# Collapse of wavefunction - how long it takes?

1. Jan 23, 2010

### paweld

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.

2. Jan 23, 2010

### Dmitry67

It is not physical, but informational process in collapse interpretations (CI, TI)
In non collapse interpretations there is no collapse at all.

3. Jan 24, 2010

### Demystifier

Instead of a collapse time, one can talk about the decoherence time. It is typically very short, much shorter than most clocks can measure.

4. Jan 24, 2010

### Dmitry67

... 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.

5. Jan 24, 2010

### f95toli

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,

6. Jan 24, 2010

### Fredrik

Staff Emeritus
The measurement results are correlated like that even when the separation is spacelike (so that different observers disagree about which measurement was first).

7. Jan 25, 2010

### Demystifier

What you are talking about is a variant of the EPR "paradox".

Yes it is.

Yes, measurements of that sort have been performed many times. They all confirm the existence of nonlocal quantum correlations.

No, the correlation can be observed even for simultaneous measurements.

8. Jan 25, 2010

### paweld

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.

9. Jan 25, 2010

### SpectraCat

10. Jan 25, 2010

### Demystifier

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

Last edited by a moderator: May 4, 2017
11. Jan 25, 2010

### paweld

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.

Last edited by a moderator: May 4, 2017
12. Jan 25, 2010

### Demystifier

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. Jan 25, 2010

### paweld

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. Jan 25, 2010

### f95toli

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. Jan 25, 2010

### paweld

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.

16. Jan 25, 2010

### f95toli

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. Jan 25, 2010

### Fredrik

Staff Emeritus
As f95toli mentioned, there's no way to use this to send even one bit of information, so there's no conflict with SR.

Experiments disagree with you. (I don't know which experiments, but I'm sure someone else does).

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. Jan 25, 2010

### f95toli

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)?

19. Jan 26, 2010

### Demystifier

Yes he does..

By the way, I met Bertlmann some 10 years ago and he really wears socks of different colors. :tongue2:

20. Jan 26, 2010

### DrChinese

Awesome story!