How does entangled particles communicate ?

[Phi for All]

!

Moderator Note

I'm happy to split any further off-topic posts to the Trash. This thread is in Quantum Theory.

[Eise]

The statement is stronger than that. There is 100% correlation of the spins, which are undetermined before the measurement. If the particles had a pre-determined spin before the measurement, you would get different results.

 

Until here I follow you, assuming that you mean that the measurements of the spins correlate 100%. Otherwise you must explain this more detailed.

 

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.

 

And here I am not sure. QM says that the spin of a particle is not determined until it is measured. Now if you say that the spin becomes down you suggest it has a determined spin. What does that mean: that a local variable in the particle flipped to down? That makes no sense of course. As far as I can see the only thing you can say according to Bell's theorem is that the measurements are correlated, not the particle themselves. I know that sounds absurd, but if you formulate it in terms of the particles themselves, you make your self 'guilty of local variables-talk'.

[swansont]

 

Until here I follow you, assuming that you mean that the measurements of the spins correlate 100%. Otherwise you must explain this more detailed.

 

 

And here I am not sure. QM says that the spin of a particle is not determined until it is measured. Now if you say that the spin becomes down you suggest it has a determined spin. What does that mean: that a local variable in the particle flipped to down? That makes no sense of course. As far as I can see the only thing you can say according to Bell's theorem is that the measurements are correlated, not the particle themselves. I know that sounds absurd, but if you formulate it in terms of the particles themselves, you make your self 'guilty of local variables-talk'.

 

Uh, no, there is no hidden variable talk here. The particle's spin is undetermined until you measure it. Once you measure it, the particle has that spin. If you measure it again, it will always have that same spin. I don't see how saying the spin is undetermined until measurement, which makes it have a certain spin, could possibly suggest a hidden variable scenario. Further, the entanglement means once we measure the spin of the first particle, we instantly know the spin of the second one — all of the information about the spins is in that one measurement.

 

 

If you want more detail on this, see http://www.scienceforums.net/topic/87347-why-hidden-variables-dont-work/

 

Also, the assumption in science is that what we measure is the real behavior. So measuring the particle to be spin down means the particle is spin down.

[Eise]

Swansont, I think I understand what you mean, but I am inclined to formulate your last sentence differently: measuring the particle spin down means that if I measure it again in the same direction as before, I will measure down again.

 

The italic part is important: if I ask you to measure the direction of the spin of a particle I give to you, but do not say in which direction I measured it, you have no way to find out that I measured spin down. Of course, if I give you thousands of particles, all measured down, you will find out: but not with one single particle.

[swansont]

Swansont, I think I understand what you mean, but I am inclined to formulate your last sentence differently: measuring the particle spin down means that if I measure it again in the same direction as before, I will measure down again.

 

The italic part is important: if I ask you to measure the direction of the spin of a particle I give to you, but do not say in which direction I measured it, you have no way to find out that I measured spin down. Of course, if I give you thousands of particles, all measured down, you will find out: but not with one single particle.

 

Yes, that's true, but it's also true that if you are traveling north and you measure again using a different coordinate system (i.e. you rotate your axes) you will get a different answer, too, so I don't think it's particularly insightful. It's also irrelevant to the question we were discussing, AFAICT.

 

It's also true that if you measure the spin along some other non-orthogonal direction, you will get a non-random distribution of up and down, just as one would expect for the spin having a determined value. As opposed to an undetermined spin, which would be random.

[Eise]

If you want more detail on this, see http://www.scienceforums.net/topic/87347-why-hidden-variables-dont-work/

As you called this link as your 'wittness a decharge' (supposing I do not understand entanglement?), I carefully listened to this great explanation. See for yourself what phrases he uses:

 

You immediately know what the same measurement of the other particle will be.

...

You will always measure opposite spin.

...

It is a though the choice of the first measurement has influenced the result of the second.

...

Now immediately you know that the other particle is spin down, if measured in the first direction.

 

And last but not least:

 

Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured, whereas other physicists believe that entangled particles can signal each other faster than light to update their hidden information when the one is measured.

 

If he is right, then you belong to the second group, that obvious believes that hidden information is changed (faster than light!) because of the measurement of the first particle.

