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Yes, I'm not sure why this is a Nature article. As far as I understand things, very few physicists who care about which interpretation of QM is correct would ch

Yes, I'm not sure why this is a Nature article. As far as I understand things, very few physicists who care about which interpretation of QM is correct would choose the Copenhagen Interpretation in this day and age. The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses. As I understand things, most physicists don't give much thought to which interpretation is correct, since any experiments to distinguish between the various interpretations are virtually impossible to do. And most physicists don't care about distinctions for which there will be no experimental evidence. Among the physicists who do care about the different QM interpretations, it is my understanding that most would go with the Everett (AKA "Many Worlds) Interpretation these days. All other interpretations that I know of are hugely problematic, but there are no significant problems at all with the Everett Interpretation. The only problem is that many people consider it to be "creepy". But not liking the best theory because it is "creepy" isn't very good science, if you ask me. Regarding there being no single-world interpretation that is logically self-consistent, I'm not convinced about this: The Bohm Interpretation, for instance, is experimentally indistinguishable from the Everett Interpretation. I.e., no matter what incredible technology and powers of QM experimentation we might develop in the future, we will never be able to do an experiment, even in theory, that tells us which of these interpretations is the right one. Consequently, it would seem that the Bohm Interpretation is logically self-consistent. The problem with the Bohm Interpretation is that it's very ad hoc and violates Occam's Razor. It only exists in order to calm our feelings about the universe being "creepy".

