Einstein and "spooky actions"

[zaybu]

Yes, and you said the way I wrote it was "in the wrong format" for dealing with entanglement. But your own source (Susskind lecture notes) used the same "format" for dealing with entanglement. Ergo, you contradict your own source. This should be your first clue.

I'm not sure what is the nature of your objection. You had posted a singlet state, my concern at that time of our discussion was about an entangled state. Hope you realize these are different things. So talking about a singlet does not mean we are talking about entangled state. They could be but not necessarily so. So bringing Susskind here won't settle the point.

I repeat: Suppose I measure the spin of one of the particles in the singlet state. Does this affect the state of the other particle, or not? For example, suppose I measure the spin of one of the particles and get "up". What is the state of the other particle? Is its state the same, or different, from what it was before I did the measurement?

I will suppose that we are talking about a setup in the line of Bell's theorem, that is, you have initially a particle at rest. It decays. Two particles are ejected: one goes towards A (Alice), the other towards B (Bob). These are entangled. Then your question would be: Alice measures an up-spin, what does Bob measure?

If that is the correct scenario you have in mind, then the answer is that Bob will measure a down-spin.

 

[Mr Spinkles]

I'm not sure what is the nature of your objection. You had posted a singlet state, my concern at that time of our discussion was about an entangled state. Hope you realize these are different things. So talking about a singlet does not mean we are talking about entangled state. They could be but not necessarily so. So bringing Susskind here won't settle the point.

From your link to Susskind's lecture notes:

"the singlet state [is] the simplest quantum system that exhibits entanglement"​

I will suppose that we are talking about a setup in the line of Bell's theorem, that is, you have initially a particle at rest. It decays. Two particles are ejected: one goes towards A (Alice), the other towards B (Bob). These are entangled. Then your question would be: Alice measures an up-spin, what does Bob measure?

If that is the correct scenario you have in mind, then the answer is that Bob will measure a down-spin.

Agreed. And what was the state of Bob's particle after Alice did her measurement, but before Bob did his measurement?

 

[zaybu]

From your link to Susskind's lecture notes:

"the singlet state [is] the simplest quantum system that exhibits entanglement"
​

I have no problem with that. I fail to see how this fits in with our disagreement.

Agreed. And what was the state of Bob's particle after Alice did her measurement, but before Bob did his measurement?

Are you asking from the POV of Bob? Bob won't know until he makes a measurement.

 

[Mr Spinkles]

I have no problem with that. I fail to see how this fits in with our disagreement.

I wrote down the singlet state and you said what I wrote was "in the wrong format" for dealing with entanglement. You were wrong, according to your own source, which used the same state and the same format for dealing with entanglement.

Are you asking from the POV of Bob? Bob won't know until he makes a measurement.

No not from the POV of Bob. The particle had a quantum state even before Bob measured anything, just as the entangled pair had a state (the singlet state) before Alice measured anything. What was that state? (Hint: it's the state where if you do a measurement, you get "down spin" with 100% probability.)

 

[zaybu]

I wrote down the singlet state and you said what I wrote was "in the wrong format" for dealing with entanglement. You were wrong, according to your own source, which used the same state and the same format for dealing with entanglement.

Perhaps that's because I had a different problem in mind than you had. I said at least twice, that there are different problems in physics, and how you would set up a solution, in this case, starting with writing down the quantum state, would depend on which problem you intend to solve. You had a singlet in mind. From my perspective, there isn't only the singlet to consider. There are many entangled cases other than the singlet state.

No not from the POV of Bob. The particle had a quantum state even before Bob measured anything, just as the entangled pair had a state (the singlet state) before Alice measured anything. What was that state? (Hint: it's the state where if you do a measurement, you get "down spin" with 100% probability.)

I'm not sure what you are out to prove. If you are trying to set this up as a case for instantaneous influence, you've got this all wrong.

If the situation started out with a pair of entangled particles being in a singlet state, that will remain so until a measurement is taken. And if Alice measures an up-spin at her end, then Bob will measure a down-spin at the other end. Saying that there is an instantaneous influence would be preposterous.