 

Also, the assumption in science is that what we measure is the real behavior. So measuring the particle to be spin down means the particle is spin down.

Measuring real behaviour, means the spin is already there before it is measured, no?

Isn't the 'revolutionary aspect' of QM that it shows that this 'assumption in science' is not correct?

 

[swansont]

As you called this link as your 'wittness a decharge' (supposing I do not understand entanglement?), I carefully listened to this great explanation. See for yourself what phrases he uses:

 

 

And last but not least:

 

 

If he is right, then you belong to the second group, that obvious believes that hidden information is changed (faster than light!) because of the measurement of the first particle.

 

Measuring real behaviour, means the spin is already there before it is measured, no?

Isn't the 'revolutionary aspect' of QM that it shows that this 'assumption in science' is not correct?

 

 

Please don't tell me what I believe, and please don't insinuate that I am lying. When I said the state is undetermined before measurement and there is no hidden information, I did not mean the opposite. When I say that we are measuring the actual/real behavior and the spin is undetermined before the measurement, I meant that, too. Not the opposite. That means what we measure represents the actual state of the particle. A particle measured to be e.g. spin down is, in fact, spin down in that measurement bases, at the moment of the measurement and afterwards, until something interacts with the particle. Meaning the measurement is not "lying" to us. If we measure again, it will again be spin down and not spin up.

 

The simplest explanation here is that you have misunderstood the video. I have no desire to go through and transcribe the video, but I assure you, the QM view is not that particles have a hidden state. It's been tested and shown to not be true. It's inherently quantum behavior and not going to be deduced from non-quantum analysis.

 

The behavior I don't understand is asking people to "correct me when I am wrong" and then proceeding to argue when corrected.

[Theoretical]

Hi Eise,

 

Nice questions. Remember though this part of the forum is strictly QM. Swansont is giving you the correct QM. Although yes there is another interpretation of the experiments ... and well I'm not allowed to even so as mention any details except that IMO it's much more convincing than QM.

[Eise]

Please don't tell me what I believe, and please don't insinuate that I am lying.

I did not tell you what you believe: I wrote: 'If he is right...'.

 

The maker of the video suggests that there are 2 possible interpretations:

• it only makes sense to talk about spin once they are measured
• entangled particles can signal each other faster than light to update their hidden information when the one is measured

You told me the particle has a spin down as soon as the other is measured up:

 

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.

That excludes the first possibility. And it fits the second.

 

That means what we measure represents the actual state of the particle. A particle measured to be e.g. spin down is, in fact, spin down in that measurement bases, at the moment of the measurement and afterwards, until something interacts with the particle.

Of course the particle has spin down at the moment it is measured spin down. But you said it already has spin down before you measured it, on basis of the measurement of the other.

 

Meaning the measurement is not "lying" to us. If we measure again, it will again be spin down and not spin up.

 

Yes, every followup measurement in the same direction will show us a spin down. But if you say that the particle already has spin down, before every subsequent measurement, you are going one step too far.

 

Say, I measure the spin of just a single particle: it has spin down. Do you say then that it already had spin down before I measured it?

 

Now it turns out, I was fooled: it was in fact an entangled particle. Laughing the other observer comes into the room, and says to me: 'I knew you would measure spin down, because I measured spin up.' Do you say now that it already had spin down before I measured it?

But I was fooled again: the other particle was measured after I did my measurement. How does this change your position about the question of the particle already was spin down before you measured it?

 

 

The simplest explanation here is that you have misunderstood the video. I have no desire to go through and transcribe the video, but I assure you, the QM view is not that particles have a hidden state. It's been tested and shown to not be true. It's inherently quantum behavior and not going to be deduced from non-quantum analysis.

This citation is a literal transcription of the video:

 

Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured, whereas other physicists believe that entangled particles can signal each other faster than light to update their hidden information when the one is measured.

 

So I really do not understand you: you say that the spin becomes down at the moment the other particle is measured, but then you claim there is no hidden state before I measure the particle.

 

The behavior I don't understand is asking people to "correct me when I am wrong" and then proceeding to argue when corrected.