ummonk 8y ago
Although I go with the Everett interpretation (which I like to think of as the "universal wave function" interpretation because it is really just a minimalist theory assuming that the laws of wavefunction evolution always apply), and the many worlds aspect is just a byproduct of the fact that a wave function left to its own devices would decohere into a bunch of practically non-interacting "worlds". It should be noted, however, that the Everett interpretation does have one issue: it's not clear why probabilities should work the way they do. There are different approaches to deriving the laws of probabilities under Everettian physics, but things very easily get metaphysical once you try to go down that road. As you point out, the Bohm Interpretation works as a single world interpretation, although it relies on reifying particles embedded in waves to essentially select a single world, which is rather ad-hoc. However, it does give us the probabilities for free, assuming any reasonable initial setup for the particles.
- platz 8y ago
  Right, with many worlds, if there is any probability of something quantum happening, with say a billion billion to one odds, it will happen with 100% certainty somewhere. And if you are that observer, how do you say it was unlucky/lucky, sine it must have happened
  - beagle3 8y ago
    It's no worse than a single world interpretation: These things happen all the time, the branch of math that deals with them is called Large Deviation Theory (and it's closely related to information theory). One of the corollaries/interpretations of Sanov's theorem is that, generally speaking, when faced with an astonishingly improbable outcome (e.g. flipping 9,000 heads and 1.000 tails out of 10,000 independent coin flips), no statistical test can differentiate between "that improbable occurence with a fair coin" and "an unfair coin" - the fair coin, when it does something improbable, with have (with overwhelming probability) a specific tilted distribution that looks unfair.
    - platz 8y ago
      But in single interpretation only one outcome happens on a trial. The full distribution does not manifest on a single draw. In MWI all the possibilities occur, which is different
      - Sharlin 8y ago
        Somebody usually wins the lottery. That doesn't change the fact that it's really unlikely for any given player to win the jackpot.
        platz 8y ago
        I guess you win. MWI is really not different from classical probability in any way.
  - MrMontyBurns 8y ago
kgwgk 8y ago
If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation? Edit: the most significant problem with the MWI, apart from the "creepy" metaphysical aspects, is the meaning and quantification of probabilities. In particular being able to derive Born's rule, which works so well in practice.
- nessus42 8y ago
  > If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation? The "Many Worlds Interpretation" is actually something of a misnomer. This is why many people prefer to call it the Everett Interpretation. It doesn't actually posit many worlds. It posits one very big complex world with very complex superpositions of state. But since your brain ends up in a superposition of states, different facets of this superposition of your brain state perceive this one big complex world, as smaller, simpler "worlds". And the term for why different pieces of this superposition of states stop having an effect on each other is called "decoherence". As for how the math works out in terms of probabilities, that is beyond me. When I studied this, the discussion was usually simplified down to a quantum coin that when flipped would come up heads 1/3 of the time and tails 2/3 of the time. This only results in two "worlds" though. A heads world and a tails world. So there was an issue that people debated at the time: Why do we perceive the .3333/.66667 probability for these two "worlds", rather than a .5/.5 probability? I must admit that I am ignorant about the current state of this debate.
  - kgwgk 8y ago
    Is saying "measurement" really much more undefined and unscientific than saying "different facets of this superposition of your brain state perceive"?
    - nessus42 8y ago
      100% yes. You cannot give a scientific accounting of "measurement" since the term isn't even defined in the Copenhagen Interpretation. There are various interpretations that attempt to define "measurement" in various ways, but those are not the Copenhagen Intepretation. As for whether you can give a scientific account of how data is processed by a brain in a superposition of states, you most certainly can. (Ever heard of "quantum computing"?) It's just complicated.
Sileni 8y ago
Just wanted to say, as someone who's been following this on the fringe for years, I really appreciate you using words that were easily searched. Really let me step through what you wrote effectively even though I didn't recognize the interpretations by name.
abeppu 8y ago
> Among the physicists who do care about the different QM interpretations, it is my understanding that most would go with the Everett (AKA "Many Worlds) Interpretation these days. In a small poll at a conference on the foundations of quantum mechanics in 2011, the Copenhagen interpretation was the most popular. This could differ from your definition of "these days" I suppose. https://arxiv.org/pdf/1301.1069.pdf https://arxiv.org/pdf/1301.1069.pdf Update: a 2016 survey by different authors found similar popularity for the Copenhagen interpretation.
- konschubert 8y ago
  I can say that this weird acceptance of the logically undefined Copenhagen interpretation was a contributor to me wanting to leave the field ...
  - AstralStorm 8y ago
    Logically undefined in what sense? Paradoxical traps or thought experiments filled with holes? MW is even more undefined in terms that we cannot even construct a measurement/singular interaction... (it is always derivative of total system state)
- nessus42 8y ago
  Well, I guess I can't argue with an unscientific poll, but it kind of boggles my mind that any scientist could take the Copenhagen Interpretation seriously. I mean it builds the term "measurement", which is an undefined and unscientific concept, right into the laws of physics. Personally, I find this to be virtually nonsensical.
