Undergrad Quantum Spin: Is it Random? Alice & Bob's Test

[DrChinese]

But if we say that measurement collapses the wavefunction, then what?

It doesn't matter, predicted results wouldn't be different.

If it doesn't help... don't choose it, for about the same reason that you generally choose not to poke yourself in the eye with a sharp stick.

The utilitarian viewpoint of scientific theory (which I usually advocate) in a nutshell!

 

[John_RB]

Suppose the two entangled objects have traveled far apart when the measurement on A is made in a laboratory on earth. Say particle B is approaching the vicinity of the moon when a signal is received requesting that a measurement is made on B. How is the orientation of particle B with respect to A known? Relative to the earth, the moon or the sun? I believe this experiment has been successfully completed over several hundred kilometres.

 

[Demystifier]

Suppose the two entangled objects have traveled far apart when the measurement on A is made in a laboratory on earth. Say particle B is approaching the vicinity of the moon when a signal is received requesting that a measurement is made on B. How is the orientation of particle B with respect to A known? Relative to the earth, the moon or the sun? I believe this experiment has been successfully completed over several hundred kilometres.

In non-relativistic QM the space is absolute, as in Newtonian mechanics. So its relative to the absolute space.

If relativistic effects were included, space would not be absolute, but spacetime would. This means that geometry of spacetime (the metric tensor) is defined everywhere, not with respect to other objects, but defined by itself. At a more advanced level, you may also wonder whether general relativity is consistent with the Mach principle (hint: no it isn't).

 

[PeterDonis]

At a more advanced level, you may also wonder whether general relativity is consistent with the Mach principle (hint: no it isn't).

This is a matter of opinion and not all physicists agree about it. In any case, discussion of it belongs in the relativity forum, not here.

 

[PeterDonis]

How is the orientation of particle B with respect to A known?

You would have to look at how the particular experiment is set up. No experiment will be able to determine the relative orientation perfectly.

In a general curved spacetime, there is no such thing as a unique "relative orientation" of spatially distant experiments. Over small enough distances, the non-uniqueness is small enough that it will be smaller than the other sources of error in the experiment so it does not need to be taken into account. This is true for experiments ranging over a few hundred kilometers on Earth. It might even be true for an experiment ranging from the Earth to the Moon.

 

[Demystifier]

In any case, discussion of it belongs in the relativity forum, not here.

I agree, but I have tried to anticipate what might be his next question.

 

[entropy1]

So when you talk about Alice's measurement giving a random result when that measurement occurs first, and then Bob's can't be random: you are talking AS IF the measurement outcomes were from completely separate and independent particles. You are in fact measuring a component/components of a combined system which is entangled.

Yes, but if you would assert that the latter (Bob's) measurement got collapsed, I figure we would have the same measurement result.

EDIT: I see you already pointed that out. So it is not decided whether B will collapse or not, which in my eyes suggests that QM will at this point realize both simultaneously!

 

[Nugatory]

Yes, but if you would assert that the latter (Bob's) measurement got collapsed, I figure we would have the same measurement result.

EDIT: I see you already pointed that out. So it is not decided whether B will collapse or not, which in my eyes suggests that QM will at this point realize both simultaneously!

You are choosing to use a collapse interpretation in a situation where it is known to work badly (because of the conflict between relativity and instantaneous collapse everywhere) and making the problem unnecessarily confusing. Just stop with this talk about collapse and look at what the math says.

(I feel as if I've said something similar before...)

 

[PeterDonis]

if you would assert that the latter (Bob's) measurement got collapsed

Then you would be having a discussion that belongs in the QM foundations and interpretations forum, not this one.