11 ms·

What’s Going on During Wave Function ‘Collapse’

161 points by howard941 7y ago

dcow 7y ago
The article feels incomplete. I don’t feel like I have a better understanding of what’s going on during wave function collapse, rather I just know QTT is a thing and people are experimenting with it and it’s yielded promising results when using more sophisticated measurement apparatus to observe an “atom”, like reversing a quantum jump mid-flight by injecting feedback into the system as it’s being measured.
- curtis 7y ago
  I barely understand any article on quantum mechanics, but now I know that QTT is a thing and people are experimenting with it. That's why I personally upvoted the article but I admit I was on the fence about it. I'm hoping someone knowledgeable will comment on the article and I might gain just a little more understanding. Maybe.
- jogundas 7y ago
  I liked some commentary in a similar direction https://physics.stackexchange.com/questions/484675/does-the-new-finding-on-reversing-a-quantum-jump-mid-flight-rule-out-any-inter/484681#484681 https://physics.stackexchange.com/questions/484675/does-the-...
  - dcow 7y ago
    Thanks for the link it covers all the details I was looking for. My favorite part: > Copenhagen and, say, many worlds just differ on how to treat branches of a superposition that have completely decohered.
malaxii 7y ago
Quantum trajectories more or less have the usual Copenhagen interpretation built in, so they don't really solve the old interpretation problems any more than decoherence does. Although they can be a nice way to understand some experiments, in particular how conditional probabilities of a set of measurement results obtained in sequence occur, I'm not sure if it's good to present it as some fundamental insight in popular articles.
- jeremyjh 7y ago
  This seems to be the opposite of what the article is saying, or implying. I read it as saying that there is no need for an interpretation, because there is no wave function collapse; the particle can be described in precise terms.
  - chrischen 7y ago
    It was buried way deep in the article, but: > But what exactly is this trajectory? One thing is clear: It’s not like a classical trajectory, meaning a path taken in space. It’s more like the path taken through the abstract space of possible states the system might have, which is called Hilbert space. QTT is not a definite prediction of classical physical trajectory.
    - simonh 7y ago
      Although classical physical trajectory is one if the possible states QTT helps us predict. It’s more than that, not less.
  - n4r9 7y ago
    To be fair in both everettian and de broglie bohm interpretations there is no physical wave function collapse. Even in neo-Copenhagen interpretations like QBism the apparent collapse is merely a reflection of an agent's belief update process.
    - kgwgk 7y ago
      QBism has a cool name, but I think it's not as revolutionary as it may seem at first sight. There are two kinds of uncertainty about a quantum system: the "classical" ignorance about what is the true quantum state and the non-determinism of the outcome of measurements even if the quantum state is perfectly known (note that only in this case the system can be described using a wave function). "Up to an overall unitary ‘readjustment’ of one’s final probabilistic beliefs [...] quantum collapse is precisely Bayesian conditionalization." I'd say that this 'readjustment' is the collapse with another name. "Quantum measurement is nothing more, and nothing less, than a refinement and a readjustment of one’s initial state of belief. [...] Let us look at two limiting cases of efficient measurements. In the first, we imagine an observer whose initial belief structure ρ = |ψ⟩⟨ψ| is a maximally sharp state of belief. By this account, no measurement whatsoever can refine it. [...] The only state change that can come about from a measurement must be purely of the mental-readjustment sort: We learn nothing new; we just change what we can predict as a consequence of the side effects of our experimental intervention. That is to say, there is a sense in which the measurement is solely disturbance." Ok, so when you do a measurement on a pure state (i.e. when the knowledge about the quantum state is maximal and cannot be refined by Bayesian updating): instead of the "wave function collapse" of “standard” QM (the wave function changes to the eigenstate corresponding to the outcome of the measurement) you have a "mental-readjustment" (because as a side effect of the measurement now you describe the system using the same wave function as in standard QM). What are the problems with the "wave function collapse" that are solved by calling it "mental readjustment"? https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf
fpsio 7y ago
Talk by Zlatko Minev on this result: https://www.youtube.com/watch?v=JGNL91MhC5A https://www.youtube.com/watch?v=JGNL91MhC5A - JQI Seminar 4/22/2019 - Zlatko Minev
viach 7y ago
Indeed very interesting, what's going on? Reading the article, the only phrase where "wave function collapse" occurred is: > What’s more, this near-complete knowledge of how the system changes smoothly over time allows researchers to “rewind the tape” and avoid the apparently irreversible “wave function collapse” of the standard quantum formalism. Please, answer this intriguing question, what's going on?
- simonh 7y ago
  I think in QTT there is no collapse event, just a series of state transitions. It looks like the concept of a collapse is a product of a less complete understanding of what is going on. That’s why the article spends so much time contrasting QTT with the view offered by Shroedingers equation. With Shroedinger there is no specific state predicted, so you have to introduce the concept of some event inducing a specific state to occur. QTT allows continuous prediction of state and even the ability to prevent or influence state transitions. At least that’s what I got out if it, although I’ll concede I think that’s a rough attempt at a summary.
d33 7y ago
I recently read a book by a cosmologist Max Tegmark in which he brought up another theory - that the function wave function doesn't really collapse, it's just that the moment of measurement lets us determine our location in the multiverse. I think this is related: https://en.wikipedia.org/wiki/Many-worlds_interpretation#Interpreting_wavefunction_collapse https://en.wikipedia.org/wiki/Many-worlds_interpretation#Int... And also an interesting read: https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality https://en.wikipedia.org/wiki/Quantum_suicide_and_immortalit...
