# Collapse of wavefunction - how long it takes?

## Main Question or Discussion Point

Let's consider the following desitegration $$\pi^0 \rightarrow e^+ + e^-$$. 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.

Demystifier
Gold Member
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.

f95toli
Gold Member
... 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,

Fredrik
Staff Emeritus
Gold Member
Let's consider the following desitegration $$\pi^0 \rightarrow e^+ + e^-$$. 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).

Demystifier
Gold Member
Let's consider the following desitegration $$\pi^0 \rightarrow e^+ + e^-$$. 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".

Is it still true.
Yes it is.

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.

Maybe we observe the correlation only because the two measurements aren't simultaneous.
No, the correlation can be observed even for simultaneous measurements.

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.

SpectraCat
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.

Demystifier
Gold Member
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 [Broken]

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See e.g.
http://prl.aps.org/abstract/PRL/v47/i7/p460_1 [Broken]
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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Demystifier
Gold Member
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?

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.

f95toli
Gold Member
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.

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 untill some time pass. So we can't predict the measurement conducted there before some time elapsed.

f95toli
Gold Member
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.

Fredrik
Staff Emeritus
Gold Member
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.

So I assume that the wavefunction of the system of particles in that part of universe will remain unchanged untill 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).

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.

f95toli
Gold Member
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 refering 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)?

Demystifier
Gold Member
Indeed, but I was refering 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. :tongue2:

DrChinese