What's so unusual about entanglement?

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  • #26
vanhees71
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Indeed experiment + theorem proves that faster-than-light communication is the only scientific explanation.


This contradicts Bell theorem so there has to be error in that reasoning.
I have no doubt about Bell theorem being correct.

No! The explanation is standard QED, not faster-than-light communication. The correlations, described by the preparation of the two photons in a polarization-entangled state, are there, because of the preparation procedure, e.g., by parametric down conversion in a birefringent crystal. Although the single-photon polarizations are maximally uncertain (state of maximal entropy, i.e., ##\hat{R}=1/2 \mathbb{1}## for the polarization state of each of the single photons in the pair) the 100% correlations are a fact due to the preparation in the entangled state, e.g., in the singlet state ##|\Psi \rangle=1/\sqrt{2}(|HV \rangle-|VH \rangle)##.

This explanation does not contradict Bell's theorem, because standard QFT is not "local realistic", it's "local quantum".
 
  • #27
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Why does this whole quantum entanglement thing impress anybody?

When you look at articles in the lay press, they tell you that there is an "instantaneous" communication from one particle to another. But as these posts clearly show, there is no communication involved at all. There is just this "correlation" that when you look at one particle, you know what the other one is. This happens as a process of deduction, not communication. If you have two gloves and you send the left one to Seattle and the right one to Boston, when you open the one in Boston and see it is a right glove, you know instantly that the one in Seattle is the left glove. Why do you know? Because gloves come in pairs left/right. If you have the right, then then the left one is elsewhere. Simple deduction, no instantaneous communication between Seattle and Boston required.

But yet, the popular press keeps on pushing this idea that there is some kind of instantaneous "communication" going on. What is worse is the implication that if we change the state of one particle, it is instantly reflected in the other - so not true. Not even the published papers make this wildly wrong claim. Obviously no communication is possible since you're just looking at the right glove in Boston, you can't turn it into a left glove and have the right glove show up in Seattle. It doesn't work that way.

So given that, why are physicists so impressed with something so obvious and completely useless for any practical purposes?

And why does the press keep on insisting this has something to do with instantaneous communication?
The press does get a good deal wrong.

The important thing about entanglement TO ME, is how it relates to the uncertainty principle. The uncertainty principle was first taught to me as a measurement problem. EG, once you measure a particles position precisely, the momentum measurement is uncertain. And a great deal of uncertainty follows the rules of measurement. The question that naturally arose was whether the uncertainty is a measurement thing, so the measurement of position rendered momentum UNMEASURABLE, or whether the uncertainty is related to the properties that a thing fundamentally has, so the measurement of position renders the momentum non-existent.

The clever thought (by Einsein, Podalsky, and Rosen, EPR) was to consider two identical particles, and measure the position of one and the momentum of the other.

Entanglement is that experiment. The two things are identical. And once you measure the momentum of particle #1, the particle #2 loses position as a fundamental property. That is why it is impressive. It shows that the uncertainty is fundamentally based on the underlying reality NOT containing a defined thing. Not something we don't know, but something far more weird, that has properties that interact with measurements in a way that is not understandable. A system that does not have fundamental property sets that we don't know, but doesn't have fundamental property sets.

In your example, there is no uncertainty. If measuring a glove pair, you can measure perfectly a left-handedness, and a right-handedness. And knowing the system was never in question. But say that anytime you measured the gloves handedness, you destroyed it and could not measure the color, or if you measured the color, you could not measure the handedness. So you could know "Blue", but not "left-handed", or you could know "right-handed", but not "Green". Now say you could know you have two entangled gloves ... why just destructively measure the color on one, and the handedness on the other. Now you have demonstrated that both properties existed as underlying elements of the glove reality, and uncertainty was based on measurement issues.

The entire fundamental issue is that when you measure the first gloves handedness, the second glove "knows" and loses color along with the first glove. The math seems to prove it (and I say seems, because I have the same reluctance to accept the proof, even though it is absolutely 100% correct).

The information that the entangled particles share is not a valid basis for communication. Somehow the entangled particles operate in a way that ignores they are in separate localities that are fundamentally disconnected. That seems to say they are instantaneously connected. But if they are, then it is in a way that we really can't exploit.

The entanglement experiments ALL agree that if you measure the same property, you get the exact expected results eg, Left in one place Right in the other. The suggestion of the experiments is that once you measure Left in one place, and know Right in the other, you cannot know color. In the glove experiment, if colors can be blue or green, and handedness can be left or right, a blue left, implies a green right as the other. And the odds can be known exactly for any set of measurements at the two locations. There is no uncertainty. What the many experiments with entangled particles keep showing, is that the odds that are exactly expected, are not the odds that are obtained. That answers a fundamental question. The instantaneous communication is speculation.
 