A correction must be convincing. Maybe you are right, but then you did not explain it well enough.

 

Can you show me where I make an error in my understanding of entanglement? Maybe that helps.

Edited December 27, 2015 by Eise

[Strange]

I did not tell you what you believe: I wrote: 'If he is right...'.

 

The maker of the video suggests that there are 2 possible interpretations:

• it only makes sense to talk about spin once they are measured
• entangled particles can signal each other faster than light to update their hidden information when the one is measured

You told me the particle has a spin down as soon as the other is measured up:

 

That excludes the first possibility. And it fits the second.

 

It fits both interpretations. That is why they are called "interpretations".

 

Once you have measured the spin, you know the spin of both particles (because they are a single system, so it only takes one measurement to know the spin of both).

[swansont]

I did not tell you what you believe: I wrote: 'If he is right...'.

 

The maker of the video suggests that there are 2 possible interpretations:

• it only makes sense to talk about spin once they are measured
• entangled particles can signal each other faster than light to update their hidden information when the one is measured

You told me the particle has a spin down as soon as the other is measured up:

 

 

That excludes the first possibility. And it fits the second.

 

 

No, actually, it doesn't.

 

 

Of course the particle has spin down at the moment it is measured spin down. But you said it already has spin down before you measured it, on basis of the measurement of the other.

When I said a particles spin is undetermined before the measurement, that means it is undetermined before the measurement and has no definite spin. I only discussed entangled pairs in one specific place, and specifically named them as particle 1 and particle 2. So I never said what you claim above. That's you misunderstanding the situation.

 

 

Yes, every followup measurement in the same direction will show us a spin down. But if you say that the particle already has spin down, before every subsequent measurement, you are going one step too far.

No, I am not. Once you have determined the spin, it has that spin in the measurement basis you have used.

 

Say, I measure the spin of just a single particle: it has spin down. Do you say then that it already had spin down before I measured it?

No. I have repeatedly said that the spin is undetermined before measurement, unless you have a way of knowing the spin already. How many times must I repeat this before you accept that I have said it?

 

 

 

Now it turns out, I was fooled: it was in fact an entangled particle. Laughing the other observer comes into the room, and says to me: 'I knew you would measure spin down, because I measured spin up.' Do you say now that it already had spin down before I measured it?

But I was fooled again: the other particle was measured after I did my measurement. How does this change your position about the question of the particle already was spin down before you measured it?

If the other person measured it first, then you particle was spin down at the moment of that other measurement. If you measure first, then it was undetermined.

 

This citation is a literal transcription of the video:

 

 

So I really do not understand you: you say that the spin becomes down at the moment the other particle is measured, but then you claim there is no hidden state before I measure the particle.

 

A correction must be convincing. Maybe you are right, but then you did not explain it well enough.

 

Can you show me where I make an error in my understanding of entanglement? Maybe that helps.

Maybe you have to stop insisting that things are wrong if you don't understand them. Maybe you need to study quantum mechanics some more so that you can meet me halfway. It's possible my explanation is lacking, but it's not only me you have misunderstood, it's the video. At some point you have to stop blaming the people explaining it to you. If you jump into the deep end of QM, it's not my fault if you can't swim.

[Eise]

Maybe you have to stop insisting that things are wrong if you don't understand them. Maybe you need to study quantum mechanics some more so that you can meet me halfway. It's possible my explanation is lacking, but it's not only me you have misunderstood, it's the video. At some point you have to stop blaming the people explaining it to you. If you jump into the deep end of QM, it's not my fault if you can't swim

But you are not explaining. Your posts here are nothing more than 'no you are wrong, I have studied physics, and it is like I say.'

 

If I give a transcription of a part of the video, and I give an argument based on that transcription, but then only only say 'No, actually, it doesn't' but do not say why, it is not much help.

 

When I said a particles spin is undetermined before the measurement, that means it is undetermined before the measurement and has no definite spin. I only discussed entangled pairs in one specific place, and specifically named them as particle 1 and particle 2. So I never said what you claim above. That's you misunderstanding the situation.

 

This is what you said:

 

The statement is stronger than that. There is 100% correlation of the spins, which are undetermined before the measurement. If the particles had a pre-determined spin before the measurement, you would get different results. 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.

