How does entangled particles communicate ?

[swansont]

Where I always make a clear distinction between 'measuring of particle 1' and 'measuring of particle 2', you just say 'measurement'.

In the case of entanglement, though, you only need to measure once to know both spin states, so that distinction is not necessary.

 

Of course nature is not tricking us. If I measure the spin of one single particle (no EPR situation), and it is down, then it really was down at the moment of measurement. But to say it was therefore down all the time is an empirically empty statement.

But we can tell if it was spin down before the measurement if we do a series of experiments and look at the statistics. We will get different results than if there was no defined spin before the measurement.

 

If the particle was spin up with a vertical detector, we will get spin up 100% of the time with that measurement basis. If we place the detector in some other orientation, the probability of getting up will vary as cos^2(theta/2). If the particle does not have a defined spin, we will get up only half the time.

 

Now this measurement has a consequence: for every subsequent measurement, I know that I will measure spin down again and again. This suggests what you are stating all the time: that the particle has spin down after my first measurement. But what I am saying is that I only know that there is a 100% correlation between the measurements. To say that the particle has spin down means that an independent observer can determine its spin without knowing its measurement history. And that is impossible: if I don't know that it was measured vertically, there is no way I find out that it has spin down when measured vertically. So I can only conclude that the measurements correlate, but empirically there is nothing special with the particle for any independent observer.

An independent observer will not get results consistent with the spin being undetermined. These experiments have been done — measuring entangled particles at nearly the same time, at a distance great enough so there can be no communication between the people.

 

See? How can I answer such a question if you do not say which measurement in which situation? What is the operational definition of 'having a definite spin'?

A particle whose spin was measured but is unknown, for example, has a definite spin.

 

What upper bound of what delay?

Which experiments?

http://www.gizmag.com/quantum-entanglement-speed-10000-faster-light/26587/

 

 

How do these experiments decide between 'a particle becoming spin down' and 'being sure that spin down will be measured'?

Explained above

[MigL]

So we have two entangled particles.

We interact with particle 1 to determine its spin, yet you say it makes no sense to talk about when the spin of particle 2 becomes determinate until we actually measure it. Even though we know what it will be determined to be when we do measure.

 

I don't want to speak for Swansont and Strange, but we seem to be in agreement that interacting with particle 1 is equivalent to interacting with particle 2, simply because that IS entanglement.

 

P.S. Don't get angry, make a better argument !

Edited December 30, 2015 by MigL

[Itoero]

but we seem to be in agreement that interacting with particle 1 is equivalent to interacting with particle 2, simply because that IS entanglement.

That's imo true and not true.

The effect is the same, but the system that leads to that effect is not...it's opposite I think.

Quantum teleportation works between entangled particles.

To teleport a particle (from entangled particle 1 to 2 for example)collapses the entanglement/wave just like measuring does.

Basically, in entanglement, the correlation in measurements does not imply causation and teleportation is about causation....while they use the same 'frame'.

The reason why it seems that interacting with particle 1 is equivalent to interacting with particle 2 is because you don't insert extra info which can teleport.

 

This is what I think.

Edited December 30, 2015 by Itoero

[Delta1212]

I think the confusion here is that Eise thinks that a theory of local hidden variables being false means that you can never have a determined state, when in fact the debate was over whether the particles always had a determined state with the apparent indeterminacy being the result of ignorance of the variables that determine the state rather than an intrinsic determinant vs only sometimes being in a determined state with the times that the state is indeterminate being the result of the state actually being undetermined rather than just being the result of ignorance on our part about what the state is.

 

Saying that a property can have a determined state is not the same thing as a local hidden variable theory, which is how I think Eise has been interpreting it, if I'm reading the thread correctly.

[reasonmclucus]

You might try to find information on a theory that entangled particles are connected through some other physical dimension of reality. I don't know if there is a formal theory or not but I have seen this suggestion made before. Such a connection might be analogous to the theoretical "short cut" through space called a "worm hole".

 

Our ability to understand the issue of other dimensions is hampered by our early indoctrination to the view that the dimensions of physical reality are length, width and height. Those of us who are lucky eventually are introduced to the idea of time as a dimension of reality.

 

Quantum physics has the concept of parallel universes which might be the equivalent of boxes within a box.involving a larger dimension. Science fiction stories sometimes deal with the possibility of moving from one "box" to another.