- pdobsan 8y ago
  The 2016 survey: https://arxiv.org/pdf/1612.00676.pdf https://arxiv.org/pdf/1612.00676.pdf Both surveys are fascinating reads. They clearly give a sense that despite of the spectacular success of QM how far we still are from the final word in that field.
rocqua 8y ago
> Regarding there being no single-world interpretation that is logically self-consistent, I'm not convinced about this: The Bohm Interpretation The paper on which this is based states in the abstract that this fails for Bohmiam Mechanics: > This conclusion extends to deterministic hidden-variable theories, such as Bohmian mechanics, for they impose a single-world interpretation.
FooHentai 8y ago
>The only problem is that many people consider it to be "creepy". That seems like an unfair characterization. The issue many have with the Everett Interpretation is that it relies on a fundamentally untestable existence of every possible universe in every possible state of being for the sake of conceptual tidiness. It's not unreasonable to apply Occam's razor and hold a measure of skepticism about this.
- nessus42 8y ago
  In my opinion, Occam's Razor does not say that you should prefer the theory that has the simplest consequences, but rather that you should accept the simplest theory that explains the observations. The Everett Interpretation is clearly the simplest theory because the deep mystery of what would cause probability waves to collapse is made completely irrelevant. There is no wave collapse to worry about or to explain.
konschubert 8y ago
> The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses. Then why is it being taught everywhere? It's been driving me crazy since forever.
- chrischen 8y ago
  I’d imagine because it is the easiest to explain.
  - nessus42 8y ago
    Also because it works 100% of the time, even if it doesn't ask or answer some questions that seem profoundly important.
gpsx 8y ago
People have an adverse reaction to the "many worlds" theory. Maybe they don't understand it? Or maybe what I think is the many world interpretation is really something else. Back when I was in theoretical physics we weren't careful about defining terms like this because, as the original comment says, many worlds vs copenhagen was not really an issue of interest, other than in a philosophical sense for people like me. And I didn't take Copenhagen seriously. If you take away any sensient beings, or whatever it is that is required to make one of these "measurements", then copoenhagen is the same as the many world interpretation. Wave functions go on evolving and there is no collapse. For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. That is strange enough for all of us. But for some reason people have trouble extending this idea to people, so that a person can be in mulitiple states at the same time (in the different "worlds", in the same sense as the electron being in different "worlds".) What made this strike home to me was when I was in graduate school and my advisor told me "There are no magic external observers. The observer is subject to quantum mechanics too. He is part of the experiment." To describe the correspondance between copenhagen and "many worlds", suppose an observer measures if an electron is spin up or spin down. In "many worlds" case his memory of the outcome is correlated with the measured state of the electron. So in the "world" where the electron is spin up (that portion of the wave function) the observer also thinks the electron was measured as spin up. And in the "world" where the electron is spin down, the observer thinks the electron was measured as spin down. In this "many worlds" case, The observer who measures the electron as spin up will not interact with the observer that measured it as spin down. For all intents in purposes, it is as if that other observer never existed. In the copenhagen case, that other observer does _not_ exist. In this interpretation, the wave function collapsed to only include the part with a single observer. In effect the observed outcome of the two interpretations is the same. The difference being one of them, the copenhagen interpretation, postulates a magical change in the wave function of the universe. (Aside - The fact that those observers will not interact is just in a practical sense, to my knowledge. I don't know if it is impossible for them to interact in theory. I don't think it is. Maybe someone else knows the answer to that.)
- monktastic1 8y ago
  > For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. I don't think most MWIers would agree with this. Normally they consider worlds to have split only once (irreversible, or approximately irreversible) decoherence has set in. An electron in the coherent state |z+> + |z-> = |x+> wouldn't qualify. In fact, this seems to be one of the biggest difficulties of the interpretation. Nobody knows whether there even is such a thing as in-principle irreversible decoherence, and if there's not, then the point at which it is "approximately irreversible" is arbitrary.
deleted 8y ago
[deleted]
IIAOPSW 8y ago
>The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses. No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter. I'm a wave function, and sometimes I have a measurement interaction with other wave functions and that causes collapse. When wave functions of random environment particles do the measurement and cause (seemingly) spontaneous collapse, we call it decoherence and its a serious drag on building quantum computers. I'm not advocating for Copenhagen, but it is not inconsistent for the reasons you stated. Outside the paper linked here, I'm unaware of any inconsistency in the Copenhagen interpretation.
- nessus42 8y ago
  > No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter. That's not the Copenhagen Interpretation. Interpretations that claim this are called "Objective-collapse" theories. Back when I cared a lot about this, the most popular Objective-collapse theory was GRW: https://en.wikipedia.org/wiki/Objective-collapse_theory https://en.wikipedia.org/wiki/Objective-collapse_theory