Where are your particles 1 and 2? I think the ambiguity lies in 'the measurement'. So let me try to rephrase what you write above:

 

a. There is 100% correlation of the spins, which are undetermined before the measurement of particle 1.

Right, I understand that, with only this catch: there is 100% correlation between the measurements of the spins.

b. If the particles had a pre-determined spin before the measurement, you would get different results.

Right, that is what the Bell-inequality is all about.

c. So in essence, particle 2 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 measurements

And this I do not understand. The part I do understand is what you said in b.: the spins are not determined before I measured particle 1. The part I do not understand is that you claim that at the moment I measure the spin of particle 1 being up, the spin of particle 2 becomes down.

The reason is that I do not understand how you can know this. The only thing you know is that if you measure particle 2 in the same direction as particle 1, you will measure down.

 

The reason I say this is, that you cannot empirically distinguish, based on your measurement of particle 2 alone:

- if particle 1 was measured before you measured particle 2

- if particle 1 was measured after you measured particle 2

- if particle 1 was measured was measured at all

 

If you cannot distinguish these, then what sense does it make to say that 'particle 2 does become spin down when the other particle is measured to be spin up', when there is no way to find out when this happens?

It fits both interpretations. That is why they are called "interpretations".

 

Of course. But only one of the interpretations would suffice, isn't it? Both being correct interpretations is a kind of overkill. The problem I have with Swansont is that the video says:

 

Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured.

 

But then when I defend this position he says I am wrong. Can you help me out?

 

Once you have measured the spin, you know the spin of both particles (because they are a single system, so it only takes one measurement to know the spin of both).

 

Your remark between the brackets helps a little. But what is wrong, when I state more precisely that, after measuring particle 1, I know what spin I will measure with particle 2 in the same direction? What is the justification to say that it becomes spin down already before the measurement or particle 2?

[swansont]

But you are not explaining. Your posts here are nothing more than 'no you are wrong, I have studied physics, and it is like I say.'

 

If I give a transcription of a part of the video, and I give an argument based on that transcription, but then only only say 'No, actually, it doesn't' but do not say why, it is not much help.

 

 

This is what you said:

 

 

Where are your particles 1 and 2? I think the ambiguity lies in 'the measurement'. So let me try to rephrase what you write above:

 

a. There is 100% correlation of the spins, which are undetermined before the measurement of particle 1.

Right, I understand that, with only this catch: there is 100% correlation between the measurements of the spins.

b. If the particles had a pre-determined spin before the measurement, you would get different results.

Right, that is what the Bell-inequality is all about.

c. So in essence, particle 2 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 measurements

And this I do not understand. The part I do understand is what you said in b.: the spins are not determined before I measured particle 1. The part I do not understand is that you claim that at the moment I measure the spin of particle 1 being up, the spin of particle 2 becomes down.

The reason is that I do not understand how you can know this. The only thing you know is that if you measure particle 2 in the same direction as particle 1, you will measure down.

Yes, the same measurement basis is assumed in that statement. If you choose another basis, you will get a distribution of results that depend on the orientation. However, it is the distribution for a particle of determined spin being measured in another orientation, because probabilities are involved. So you would have to do the experiment multiple times to see this.

 

All of that is discussed in the video.

 

 

The reason I say this is, that you cannot empirically distinguish, based on your measurement of particle 2 alone:

- if particle 1 was measured before you measured particle 2

- if particle 1 was measured after you measured particle 2

- if particle 1 was measured was measured at all

 

If you cannot distinguish these, then what sense does it make to say that 'particle 2 does become spin down when the other particle is measured to be spin up', when there is no way to find out when this happens?

 

 

Of course. But only one of the interpretations would suffice, isn't it? Both being correct interpretations is a kind of overkill. The problem I have with Swansont is that the video says:

 

 

But then when I defend this position he says I am wrong. Can you help me out?

 

I can only find instances of that statement where you subsequently told me (incorrectly) what I believe. I don't see where you have defended that position.