 

It would be conceivable that someone could develop a theory showing how particles that are distant in our 3-dimensional world could be connected in some other type of dimensional configuration. I don't know what the physical characteristics of such dimensions would be but it would be plausible mathematically.

[MigL]

Maybe 'equivalent' wasn't the right choice of wording.

The interaction/experiment/observatioin collapses the wave function.

The wave function is common to both entangled particles.

Collapsing it in one 'place' collapses it everywhere, including the second particle.

The fact that we haven't formally 'observed' the state of the second particle doesn't make it any less true.

[Eise]

A particle whose spin was measured but is unknown, for example, has a definite spin.

 

Only if you know in what direction it was measured. If I send particles to you, one by one, and sometimes I measure their spin, sometimes I do, but I do not say in which direction I measured, you cannot distinguish between which particles I have measured, and which I didn't. So having a 'definite spin' is not intrinsically for the particle: it is at most intrinsically to the complete experimental setup.

 

In EPR-like experiments, if I vary the distance of the particle source, I cannot decide on my measurements of particle 2 alone if I measured the spin first, or if the measurement of particle 1 was first. Even if I compare my list of measurements of particle 2 afterwards with the list of measurements of particle 1, I cannot decide which was measured first: I only see their correlation. If I cannot decide this, what sense does it make to speak of the moment one of the particles becomes spin down?

 

An independent observer will not get results consistent with the spin being undetermined. These experiments have been done — measuring entangled particles at nearly the same time, at a distance great enough so there can be no communication between the people.

No. It is only when comparing the measurements, that I can see that the measurements are correlated. And that this correlation is faster than light. But I cannot say when particle 2 became down. Only when particle 1 is measured first, I can say that this measurement determined the measurement of particle 2.

I think the confusion here is that Eise thinks that a theory of local hidden variables being false means that you can never have a determined state, when in fact the debate was over whether the particles always had a determined state with the apparent indeterminacy being the result of ignorance of the variables that determine the state rather than an intrinsic determinant vs only sometimes being in a determined state with the times that the state is indeterminate being the result of the state actually being undetermined rather than just being the result of ignorance on our part about what the state is.

 

Saying that a property can have a determined state is not the same thing as a local hidden variable theory, which is how I think Eise has been interpreting it, if I'm reading the thread correctly.

That is an interesting point... But I must confess I have seriously troubles to understand your first paragraph, which is in fact one single sentence. I would be glad if you could formulate your point a bit more extensive and clear. (Really! I am glad somebody tries to understand where the confusion lies.)

 

But on your second sentence: I think this 'determinate state' is context dependent. If I know that particle's 1 spin is measured vertically, and it is up, then I know, when I measure also in the vertical direction, that I will measure down. Is that what you mean with a determinate state?

 

But then, when I don't know in what direction particle 1 was measured, how can I find out if particle 2 is in a determined state? What is a determined state when it is dependent on the measuring context?

 

Swansont says I can find out by measuring many particles (which he forgot I already mentioned before), but that is not the point in EPR-like situations (if it was, and I was using photons, I could use my polaroid sunglasses to prove E, P and R were wrong...).

Edited December 31, 2015 by Eise

[swansont]

Only if you know in what direction it was measured. If I send particles to you, one by one, and sometimes I measure their spin, sometimes I do, but I do not say in which direction I measured, you cannot distinguish between which particles I have measured, and which I didn't. So having a 'definite spin' is not intrinsically for the particle: it is at most intrinsically to the complete experimental setup.

Statistically I can tell if you had measured particles to give them a definite spin. I never claimed it was an intrinsic property of the particle. You have to measure it somehow. Of course it depends on experimental setup.

 

 

In EPR-like experiments, if I vary the distance of the particle source, I cannot decide on my measurements of particle 2 alone if I measured the spin first, or if the measurement of particle 1 was first. Even if I compare my list of measurements of particle 2 afterwards with the list of measurements of particle 1, I cannot decide which was measured first: I only see their correlation. If I cannot decide this, what sense does it make to speak of the moment one of the particles becomes spin down?

Because you can do experiments where you test this. That you choose to a different experiment doesn't change that.