ummonk 8y ago

Although I go with the Everett interpretation (which I like to think of as the "universal wave function" interpretation because it is really just a minimalist theory assuming that the laws of wavefunction evolution always apply), and the many worlds aspect is just a byproduct of the fact that a wave function left to its own devices would decohere into a bunch of practically non-interacting "worlds". It should be noted, however, that the Everett interpretation does have one issue: it's not clear why probabilities should work the way they do. There are different approaches to deriving the laws of probabilities under Everettian physics, but things very easily get metaphysical once you try to go down that road. As you point out, the Bohm Interpretation works as a single world interpretation, although it relies on reifying particles embedded in waves to essentially select a single world, which is rather ad-hoc. However, it does give us the probabilities for free, assuming any reasonable initial setup for the particles.

platz 8y ago
Right, with many worlds, if there is any probability of something quantum happening, with say a billion billion to one odds, it will happen with 100% certainty somewhere. And if you are that observer, how do you say it was unlucky/lucky, sine it must have happened
- beagle3 8y ago
  It's no worse than a single world interpretation: These things happen all the time, the branch of math that deals with them is called Large Deviation Theory (and it's closely related to information theory). One of the corollaries/interpretations of Sanov's theorem is that, generally speaking, when faced with an astonishingly improbable outcome (e.g. flipping 9,000 heads and 1.000 tails out of 10,000 independent coin flips), no statistical test can differentiate between "that improbable occurence with a fair coin" and "an unfair coin" - the fair coin, when it does something improbable, with have (with overwhelming probability) a specific tilted distribution that looks unfair.
  - platz 8y ago
    But in single interpretation only one outcome happens on a trial. The full distribution does not manifest on a single draw. In MWI all the possibilities occur, which is different
    - Sharlin 8y ago
      Somebody usually wins the lottery. That doesn't change the fact that it's really unlikely for any given player to win the jackpot.
      - platz 8y ago
        I guess you win. MWI is really not different from classical probability in any way.
- MrMontyBurns 8y ago

kgwgk 8y ago

If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation? Edit: the most significant problem with the MWI, apart from the "creepy" metaphysical aspects, is the meaning and quantification of probabilities. In particular being able to derive Born's rule, which works so well in practice.

nessus42 8y ago
> If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation? The "Many Worlds Interpretation" is actually something of a misnomer. This is why many people prefer to call it the Everett Interpretation. It doesn't actually posit many worlds. It posits one very big complex world with very complex superpositions of state. But since your brain ends up in a superposition of states, different facets of this superposition of your brain state perceive this one big complex world, as smaller, simpler "worlds". And the term for why different pieces of this superposition of states stop having an effect on each other is called "decoherence". As for how the math works out in terms of probabilities, that is beyond me. When I studied this, the discussion was usually simplified down to a quantum coin that when flipped would come up heads 1/3 of the time and tails 2/3 of the time. This only results in two "worlds" though. A heads world and a tails world. So there was an issue that people debated at the time: Why do we perceive the .3333/.66667 probability for these two "worlds", rather than a .5/.5 probability? I must admit that I am ignorant about the current state of this debate.
- kgwgk 8y ago
  Is saying "measurement" really much more undefined and unscientific than saying "different facets of this superposition of your brain state perceive"?
  - nessus42 8y ago
    100% yes. You cannot give a scientific accounting of "measurement" since the term isn't even defined in the Copenhagen Interpretation. There are various interpretations that attempt to define "measurement" in various ways, but those are not the Copenhagen Intepretation. As for whether you can give a scientific account of how data is processed by a brain in a superposition of states, you most certainly can. (Ever heard of "quantum computing"?) It's just complicated.

Sileni 8y ago

Just wanted to say, as someone who's been following this on the fringe for years, I really appreciate you using words that were easily searched. Really let me step through what you wrote effectively even though I didn't recognize the interpretations by name.

abeppu 8y ago

> Among the physicists who do care about the different QM interpretations, it is my understanding that most would go with the Everett (AKA "Many Worlds) Interpretation these days. In a small poll at a conference on the foundations of quantum mechanics in 2011, the Copenhagen interpretation was the most popular. This could differ from your definition of "these days" I suppose. https://arxiv.org/pdf/1301.1069.pdf https://arxiv.org/pdf/1301.1069.pdf Update: a 2016 survey by different authors found similar popularity for the Copenhagen interpretation.

konschubert 8y ago
I can say that this weird acceptance of the logically undefined Copenhagen interpretation was a contributor to me wanting to leave the field ...
- AstralStorm 8y ago
  Logically undefined in what sense? Paradoxical traps or thought experiments filled with holes? MW is even more undefined in terms that we cannot even construct a measurement/singular interaction... (it is always derivative of total system state)
nessus42 8y ago
Well, I guess I can't argue with an unscientific poll, but it kind of boggles my mind that any scientist could take the Copenhagen Interpretation seriously. I mean it builds the term "measurement", which is an undefined and unscientific concept, right into the laws of physics. Personally, I find this to be virtually nonsensical.
pdobsan 8y ago
The 2016 survey: https://arxiv.org/pdf/1612.00676.pdf https://arxiv.org/pdf/1612.00676.pdf Both surveys are fascinating reads. They clearly give a sense that despite of the spectacular success of QM how far we still are from the final word in that field.

rocqua 8y ago

> Regarding there being no single-world interpretation that is logically self-consistent, I'm not convinced about this: The Bohm Interpretation The paper on which this is based states in the abstract that this fails for Bohmiam Mechanics: > This conclusion extends to deterministic hidden-variable theories, such as Bohmian mechanics, for they impose a single-world interpretation.

