Einstein and "spooky actions"

[zaybu]

I wouldn't worry about assumptions in my mind, I would focus on the plain meaning of words on the page. The words on the page I wrote said Alice's measurement changes the state of both particles, from singlet to something else, without even considering additional measurements by Bob. You angrily shouted agreement. Were you wrong? Yes or no?

This is the post I had agreed: http://www.religiousforums.com/forum/3328827-post226.html

And here is the full post:

Originally Posted by Mr Spinkles
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.

MY ANSWER

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.

Where is it written in that post of yours: Alice's measurement changes the state of both particles?

 

[PolyHedral]

Right. That is what is meant by "nonlocality" in quantum mechanics.

And, as mentioned, it produces a paradox when you have relativistic observers moving around. (What state is Bob's particle really in if different observers disagree?)

 

[Mr Spinkles]

This is the post I agree: http://www.religiousforums.com/forum/3328827-post226.html

And here is the full post:

MY ANSWER

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

Where is it written in that post of yours: Alice's measurement changes the state of both particles?

If you would read with as much enthusiasm as you scream, you would have the answer to your question. Please carefully read the parts highlighted in red, below:

(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.

 

[zaybu]

If you would read with as much enthusiasm as you scream, you would have the answer to your question. Please carefully read the parts highlighted in red, below:
Originally Posted by Mr Spinkles
(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.

So I am right, you never wrote Alice's measurement changes the state of both particles. But that's what was on your mind. And then you assumed that I had agreed to what was on your mind.

 

[zaybu]

That sounds like you're saying there's some "reality" underlying the wavefunction which allows the WF to be relative with POV.

So what is it?

I'm glad that you spotted that. Indeed, using the old language of "wave is real", or "wave collapse", you get tied up into weird interpretation. It's better to get rid of this baggage that comes from an era when QM was still on the pioneer frontier, and people were trying to make sense of it.

In the scenario under discussion, each of the observers, Alice and Bob, will look at a state vector in a 2-D Hilbert Space that describes their own particle with probabilitie: 50% up, 50% down. When they get together and compare their data, they will see a correlation. They can then conclude that this "entanglement" is due to the law of conservation of angular momentum.

There's no need for further interpretation.

 

[Mr Spinkles]

And, as mentioned, it produces a paradox when you have relativistic observers moving around. (What state is Bob's particle really in if different observers disagree?)

Well first notice that if Bob waits long enough to do his measurement, there is no disagreement between relativistic observers. It's only when the events occur close enough in time that relativistic observers start disagreeing. Secondly, relativity already has such paradoxes, before we even consider quantum mechanics. In the pole-and-barn or ladder paradox, for example, was the pole ever really entirely inside the barn? Third, quantum mechanics allows some wiggle room on the state of particles, as I said, before we even consider relativity. So as with many cases in special relativity it's an apparent, but not actual, paradox.

 

[Mr Spinkles]

So I am right, you never wrote Alice's measurement changes the state of both particles. But that's what was on your mind. And then you assumed that I had agreed to what was on your mind.

No. Read it again, carefully this time.

 

[PolyHedral]

Well first notice that if Bob waits long enough to do his measurement, there is no disagreement between relativistic observers. It's only when the events occur close enough in time that relativistic observers start disagreeing. Secondly, relativity already has such paradoxes, before we even consider quantum mechanics. In the pole-and-barn or ladder paradox, for example, was the pole ever really entirely inside the barn? Third, quantum mechanics allows some wiggle room on the state of particles, as I said. So as with many cases in special relativity it's an apparent, but not actual, paradox.

Relativity provides an objective reality, i.e. the 4D spacetime governed by spacetime intervals. QM does not provide any mechanism for reconciling disagreements over which vector describes a particle, i.e. predicted observables. Put another way, Alice and Bob are predicting different things from a measurement, in light of events that have not happened yet from the perspective of the measurement.

 

[Mr Spinkles]

Relativity provides an objective reality, i.e. the 4D spacetime governed by spacetime intervals. QM does not provide any mechanism for reconciling disagreements over which vector describes a particle, i.e. predicted observables. Put another way, Alice and Bob are predicting different things from a measurement, in light of events that have not happened yet from the perspective of the measurement.

I don't follow you.

 

[zaybu]

No. Read it again, carefully this time.

I did, and you didn't. But keep up with the pretentions when we all know you've lost this debate.

 

[zaybu]

Relativity provides an objective reality, i.e. the 4D spacetime governed by spacetime intervals. QM does not provide any mechanism for reconciling disagreements over which vector describes a particle, i.e. predicted observables. Put another way, Alice and Bob are predicting different things from a measurement, in light of events that have not happened yet from the perspective of the measurement.

I did provide a more objective interpretation of QM here: http://www.religiousforums.com/forum/3329785-post265.html

It's less fun than Spinkles woowoo interpretation, but you don't get tangled up with unnecessary baggage.

 

[PolyHedral]

I don't follow you.

Which part? That an event that has not happened yet (from Bob's POV) influences the measurement? Or that STR has a mechanism for unifying all the measurements, whereas QM does not?

 

[zaybu]

Which part? That an event that has not happened yet (from Bob's POV) influences the measurement?

Spinkles has written a state vector from a God's POV. Alice or Bob can't. They can only write a state vector for the particle they are each measuring.

 

[PolyHedral]

Spinkles has written a state vector from a God's POV. Alice or Bob can't. They can only write a state vector for the particle they are each measuring.

So Alice can't make a statement about a particle Bob has... why? Because it's too far away? Because it's the other half of an entangled pair she's measured?

 

[Mr Spinkles]

I did, and you didn't. But keep up with the pretentions when we all know you've lost this debate.