 

[Reptillian]

No not from the POV of Bob. The particle had a quantum state even before Bob measured anything, just as the entangled pair had a state (the singlet state) before Alice measured anything. What was that state? (Hint: it's the state where if you do a measurement, you get "down spin" with 100% probability.)

It gets even more interesting when Alice and ol' Bobbie are spacelike separated. Then the probabilities involved seem to be frame dependent as there are some frames where the events are simultaneous, some where A occurs before B, and some in which B occurs before A. A nice first approach to a more relativistic quantum theory might involve making particle states frame dependent.

 

[zaybu]

It gets even more interesting when Alice and ol' Bobbie are spacelike separated. Then the probabilities involved seem to be frame dependent as there are some frames where the events are simultaneous, some where A occurs before B, and some in which B occurs before A.

It turns out that what they will measure does not depend on which frame is chosen, or who measure first, or whether they measure at the same time.

A nice first approach to a more relativistic quantum theory might involve making particle states frame dependent.

I'm afraid it's not that simple. In QFT, plugging in GR, you get gravitons interacting with gravitons, producing more gravitons, which in turn produce more gravitons, and so on. What you get is a runaway process, and you can't get rid of those infinities.

 

[PolyHedral]

It turns out that what they will measure does not depend on which frame is chosen, or who measure first, or whether they measure at the same time.

Physically, why not?

 

[Mr Spinkles]

There are many entangled cases other than the singlet state.

Yep. But when you said what I wrote down was wrong, you were wrong. Then you wrote down something that is actually wrong (not a singlet state, as you claimed, it's not even a sensible entangled two-particle state). You won't find what you wrote down in your own sources, the way you described it--that should be your second clue.

I'm not sure what you are out to prove. ...

Humor me. Are you unwilling, or unable to answer the question?

If the situation started out with a pair of entangled particles being in a singlet state, that will remain so until a measurement is taken.

Right. My question is about what the state is after a measurement. Please answer it.

 

[zaybu]

Physically, why not?

Then you would really have a spooky action at a distance.

Consider so far the scenario that we've had so far: Alice measures an up-spin, and Bob a down-spin. Now, instead of Alice staying at rest wrt Bob, she moves (either to the right or left, doesn't matter) and suppose she now measures a down-spin!That would mean that her moving caused the particle to flip over. We know that we can cause a flip by passing the particle in a magnetic field, iow, we apply a force on it, but here, Alice only moved. So unless she is carrying a magnetic field, the result of her measurement will be the same, whether she's moving or not.

 

[zaybu]

Humor me. Are you unwilling, or unable to answer the question?

Which part of "If you are trying to set this up as a case for instantaneous influence, you've got this all wrong" don't you understand?

Right. My question is about what the state is after a measurement. Please answer it.

We've done that. Which part of "if Alice measures an up-spin, Bob will measure a down-spin," don't you understand?

 

[Mr Spinkles]

Which part of "if Alice measures an up-spin, Bob will measure a down-spin," don't you understand?

I understand it, but it doesn't answer my question. I'm not asking what Bob will measure. I'm asking about the state of the particle: what is it after Alice's but before Bob's measurement?

 

[PolyHedral]

Then you would really have a spooky action at a distance.

Consider so far the scenario that we've had so far: Alice measures an up-spin, and Bob a down-spin. Now, instead of Alice staying at rest wrt Bob, she moves (either to the right or left, doesn't matter) and suppose she now measures a down-spin!That would mean that her moving caused the particle to flip over. We know that we can cause a flip by passing the particle in a magnetic field, iow, we apply a force on it, but here, Alice only moved. So unless she is carrying a magnetic field, the result of her measurement will be the same, whether she's moving or not.

But that's not what I mean.

Say Alice and Bob are spacelike seperated and have one half of an entangled pair each. Alice measures spin-up, and Bob measures spin-down, but Carol sees Alice measure first, Dave sees Bob measure first, and Emma sees them both measure at the same time.

So, what's the explanation for the spins being correlated like that? Quantum magic?

 

[zaybu]

But that's not what I mean.

Say Alice and Bob are spacelike seperated and have one half of an entangled pair each. Alice measures spin-up, and Bob measures spin-down, but Carol sees Alice measure first, Dave sees Bob measure first, and Emma sees them both measure at the same time.

