When does entangled collapse happen?

[Eelco]

I have a question I havnt been able to figure out by reading or googling:

If we have an entangled pair of particles, and we measure one, thus collapsing its wavefunction, 'when' does the other particle collapse?

'Instantly' begs the question of 'in which reference frame?' For instance, 'instantly' as seen from their combined center of mass.

Is this experimentally measurable? Or is this an altogether meaningless question? That is, all that matters and all you ever know is that the particles have consistent histories?

Im tending towards the latter conclusion, but I am not sure.

 

[nismaratwork]

For clarification, are you talking about the destruction of the entangled state, or wavefunction collapse or both? In addition, have you looked at the concept of decoherence at all?

 

[Eelco]

Both, now that you mention it, but I was thinking of purely collapse originally.

I am superficially aware of decoherence, but I am unsure how it would answer my question. If it does, can you sketch in a few sentences how?

 

[Tomsk]

I think your latter conclusion is right. If you have two entangled particles A and B, and you measure A, nothing actually travels from B to you. So there isn't really any sort of speed involved. You gain some information from A, which tells you about B, but only because you already knew A and B were entangled. The other thing is, wavefunction collapse is an observer dependent phenomenon. If you make a measurement on A, and let's say I am spatially separated from you, then I don't gain the information about A that you did, so I can't say that A's wavefunction has collapsed.

Another thing is, it is only possible to say that a measurement happened at time t with a probability P(t): http://arxiv.org/abs/quant-ph/9802020

edit: Also I'm not sure decoherence is necessary, since the result here is independent of any sort of mechanism or explanation of wavefunction collapse.

 

[Eelco]

The reason why I asked is that it might make objective versus subjective wavefunction collapse an experimental matter; but I am not sure even if it should.

On a related note: decoherence doesn't make a lot of sense to me as a solution to the measurement problem (and many people dispute it is). The Schrödinger equation is fundamentally dissipative, even in a vacuum. If there is never any kind of objective contraction of the wavefunction, how is it that there is still anything resembling locality in this universe after billions of years?

Purely based on this though experiment, id say objective collapse or some other anti-diffusive process must be required to make sense of the world, or am I missing something? Or is there a generalized theorem that says objective and subjective collapse are experimentally indistinguishable anyway?

 

[Demystifier]

I have a question I havnt been able to figure out by reading or googling:

If we have an entangled pair of particles, and we measure one, thus collapsing its wavefunction, 'when' does the other particle collapse?

'Instantly' begs the question of 'in which reference frame?' For instance, 'instantly' as seen from their combined center of mass.

Is this experimentally measurable? Or is this an altogether meaningless question? That is, all that matters and all you ever know is that the particles have consistent histories?

Im tending towards the latter conclusion, but I am not sure.

There are many different interpretations of QM, and each provides different answers to these questions. Thus, there is no unique answer.

Personally, I like interpretations in which the collapse does not occur at all. Instead, decoherence combined with some additional assumptions makes an illusion of collapse. (Examples of such interpretations are Bohmian and many-world interpretations.)

 

[Demystifier]

On a related note: decoherence doesn't make a lot of sense to me as a solution to the measurement problem (and many people dispute it is). The Schrödinger equation is fundamentally dissipative, even in a vacuum. If there is never any kind of objective contraction of the wavefunction, how is it that there is still anything resembling locality in this universe after billions of years?

Decoherence does not cause an objective contraction, but it does cause something similar. It causes an objective split of wave function into many branches, each of which is contracted. Decoherence does not happen in the vacuum, but it happens when the system interacts with MANY particles in the environment. What decoherence does not explain is how only one of these many branches is picked out. Possible explanations are provided by the Bohmian and many-world interpretations.

 

[zonde]

If we have an entangled pair of particles, and we measure one, thus collapsing its wavefunction, 'when' does the other particle collapse?

'Instantly' begs the question of 'in which reference frame?' For instance, 'instantly' as seen from their combined center of mass.

Is this experimentally measurable? Or is this an altogether meaningless question? That is, all that matters and all you ever know is that the particles have consistent histories?

Im tending towards the latter conclusion, but I am not sure.

In case of entanglement wavefunction collapse is just an update of information.
I think most illustrative are entanglement swapping experiments where entanglement between two (non-entangled) particles is determined by joined measurement of they entangled partners.

 

[DrChinese]

If there is never any kind of objective contraction of the wavefunction, how is it that there is still anything resembling locality in this universe after billions of years?

What makes you think that there is?

The Bohmians say that the hidden variables are in the present, and they are non-local.

Of course, the determinists think the hidden variables are in the past somewhere (although Bell seems to rule this out). And there are some that think the hidden variables reside in the future, which would be non-locality of a different kind.