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  • #28
zonde
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The explanation is standard QED, not faster-than-light communication.
You mean that QED explains why measurements of entangled particles can violate Bell inequalities?

The correlations, described by the preparation of the two photons in a polarization-entangled state, are there, because of the preparation procedure, e.g., by parametric down conversion in a birefringent crystal. Although the single-photon polarizations are maximally uncertain (state of maximal entropy, i.e., ##\hat{R}=1/2 \mathbb{1}## for the polarization state of each of the single photons in the pair) the 100% correlations are a fact due to the preparation in the entangled state, e.g., in the singlet state ##|\Psi \rangle=1/\sqrt{2}(|HV \rangle-|VH \rangle)##.
You have given Hilbert space QM description of entangled state. Hilbert space QM is non-local. So what argument do you want to make by that?

[mentor's note - inappropriate comment deleted]
 
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  • #29
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"If you have two gloves and you send the left one to Seattle and the right one to Boston, when you open the one in Boston and see it is a right glove, you know instantly that the one in Seattle is the left glove."

This is not entanglement. This is an example of classical correlation, with local hidden variables.

If you put each glove in a box, you could write down under the box whether it contains the right or left glove.
Such written informations are the local hidden variables.

The situation is very different for quantum entanglement.
If you put 2 correlated particles in two boxes, you could not beforehand write down on the boxes what the measurements results would be, as the Bell's theorem (Bell's inequalities) clearly prove.
 
  • #30
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Would it be far fetched to say that the quantum world becomes classical(-like) via entanglement? We always receive inputs via interactions which are known to break entanglement and the world is entangled most of the time.
 
  • #31
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Would it be far fetched to say that the quantum world becomes classical(-like) via entanglement? We always receive inputs via interactions which are known to break entanglement and the world is entangled most of the time.

It might be better to say that the quantum world becomes classical because of decoherence. You are right that entanglement is involved - a particle interacts with a macroscopic detector and the two end up in an entangled state (the non factorizable superposition of "particle is in state X and detector reads X" and "particle is in state Y and detector reads Y"). Then decoherence causes this superposition to rapidly evolve into one of the two classical states: We measured X or we measured Y.

(There is a mathematical subtlety about proper versus improper mixed states here.... let's not go there right now please?).
 
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  • #32
Some classically unexpected results of quantum theory used to bother me a lot when I was younger. But now that I am in my sixties, my lack of understanding no longer seems so disturbing. I think this is because I no longer imagine that I will ever be able to "logically" explain everything in the world either to myself or to others. In particular, it seems clear that we have no basic understanding of time, either in Special or General Relativity, nor in any part of quantum theory, nor even in Newton's theory of gravitation. Each of these theories seem to violate our expectations about time in one way or another. Since we -- all people, as far as I know -- have as yet no good way to think about the key concept of time, I no longer feel alone in not understanding any particularly surprising results of quantum theory or relativity. I can still be astonished, but I no longer feel defeated by things I cannot understand or explain.

One more point: I have learned about so many utterly amazing things in our world/universe that a few extra inexplicable correlations are nowhere near the most surprising things I know about. As a simple example, it is overwhelmingly astonishing to simply survey the orders of magnitude we can observe in both time and space. Parts of our universe are so much bigger -- and parts so much smaller -- and parts so much slower -- and parts so much faster than we can ever hope to directly perceive! We are tiny and insignificant and probably will never understand very much of what we know exists. And that I find very wonderful.
 
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  • #33
zonde
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This explanation does not contradict Bell's theorem, because standard QFT is not "local realistic", it's "local quantum".
The end result of your implied QFT explanation is based on "clicks in detectors", right? Well these "clicks in detectors" is all the "local realism" it takes for Bell's theorem to be relevant.
 
  • #34
stevendaryl
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The end result of your implied QFT explanation is based on "clicks in detectors", right? Well these "clicks in detectors" is all the "local realism" it takes for Bell's theorem to be relevant.

I wouldn't say that. A typical physics experiment consists of three steps:
  1. Setting up the system in a particular state.
  2. Letting the system evolve in time.
  3. Performing a measurement on the system.
Steps 1 and 3 involve macroscopic objects: Particle sources and particle detectors, and maybe bubble chambers, etc. Step 2 involves microscopic objects: undetected electrons or photons or whatever.

Bell's theorem is about the issue of whether you can explain the statistical observations for Step 3 by assuming definite, but unmeasured, properties for the microscopic objects involved in Step 2. The clicks in detectors are realistic, but take place in Step 3.
 
