Einstein and "spooky actions"

[Mr Spinkles]

Yes she can know from two factors: her own measurement and conservation of angular momentum. So she knows that WHEN Bob will make his measurement what his observation will be. But knowing that doesn't affect Bob's particle, no more that my knowledge of the sun rising tomorrow in the East will affect that event.

Note: Bob's particle is in the singlet only until he makes the measurement, after that, it is no longer in that state, but the down spin.

No, zaybu. When Alice measures one particle and its state changes from singlet to spin-up (or down), she also knows the state of the other particle changes from singlet to spin-down (or up). That is why Alice knows, after her measurement, that any future measurement of the other particle will get spin-down with 100%, not 50% certainty. That's the whole point of entanglement.

Alice's observation is that her particle has an up spin. How's that incompatible?

But Alice isn't simply measuring her particle all by itself. Alice is measuring one part of an entangled system of particles, namely two particles in the singlet state. Write out the singlet state (correctly). It's the sum of two eigenkets, (up/down) and (down/up). When Alice measures the first particle as spin-up, she knows the combined two-particle state has "collapsed" into the eigenstate (up/down). That eigenstate means the first particle is spin-up AND the second particle is spin-down. In other words, neither particle is in the singlet state anymore. That is why Alice knows future measurements of the second particle will get spin-down, with 100% certainty.

Do you still not realize you are wrong here? Would math help you, or not?

 

[zaybu]

No, zaybu. When Alice measures one particle and its state changes from singlet to spin-up (or down), she also knows the state of the other particle changes from singlet to spin-down (or up). That is why Alice knows, after her measurement, that any future measurement of the other particle will get spin-down with 100%, not 50% certainty. That's the whole point of entanglement.

I agree with that, she "knows... any future measurement".

What I didn't agree before is that you said in post # 219, she "has changed the state of the other particle". These are completely different statements.

http://www.religiousforums.com/forum/3328575-post210.html

She knows what will be Bob's measurement, but her knowledge has no effect on the outcome at Bob's end. Can you get that straight? I am running out of patience, repeating the same thing over and over.

 

[PolyHedral]

Alice's observation is that her particle has an up spin. How's that incompatible?

No, zaybu. When Alice measures one particle and its state changes from singlet to spin-up (or down), she also knows the state of the other particle changes from singlet to spin-down (or up). That is why Alice knows, after her measurement, that any future measurement of the other particle will get spin-down with 100%, not 50% certainty. That's the whole point of entanglement.

That, essentially. According to you, Alice's measurement does not disturb Bob's particle, which is still in the singlet state. Fine.
But Bob's particle being in a singlet state implies Bob's measurement will return two possible values - Alice knows this is not true.

Alice and Bob now disagree over the state of what's in reality one particle. That makes no sense. Both Alice and Bob (and Carol, Dave and Emma) saw everything that went into the pair's preperation, so how they can possibly disagree about what state its in?

 

[zaybu]

That, essentially. According to you, Alice's measurement does not disturb Bob's particle, which is still in the singlet state. Fine.
But Bob's particle being in a singlet state implies Bob's measurement will return two possible values - Alice knows this is not true.

No, Bob's measurement will give only one result, not two. Sure, as long he hasn't made the measurement, there are two possibilities as far as he is concerned. And he won't know until he does make that measurement. OTOH, Alice knows what will be that measurement, but unless she can send a signal faster than light, she can't make Bob aware of that result.

The error that Spinkles is making is that he believes that Alice's knowledge of Bob's measurement will affect Bob's particle, which is totally false. My knowledge of the sun rising tomorrow in the East won't affect that event.

 

[Mr Spinkles]

I agree with that, she "knows... any future measurement".

What I didn't agree before is that you said in post # 219, she "has changed the state of the other particle". These are completely different statements.

They are different statements, but they are related and they are both true.

To see this, ask yourself the following questions:

(1) Before Alice's measurement: What is the quantum state of the particles? Answer: 1/sqrt(2) x [(up/down) + (down/up)]. What is the spin of the first particle? Undefined/unknown. What is the spin of the second particle? Undefined/unknown.

(2) After Alice's measurement: What is the state of the particles? Answer: 50% of the time, Alice gets (up/down). If that's what she gets, then what is the spin of the first particle? Up. What is the spin of the second particle? Down. The other 50% of the time, Alice gets (down/up). In that case, what is the spin of the first particle? Down. What is the spin of the second particle? Up.