kgwgk 7y ago
Previous discussions: https://news.ycombinator.com/item?id=20105091 https://news.ycombinator.com/item?id=20105091 https://news.ycombinator.com/item?id=20116267 https://news.ycombinator.com/item?id=20116267
phkahler 7y ago
I always say nothing is going on during wave function collapse. You can't tell the difference between a particle whose wave function has collapsed and one that hasn't. Since there is no change it seems nothing actually happened ;-)
- antonvs 7y ago
  > You can't tell the difference between a particle whose wave function has collapsed and one that hasn't. Could you expand on this? For example, in the double slit experiment, it's easy to observe the effects of "uncollapsed" (superposition) behavior. Perhaps you're saying that we can't observe a particle without collapse occurring (from a Copenhagen perspective), so we never directly observe an uncollapsed particle. But we can observe the distinct effects that uncollapsed particles have, which seems to undermine your claim.
  - monocasa 7y ago
    The double slit experiment has been replicated in fully classical macroscopic systems. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97.154101 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97...
    - Iv 7y ago
      Except that a droplet is not a single particle. We have known for a long time that a droplet is divisible and can spread as a wave. Quantum theory states the existence of quantums: indivisible parts of matter and energy.
      - monocasa 7y ago
        The interaction in that experiment doesn't have the droplet dividing or dissolving into the wave. For the purposes of that experiment, the droplet is a quantum unit.
      - mrkstu 7y ago
        So all quantum vs classical differences could be reduced to what we are measuring isn’t ‘atomic’ in the Greek classical sense?
lisper 7y ago
> “Quantum trajectory theory makes predictions that are impossible to make with the standard formulation,” Devoret said. That is bullshit. If it were true, the predictions made by QTT would be tested and if they came out as predicted it would be the biggest news in physics since the Aspect experiment. What is "going on" during wave function "collapse" has been known for decades: entanglement and decoherence. That's all. I am really getting sick and tired of these sensationalistic headlines. (I'm also not entirely sober. It is the Fourth of July after all.)
- Misdicorl 7y ago
  Hear hear. These articles and really entire subfield of 'physics' appears to be philosophy trying to masquerade as a real science
- dwaltrip 7y ago
  > What is "going on" during wave function "collapse" has been known for decades: entanglement and decoherence. That's all. Wasn't it a famous physicist who said "If you think you understand Quantum Mechanics then you don't understand Quantum Mechanics"? I'm not defending this article, but I was under the impression that the proper interpretation of quantum mechanics / QFT is far from a settled matter among physicists.
  - MereInterest 7y ago
    It's not so much that it is settled as everybody knows that it isn't a useful topic of conversation. All the various interpretations have the same math behind them, give the same prediction, and have the same observable behavior. Since there isn't anything to distinguish them, they are all equally valid. In practice, things tend to go with the "Shut up and calculate"[1] philosophy. [1] https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#shutup https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...
    - dwaltrip 7y ago
      I've heard that idiom, and I'm sure it has its use. I don't think we should stop calculating, by any means. However, there are individuals in the field who think new insights are possible through continued attempts to improve our interpretations (in addition to continued development of the theory as expressed through robust math). I don't have a source handy, but I've heard the physicist Sean Carroll express this idea on his podcast, Mindscape. Numbers by themselves don't mean anything (for humans at least). The meaning is found in how the numbers relate to the concepts that we make use of.
Simon_says 7y ago
Has nobody at Quanta Magazine heard of the Many Worlds Interpretation? I'm tired of these awful articles from Quanta.
- TheOtherHobbes 7y ago
  This 'awful article' is about experimental tests of a working theory. Can you provide a reference explaining how MWI might be falsified experimentally?
  - Simon_says 7y ago
    MWI can be differentiated from the Copenhagen Interpretation via experiment as soon as the Copenhagen Interpretation decides what a "measurement" is, when it happens, and when it doesn't. As it is now, the Copenhagen Interpretation is an incomplete theory because it doesn't give you a formal algorithmic way to know when to apply the measurement operator. The answer is whatever it needs to be to make the answer come out right. To answer your question more directly, MWI could be falsified in a lot of ways, but one would be if structures larger than a certain size don't self-interfere.
    - kgwgk 7y ago
      Is the MWI more complete in this regard? How does it decide what a "branching event" is, when it happens, and when it doesn't?
      - Simon_says 7y ago
        Yes it is. MWI's only rule is that reality evolves according the the Schrödinger equation. It's what would get if you took collapse out of the Copenhagen Interpretation. The idea of "branching" is a human-legible metaphor for describing the resultant behavior. But the math doesn't have branching.
        kgwgk 7y ago
bencollier49 7y ago
Stopped reading half way through, as it felt like the author didn't understand what he was writing about. What hope do I have reading the article, then?
- jng 7y ago
  It took me a long time to understand two's complement arithmetic for negative numbers in binary. Read several explanations on books and magazines (this was before the internet existed), couldn't understand it. Then one day it clicked. Awesome trick. I went back to some of the explanations that hadn't helped me understand it, and I could clearly see the author didn't understand it themselves. The world is too full of that.
konschubert 7y ago
Nobody really cared about this question when I was studying physics. It drove me crazy. The amount of handwaving was insane.