 

No. It is only when comparing the measurements, that I can see that the measurements are correlated. And that this correlation is faster than light. But I cannot say when particle 2 became down. Only when particle 1 is measured first, I can say that this measurement determined the measurement of particle 2.

And you can do that measurement arbitrarily soon after that of particle 1.

 

That is an interesting point... But I must confess I have seriously troubles to understand your first paragraph, which is in fact one single sentence. I would be glad if you could formulate your point a bit more extensive and clear. (Really! I am glad somebody tries to understand where the confusion lies.)

 

But on your second sentence: I think this 'determinate state' is context dependent. If I know that particle's 1 spin is measured vertically, and it is up, then I know, when I measure also in the vertical direction, that I will measure down. Is that what you mean with a determinate state?

 

But then, when I don't know in what direction particle 1 was measured, how can I find out if particle 2 is in a determined state? What is a determined state when it is dependent on the measuring context?

You can't with a single pair. You need to do multiple measurements. We've been over this before.

 

Swansont says I can find out by measuring many particles (which he forgot I already mentioned before), but that is not the point in EPR-like situations (if it was, and I was using photons, I could use my polaroid sunglasses to prove E, P and R were wrong...).

That's rather presumptuous. Who are you to say what the point of an EPR-like situation is? If one were doing a test of this, multiple measurements would be exactly the point of the experiment.

[Eise]

I never claimed it was an intrinsic property of the particle.

 

I think you did, when you said that the spin of the particle becomes down. If you mean with 'becoming down' that 'will measure down from then on' I agree with you. Otherwise I do not.

 

Because you can do experiments where you test this. That you choose to a different experiment doesn't change that.

 

And you can do that measurement arbitrarily soon after that of particle 1.

 

You can't with a single pair. You need to do multiple measurements. We've been over this before.

 

Exactly. But you cannot give an experimental setup, that justifies your statement that a single particle becomes spin down at the moment its entangled counterpart is measured. You can only say that from the moment its counterpart is measured having spin up, it is determined that it will be measured spin down.

[swansont]

I think you did, when you said that the spin of the particle becomes down. If you mean with 'becoming down' that 'will measure down from then on' I agree with you. Otherwise I do not.

 

That's not an intrinsic property, and I never used the word intrinsic.

 

Spin is an intrinsic property. The value of the spin projection is not (other than spin zero). It can be changed, so how can it be?

 

Exactly. But you cannot give an experimental setup, that justifies your statement that a single particle becomes spin down at the moment its entangled counterpart is measured. You can only say that from the moment its counterpart is measured having spin up, it is determined that it will be measured spin down.

Because we can do the experiments to confirm this. It's always spin down in that basis. If measured in another basis, the statistics are consistent with it being spin down in the original basis. Plus, angular momentum is conserved. Do you have any scientific reasoning to buttress your position? Bald assertion (even repeated bald assertion) doesn't count for anything.

[Eise]

Because we can do the experiments to confirm this. It's always spin down in that basis. If measured in another basis, the statistics are consistent with it being spin down in the original basis. Plus, angular momentum is conserved. Do you have any scientific reasoning to buttress your position? Bald assertion (even repeated bald assertion) doesn't count for anything.

 

Please mention the experiment(s), with a link please.

[swansont]

 

Please mention the experiment(s), with a link please.

 

 

So, that's a "no" for my question?

 

Citations are in the references

https://en.wikipedia.org/wiki/Bell_test_experiments

[Eise]

So, that's a "no" for my question?

No. I found one myself:

 

We report on a new kind of experimental investigations of the tension between quantum nonlocally and relativity. Entangled photons are sent via an optical fiber network to two villages near Geneva, separated by more than 10 km where they are analyzed by interferometers. The photon pair source is set as precisely as possible in the center so that the two photons arrive at the detectors within a time interval of less than 5 ps (corresponding to a path length difference of less than 1 mm). One detector is set in motion so that both detectors, each in its own inertial reference frame, are first to do the measurement! The data always reproduces the quantum correlations, making it thus more difficult to consider the projection postulate as a compact description of real collapses of the wave-function.

 

Simply said: if the detectors are at nearly the same distance from the source, then an inertial frame can be found in which the order of detection can be opposite, then in the laboratory frame. So I have no idea how you can say what the moment is that particle 2 becomes spin down when the other one is measured.