This is why Alice knows with 100% certainty what a future measurement of the other particle will be (whether the measurement is done by Bob, Alice, or anyone). Because she knows the other particle is NO LONGER in the singlet state, which is an entangled state consisting of the sum of two eigenstates. Alice knows that the second particle, after her measurement, is now in just ONE of those spin eigenstates (and by looking at what she measured she knows which eigenstate it is in).

Do you disagree with this? It is elementary QM.

She knows what will be Bob's measurement, but her knowledge has no effect on the outcome at Bob's end.

Let's not get ahead of ourselves. We don't even need to think about Bob yet since you are still confused about what happens when Alice is all by herself. Let's just say, for the moment, that Alice measures the first particle, and then considers what the state of both particles is afterward.

 

[zaybu]

They are different statements, but they are related and they are both true.

To see this, ask yourself the following questions:

(1) Before Alice's measurement: What is the quantum state of the particles? Answer: 1/sqrt(2) x [(up/down) + (down/up)]. What is the spin of the first particle? Undefined/unknown. What is the spin of the second particle? Undefined/unknown.

(2) After Alice's measurement: What is the state of the particles? Answer: 50% of the time, Alice gets (up/down). If that's what she gets, then what is the spin of the first particle? Up. What is the spin of the second particle? Down. The other 50% of the time, Alice gets (down/up). In that case, what is the spin of the first particle? Down. What is the spin of the second particle? Up.

This is why Alice knows with 100% certainty what a future measurement of the other particle will be (whether the measurement is done by Bob, Alice, or anyone). Because she knows the other particle is NO LONGER in the singlet state, which is an entangled state consisting of the sum of two eigenstates. Alice knows that the second particle, after her measurement, is now in just ONE of those spin eigenstates (and by looking at what she measured she knows which eigenstate it is in).

Do you disagree with this? It is elementary QM.

YES. WE DON'T DISAGREE ON THIS. CAN YOU GET THIS STRAIGHT?

You don't even understand where we disagree. I have to conclude that I'm talking way over your head.

 

[Mr Spinkles]

Btw Polyhedral I believe your thought-experiment about different relativistic frames doesn't actually lead to any contradictions.

The error that Spinkles is making is that he believes that Alice's knowledge of Bob's measurement will affect Bob's particle, which is totally false.

Alice's measurement does affect both particles. However, you are right to suggest that nothing allows Bob to know that the state of his particle has changed from a singlet state to an eigenstate, since he measures the same thing (50% up, 50% down) in either case. It is not until after Alice and Bob get together and compare their measurements that they realize and agree Alice WAS affecting the state of Bob's particle. This is a subtlety which cannot be appreciated unless you understand elementary QM, and you can patiently and carefully go through the logic; and thus far you have not demonstrated that you can do this.

 

[PolyHedral]

No, Bob's measurement will give only one result, not two. Sure, as long he hasn't made the measurement, there are two possibilities as far as he is concerned. And he won't know until he does make that measurement. OTOH, Alice knows what will be that measurement, but unless she can send a signal faster than light, she can't make Bob aware of that result.

So, after Alice's measurement, what state is the particle actually in? It can't be singlet superposition, because Alice knows that Bob will always measure the opposite of what she did. It also can't be an eigenstate, because there's no time for Alice's measurement to have an effect on Bob's, and since Bob's particle was originally in the superpositional state, it should still be.

Btw Polyhedral I believe your thought-experiment about different relativistic frames doesn't actually lead to any contradictions.

The contradiction is in different people disagreeing about what states the particles are in, which I don't think is allowed within QM.

 

[Mr Spinkles]

YES. WE DON'T DISAGREE ON THIS. CAN YOU GET THIS STRAIGHT?

Okay. So, if you agree with all of that, then you must agree that when Alice made her measurement, she has indeed "changed the state of the other particle". Correct?

It seems obvious but I have to ask, because in your previous post, you denied it.

 

[zaybu]

Alice WAS affecting the state of Bob's particle.

You can't justify this in anyway. And as such, it is just speculation on your part. Knowledge of a state does not affect the state of particle. If it did, it would be absolutely, totally ridiculous. You might as well believe in voodoo magic.

 

[Mr Spinkles]

The contradiction is in different people disagreeing about what states the particles are in, which I don't think is allowed within QM.

No in QM they only have to agree on the outcomes of measurements. I think both you and zaybu may be confused by the word "state", which strictly refers to the quantum state of a particle, but is sometimes used loosely to refer to the outcome of a measurement or to refer to only those states which are eigenstates.

