Collapse of wavefunction - how long it takes?

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Discussion Overview

The discussion centers around the concept of wavefunction collapse, particularly in the context of quantum entanglement and the measurement of particle properties, such as spin. Participants explore various interpretations of wavefunction collapse, the implications of nonlocality, and the time it takes for quantum states to decohere or collapse, with references to specific experiments and theoretical implications.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question whether the correlation of spins between entangled particles remains valid when they are far apart, suggesting that measurements may not be simultaneous.
  • Others argue that non-collapse interpretations of quantum mechanics do not involve a collapse at all.
  • A distinction is made between collapse time and decoherence time, with some noting that decoherence can occur very quickly, while others provide examples where it takes longer, such as in superconductive systems.
  • Participants discuss the implications of measurements confirming nonlocal quantum correlations, with some expressing skepticism about the nature of nonlocality and its experimental validation.
  • There is a debate about whether the collapse of the wavefunction can be considered instantaneous or if it propagates with a finite velocity, with some suggesting that this has implications for momentum conservation.
  • Some participants reference the EPR paradox and Bell's theorem, discussing how these concepts relate to the observed correlations in quantum experiments.
  • Concerns are raised about the interpretation of experimental results and whether they definitively prove or disprove the nature of wavefunction collapse.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of wavefunction collapse and nonlocality, with no clear consensus reached. Some agree on the existence of nonlocal correlations, while others remain skeptical and question the interpretations of experimental results.

Contextual Notes

Participants highlight limitations in current understanding, such as the dependence on definitions of collapse and the challenges in measuring the time of collapse or decoherence accurately. There are also references to specific experiments that may not fully address the questions raised about instantaneous collapse.

Who May Find This Useful

This discussion may be of interest to those studying quantum mechanics, particularly in the areas of entanglement, wavefunction collapse, and interpretations of quantum theory.

paweld
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Let's consider the following desitegration [tex]\pi^0 \rightarrow e^+ + e^-[/tex]. If measure the spin of electron we know the spin of positron but what if particles are far away. Is it still true. Does anyone measure the correlation of spins if the particles are really far off. Maybe we observe the correlation only because the two measurements aren't simultaneous.
 
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It is not physical, but informational process in collapse interpretations (CI, TI)
In non collapse interpretations there is no collapse at all.
 
Instead of a collapse time, one can talk about the decoherence time. It is typically very short, much shorter than most clocks can measure.
 
... but not always, for example, in the experiment with a superconductive ring with 2 opposite currents in suerposition. Because of the superconductivity, it can take minutes for the superposition to dissapear.
 
Dmitry67 said:
... but not always, for example, in the experiment with a superconductive ring with 2 opposite currents in suerposition. Because of the superconductivity, it can take minutes for the superposition to dissapear.

No, in flux qubits (which is what you are describing) the coherence times are typically tens of nanoseconds, maybe a microsecond or two at most.

But a microsecond is still quite a a long time (most experiments are done using microwave electronics so the "time resolution" of the experiment is less than a ns) so this is still reasonably easy to measure,
 
paweld said:
Let's consider the following desitegration [tex]\pi^0 \rightarrow e^+ + e^-[/tex]. If measure the spin of electron we know the spin of positron but what if particles are far away. Is it still true.
The measurement results are correlated like that even when the separation is spacelike (so that different observers disagree about which measurement was first).
 
paweld said:
Let's consider the following desitegration [tex]\pi^0 \rightarrow e^+ + e^-[/tex]. If measure the spin of electron we know the spin of positron but what if particles are far away.
What you are talking about is a variant of the EPR "paradox".

paweld said:
Is it still true.
Yes it is.

paweld said:
Does anyone measure the correlation of spins if the particles are really far off.
Yes, measurements of that sort have been performed many times. They all confirm the existence of nonlocal quantum correlations.

paweld said:
Maybe we observe the correlation only because the two measurements aren't simultaneous.
No, the correlation can be observed even for simultaneous measurements.
 
Demystifier said:
Yes, measurements of that sort have been performed many times. They all confirm the existence of nonlocal quantum correlations.
Could you give some examples. I wonder how people try to measure it. I simply still quite scepticall about it. Nonlocality seems to be too peculiar for me. Thanks.
 
paweld said:
Could you give some examples. I wonder how people try to measure it. I simply still quite scepticall about it. Nonlocality seems to be too peculiar for me. Thanks.

