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This write up not only oversimplifies, but it totally neglects one of the more interesting motivations of the original paper [1]. The authors are probing whethe

This write up not only oversimplifies, but it totally neglects one of the more interesting motivations of the original paper [1]. The authors are probing whether or not quantum mechanics is consistent with a single-world interpretation---that is, whether or not there is a unique reality. Formally, the claim is that there is no physical theory that is (1) consistent with QM, (2) consistent with a single-world interpretation, and (3) logically self-consistent. 1. https://arxiv.org/pdf/1604.07422.pdf https://arxiv.org/pdf/1604.07422.pdf

cs702 8y ago
The paper's title makes the claim succinctly: "Single-world interpretations of quantum theory cannot be self-consistent." Nature's headline does a terrible job at conveying this, really. I would have expected better from the people who edit headlines there.
- deleted 8y ago
  [deleted]
phkahler 8y ago
>> Formally, the claim is that there is no physical theory that is (1) consistent with QM, (2) consistent with a single-world interpretation, and (3) logically self-consistent. Does pilot wave theory fit the bill?
- nanofortnight 8y ago
  Pilot wave theory (also known as Bohmian mechanics) imposes a single-world interpretation.
  - phkahler 8y ago
    My understanding is that it meets all 3 criteria. Hence the assertion that there is no theory that meets all 3 would be false.
    - yorwba 8y ago
      According to the paper, Bohmian mechanics violates assumption (Q) (the first one), which is not exactly "consistent with QM", but rather "If an agent knows the state of a quantum system S and knows that a measurement of x applied to the state yields ξ with probability 1, then the agent knows that x = ξ". Bohmian mechanics violates it by not applying to arbitrary subsystems, but only to the universe as a whole (i.e. parts of the universe cannot be treated as quantum-mechanical systems themselves, because they don't have their own pilot waves)
      - phkahler 8y ago
        Hence non-locality right?
      - catawbasam 8y ago
        re "only to the universe as a whole"-- “The etymology of the word ‘universe’ can be traced back to the use of the Old French univers, in the twelfth century, which derives from the earlier Latin universum. This word is created from unus, meaning ‘one’, and ‘versus’, the past participle of the verb vertere, meaning ‘to turn, rotate, roll or change’. So we have a literal meaning of everything ‘turned into one’ or ‘rolled into one’.” from John D. Barrow’s “The Book of Universes.” Sounds like Bohm is taking the 'uni' in universe seriously, while the paper starts by assuming there is no fundamental unity in the universe.
        Koshkin 8y ago
        The Latin word may have been invented less than a thousand years ago: "...dicuntur universum sive omne : quia universum est unum versum in omnia..."
      - mjevans 8y ago
        I find that to be logical, as it seems like it would be incredibly difficult to dis-entangle an observer from the minute fluctuation and relationships with every other bit of energy in the universe.
      - ummonk 8y ago
nessus42 8y ago
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.
dang 8y ago
We changed the URL from https://www.nature.com/articles/d41586-018-06749-8 https://www.nature.com/articles/d41586-018-06749-8, which people were complaining about, to one that at least a few readers seemed to like better.
electrograv 8y ago
Usually when there's a paradox like this, it means we've chosen bad axioms somewhere. Even things that seem intuitively self-evident can prove untrue. For example, what if time does not necessarily flow in a single direction at the quantum scale; what if instead, the ground level of physical reality is a timeless information graph / equation that is 'solved by the universe'? I'm no physicist and no nearly nothing about the real math of QM, but every time I read these lay-explanations of "quantum weirdness" and "wave function collapse", I get this strange feeling that we're thinking about time all wrong: What if unidirectional time is an illusion? What if causality (and inference) is an illusion (thus explaining how hard it is to capture it mathematically)?
- AstralStorm 8y ago
  The specific axioms seem to be at fault in both thought experiments in step 0.00. Iterated experiment presumes creating fully known fixed same state psi. Wiener Friends experiment makes the F1 magically know (memorize) state of quantum RNG without measuring it and without being entangled. Both experiments require cloning which is forbidden.
- comboy 8y ago
  > What if causality (and inference) is an illusion You may want to check David Hume.
jondubois 8y ago
If the single-world interpretation is not true, wouldn't it mean that quantum physics is useless? What use is a quantum computer if the algorithm shows different results to different users?
- szemet 8y ago
  And what the use of a noisy channel if it may changes bits sent over it? ;) You just need to have proper error correction. You are right - of course - if QM is completely noisy, or you can't have proper error correction (due to some physical limitation for example). E.g. for quatum computation some people believes so: https://www.quantamagazine.org/gil-kalais-argument-against-quantum-computers-20180207/ https://www.quantamagazine.org/gil-kalais-argument-against-q...
- Sharlin 8y ago
  Quantum computers are intrinsically stochastic machines. Quantum algorithms are about hedging the bets so that you get the "correct" answer at some p>0.5. Then you just repeat the computation until you're reasonably sure you have the right answer. In the Copenhagen interpretation the wavefunction magically "collapses" to one outcome, in Everett interpretation the wavefunction decoheres producing both outcomes in separate "worlds".