161 points by howard941 7y ago

dcow 7y ago

The article feels incomplete. I don’t feel like I have a better understanding of what’s going on during wave function collapse, rather I just know QTT is a thing and people are experimenting with it and it’s yielded promising results when using more sophisticated measurement apparatus to observe an “atom”, like reversing a quantum jump mid-flight by injecting feedback into the system as it’s being measured.

curtis 7y ago
I barely understand any article on quantum mechanics, but now I know that QTT is a thing and people are experimenting with it. That's why I personally upvoted the article but I admit I was on the fence about it. I'm hoping someone knowledgeable will comment on the article and I might gain just a little more understanding. Maybe.
jogundas 7y ago
I liked some commentary in a similar direction https://physics.stackexchange.com/questions/484675/does-the-new-finding-on-reversing-a-quantum-jump-mid-flight-rule-out-any-inter/484681#484681 https://physics.stackexchange.com/questions/484675/does-the-...
- dcow 7y ago
  Thanks for the link it covers all the details I was looking for. My favorite part: > Copenhagen and, say, many worlds just differ on how to treat branches of a superposition that have completely decohered.

malaxii 7y ago

Quantum trajectories more or less have the usual Copenhagen interpretation built in, so they don't really solve the old interpretation problems any more than decoherence does. Although they can be a nice way to understand some experiments, in particular how conditional probabilities of a set of measurement results obtained in sequence occur, I'm not sure if it's good to present it as some fundamental insight in popular articles.