 

Your remark between the brackets helps a little. But what is wrong, when I state more precisely that, after measuring particle 1, I know what spin I will measure with particle 2 in the same direction? What is the justification to say that it becomes spin down already before the measurement or particle 2?

Because angular momentum is conserved. Even in the hidden variable scenario, the spins have to have their anti-correlation.

 

Also, please not that there are more than these two scenarios: the QM picture and hidden variables are the two that Bell's inequality discusses whether the particles' spins are undetermined prior to measurement or if they have a determined value before measurement. The issue of communication is not that same issue.

[Eise]

Yes, the same measurement basis is assumed in that statement. If you choose another basis, you will get a distribution of results that depend on the orientation. However, it is the distribution for a particle of determined spin being measured in another orientation, because probabilities are involved. So you would have to do the experiment multiple times to see this.

 

All of that is discussed in the video.

 

Yes, I know that and I understand that. But it seems you have only read my last sentence, and then you just say something we are not discussing.

 

My point is that it is impossible to empirically distinguish between the situations where one measures the spin of particle 2 before or after the measurement of particle 1. That is the reason that I say that at the moment you measure particle 1, you know what will be measured at particle 2. That is a proposition that is independent of the order of measurements. Say I am the observer of particle 1, independently of exactly when you measured particle 2, I know you will have measured particle 2.

 

I can only find instances of that statement where you subsequently told me (incorrectly) what I believe. I don't see where you have defended that position.

 

Then you are not reading very well. Once again:

 

The video says, literally:

 

Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured.

I defend this point, and you say I am wrong. If I am wrong, then the video is wrong, or at least not precise enough.

 

E.g. one could change it as follows:

 

...it only makes sense to talk about spin once one of them is measured.

 

Is that what you mean?

 

Because angular momentum is conserved.

 

OK, I understand that.

 

Even in the hidden variable scenario, the spins have to have their anti-correlation.

 

So do you defend a hidden variable scenario? Or what do you mean by 'Even...'?

 

Also, please note that there are more than these two scenarios: the QM picture and hidden variables are the two that Bell's inequality discusses whether the particles' spins are undetermined prior to measurement or if they have a determined value before measurement. The issue of communication is not that same issue.

 

OK. But AFAIK the experiments done in Bell-like situations show at least that there are no local hidden variables.

 

And again you are unclear: you say the "particles' spins", and just 'measurement'. Do you mean I measure particle 1, and then the particles' spins are determined? And -BTW- by using the word 'determined' you introduce just another version of this unclarity:

• If you say that determined means that it is fixed that I will measure spin down at particle 2, I agree.
• If you say that determined means that the spin of particle 2 becomes down at the moment I measure spin up at particle 1, I do not agree, because there is no empirical way to find out when this happens. If I for example change the distance of my second measuring device, I just will always find the same correlation between measurements.

[swansont]

Yes, I know that and I understand that. But it seems you have only read my last sentence, and then you just say something we are not discussing.

 

I responded to something you have (repeatedly) written, so it seems that we are discussing it.

 

My point is that it is impossible to empirically distinguish between the situations where one measures the spin of particle 2 before or after the measurement of particle 1. That is the reason that I say that at the moment you measure particle 1, you know what will be measured at particle 2. That is a proposition that is independent of the order of measurements. Say I am the observer of particle 1, independently of exactly when you measured particle 2, I know you will have measured particle 2.

The order doesn't matter. You will still have the correlation.

 

 

Then you are not reading very well. Once again:

 

The video says, literally:

 

 

I defend this point, and you say I am wrong. If I am wrong, then the video is wrong, or at least not precise enough.

I will repeat: I do not see where you have defended that point. I see where you have quoted the video, but the dialog that follows has not been you defending that point, even in this response.

 

 

E.g. one could change it as follows:

 

...it only makes sense to talk about spin once one of them is measured.

 

 

Is that what you mean?

Both spins are known with the one measurement, so I don't see that the statements are different.

 

 

So do you defend a hidden variable scenario? Or what do you mean by 'Even...'?

I have NEVER "defended" the hidden variable scenario. Why do you keep saying this in various forms? I have repeatedly pointed out that it is wrong. However, the proposal exists, and one may analyze it and show that it is in fact wrong. Much like one might mention the purported properties of phlogiston without defending it as a valid theory, or acknowledge that a few people think there is a flat earth and discuss their reasoning. Analysis is not a defense of their position.