So, what's the explanation for the spins being correlated like that? Quantum magic?

No, conservation of angular momentum. Initially, a particle was at rest, that is, momentum = 0, angular momentum (spin) =0. It decays into two particles, one going to the left, the other to the right (conservation of momentum).If particle going to the left (towards Alice) has spin up, particle going to Bob has spin down (conservation of spin). Carol sees Alice seeing spin up. Dave sees Bob seeing spin down. Emma sees up at A, and down at B.

Sorry, no magic.:sorry1:

 

[zaybu]

I understand it, but it doesn't answer my question. I'm not asking what Bob will measure. I'm asking about the state of the particle: what is it after Alice's but before Bob's measurement?

As for Bob, before he measures, he can only know that it can be 50% chance that it will be up, and 50% down. But once he takes his measurement, it will be down. Note: it makes no different whether he makes his measurement before Alice or after, or at the same time (see post #194).

 

[Mr Spinkles]

As for Bob, before he measures, he can only know that it can be 50% chance that it will be up, and 50% down. But once he takes his measurement, it will be down. Note: it makes no different whether he makes his measurement before Alice or after (see post #194).

Right. But before Bob measures, Alice knows what the state of Bob's particle is, and she knows that he has a 100% chance of measuring spin down. What is that state?*
*Let's continue to assume Alice measures first and gets spin up, for convenience, since it doesn't matter.

 

[Reptillian]

Then you would really have a spooky action at a distance.

Consider so far the scenario that we've had so far: Alice measures an up-spin, and Bob a down-spin. Now, instead of Alice staying at rest wrt Bob, she moves (either to the right or left, doesn't matter) and suppose she now measures a down-spin!That would mean that her moving caused the particle to flip over. We know that we can cause a flip by passing the particle in a magnetic field, iow, we apply a force on it, but here, Alice only moved. So unless she is carrying a magnetic field, the result of her measurement will be the same, whether she's moving or not.

I don't think it's all that far fetched...after all, we have time dependent quantum mechanics in which states change as a function of time, why not as a function of space, time, or motion? It would take some work...we'd probably have to change the quantum theory a bit... since space and motion are operators and not parameters, but it seems like a relativistic quantum theory should treat space and time on equal footing anyway.

But that's not what I mean.

Say Alice and Bob are spacelike seperated and have one half of an entangled pair each. Alice measures spin-up, and Bob measures spin-down, but Carol sees Alice measure first, Dave sees Bob measure first, and Emma sees them both measure at the same time.

So, what's the explanation for the spins being correlated like that? Quantum magic?

So there's a theoretical starting point....you know what the states should be for Alice, Bob, Carol, Dave, and Emma, and what the relationship is between their positions and momenta in spactime...my biggest concern is that wavefuntion collapse isn't a continuous process. There's no smooth transition between frames. Either the wavefunction has collapsed and the particle has changed states, or it hasn't. Perhaps electron indistinguishability can offer an escape?

 

[zaybu]

but it seems like a relativistic quantum theory should treat space and time on equal footing anyway.

We do. It's called Quantum Field Theory ( QFT).

 

[zaybu]

Right. But before Bob measures, Alice knows what the state of Bob's particle is, and she knows that he has a 100% chance of measuring spin down. What is that state?*
*Let's continue to assume Alice measures first and gets spin up, for convenience, since it doesn't matter.

If Alice knows what the state of Bob's particle is, ( which will be a down-spin for Bob), then that's what it is, a down-spin state. From Bob's POV, who hasn't measured yet, and doesn't know what Alice has measured, then as far as he's concerned, the particle is still in its initial state, the singlet - 50% chance that it will be up, and 50% down.

We are repeating ourselves. There's not much else to add to this scenario. What's your point?

 

[Mr Spinkles]

If Alice knows what the state of Bob's particle is, ( which will be a down-spin for Bob), then that's what it is, a down-spin state.

Great. So before Alice does a measurement, Bob's particle is in the singlet state. After Alice does a measurement, Bob's particle is no longer in the singlet state. It's now in the down (or up) spin state. Right?