jeremyjh 7y ago
This seems to be the opposite of what the article is saying, or implying. I read it as saying that there is no need for an interpretation, because there is no wave function collapse; the particle can be described in precise terms.
- chrischen 7y ago
  It was buried way deep in the article, but: > But what exactly is this trajectory? One thing is clear: It’s not like a classical trajectory, meaning a path taken in space. It’s more like the path taken through the abstract space of possible states the system might have, which is called Hilbert space. QTT is not a definite prediction of classical physical trajectory.
  - simonh 7y ago
    Although classical physical trajectory is one if the possible states QTT helps us predict. It’s more than that, not less.
- n4r9 7y ago
  To be fair in both everettian and de broglie bohm interpretations there is no physical wave function collapse. Even in neo-Copenhagen interpretations like QBism the apparent collapse is merely a reflection of an agent's belief update process.
  - kgwgk 7y ago
    QBism has a cool name, but I think it's not as revolutionary as it may seem at first sight. There are two kinds of uncertainty about a quantum system: the "classical" ignorance about what is the true quantum state and the non-determinism of the outcome of measurements even if the quantum state is perfectly known (note that only in this case the system can be described using a wave function). "Up to an overall unitary ‘readjustment’ of one’s final probabilistic beliefs [...] quantum collapse is precisely Bayesian conditionalization." I'd say that this 'readjustment' is the collapse with another name. "Quantum measurement is nothing more, and nothing less, than a refinement and a readjustment of one’s initial state of belief. [...] Let us look at two limiting cases of efficient measurements. In the first, we imagine an observer whose initial belief structure ρ = |ψ⟩⟨ψ| is a maximally sharp state of belief. By this account, no measurement whatsoever can refine it. [...] The only state change that can come about from a measurement must be purely of the mental-readjustment sort: We learn nothing new; we just change what we can predict as a consequence of the side effects of our experimental intervention. That is to say, there is a sense in which the measurement is solely disturbance." Ok, so when you do a measurement on a pure state (i.e. when the knowledge about the quantum state is maximal and cannot be refined by Bayesian updating): instead of the "wave function collapse" of “standard” QM (the wave function changes to the eigenstate corresponding to the outcome of the measurement) you have a "mental-readjustment" (because as a side effect of the measurement now you describe the system using the same wave function as in standard QM). What are the problems with the "wave function collapse" that are solved by calling it "mental readjustment"? https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf

fpsio 7y ago

Talk by Zlatko Minev on this result: https://www.youtube.com/watch?v=JGNL91MhC5A https://www.youtube.com/watch?v=JGNL91MhC5A - JQI Seminar 4/22/2019 - Zlatko Minev

viach 7y ago

Indeed very interesting, what's going on? Reading the article, the only phrase where "wave function collapse" occurred is: > What’s more, this near-complete knowledge of how the system changes smoothly over time allows researchers to “rewind the tape” and avoid the apparently irreversible “wave function collapse” of the standard quantum formalism. Please, answer this intriguing question, what's going on?

simonh 7y ago
I think in QTT there is no collapse event, just a series of state transitions. It looks like the concept of a collapse is a product of a less complete understanding of what is going on. That’s why the article spends so much time contrasting QTT with the view offered by Shroedingers equation. With Shroedinger there is no specific state predicted, so you have to introduce the concept of some event inducing a specific state to occur. QTT allows continuous prediction of state and even the ability to prevent or influence state transitions. At least that’s what I got out if it, although I’ll concede I think that’s a rough attempt at a summary.

d33 7y ago

I recently read a book by a cosmologist Max Tegmark in which he brought up another theory - that the function wave function doesn't really collapse, it's just that the moment of measurement lets us determine our location in the multiverse. I think this is related: https://en.wikipedia.org/wiki/Many-worlds_interpretation#Interpreting_wavefunction_collapse https://en.wikipedia.org/wiki/Many-worlds_interpretation#Int... And also an interesting read: https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality https://en.wikipedia.org/wiki/Quantum_suicide_and_immortalit...

kgwgk 7y ago

Previous discussions: https://news.ycombinator.com/item?id=20105091 https://news.ycombinator.com/item?id=20105091 https://news.ycombinator.com/item?id=20116267 https://news.ycombinator.com/item?id=20116267

phkahler 7y ago

I always say nothing is going on during wave function collapse. You can't tell the difference between a particle whose wave function has collapsed and one that hasn't. Since there is no change it seems nothing actually happened ;-)

antonvs 7y ago
> You can't tell the difference between a particle whose wave function has collapsed and one that hasn't. Could you expand on this? For example, in the double slit experiment, it's easy to observe the effects of "uncollapsed" (superposition) behavior. Perhaps you're saying that we can't observe a particle without collapse occurring (from a Copenhagen perspective), so we never directly observe an uncollapsed particle. But we can observe the distinct effects that uncollapsed particles have, which seems to undermine your claim.
- monocasa 7y ago
  The double slit experiment has been replicated in fully classical macroscopic systems. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97.154101 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97...
  - Iv 7y ago
    Except that a droplet is not a single particle. We have known for a long time that a droplet is divisible and can spread as a wave. Quantum theory states the existence of quantums: indivisible parts of matter and energy.
    - monocasa 7y ago
      The interaction in that experiment doesn't have the droplet dividing or dissolving into the wave. For the purposes of that experiment, the droplet is a quantum unit.
    - mrkstu 7y ago
      So all quantum vs classical differences could be reduced to what we are measuring isn’t ‘atomic’ in the Greek classical sense?