 

 

OK. But AFAIK the experiments done in Bell-like situations show at least that there are no local hidden variables.

Yes, precisely. The discussion of hidden variables in the Bell experiment does not address non-locality, i.e. faster-than-light communication. That is a separate issue.

 

And again you are unclear: you say the "particles' spins", and just 'measurement'. Do you mean I measure particle 1, and then the particles' spins are determined? And -BTW- by using the word 'determined' you introduce just another version of this unclarity:

• If you say that determined means that it is fixed that I will measure spin down at particle 2, I agree.
• If you say that determined means that the spin of particle 2 becomes down at the moment I measure spin up at particle 1, I do not agree, because there is no empirical way to find out when this happens. If I for example change the distance of my second measuring device, I just will always find the same correlation between measurements.

If the particles are entangled, then both spins are known as soon as either is measured.

 

And yes, you can determine when this happens, because you can eliminate the particles having a definite spin before the measurement. That's the whole point of the Bell experiment.

[Eise]

I will repeat: I do not see where you have defended that point. I see where you have quoted the video, but the dialog that follows has not been you defending that point, even in this response.

I did here:

 

the only thing you can say according to Bell's theorem is that the measurements are correlated, not the particle themselves.

The video says at this point:

 

it only makes sense to talk about spin once they are measured

So what is the difference?

 

 

And yes, you can determine when this happens, because you can eliminate the particles having a definite spin before the measurement. That's the whole point of the Bell experiment.

'eliminate the particles having a definite spin before the measurement'? No idea what you mean. Can you please explain?

[MigL]

Take a coin and split it down the middle such that one half is the 'head' part and the other half, the 'tail' part.

Place both halves in envelopes such that you don't know which went into which envelope.

 

Can you say anything about the state of the half in either envelope, Eise ?

Are their states not indeterminate ?

 

Now move one envelope a great distance from the other ( M-31 in Andromeda if you so desire ).

Now I perform an 'experiment' by opening the local envelope, and find the half coin to be in one particular state, 'heads' or 'tails'.

I then know the state of the other non-local half, as it is the opposite of the local state.

 

This is fully repeatable and implies no FTL communication.

And I realize this is a simplification of entanglement. but it is an example of t a correlation that doesn't force you to choose between pre-determined variables and communication.

[swansont]

I did here:

I don't see how

 

"the only thing you can say according to Bell's theorem is that the measurements are correlated, not the particle themselves."

 

is a defense of

 

"Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured."

 

 

As to the former, I addressed that several posts back.

 

 

The video says at this point:

 

 

So what is the difference?

As I stated before, I don't see a difference. So I'm the wrong one to ask.

 

 

'eliminate the particles having a definite spin before the measurement'? No idea what you mean. Can you please explain?

If the particles have a definite state you get one set of results. If they don't, you get different results. The experiments exclude the former case. If you want more details, perhaps you should peruse a thread on Bell tests, or start a new one, since this thread is about the purported communication of entangled particles, and we have been straying from that. But it's explained in the video I linked to.

[Eise]

Are their states not indeterminate ?

 

No. They are determinate. I only don't know which half is in which envelope. That is exactly the 'revolutionary' insight of the Bell-inequality: that no theory with local variables can reproduce the predictions of QM. And so even hidden variables won't do. In your example there are hidden local variables, so your explanation is worthless.

 

Sorry for my anger, but I get tired of Swansont (and now you) thinking that I do not understand the Bell theorem and its consequences, only because I give a different interpretation then he does. Possibly you mix me up with the OP.

I don't see how

 

"the only thing you can say according to Bell's theorem is that the measurements are correlated, not the particle themselves."

 

is a defense of

 

"Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured."

 

I did not say one was a defence of the other. I am saying that it is the viewpoint I defend.

 

As I stated before, I don't see a difference. So I'm the wrong one to ask.