 

https://en.wikipedia.org/wiki/Bell_test_experiments

These are just tests of the violation of the Bell-inequalities by QM. That is not our discussion.

Our discussion is the question if it is justified to say:

 

So in essence, the particle does become spin down when the other particle is measured to be spin up, because we know the specific spins could not have been in place before the measurement.

 

(Bold by me)

 

Given the experiment I linked to, even I was wrong: according to this experiment it is not even justified to say that the measurement of particle 1 determines from that moment on what the measurement of the particle 2 will be, because an inertial frame can be found in which the order of measurements was opposite. The only thing we can conclude is that the measurements are correlated, nothing more.

Edited January 1, 2016 by Eise

[swansont]

No. I found one myself:

 

 

Simply said: if the detectors are at nearly the same distance from the source, then an inertial frame can be found in which the order of detection can be opposite, then in the laboratory frame. So I have no idea how you can say what the moment is that particle 2 becomes spin down when the other one is measured.

 

How does that experiment contradict what I said? Not confirming is not the same as contradicting — a physics experiment is designed to test specific aspects of physics, not all of them. The experiment is consistent with what I was saying.

 

These are just tests of the violation of the Bell-inequalities by QM. That is not our discussion.

Well, yes it is. You don't appear to recognize it, though. Stop moving the goalposts around. You asked several things. It's disingenuous to take an answer to one question and complain that it's not the answer to another one. Bell tests are all about whether the particle has a definite spin, and further, you asked me to provide experiments in response to

 

"Because we can do the experiments to confirm this. It's always spin down in that basis. If measured in another basis, the statistics are consistent with it being spin down in the original basis. Plus, angular momentum is conserved."

 

Several (if not all) of those experiments show precisely this. And it's how we know entangled spins were not determined before the measurement.

 

Our discussion is the question if it is justified to say:

 

 

(Bold by me)

 

Given the experiment I linked to, even I was wrong: according to this experiment it is not even justified to say that the measurement of particle 1 determines from that moment on what the measurement of the particle 2 will be, because an inertial frame can be found in which the order of measurements was opposite. The only thing we can conclude is that the measurements are correlated, nothing more.

That they are correlated, and the way they are correlated, tells us a lot. Does the article claim to violate the Bell test (such that there are hidden variables)? No, it doesn't.

 

So, in summary: Bell experiments tell us that the spins are not determined before measurement. Conservation of angular momentum and experimental confirmation tell us that the spins are determined when any particle is measured.

[Eise]

That they are correlated, and the way they are correlated, tells us a lot. Does the article claim to violate the Bell test (such that there are hidden variables)? No, it doesn't.

Did I suggest something else? This is tiresome, Swansont. I did not present this article as proof that that the other experiments are wrong or something. It shows that the correlations as predicted by QM are correct, even when viewed from different inertial frames.

 

So, in summary: Bell experiments tell us that the spins are not determined before measurement.

 

Right. I fully agree and never denied it. Somehow you seem to think I do.

 

Conservation of angular momentum and experimental confirmation tell us that the spins are determined when any particle is measured.

Right. I even agree with this.

 

But the statement above is more general than the one you originally did:

 

So in essence, the particle does become spin down when the other particle is measured to be spin up.

Let's try once again:

• detector 1 measures the spin of particle 1 as spin up
• detector 2 measures the spin of particle 2 as spin down

• From inertial frame A particle 1 is measured first: so according to you from that moment on the spin of particle 2 becomes spin down. This of course also means that it becomes spin down in front of detector 2.

• From inertial frame B particle 2 is measured first: so according to you from that moment on the spin of particle 1 becomes spin up. This of course also means that it becomes spin up in front of detector 1.

This means that in a. the measurement of particle 1 determines the spin of particle 2, but in b. measurement 2 determines the spin of particle 1. In my eyes that makes no sense, as from both inertial frames we look at the same experiment. Therefore I conclude that the only thing we are justified to say is that the measurements are correlated, not that one determines the other.

 

So, now instead of just saying I am wrong, or do not understand QM or Bell-experiments, say what my confusion is, and why clarification of this confusion leads to the justification of your statement that particle 2 becomes down when the measurement of particle 1 is measured up.