Google "Einstein-Podolsky-Rosen Paradox", or simply EPR paradox. Google "Bell's Inequalilty" and "Bell's theorem" too while you're at it. Lotsa good stuff to read. You are not the only one confused/concerned/skeptical about it. :smile:
 
  • #10
paweld said:
Could you give some examples. I wonder how people try to measure it. I simply still quite scepticall about it. Nonlocality seems to be too peculiar for me. Thanks.
See e.g.
http://prl.aps.org/abstract/PRL/v47/i7/p460_1
 
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  • #11
Demystifier said:
See e.g.
http://prl.aps.org/abstract/PRL/v47/i7/p460_1

This experiment shows only that Bell's inequality isn't fulfilled and quantum predictions are correct. But I think that one can't say that it proves that collapsion of wavefunction of two photons emited in singlet state is immidiate. They measure time difference only when distance between two polarizers was 6.5 m and in this situation the possible delay of wavefunction collapsion is about 0.2 ns. I don't think they could measure it - they neither prove nor disprove it! In my opinion one should increase the distance between the places of measurement to make this possible phenomena measurable.
 
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  • #12
paweld said:
But I think that one can't say that it proves that collapsion of wavefunction of two photons emited in singlet state is immidiate.
Do you think that it proves that the collapse happens with a velocity faster than the velocity of light?
And if you do, do you agree that then there is a Lorentz frame in which it is instantaneous?
 
  • #13
I don't know what should I think. If collapsion is not instantaneous e.g. momentum conservation is broken. On the other hand if it is instantaneous, for some observer the collapsion will be before measurement. Probably momentum and other quantities are only approximatelly conserved and if we masure something in one part of a system collapsion will be propagating with finite veliocity to different parts of the system and only after some time the measurements conducted there will confirm for example conservation of momentum.
 
  • #14
paweld said:
collapsion will be propagating with finite veliocity to different parts of the system and only after some time the measurements conducted there will confirm for example conservation of momentum.

You can't think of a "propagating collapse", at least not if QM is correct.
Firstly, the whole concept of a "instantaneous collapse" as described in pop-sci is flawed for reasons that have already been described in this thread and others.
Secondly, remember that we are talking about properties of the system here; we can't talk about individual particles in the classical sense until after the measurement.
 
  • #15
f95toli said:
we can't talk about individual particles in the classical sense until after the measurement.

But we can measure e.g. spin of one particle and as far as quantum mechanics is correct we know without measurement the spin of second particle (if they were in singlet state). So teoreticaly even if second particle is in other part of the universe we know its state - I don't think it's possible. So I assume that the wavefunction of the system of particles in that part of universe will remain unchanged until some time pass. So we can't predict the measurement conducted there before some time elapsed.
 
  • #16
Have you ever heard of Bertlmann's socks?

Also, remember that we can NOT use entanglement to transfer information faster than light, meaning none of these experiments require FTL communication to work.
Hence, there is no violation of SR if that is what you are worried about.
 
  • #17
paweld said:
So teoreticaly even if second particle is in other part of the universe we know its state - I don't think it's possible.
As f95toli mentioned, there's no way to use this to send even one bit of information, so there's no conflict with SR.

paweld said:
So I assume that the wavefunction of the system of particles in that part of universe will remain unchanged until some time pass. So we can't predict the measurement conducted there before some time elapsed.
Experiments disagree with you. (I don't know which experiments, but I'm sure someone else does).

f95toli said:
Have you ever heard of Bertlmann's socks?
I would say that that's the incorrect description of what's going on in an EPR experiment, and that it's disproved by Bell inequality violations.
 
  • #18
Fredrik said:
I would say that that's the incorrect description of what's going on in an EPR experiment, and that it's disproved by Bell inequality violations.

Indeed, but I was referring to the Bell's paper "Bertlmann's socks and the Nature of Reality", where he discusses the EPR paradox.
I haven't read the paper in years, but doesn't he explain quite well why the correlations seen in QM are quite different from "classical" correlations (such as the socks)?
 
  • #19
f95toli said:
Indeed, but I was referring to the Bell's paper "Bertlmann's socks and the Nature of Reality", where he discusses the EPR paradox.
I haven't read the paper in years, but doesn't he explain quite well why the correlations seen in QM are quite different from "classical" correlations (such as the socks)?
Yes he does..

By the way, I met Bertlmann some 10 years ago and he really wears socks of different colors. :-p
 
  • #20
Demystifier said:
Yes he does..

By the way, I met Bertlmann some 10 years ago and he really wears socks of different colors. :-p

Awesome story!
 

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