 

Right. That means my viewpoint was valid all the time. Or the video is wrong, when it says:

 

Some physicists see them (Eise: i.e. results of experiments with entangled particles) as evidence that there is no hidden information in quantum particles, and it only makes sense to talk about spin once they are measured.

I already noticed that the key difference between us is what here is meant with 'once they are measured'. I interpret 'they' as plural, i.e. when both particles are measured. You say that the spin of one particle is measured, you know the spin of the other. But I see that as a logical consequence (because of the law of preservation of angular momentum), not as a measurement. Of course you can be sure as if you measured it directly, but you did not measure it. But you can be sure that if you measure the spin of particle 2, you will measure down, because you measured the other one up. So for all practical purposes, you can treat the particle as having spin down. But that does not mean it has spin down.

 

But as said before, you go one step further:

 

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.

 

You seem to know when particle 2 becomes down. (Of course I know the particle did not have those spins from the beginning. (Really, you seem to think I am stupid)). But I don't know how you can know when the spin of particle 2 becomes down, if there is no way to empirically verify this. If I change the distance of my 'particle 2 measurement' device from the source, the only thing I will ever notice is that the measurements are correlated, nothing else.

 

If the particles have a definite state you get one set of results. If they don't, you get different results. The experiments exclude the former case. If you want more details, perhaps you should peruse a thread on Bell tests, or start a new one, since this thread is about the purported communication of entangled particles, and we have been straying from that. But it's explained in the video I linked to.

 

I know that. I thought you meant something else, but was not sure.

 

The relevance for the topic is what Kris_o_O wrote here:

 

Ok this makes sense, so you entangle 2 particles and separate them one billion light years apart. measure one and it's spin up, how does the other particle become spin down instantaneous?

My point is that one does not know if it is instantaneous. We only know that the measurements correlate, so if you measure the spin of particle 1, you know what a measurement of the spin of particle 2 will be (or was).

Edited December 29, 2015 by Eise

[swansont]

Sorry for my anger, but I get tired of Swansont (and now you) thinking that I do not understand the Bell theorem and its consequences, only because I give a different interpretation then he does. Possibly you mix me up with the OP.

Perhaps because when you say "No idea what you mean.", it implies you don't understand.

 

 

I already noticed that the key difference between us is what here is meant with 'once they are measured'. I interpret 'they' as plural, i.e. when both particles are measured. You say that the spin of one particle is measured, you know the spin of the other. But I see that as a logical consequence (because of the law of preservation of angular momentum), not as a measurement. Of course you can be sure as if you measured it directly, but you did not measure it. But you can be sure that if you measure the spin of particle 2, you will measure down, because you measured the other one up. So for all practical purposes, you can treat the particle as having spin down. But that does not mean it has spin down.

Then this really has nothing to do with the Bell experiment, or even QM. It's the premise underlying all science that what we measure is the actual behavior, i.e. that nature is not tricking us.

 

 

But as said before, you go one step further:

 

 

You seem to know when particle 2 becomes down. (Of course I know the particle did not have those spins from the beginning. (Really, you seem to think I am stupid)). But I don't know how you can know when the spin of particle 2 becomes down, if there is no way to empirically verify this. If I change the distance of my 'particle 2 measurement' device from the source, the only thing I will ever notice is that the measurements are correlated, nothing else.

Since you're so conversant with the Bell test, perhaps you can figure it out. What happens to your results if the particle has a definite spin at the time of measurement?

 

I know that. I thought you meant something else, but was not sure.

You know that, and yet you persist in asking a question that suggests you do not.

 

My point is that one does not know if it is instantaneous. We only know that the measurements correlate, so if you measure the spin of particle 1, you know what a measurement of the spin of particle 2 will be (or was).

One could do an experiment where you try and measure the two at the same time, separated by a large distance. You could put an upper bound on the delay, and confirm if it's not instantaneous. And people have done such experiments.

[Eise]

Perhaps because when you say "No idea what you mean.", it implies you don't understand.

Perhaps because you do not express yourself clearly enough to be understood?

 

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

 

Then this really has nothing to do with the Bell experiment, or even QM. It's the premise underlying all science that what we measure is the actual behavior, i.e. that nature is not tricking us.

 

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.

 

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.