 

I think I am pretty precise in why I think what I think, so I would appreciate it if you make some effort in showing where according to you my error lies.

 

[swansont]

Did I suggest something else? This is tiresome, Swansont. I did not present this article as proof that that the other experiments are wrong or something. It shows that the correlations as predicted by QM are correct, even when viewed from different inertial frames.

Yes, you did suggest something else. You suggested that we can't tell if a particle has a definite spin before detection. This experiment contradicts that claim. The correlations predicted by QM are that there are no local hidden variables. That there is no definite spin before detection. i.e. the opposite of what you had suggested.

 

 

Right. I fully agree and never denied it. Somehow you seem to think I do.

Oh, bullshit. You keep saying there is no way to tell when the particle attains the spin it was measured to have. Since it has to have that spin once we've measured it, the only other option is that it already had that spin. And if it's an entangled pair, we can measure the second particle to confirm its spin (though we know it already)with an arbitrarily short delay between measurements, so we can confirm that it has its spin at that time. So again, the only other option is that it already had that spin before the measurement. Which is excluded by the Bell test.

 

So if you agree with that, why is it you have insisted that we can't tell when a particle attains the spin we measure it to have? What is the other option here?

 

Let's try once again:

• detector 1 measures the spin of particle 1 as spin up
• detector 2 measures the spin of particle 2 as spin down

• From inertial frame A particle 1 is measured first: so according to you from that moment on the spin of particle 2 becomes spin down. This of course also means that it becomes spin down in front of detector 2.

• From inertial frame B particle 2 is measured first: so according to you from that moment on the spin of particle 1 becomes spin up. This of course also means that it becomes spin up in front of detector 1.

This means that in a. the measurement of particle 1 determines the spin of particle 2, but in b. measurement 2 determines the spin of particle 1. In my eyes that makes no sense, as from both inertial frames we look at the same experiment. Therefore I conclude that the only thing we are justified to say is that the measurements are correlated, not that one determines the other.

 

It doesn't matter which one is measured first. That's the whole crux of the weirdness of entanglement.

 

We already know that the results are acausal — any purported signal that might have been sent between particles would have to exceed c. From what I can tell, what this experiment shows is that same effect but in a different way.

 

So, now instead of just saying I am wrong, or do not understand QM or Bell-experiments, say what my confusion is, and why clarification of this confusion leads to the justification of your statement that particle 2 becomes down when the measurement of particle 1 is measured up.

 

I think I am pretty precise in why I think what I think, so I would appreciate it if you make some effort in showing where according to you my error lies.

The experiment is cast in terms of the particles' frames. The observer is in only one frame, and once the observer measures one spin, s/he has determined both spins. This experiment is demonstrating a separate effect, as I just explained.

[Eise]

So Wikipedia is wrong here (hey, of course Wikipedia can be wrong, I know that).

The distance and timing of the measurements can be chosen so as to make the interval between the two measurements spacelike, i.e. from any of the two measuring events to the other a message would have to travel faster than light. Then, according to the principles of special relativity, it is not in fact possible for any information to travel between two such measuring events—it is not even possible to say which of the measurements came first, as this would depend on the inertial system of the observer. Therefore the correlation between the two measurements cannot appropriately be explained as one measurement determining the other: different observers would disagree about the role of cause and effect.

Italic and bold by me.

If you think I am saying something differently let me know. Or correct the Wikipedia article. Or explain to me what the difference is between determining and causing.

 

('To determine' has a gross ambiguity: it can mean, in daily context, 'to find out, to ascertain', or 'to settle, to direct'. Maybe we should take care in what we mean here, because it seems as if in QM both meanings are practically the same...)

Since it has to have that spin once we've measured it, the only other option is that it already had that spin.

Hmmmm. I think this is the point where we disagree. Classically you're surely right.

 

I'll think over the rest of your post. I might come back at some points.

So if you agree with that, why is it you have insisted that we can't tell when a particle attains the spin we measure it to have? What is the other option here?

That we do not know.

 

The only thing we know is that the particles had no definite spin before the measurement of one of these particles: that is ruled out by Bell's theorem. What we do know is what we will measure if we already happen to know what was measured at detector 1. And there is a gap between these two, which we cannot fill up, even if we can make the gap as small as you want.

