8 ms·

Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction o

Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction of all physicists. Thus we have articles like this :/ Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.

macspoofing 6y ago
>Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Oh yeah? Then what is the physical explanation for collapse?
- btilly 6y ago
  What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of hand. But as we create ever more complex but controlled systems, we can perform ever more elaborate experiments verifying that quantum mechanics works exactly as predicted. At some point if we take seriously the idea that the most successful scientific theory of all time is an accurate description of ourselves, then we have to accept that perhaps there is no collapse after all.
  - tsimionescu 6y ago
    But this doesn't answer the question. If you claim that all of these possible observers 'exist', how does this have a physical meaning? This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?
    - akvadrako 6y ago
      Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered. But anyway, the other worlds do effect our world - that's why we get interference patterns in double slit experiments.
      - ClumsyPilot 6y ago
        > "Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered." This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions. It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory must be falsifiable -> i.e. measurable and provable / disprovable somehow. You should at least point people to reading material before making fun of them.
        akvadrako 6y ago
        I wasn’t making fun of anyone but your “obvious“ reading seems wrong. I’m pretty sure he suggested that something only exists physically if it has some measurable effect on us.
      - tsimionescu 6y ago
        In physical terms, we do generally define existence that way - for example, we say that time and space didn't exist 'before' the big bang, because there was nothing that could have a position or change. I was thinking of the same notion of existence and how it can be applied to MWI - essentially existence in the physical sense must mean that something is measurable, that it has some effect on the world (perhaps in the past or in the future).
        akvadrako 6y ago
        I don't think we do define existence that way. Say you and your friend both go to opposite ends of the visible universe; due to inflation you'll never be able to communicate again. I suspect most people would say their friend continues to exist. This is very analogous to the many worlds situation.
  - gmkiv 6y ago
    I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this. Does it?
    - LittleTester 6y ago
      No it can't. There have been many attempts and they don't work. The Born rule is independent of unitary evolution. The closest one can get is to declare that the quantum state is fundamentally a statistical object (i.e. the only information in it is observation probabilities) and then with certain assumptions about the size of the state space you can show that the Born rule is the only possible rule for connecting the state to statistics consistent with the unitary dynamics. So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.
    - Enginerrrd 6y ago
      Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?
      - LittleTester 6y ago
        It's not unitary evolution breaking down, just that the Born rule isn't a consequence of unitary evolution. They're separate independent hypotheses. In most derivations of QM from an axiomatic basis they're consequences of separate combinations of axioms.
        Enginerrrd 6y ago
        Thanks. Do you by chance have a good source for a gentle introduction into axiomatic QM? Like undergrad level is fine, I've taken basic QM and worked through Griffith's intro book on my own, and I've had a lot of math. I'd love to read more but my google results aren't turning up a good definitive introduction.
  - LittleTester 6y ago
    Well in the most common family of interpretations "collapse" isn't an actual physical process, just Bayesian updating. So you wouldn't expect to find physical evidence of it in that sense. It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state. There's very little motivation for reading the quantum state "ontically" in the way you are doing.
- nabla9 6y ago
  Wave function collapse is not experimentally verified or observed physical phenomenon (so far). It's postulated by some interpretations of QM. Apparent wave function collapse can be explained using quantum decoherence.
GuB-42 6y ago
I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"? The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.
- kanzenryu2 6y ago
  The first quote dates to the 1960s before decoherence was understood.
  - LittleTester 6y ago
    Decoherence doesn't really solve these issues. It gives you an approximately diagonal density matrix for the macroscopic degrees of freedom, but: (a) Not exactly diagonal (b) There isn't a unique decomposition of the macroscopic density matrix even after decoherence thus it cannot be taken as simply ignorance of some set of macrostates. You need something stronger, namely superselection or irreversibility.
    - alpineidyll3 6y ago
      That's correct, But there are numerous good methods to propogate irreversible dynamics in qm. Many of these are exact in the limit of a noninteracting bath of bilinearly coupled oscillators which is sufficient to describe a measurement collapse. There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.
      - Xcelerate 6y ago
        > There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated. Then there are a lot of Nobel prize winning physicists who would love to be enlightened about how simple the mystery actually is.

