Wave function collapse

  • #1
quantum philosopher
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TL;DR Summary
When entities interact with other entities their wave function collapse.
Let's take the example of an electron in of a hydrogen atom . It continuously interact with the proton of the hydrogen atom . Both the proton and electron are continuous interacting with each other. It is said that wave function collapse when it is being observed or observers interact with it. So does this means the wave function of the electron and proton are always in a collapsed state ( Dirac delta function)?
 
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  • #2
I think that when people say that "interaction is measurement", they mean it in the von Neumann sense of measurement. The idea that if particle A is entangled with particle B, if one traces out over particle B states, the particle A "sees" a very classical state. So in this sense you need an entangling interaction (not just any interaction) for the proton to "measure" the electron (or the electron to "measure" the proton), but that does not mean that for an external agent like you and me, that the whole system has been measured. For everything to be measured also for you, you have to interact (with an entangling interaction) with the system. This idea is one of the elements of many-worlds theory.

Note that there are many alternative ways to see (or not see) the measurement depending on the interpretation of quantum mechanics.

Note also that collapse (the effect of getting a single result) requires more than that.
 
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  • #3
Do you mean that when wave functions collapse , they collapse only for the observers who observe them? If we were the proton we will see that the electron as a collapsed wavefunction always?
 
  • #4
quantum philosopher said:
Do you mean that when wave functions collapse , they collapse only for the observers who observe them? If we were the proton we will see that the electron as a collapsed wavefunction always?
What does "see that the electron as a collapsed wavefunction" mean???? Please be operationallly specific. It means nothing to me.
 
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  • #5
quantum philosopher said:
It is said...
Said where? Without knowing where you heard that we have no way of knowing whether you are misunderstanding what you've heard or are have been hearing from an unreliable source.
...that wave function collapse when it is being observed or observers interact with it.
That's not quite right. Only those interactions with the environment that are thermodynamically irreversible lead to collapse; when a measuring device is part of that environment it is not uncommon to use the word "observation" to describe the interaction. So all observations are interactions, but all interactions are observations. The key concept here is decoherence; you can google for "Quantum decoherence" or give David Lindley's layman-friendly book "Where does the weirdness go?" a try.

"Observer" is a red herring, and has been for many decades.
So does this means the wave function of the electron and proton are always in a collapsed state ( Dirac delta function)?
No.
And even if we did have a particle whose state is given by a delta function, say a free particle whose position is exactly known and therefore its wave function is a delta function of position (of course this state is not physically realizable, but it is a convenient example) it would be incorrect to say that its state is "collapsed" - that state is still an superposition of momentum eigenstates.
 
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  • #6
quantum philosopher said:
Do you mean that when wave functions collapse , they collapse only for the observers who observe them? If we were the proton we will see that the electron as a collapsed wavefunction always?
Sorry. See edit, I misused the terms measurement and collapse. Yeah, the proton after interaction "measures" the electron as being in a classical statistical mixture. No collapse yet.

But again this is just a question of ill-defined language, like if we were anthropomorphizing the particles. We need math here, do you know density matrices?

Here is a link to what I mean: https://www.quantum.umb.edu/Jacobs/QMT/QMT_Chapter1.pdf
 
  • #7
quantum philosopher said:
TL;DR Summary: When entities interact with other entities their wave function collapse.

Let's take the example of an electron in of a hydrogen atom . It continuously interact with the proton of the hydrogen atom . Both the proton and electron are continuous interacting with each other. It is said that wave function collapse when it is being observed or observers interact with it. So does this means the wave function of the electron and proton are always in a collapsed state ( Dirac delta function)?
Freeman Dyson in “THE COLLAPSE OF THE WAVE FUNCTION” in John Brockman’s book “This Idea Must Die: Scientific Theories That Are Blocking Progress (Edge Question Series)” (New York, NY, USA: HarperCollins (2015)):

Fourscore and eight years ago, Erwin Schrödinger invented wave functions as a way to describe the behavior of atoms and other small objects. According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes
uncertainty. There is no mystery here.

Unfortunately, people writing about quantum mechanics often use the phrase “collapse of the wave function” to describe what happens when an object is observed. This phrase gives a misleading idea that the wave function itself is a physical object. A physical object can collapse when it bumps into an obstacle. But a wave function cannot be a physical object. A wave function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave function. When new knowledge displaces ignorance, the wave function does not collapse; it merely becomes irrelevant.
 
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  • #8
I read you all and I think I should first think a lot about what is going on so that basics become clear to me
 
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  • #9
Moderator's note: Thread moved to the interpretations subforum since "wave function collapse" is an interpretation-dependent concept.
 
  • #10
quantum philosopher said:
It is said that wave function collapse when it is being observed or observers interact with it.
"It is said" where? Please give a specific reference.
 
  • #11
Our teacher said
 
  • #12
Alot can be said about that "defintions" of states and measurements are cast in the language of linear algebra as the mathematical structure used in QM is based on assuming the "wavefunctions" form linear spaces...even with that in place people do "interpret"..

Assmuing you are approaching this from philosophy, I will just jump right into what I am GUESSING is your question as...
quantum philosopher said:
Both the proton and electron are continuous interacting with each other. It is said that wave function collapse when it is being observed or observers interact with it. So does this means the wave function of the electron and proton are always in a collapsed state ( Dirac delta function)?
quantum philosopher said:
Do you mean that when wave functions collapse , they collapse only for the observers who observe them? If we were the proton we will see that the electron as a collapsed wavefunction always?
I think you draw the conclusion, based on various things you've read and heard, that "observation" implies "interaction by an observer", but at the same time parts of the quantym system presumable constantly "interact" with each other.

If you think that observation ~ interaction by observer, then your question makes sense, and it makes sense to draw the conclusion that the collapse is observer dependent. I think this is not bad understnading, but there are lots of problem with this, so it's also fair to say that it is wrong, but why?

It's because in QM all "observers" are assumed to share a common "macroscopic reality" where they can interact classically. This breaks down badly if we apply the observer to a part of the quantum system. For example if we consider a proton to be an observer. In terms of QM, as we know it, a proton can not apply the QM apparatous of preparation and acquiring statistics from it's electron interaction; AND then put that in the marcroscopic information that can be "copied" etc. IT just doesn't work, neither in theory nor in practice.

But then the confusion ahead of us is sill, what is the difference or scale between an observer making observations and a subsystme just interacting with another system? This is an open problem in QM, an the "inside formulation" of a theory of measurment that MIGHT MAYBE make sense of considering a proton as an observer, simply does not exist yet.

So if this is your core question, even if you can learn alot more by looking at some formal quantum texts, this "philosophical" or "conceptual" or "incoherencet reasoning" issue is an open question, and you will not find the answer to THAT in a standard QM textbook. The "best answer" at hand is probably decoherence, to say that everything in quanutm, but that has it's own problem and does not satisfy everyones taste.

I am asking myself the same question, and still searching for the answer.

/Fredrik
 
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