FooHentai 8y ago

>The only problem is that many people consider it to be "creepy". That seems like an unfair characterization. The issue many have with the Everett Interpretation is that it relies on a fundamentally untestable existence of every possible universe in every possible state of being for the sake of conceptual tidiness. It's not unreasonable to apply Occam's razor and hold a measure of skepticism about this.

nessus42 8y ago
In my opinion, Occam's Razor does not say that you should prefer the theory that has the simplest consequences, but rather that you should accept the simplest theory that explains the observations. The Everett Interpretation is clearly the simplest theory because the deep mystery of what would cause probability waves to collapse is made completely irrelevant. There is no wave collapse to worry about or to explain.

konschubert 8y ago

chrischen 8y ago
I’d imagine because it is the easiest to explain.
- nessus42 8y ago
  Also because it works 100% of the time, even if it doesn't ask or answer some questions that seem profoundly important.

gpsx 8y ago

People have an adverse reaction to the "many worlds" theory. Maybe they don't understand it? Or maybe what I think is the many world interpretation is really something else. Back when I was in theoretical physics we weren't careful about defining terms like this because, as the original comment says, many worlds vs copenhagen was not really an issue of interest, other than in a philosophical sense for people like me. And I didn't take Copenhagen seriously. If you take away any sensient beings, or whatever it is that is required to make one of these "measurements", then copoenhagen is the same as the many world interpretation. Wave functions go on evolving and there is no collapse. For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. That is strange enough for all of us. But for some reason people have trouble extending this idea to people, so that a person can be in mulitiple states at the same time (in the different "worlds", in the same sense as the electron being in different "worlds".) What made this strike home to me was when I was in graduate school and my advisor told me "There are no magic external observers. The observer is subject to quantum mechanics too. He is part of the experiment." To describe the correspondance between copenhagen and "many worlds", suppose an observer measures if an electron is spin up or spin down. In "many worlds" case his memory of the outcome is correlated with the measured state of the electron. So in the "world" where the electron is spin up (that portion of the wave function) the observer also thinks the electron was measured as spin up. And in the "world" where the electron is spin down, the observer thinks the electron was measured as spin down. In this "many worlds" case, The observer who measures the electron as spin up will not interact with the observer that measured it as spin down. For all intents in purposes, it is as if that other observer never existed. In the copenhagen case, that other observer does _not_ exist. In this interpretation, the wave function collapsed to only include the part with a single observer. In effect the observed outcome of the two interpretations is the same. The difference being one of them, the copenhagen interpretation, postulates a magical change in the wave function of the universe. (Aside - The fact that those observers will not interact is just in a practical sense, to my knowledge. I don't know if it is impossible for them to interact in theory. I don't think it is. Maybe someone else knows the answer to that.)

monktastic1 8y ago
> For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. I don't think most MWIers would agree with this. Normally they consider worlds to have split only once (irreversible, or approximately irreversible) decoherence has set in. An electron in the coherent state |z+> + |z-> = |x+> wouldn't qualify. In fact, this seems to be one of the biggest difficulties of the interpretation. Nobody knows whether there even is such a thing as in-principle irreversible decoherence, and if there's not, then the point at which it is "approximately irreversible" is arbitrary.

deleted 8y ago

[deleted]

IIAOPSW 8y ago

>The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses. No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter. I'm a wave function, and sometimes I have a measurement interaction with other wave functions and that causes collapse. When wave functions of random environment particles do the measurement and cause (seemingly) spontaneous collapse, we call it decoherence and its a serious drag on building quantum computers. I'm not advocating for Copenhagen, but it is not inconsistent for the reasons you stated. Outside the paper linked here, I'm unaware of any inconsistency in the Copenhagen interpretation.

nessus42 8y ago
> No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter. That's not the Copenhagen Interpretation. Interpretations that claim this are called "Objective-collapse" theories. Back when I cared a lot about this, the most popular Objective-collapse theory was GRW: https://en.wikipedia.org/wiki/Objective-collapse_theory https://en.wikipedia.org/wiki/Objective-collapse_theory