What causes wavefunction collapse?

  • #26
Fra
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We haven't adequately shown, to my understanding, that the predictive qualities of QM exist outside of measurement.
Not sure what you mean my predictive qualities without measurement. Assuming you aren't speaking of realism, then I agree in this sense:

What we do not yet have, is to show that the ACTION of any composite system, can be EXPLAINED from inferencial first principles in terms of thinking of the interactions as the parts performing measurements on each other, and responding accordingly. This is the idea behind the rational action I mentioned. But this is a conjecture and/or interpretation, it remains to see in the future if explicit unification of actions can be found in this way.

So far, we just come up with a classical hamiltonian or lagrangian and "quantize it" as per some ad hoc rule. This clearly is deeply unsatisfactory.

If so, it would be an indirect confirmation of "measurements as interactions" without actually performing the measurement would be exactly that: if we can perform measurements after some duration of a composite system, then if our predicttions are right (ie if the action/hamiltonian following from rational action is right) then it means the abstraction of explaing physical interaction terms of "negotating observers performing measuremetns on each other" has predictive value. So when worked out, this idea is potentially falsifiable, thus rendering it more than just a pure interpretation.

But this what I suggest is not a restoration of realism at all. It's rather attacking the realism of ensembles and statistics, which only makes sense for small subsystems, where the statistics is encoded in a classical environment and all classical external observers can agree. But this is a special case. And it does not give hints to unification of interactions.

/Fredrik
 
  • #27
K^2
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Fra, when you divide the universe into observer and observed, yes, it all falls into places. But that sort of division only naturally follows from MWI. In Copenhagen, there is no apparent reason to it.
JordanL said:
It's my understanding that this assumes realism holds, which Bell's theorem tells us it might not.

We haven't adequately shown, to my understanding, that the predictive qualities of QM exist outside of measurement.
Predictive powers of QM need not hold outside of measurement. If you can't measure it, it does not matter whether our description of it is consistent with actual dynamics, as long as all observables agree with prediction.

That's kind of the whole point of QM. We assume the dynamics of whatever's actually there is linear. Then we replace whatever's actually there with wave function and pretend it is real, but there is absolutely no reason it should be, and it doesn't matter if it is. As long as superposition principle holds, we're good.
 
  • #28
Fra
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Fra, when you divide the universe into observer and observed, yes, it all falls into places. But that sort of division only naturally follows from MWI.
I'm not a MWI fan, but most that talk about MWI talk about non-interacting branches. Ie. truly multiverses.

The picture I tried to paint, is with interacting views! That's the whole point, otherwise it gets empty. So there are many INTERACTING observers, not many non-interacting worlds (whatever that even means, I don't know).

/Fredrik
 
  • #29
K^2
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From perspective of one observer, another is just a quantum system. Consider Shroedinger's Cat. The cat is an observer, yet from perspective of outside observer, it does not collapse the wave function of the radioactive atom. It merely goes into an entangled superposition state with it. From perspective of the cat, the atom's wave function has clearly collapsed.

And before you ask, yes, we have experiments that confirm this behavior. Look at Quantum Eraser.
 
  • #30
Fra
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142
The cat is an observer, yet from perspective of outside observer, it does not collapse the wave function of the radioactive atom. It merely goes into an entangled superposition state with it. From perspective of the cat, the atom's wave function has clearly collapsed.
Yes, agreed. As long as the outside observer performs no measurement, the cat-atom "complex" is expected to evolve in unitarily.

But my point is that there is no conflict in this. We can't compare the views of the cat and the views of the outside observer and think they don't match. The merging of two views, requires them to be communicated to the same observer sort of like we need to parallelltransport vectors from one tangent space to another one before we can even define the concept of angles.

The picture is similar, except that there is no objective "transformation" from observer to observer. (Except for special cases! for example SR observers).

Instead this transformation is a physical process, that takes time and according to my conjecture may help explain interactions, rather than cause problems.

/Fredrik
 
  • #31
314
18
The cat is an observer, yet from perspective of outside observer, it does not collapse the wave function of the radioactive atom. It merely goes into an entangled superposition state with it.
That is provided the cat is perfectly isolated from the outside environment, which is not usually the case and which would be extremely hard/impossible to achieve for macroscopic objects of reasonable size.
Penrose makes an argument that any macroscopic object in superposition would necesserily interact with other objects through gravity. And we don't have http://en.wikipedia.org/wiki/The_First_Men_in_the_Moon" [Broken] to make the box out of.
 
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  • #32
K^2
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Delta Kilo, the argument that there is no way to isolate the system is not particularly appealing, since we can always improve the isolation and reduce time scales.

But my point is that there is no conflict in this. We can't compare the views of the cat and the views of the outside observer and think they don't match. The merging of two views, requires them to be communicated to the same observer sort of like we need to parallelltransport vectors from one tangent space to another one before we can even define the concept of angles.
That's fine, but again, you are looking at collapse as a matter-of-perspective thing, and that's basically MWI.
 
  • #33
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K^2: What I meant was that "realism" as discussed in EPR is, to my understanding, defined as predictability without measurement, (or at least without precise, definitive measurement).

It's my understanding that this is why Einstein was never much of a fan of QM in the first place, because he felt it pointed to an inherent unrealism of our universe.

But if I am misinformed, please correct me. That's why I'm here. :)
 
  • #34
DrChinese
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K^2: What I meant was that "realism" as discussed in EPR is, to my understanding, defined as predictability without measurement, (or at least without precise, definitive measurement).
The EPR elements of reality, yes that is as good a definition as you can get. If you can predict, without disturbing the system, the outcome of a measurement, then there should be a physical element of reality associated with the observable. A lot of people prefer other definitions, for reasons that elude me. Kinda like slicing hairs to me. But the EPR definition is strong enough to have survived anyway.
 

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