Comment by platz | Hacker News Reader

Comment by platz | Hacker News Reader

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> 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 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'

> OP's post was edited

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?

> the main difference is the difference in ontology

Sure, I'm not sure how much Science has to say about ontology but I'm interested in the outcome.

> 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.

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.

Deutsch may believe that. (Heck, I believe it). It's a natural interpretation of the MWI wavefunction. But it is an interpretation, a layer on top of the physical claims, a model - not a mandatory part of MWI. Deutsch may interpret a given wavefunction as describing a particular ensemble of 'tangible' and 'shadow' objects, but someone who interprets that same wavefunction a different way (but agrees on the same wavefunction, and expects it to evolve according to the same laws of quantum mechanics) will still generate the same experimental predictions and, more to the point, still be doing MWI.

That makes MWI a "no true parallel Scotsman" interpretation.

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.

> 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.

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.

I think Deutsch is doing a very good job of explaining exactly the concept I'm trying to explain.

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.

I'm not complaining about anything. I'm simply observing that Deutsch and other prominent advocates of the MWI do not consider multiple worlds to be metaphorical. They insist in no uncertain terms that they are real, and go to great lengths to justify this position. There is absolutely no ambiguity with respect to what they believe.

MWI provides a clear, comprehensive explanation for all of the “weird” quantum phenomenon. Beyond that, this quote from FoR [1] convinced me:

“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/...

I'm sorry to downvote you, but I think this is both a misrepresentation of MWI and a distraction from the conversation that we're trying to have.

"Interpretations of quantum mechanics, unlike Gods, are not jealous, and thus it is safe to believe in more than one at the same time. So if the many-worlds interpretation makes it easier to think about the research you’re doing in April, and the Copenhagen interpretation makes it easier to think about the research you’re doing in June, the Copenhagen interpretation is not going to smite you for praying to the many-worlds interpretation. At least I hope it won’t, because otherwise I’m in big trouble."

- Peter Shor (https://www.scottaaronson.com/blog/?p=218#comment-5753)

If one die can have 6 values, how is it possible that two identical dice have 21 configurations? Where are those additional configurations coming from if there are only 12 possible values in the pair of dice?

This is a stupid argument, comparable to "if humans evolved from apes why are there still apes?"

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.

Can you explain how qbits work then? How exactly do they take on multiple values at the same time?

Not in an HN comment, but there's a lot of material available on the internet. Search for "introduction to quantum computing."

There seems to be some confusion here, based around the usage of words.

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?

No it isn't. Copenhagenists insist that the wave function "collapses" upon "measurement" and so measurements only have one outcome in point of metaphyisical fact. Many-worlders insist that the wave function does not collapse, and hence measurements have many outcomes in point of metaphyiscal fact.

You may consider this a distinction without a difference, but the fact of the matter is that people do argue about this. A lot.

Your usage of "many outcomes" is what may be causing contention/confusion for those looking for answers in what is described by the mathematical equations, and scientific theories. If you want to justify "many outcomes" on metaphysical grounds, that is not a scientific conversation.

Also "metaphysical fact" is a questionable term.

Confused. Consider a light beam with only one photon at a time (very dim) measured by a photoreceptor. Put a 2nd photoreceptor after the first - it detects no photons.

So detecting the first, definitely 'collapsed the wave function'. This is not an unknown result in physics.

I agree that it's an important difference! And it matters a lot. I am just saying that the concept of "copies" is no more mysterious than the concept of "superposition", and if you can accept the latter then I don't see why you complain about the former.

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 reason I can accept superposition is that I have first-hand access to experimental evidence of it. I can see interference fringes with my own eyes. By way of contrast, not only do I not have any direct evidence of the existence of copies of me, I cannot possibly have such evidence. And this is not a technological limitation. This is a limit predicted by the theory itself.

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-complicated entanglements and superpositions within that wave function, of which we—for some reason—can only observe one position of that superposition.

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 metaphysical fact, many worlds.

If you agree on that math/science then this point you're adamant about whether it is or is not a "world" reduces to a semantics/ontology/philosophy question, which we're not going to get further clarity on in a thread about trying to gain a proper or better understanding as laypeople of the scientific theories, unless you want to advance the conversation to the understanding of those scientific theories into a semantic/ontological/philosophic realm.

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.

> 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.

Sorry for the slow response. Been a busy couple of days :)

> 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.

> "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.

Is the collapse of the wave function related to consciousness? How would we construct a test to verify that?

"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.

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.

Ta. As far as I can tell a lack of an explanation for the Born rule is a feature of all quantum interpretations, not just many worlds. It's pretty much the central mystery of quantum mechanics.

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