 

Since you're so conversant with the Bell test, perhaps you can figure it out. What happens to your results if the particle has a definite spin at the time of measurement?

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'?

 

One could do an experiment where you try and measure the two at the same time, separated by a large distance. You could put an upper bound on the delay, and confirm if it's not instantaneous. And people have done such experiments.

What upper bound of what delay?

Which experiments?

 

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

[Strange]

 

I already noticed that the key difference between us is what here is meant with 'once they are measured'. I interpret 'they' as plural, i.e. when both particles are measured. You say that the spin of one particle is measured, you know the spin of the other. But I see that as a logical consequence (because of the law of preservation of angular momentum), not as a measurement. Of course you can be sure as if you measured it directly, but you did not measure it. But you can be sure that if you measure the spin of particle 2, you will measure down, because you measured the other one up. So for all practical purposes, you can treat the particle as having spin down. But that does not mean it has spin down.

 

This is where you are going wrong. It only takes one measurement to make the spin of the pair determined because they are entangled.

 

That is what entangled means: both particles are described by a single wavefunction. Any measurement applies to the pair, not just to one. That is what theory tells us (and what EPR objected to). That is what Bell's theorem tests. Adn that is what experiment is consistent with.

 

If you want to invent a new form of quantum theory where this is the case, then good luck.

[Eise]

If you want to invent a new form of quantum theory where this is the case, then good luck.

I do not want to invent a new form of QM. I want to retract to the only point we can really know: that the measurements of particle 1 and particle 2 are correlated; that we cannot look behind the scenes of quantum measurements.

 

This is where you are going wrong. It only takes one measurement to make the spin of the pair determined because they are entangled.

I understand that, so I am not so clear where I say something that contradicts this. Maybe you can spell this out?

 

In my opinion, the problem lies in the word 'determined'.

 

If I measure particle 1, then I immediately know what a measurement of particle 1 will be (or has been, it could already have been measured before I measured mine. I can only know this if I compare lists of observations from measure station 1 and measure station 2).

 

So, as I see it, Swansont (and you?) on one side, and I on the other, understand different things under 'determined'. Say, particle 1 is measured first, and its spin is up.

 

- Then I say from that moment on I know what a future measurement on the spin of particle 2 will be: down.

- Swansont says: at that moment the spin of particle 2 becomes down.

 

I state that both are empirically equivalent, because any experiment to try to prove the second, implies a measurement. And that is exactly the first statement.

 

The same holds for single particle measurements, as I described above.

 

I try to translate this in English (Anton Zeilinger, Einsteins Spuk, page 208):

 

The other possibility would be to give up the image of a reality that exists independently of us in all its properties. That would mean that we, by our measurements, by our decision what to measure, have an essential influence on what can be real. There are hints that this is the better answer.

 

Then he refers to Kochen and Specker. (BTW, the first possibility Zeilinger mentions is to give up locality. Zeilinger notices that this option is the most popular under physicists...)

 

PS On reading about Zeilinger's professional work, I encountered (for the first time) the Leggett inequality. From Wikipedia:

 

The Leggett inequalities are violated by quantum mechanical theory. The results of experimental tests in 2007 and 2010 have shown agreement with quantum mechanics rather than the Leggett inequalities. Given that experimental tests of Bell's inequalities have ruled out local realism in quantum mechanics, the violation of Leggett's inequalities is considered to have falsified realism in quantum mechanics.

Edited December 29, 2015 by Eise

[Strange]

I understand that, so I am not so clear where I say something that contradicts this. Maybe you can spell this out?

 

You appeared to be saying that the spin of the second particle was undetermined until it was measured.

 

 

I state that both are empirically equivalent, because any experiment to try to prove the second, implies a measurement.

 

They might be empirically equivalent but one is consistent with the quantum theory and the other isn't.

[Eise]

You appeared to be saying that the spin of the second particle was undetermined until it was measured.

No. I am saying it makes no sense to talk about the moment that the spin of particle 2 becomes down.

 

They might be empirically equivalent but one is consistent with the quantum theory and the other isn't.

 

Which is inconsistent with QT, and why? Do you have empirical support for this?

Edited December 29, 2015 by Eise