Edited January 2, 2016 by Eise

[swansont]

So Wikipedia is wrong here (hey, of course Wikipedia can be wrong, I know that).

 

 

Italic and bold by me.

Different definition of determine. You neglected to italicize the sentence following that clarifies the details.

 

 

 

If you think I am saying something differently let me know. Or correct the Wikipedia article. Or explain to me what the difference is between determining and causing.

To determine: to ascertain or establish as opposed to to cause (def 2 vs def 1 in my dictionary) I can determine that a coin is heads up by looking at it, but I did not cause it to be heads up by looking at it. The measurement in QM only removes the superposition.

 

('To determine' has a gross ambiguity: it can mean, in daily context, 'to find out, to ascertain', or 'to settle, to direct'. Maybe we should take care in what we mean here, because it seems as if in QM both meanings are practically the same...)

No, as I pointed out above.

 

The only thing we know is that the particles had no definite spin before the measurement of one of these particles: that is ruled out by Bell's theorem. What we do know is what we will measure if we already happen to know what was measured at detector 1. And there is a gap between these two, which we cannot fill up, even if we can make the gap as small as you want.

Precisely. And that precludes a claim that we don't know when it had a definite spin, it only limits the precision with which we can make it, but that's true of all experimental physics. And the theory, of course, removes that issue.

[Eise]

Precisely. And that precludes a claim that we don't know when it had a definite spin, it only limits the precision with which we can make it, but that's true of all experimental physics.

OK. I understand that.

 

For the rest you are so short in your answers, that it does not help me to understand the points you are making.

 

Maybe somebody else can explain this? How can we say that from one frame of reference event 1 determines event 2, but from another that event 2 determines event 1?

[swansont]

OK. I understand that.

 

For the rest you are so short in your answers, that it does not help me to understand the points you are making.

When you claim to understand something, I take you at your word. Bust such claims make it hard to identify what it is that you need explained to you.

 

 

Maybe somebody else can explain this? How can we say that from one frame of reference event 1 determines event 2, but from another that event 2 determines event 1?

That would be relativity. Time is relative to the frame in which the observer is in. And in this context it might be better to simply say "precede", since "determine" seems to be causing problems, and to bring it up in an appropriate thread.

[Eise]

Because I feel treated as a stupid moron here, I asked the question on another (German, sorry) physics forum. There, one of the moderators answered me in the following way.

 

First he gave me link to this article, in which following phrases can be found (you can check if I have taken them out of context, but I do not believe so):

 

When does the co-collapse of an entangled particle occur?
...
This makes explicit that a measurement on one particle does not at all influence the other one.
...
If the measurement is nothing but an isolated event in space time, there is no point whatsoever in trying to associate a spatial slice to it.
...
the only place where something physical happens is the place of the measurement

 

After I explained Swansont's line of argument he reacted with the remark that one can say it like this (... the particle becomes spin down at the moment the other particle is measured), but that it is an unfortunate expression prone to lead to misunderstanding because it implies a wrong picture of what is happening.

 

I also happily see that you changed 'determine' in 'precede'. That takes at least some of the misunderstanding away.

Edited January 4, 2016 by Eise

[swansont]

Because I feel treated as a stupid moron here, I asked the question on another (German, sorry) physics forum. There, one of the moderators answered me in the following way.

 

First he gave me link to this article, in which following phrases can be found (you can check if I have taken them out of context, but I do not believe so):

 

 

After I explained Swansont's line of argument he reacted with the remark that one can say it like this (... the particle becomes spin down at the moment the other particle is measured), but that it is an unfortunate expression prone to lead to misunderstanding because it implies a wrong picture of what is happening.

 

I also happily see that you changed 'determine' in 'precede'. That takes at least some of the misunderstanding away.

 

The context is that in delayed choice experiments, as well as the relativity-related experiment, is that there is a simultaneity issued that is not present in the original discussion. I had not anticipated that the conversation would go in that direction. Precede is a better term, yes, but then again, I never used "determine" in this context anyway. I used "undetermined" a number of times, but not in any way that I would think would be construed with causality. I thought I was clear that there was not a causal relationship involved (considering that this would require information having to exceed c)

 

Did you ask about the statement that a single observer knows the spins of both particles as the result of the measurement of either one?