lisper 7y ago

> “Quantum trajectory theory makes predictions that are impossible to make with the standard formulation,” Devoret said. That is bullshit. If it were true, the predictions made by QTT would be tested and if they came out as predicted it would be the biggest news in physics since the Aspect experiment. What is "going on" during wave function "collapse" has been known for decades: entanglement and decoherence. That's all. I am really getting sick and tired of these sensationalistic headlines. (I'm also not entirely sober. It is the Fourth of July after all.)

Misdicorl 7y ago
Hear hear. These articles and really entire subfield of 'physics' appears to be philosophy trying to masquerade as a real science
dwaltrip 7y ago
> What is "going on" during wave function "collapse" has been known for decades: entanglement and decoherence. That's all. Wasn't it a famous physicist who said "If you think you understand Quantum Mechanics then you don't understand Quantum Mechanics"? I'm not defending this article, but I was under the impression that the proper interpretation of quantum mechanics / QFT is far from a settled matter among physicists.
- MereInterest 7y ago
  It's not so much that it is settled as everybody knows that it isn't a useful topic of conversation. All the various interpretations have the same math behind them, give the same prediction, and have the same observable behavior. Since there isn't anything to distinguish them, they are all equally valid. In practice, things tend to go with the "Shut up and calculate"[1] philosophy. [1] https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#shutup https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...
  - dwaltrip 7y ago
    I've heard that idiom, and I'm sure it has its use. I don't think we should stop calculating, by any means. However, there are individuals in the field who think new insights are possible through continued attempts to improve our interpretations (in addition to continued development of the theory as expressed through robust math). I don't have a source handy, but I've heard the physicist Sean Carroll express this idea on his podcast, Mindscape. Numbers by themselves don't mean anything (for humans at least). The meaning is found in how the numbers relate to the concepts that we make use of.

Simon_says 7y ago

Has nobody at Quanta Magazine heard of the Many Worlds Interpretation? I'm tired of these awful articles from Quanta.

TheOtherHobbes 7y ago
This 'awful article' is about experimental tests of a working theory. Can you provide a reference explaining how MWI might be falsified experimentally?
- Simon_says 7y ago
  MWI can be differentiated from the Copenhagen Interpretation via experiment as soon as the Copenhagen Interpretation decides what a "measurement" is, when it happens, and when it doesn't. As it is now, the Copenhagen Interpretation is an incomplete theory because it doesn't give you a formal algorithmic way to know when to apply the measurement operator. The answer is whatever it needs to be to make the answer come out right. To answer your question more directly, MWI could be falsified in a lot of ways, but one would be if structures larger than a certain size don't self-interfere.
  - kgwgk 7y ago
    Is the MWI more complete in this regard? How does it decide what a "branching event" is, when it happens, and when it doesn't?
    - Simon_says 7y ago
      Yes it is. MWI's only rule is that reality evolves according the the Schrödinger equation. It's what would get if you took collapse out of the Copenhagen Interpretation. The idea of "branching" is a human-legible metaphor for describing the resultant behavior. But the math doesn't have branching.
      - kgwgk 7y ago

bencollier49 7y ago

Stopped reading half way through, as it felt like the author didn't understand what he was writing about. What hope do I have reading the article, then?

jng 7y ago
It took me a long time to understand two's complement arithmetic for negative numbers in binary. Read several explanations on books and magazines (this was before the internet existed), couldn't understand it. Then one day it clicked. Awesome trick. I went back to some of the explanations that hadn't helped me understand it, and I could clearly see the author didn't understand it themselves. The world is too full of that.

konschubert 7y ago

Nobody really cared about this question when I was studying physics. It drove me crazy. The amount of handwaving was insane.