 

[Mr Spinkles]

Alice WAS affecting the state of Bob's particle.

You can't justify this in anyway. And as such, it is just speculation on your part. Knowledge of a state does not affect the state of particle. If it did, it would be absolutely, totally ridiculous.

It's not "knowledge" per se that affects the state of a particle, but the physical interaction which we call measurement. But again let's not get ahead of ourselves, you're still struggling with basic concepts. Let's review what you screamed your agreement to in all caps in post #226:

(1) Before Alice's measurement: ... What is the spin of the second particle? Undefined/unknown.

(2) After Alice's measurement: ... 50% of the time ... What is the spin of the second particle? Down. ... The other 50% of the time ... What is the spin of the second particle? Up.

Ergo, 100% of the time, Alice's measurement changes the state of the second particle. According to what you screamed in all caps. Yes or no?

 

[zaybu]

No in QM they only have to agree on the outcomes of measurements. I think both you and zaybu may be confused by the word "state", which strictly refers to the quantum state of a particle, but is sometimes used loosely to refer to the outcome of a measurement or to refer to only those states which are eigenstates.

There's no confusion about the meaning of a state. The disagreemernt is that you believe that having knowledge of a state will affect the state of a particle, and that's not what QM says. Your belief lies outside the framework of QM. The burden of proof on your shoulder to prove that belief. Not mine.

 

[Mr Spinkles]

There's no confusion about the meaning of a state. The disagreemernt is that you believe that having knowledge of a state will affect the state of a particle, and that's not what QM says.

Let's first establish what zaybu says, before worrying about what QM says. You have contradicted yourself several times now, let's see if you can clear up the contradiction. See my previous post, #232.

 

[zaybu]

Ergo, 100% of the time, Alice's measurement changes the state of the second particle. According to what you screamed in all caps. Yes or no?

You can repeat that ad infinitum but it won't make it true. The second particle will turn up only if Bob measures it. What if he doesn't measure??? Remember, measuring interfers with the state of the particle. No measurement, no interference, and that second particle stays as singlet. So her knowledge of her own measurement does NOT change the state of the second particle.

 

[idav]

Let's first establish what zaybu says, before worrying about what QM says. You have contradicted yourself several times now, let's see if you can clear up the contradiction. See my previous post, #232.

Pardon me, just trying to follow along. From what I gather, zaybu takes issue with your statement that particle 1 affects the state of the particle 2. When all that is happening is the first measurement tells you the second measurement by process of elimination, which is what zaybu agreed with.

 

[Mr Spinkles]

So her knowledge of her own measurement does NOT change the state of the second particle.

So, when you shrieked agreement in all caps in post #226, you were mistaken? You don't actually agree?

 

[zaybu]

So, when you shrieked agreement in all caps in post #226, you were mistaken? You don't actually agree?

You don't seem to get it. In post 226, in which I agree, you don't mention anywhere that Alice's knowledge affect the second particle. That is why I agree with that post. You still don't understand our disagreement. I have absolutely no problem with Alice knowing that Bob will measure a spin up. Can you get that for once and for all? It's your claim that her knowledge will affect the second particle that I disagree with.

BTW, I'm still waiting your answer to post 235.

 

[Mr Spinkles]

Pardon me, just trying to follow along. From what I gather, zaybu takes issue with your statement that particle 1 affects the state of the particle 2.

When they are in an entangled state this can happen can, yes. Why do you think it's called "entanglement"?

When all that is happening is the first measurement tells you the second measurement by process of elimination, which is what zaybu agreed with.

That's not "all that is happening" according to QM. Measurement changes the state of the system measured, in general. When you measure the spin of one particle in a singlet state, it is no longer in a singlet state, and therefore the other particle cannot be in that state, either. Zaybu angrily shouted agreement with this, now he's not sure.

 

[Mr Spinkles]

You don't seem to get it. In post 226, in which I agree, you don't mention anywhere that Alice's knowledge affect the second particle. That is why I agree with that . You still don't understand our disagreement. I have absolutely no problem with Alice knowing that Bob will measure a spin up. Can you get that for once and for all? It's your claim that her knowledge will affect the second particle that I disagree with.

Where did I claim "her knowledge" affects anything? If you stand by what you shouted in all caps, then you must agree that Alice's measurement changes the state of the second particle. That is, if she measures, then the second particle will have a state that is different from what its state was before she measured. Do you agree, or not?