perform a unitary transformation and the other a lossy projection?
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-complicated entanglements and superpositions within that wave function, of which we—for some reason—can only observe one position of that superposition.
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.
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.
> No, they wouldn't. They don't. See my response to lmm here
I don't think lmm and I disagree.
I said "[The underlying principle of Many Worlds] is that there is a singular wave function, which is the true reality". lmm says: "The only thing that MWI demands we take as metaphysically real is the wavefunction".
I think this is expressing the same thing in different terms.
I said: "The things we refer to as "worlds" are massively-complicated entanglements and superpositions within that wave function". lmm says: "Given a wavefunction like 1/sqrt2(|experimenter who observed a live cat> + |experimenter who observed a dead cat>) ...". Again, I think our descriptions are in fairly close agreement.
I think lmm's description is a good summary of how the "worlds" in Many Worlds are viewed by Many Worlds advocates: "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". The "worlds" aren't actually fundamental to the theory, but rather an emergent phenomena from them.
The reason for the label "Many Worlds" is not because they are the fundamental concept. It's because it's the most... interesting element from a pop-sci marketing angle.
Is the collapse of the wave function related to consciousness? How would we construct a test to verify that?
>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.
I know MWI has significant gaps, this appears to be one of the biggest.
thank you for citing that paper directly.
MWI doesn't really solve either the locality or the FTL problem. If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction.
That's a rather larger and more serious signalling and information transfer problem than the much smaller-scale non-locality required to explain entanglement.
Couldn't MWI represented with a probability cloud? Maybe the many-worlds exists as a fixed-volume probability on all states in the universe.
From this perspective, MWI would be similar to a multi-universal annealing system, where different quantum outcomes result in fractures within the probability cloud (as opposed to an expansion of the tree-graph).
My complaint is exactly that the Copenhagen interpretation introduces that unnecessary FTL signalling requirement. Under the MWI, nothing weird is going on in the EPR thought experiment. But if you believe that a collapse occurs when we measure system A, then that collapse has to somehow occur non-locally and/or be FTL-transmitted between A and B.
> If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction.
You don't; to talk about it in those terms is putting the cart before the horse. You have to believe that a wavefunction evolves in the usual way and represents the thing that it usually represents. You have to believe that when a pure system interacts with a system in superposition then the result is two systems that are entangled and in superposition - something you presumably already believe if you believe in any kind of QM at all. You only have to believe that a new universe appears instantaneously if your interpretation of putting a particle into superposition is that a new particle appears instantaneously, which is not an approach I've ever heard advocated.
I’m with Vaidman on this, who is one of the best known supporters of MW, which is that the clearest solution is just to postulate the connection between psi magnitude and subjective experience. It’s an empirical discovery that works.
It's like when GR can "recover" F=MA, despite not having assumed F=MA that as an axiom.
Yes, he did:
> MWI is not incompatible with postulating the Born rule if you want to
"Postulate" (as a noun) and "axiom" are synonyms.
>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
For a recent high-quality example: https://www.nature.com/articles/s41467-019-09348-x