cs702 8y ago

The paper's title makes the claim succinctly: "Single-world interpretations of quantum theory cannot be self-consistent." Nature's headline does a terrible job at conveying this, really. I would have expected better from the people who edit headlines there.

deleted 8y ago
[deleted]

phkahler 8y ago

>> Formally, the claim is that there is no physical theory that is (1) consistent with QM, (2) consistent with a single-world interpretation, and (3) logically self-consistent. Does pilot wave theory fit the bill?

nanofortnight 8y ago
Pilot wave theory (also known as Bohmian mechanics) imposes a single-world interpretation.
- phkahler 8y ago
  My understanding is that it meets all 3 criteria. Hence the assertion that there is no theory that meets all 3 would be false.
  - yorwba 8y ago
    According to the paper, Bohmian mechanics violates assumption (Q) (the first one), which is not exactly "consistent with QM", but rather "If an agent knows the state of a quantum system S and knows that a measurement of x applied to the state yields ξ with probability 1, then the agent knows that x = ξ". Bohmian mechanics violates it by not applying to arbitrary subsystems, but only to the universe as a whole (i.e. parts of the universe cannot be treated as quantum-mechanical systems themselves, because they don't have their own pilot waves)
    - phkahler 8y ago
      Hence non-locality right?
    - catawbasam 8y ago
      re "only to the universe as a whole"-- “The etymology of the word ‘universe’ can be traced back to the use of the Old French univers, in the twelfth century, which derives from the earlier Latin universum. This word is created from unus, meaning ‘one’, and ‘versus’, the past participle of the verb vertere, meaning ‘to turn, rotate, roll or change’. So we have a literal meaning of everything ‘turned into one’ or ‘rolled into one’.” from John D. Barrow’s “The Book of Universes.” Sounds like Bohm is taking the 'uni' in universe seriously, while the paper starts by assuming there is no fundamental unity in the universe.
      - Koshkin 8y ago
        The Latin word may have been invented less than a thousand years ago: "...dicuntur universum sive omne : quia universum est unum versum in omnia..."
    - mjevans 8y ago
      I find that to be logical, as it seems like it would be incredibly difficult to dis-entangle an observer from the minute fluctuation and relationships with every other bit of energy in the universe.
    - ummonk 8y ago

nessus42 8y ago

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.

dang 8y ago

We changed the URL from https://www.nature.com/articles/d41586-018-06749-8 https://www.nature.com/articles/d41586-018-06749-8, which people were complaining about, to one that at least a few readers seemed to like better.

electrograv 8y ago

Usually when there's a paradox like this, it means we've chosen bad axioms somewhere. Even things that seem intuitively self-evident can prove untrue. For example, what if time does not necessarily flow in a single direction at the quantum scale; what if instead, the ground level of physical reality is a timeless information graph / equation that is 'solved by the universe'? I'm no physicist and no nearly nothing about the real math of QM, but every time I read these lay-explanations of "quantum weirdness" and "wave function collapse", I get this strange feeling that we're thinking about time all wrong: What if unidirectional time is an illusion? What if causality (and inference) is an illusion (thus explaining how hard it is to capture it mathematically)?

AstralStorm 8y ago
The specific axioms seem to be at fault in both thought experiments in step 0.00. Iterated experiment presumes creating fully known fixed same state psi. Wiener Friends experiment makes the F1 magically know (memorize) state of quantum RNG without measuring it and without being entangled. Both experiments require cloning which is forbidden.
comboy 8y ago
> What if causality (and inference) is an illusion You may want to check David Hume.

jondubois 8y ago

If the single-world interpretation is not true, wouldn't it mean that quantum physics is useless? What use is a quantum computer if the algorithm shows different results to different users?

szemet 8y ago
And what the use of a noisy channel if it may changes bits sent over it? ;) You just need to have proper error correction. You are right - of course - if QM is completely noisy, or you can't have proper error correction (due to some physical limitation for example). E.g. for quatum computation some people believes so: https://www.quantamagazine.org/gil-kalais-argument-against-quantum-computers-20180207/ https://www.quantamagazine.org/gil-kalais-argument-against-q...
Sharlin 8y ago
Quantum computers are intrinsically stochastic machines. Quantum algorithms are about hedging the bets so that you get the "correct" answer at some p>0.5. Then you just repeat the computation until you're reasonably sure you have the right answer. In the Copenhagen interpretation the wavefunction magically "collapses" to one outcome, in Everett interpretation the wavefunction decoheres producing both outcomes in separate "worlds".