To call this a debate would be to suggest that your childish screaming is an argument. It is not.

Let's use plain English and elementary logic. In the post you shouted agreement to, before Alice's measurement, Bob's particle is in the singlet state with spin "Undefined/unknown". After Alice's measurement, Bob's particle is "NO LONGER in the singlet state" with spin "Down".

Ergo ... the state of Bob's particle changed.

Does the state always change if Alice measures? Yes.

Does the state change if no one measures? No.

Ergo ... "Alice's measurement changes the state of Bob's particle". That's how the English language summarizes the above pieces of information.

By the way, how can you scream agreement that after Alice's measurement Bob's particle is "NO LONGER in the singlet state" when you said "If Bob makes no measurement, his particle stays in a singlet state"? Yet another contradiction on your part. Perhaps less shouting, and more care, would help you avoid self-contradiction.

And I notice you dodged my post # 257. A wise choice.

 

[zaybu]

Let's use plain English and elementary logic. In the post you shouted agreement to, before Alice's measurement, Bob's particle is in the singlet state with spin "Undefined/unknown". After Alice's measurement, Bob's particle is "NO LONGER in the singlet state" with spin "Down".

Ergo ... the state of Bob's particle changed. Does the state always change if Alice measures? Yes

.

Note to all who are reading this: these statements from Spinkles were not in the original post to which I had agreed. Just for the record.

Now I will answer that last statement.

Bob's particle doesn't change state until a measurement is taken by Bob. Anything else is pure speculation.

 

[PolyHedral]

Bob's particle doesn't change state until a measurement is taken by Bob. Anything else is pure speculation.

So it's speculation on Alice's part that momentum is conserved. Gotcha.

 

[zaybu]

So Alice can't make a statement about a particle Bob has... why? Because it's too far away? Because it's the other half of an entangled pair she's measured?

Suppose Alice measuring the spin of particles coming at her with a detector. Suppose they are coming at her one by one. So she records their spin, which might look like:

UP, DOWN, UP, DOWN, DOWN, UP, UP, DOWN, DOWN UP, ....

She notices that 50% are UP, and 50% are down. According to QM, this is what she should get.

Bob does the same at his end. He gets:

DOWN, UP,DOWN, UP,UP, DOWN,DOWN, UP,UP, DOWN, ...

He also notices that 50% are UP, and 50% are down. And according to QM, this is what he should get.

Then one day, Alice and Bob get together and compare their data. Not only their data agrees with QM, but there's a correlation between them; every time Alice measured an UP spin, Bob measured a corresponding DOWN spin, and vice versa. So they wonder why.

So sometime later, a clever physicist points out to them that the particles they were measured came from a common source which was at rest and was decaying, giving off particles going in opposite direction with opposite spin.

They now know the reason behind this correlation.

So it's speculation on Alice's part that momentum is conserved. Gotcha.

 

[Mr Spinkles]

Which part? That an event that has not happened yet (from Bob's POV) influences the measurement? Or that STR has a mechanism for unifying all the measurements, whereas QM does not?

(1) An event Bob is unaware of affects Bob's measurement, yes. I don't see what the "problem" is there. (2) I don't know what you mean by "unifying all measurements". In special relativity, there are some invariant things all observers have to agree on, but they can disagree on other things, such as the length of a pole, the timing of events or even the temporal order of events in some cases.

Let's say in one frame S, Alice and Bob measure simultaneously. In this frame, they both measure a particle in the singlet state. The outcome is that Alice and Bob each measure up 50% of the time, and down 50% of the time. In addition, when they compare their data they see it is anti-correlated. Every time Alice measured up, Bob measured down, and vice-versa.

In another frame S', Alice measures before Bob. In this frame, Alice measures a particle in the singlet state, but Bob measures a particle in a spin eigenstate. The outcome is the same as it was in frame S: once again, Alice and Bob each measure up 50% of the time, and down 50% of the time. As before, when they compare their data they see it is anti-correlated.

Supposing the spacetime interval between Alice and Bob is spacelike, then we can also have a frame S'' in which Bob measures before Alice. In this frame, it is Bob who measures a particle in the singlet state, and Alice measures a particle in a spin eigenstate. Again the outcomes are the same.

So, what can we say about all this? Well, everyone in all frames agree on the objectively established outcomes. Naturally, the sentences in bold in each paragraph bother you. There is no help for it; in both relativity by itself and in QM by itself, some things can be observer-dependent in a way that is counter-intuitive and bothersome. For example, in relativity observers can disagree on the temporal order of events. Was the state of the universe A, then B, or was it B, then A? Observers can disagree about this according to relativity. It is therefore not surprising that these intuitively bothersome things continue crop up when you add QM to relativity.

I hasten to point out that the other thing we can say about all this is that all observers agree Bob and Alice's particles influenced each other in a nonlocal way. In frame S, Alice's particle affected Bob's. In frame S', they affected each other. In frame S'', Bob's affected Alice's. But everyone agrees there was an instantaneous effect of one on the other, in spite of their being spatially separated. How else do the observers explain the perfect (anti-)correlations in their data? (Other than by postulating that QM is a wrong or incomplete theory, I mean ... Bell already showed us that is unlikely to help us.)

In other words, the ambiguities that you correctly point out as introduced by relativity cannot rescue zaybu's shrill whining against nonlocality.

 

[Mr Spinkles]

Bob's particle doesn't change state until a measurement is taken by Bob. Anything else is pure speculation.

Emphasis added. This contradicts what I said in the post which you shrieked agreement to, namely that Bob's particle is "NO LONGER in the singlet state" after Alice's measurement. Do you stand by your shouted agreement with that, or not? Please make this clear.