macspoofing 6y ago

>Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Oh yeah? Then what is the physical explanation for collapse?

btilly 6y ago
What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of hand. But as we create ever more complex but controlled systems, we can perform ever more elaborate experiments verifying that quantum mechanics works exactly as predicted. At some point if we take seriously the idea that the most successful scientific theory of all time is an accurate description of ourselves, then we have to accept that perhaps there is no collapse after all.
- tsimionescu 6y ago
  But this doesn't answer the question. If you claim that all of these possible observers 'exist', how does this have a physical meaning? This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?
  - akvadrako 6y ago
    Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered. But anyway, the other worlds do effect our world - that's why we get interference patterns in double slit experiments.
    - ClumsyPilot 6y ago
      > "Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered." This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions. It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory must be falsifiable -> i.e. measurable and provable / disprovable somehow. You should at least point people to reading material before making fun of them.
      - akvadrako 6y ago
        I wasn’t making fun of anyone but your “obvious“ reading seems wrong. I’m pretty sure he suggested that something only exists physically if it has some measurable effect on us.
    - tsimionescu 6y ago
      In physical terms, we do generally define existence that way - for example, we say that time and space didn't exist 'before' the big bang, because there was nothing that could have a position or change. I was thinking of the same notion of existence and how it can be applied to MWI - essentially existence in the physical sense must mean that something is measurable, that it has some effect on the world (perhaps in the past or in the future).
      - akvadrako 6y ago
        I don't think we do define existence that way. Say you and your friend both go to opposite ends of the visible universe; due to inflation you'll never be able to communicate again. I suspect most people would say their friend continues to exist. This is very analogous to the many worlds situation.
- gmkiv 6y ago
  I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this. Does it?
  - LittleTester 6y ago
    No it can't. There have been many attempts and they don't work. The Born rule is independent of unitary evolution. The closest one can get is to declare that the quantum state is fundamentally a statistical object (i.e. the only information in it is observation probabilities) and then with certain assumptions about the size of the state space you can show that the Born rule is the only possible rule for connecting the state to statistics consistent with the unitary dynamics. So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.
  - Enginerrrd 6y ago
    Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?
    - LittleTester 6y ago
      It's not unitary evolution breaking down, just that the Born rule isn't a consequence of unitary evolution. They're separate independent hypotheses. In most derivations of QM from an axiomatic basis they're consequences of separate combinations of axioms.
      - Enginerrrd 6y ago
        Thanks. Do you by chance have a good source for a gentle introduction into axiomatic QM? Like undergrad level is fine, I've taken basic QM and worked through Griffith's intro book on my own, and I've had a lot of math. I'd love to read more but my google results aren't turning up a good definitive introduction.
- LittleTester 6y ago
  Well in the most common family of interpretations "collapse" isn't an actual physical process, just Bayesian updating. So you wouldn't expect to find physical evidence of it in that sense. It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state. There's very little motivation for reading the quantum state "ontically" in the way you are doing.
nabla9 6y ago
Wave function collapse is not experimentally verified or observed physical phenomenon (so far). It's postulated by some interpretations of QM. Apparent wave function collapse can be explained using quantum decoherence.

GuB-42 6y ago

I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"? The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.

kanzenryu2 6y ago
The first quote dates to the 1960s before decoherence was understood.
- LittleTester 6y ago
  Decoherence doesn't really solve these issues. It gives you an approximately diagonal density matrix for the macroscopic degrees of freedom, but: (a) Not exactly diagonal (b) There isn't a unique decomposition of the macroscopic density matrix even after decoherence thus it cannot be taken as simply ignorance of some set of macrostates. You need something stronger, namely superselection or irreversibility.
  - alpineidyll3 6y ago
    That's correct, But there are numerous good methods to propogate irreversible dynamics in qm. Many of these are exact in the limit of a noninteracting bath of bilinearly coupled oscillators which is sufficient to describe a measurement collapse. There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.
    - Xcelerate 6y ago
      > There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated. Then there are a lot of Nobel prize winning physicists who would love to be enlightened about how simple the mystery actually is.