  • #35
entropy1
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It might be better to say that the quantum world becomes classical because of decoherence. You are right that entanglement is involved - a particle interacts with a macroscopic detector and the two end up in an entangled state (the non factorizable superposition of "particle is in state X and detector reads X" and "particle is in state Y and detector reads Y"). Then decoherence causes this superposition to rapidly evolve into one of the two classical states: We measured X or we measured Y.

(There is a mathematical subtlety about proper versus improper mixed states here.... let's not go there right now please?).

It's worth taking a moment to say exactly what is meant by "communication" here. Whatever it is, it can't be causal, because when the two measurements are spacelike-separated we can interpret the correlation either as Bob's measurement influencing Alice's or as Alice's measurement influencing Bob's.

I totally agree. If there is no way to discern the direction of causality in relativistic sense, then causality is relative, and can't be identified: A does not 'influence' B and neither vice-versa, for it can't be discerned! Correlations are a informational phenomenon, that is to say, only when the necessary information is gathered and investigated, the correlation emerges. Information is not linked to specific matter or energy, until it is measured and determined. It 'is' nowhere in particular. Therefore it can't 'travel' (faster than light) in that sense. I hope I interpreted you right.
 
  • #36
morrobay
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If that were all there was to it - one observer measures spin-up on a given axis, the other must measure spin-down because entangled particles come in up/down pairs just as gloves come left/right pairs - you would be right and entanglement would be no big deal.

But that's not all there is to it.

Consider a pair of entangled spin-1/2 particles; we create the pair and then send one member of the pair to each of two observers (traditionally named Alice and Bob). Suppose they measure the spin along different axes? Quantum mechanics says, and experiment confirms, that if Alice measures spin-up on her axis, then the probability that the Bob will measure spin-up on his axis is ##\sin^2\frac{\alpha-\beta}{2}## where ##\alpha## and ##\beta## are Alice's and Bob's angle settings. You can verify that when they both use the same angle the probability of them both getting the same result is zero, just as with the gloves.

Now if you look at the formula you will notice somethingly profoundly weird about it, something that doesn't happen with the gloves: if Alice changes the angle at which she chooses to measure, it will change the probability of Bob getting a given result even though he hasn't changed anything in his setup. Alice can even change her setup while the two particles are in flight and Bob and Bob's particle are light-years away, with Bob's particle just centimeters away from his detector - and Bob's probabilities will change. That's what makes entanglement interesting.

It is possible to prove (google for "Bell's theorem", and check out the web site maintained by our own DrChinese) that if QM is correct Bob's results cannot be determined just from the setting of his detector and the properties the two particles had when they were created. One way or another, you have to include Alice's setting as well.

Would it be possible for hidden variables or pre existing values that were created at source to explain probabilities for above case ?
Ie that for every detector angle Alice selects (space.like) there is a corresponding pre existing outcome for Bob
I understand that Bell disproves this but that is not the question here regarding entanglement.
 
  • #37
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Indeed experiment + theorem proves that faster-than-light communication is the only scientific explanation.


This contradicts Bell theorem so there has to be error in that reasoning.
I have no doubt about Bell theorem being correct.
It is arguably true that there is no communication between entangled particles, after all EPR paradox satisfies two-way no signal. There is no way to tell what happened to one particle by measuring the other. Bell inequality is based on two assumptions. One assumption is no FTL causality, the other is that the state of a physical system (as many pieces of information) is always bond to local points in space-time (events).
EPR paradox doesn't necessarily breaks the first one. It probably just breaks the second one.
Maybe when two quantum measurements happen at two different events, the result is delivered by quantum information that is undefined by space and time.
 
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  • #38
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Entanglement does have useful purposes (though perhaps not practical).
Quantum Entanglement totally does have practical applications. The University of Geneva teleported a particle entangled with another particle over the distance that they were entangled. They actually didn't "teleport" it, the entangled particle on the other end just copied it. Now scientists are coming ever closer to teleporting more than one particle matter over a longer distance.
 
  • #39
zonde
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Bell inequality is based on two assumptions. One assumption is no FTL causality, the other is that the state of a physical system (as many pieces of information) is always bond to local points in space-time (events).
EPR paradox doesn't necessarily breaks the first one. It probably just breaks the second one.
You view the two assumptions as independent. But they are not exactly that way.
Lets consider measurement of entangled particles with the same measurement settings. In that case we have perfect anticorrelation (or correlation depending on type of entanglement) for detected pairs. So we say that this certainty of detection outcome at one end (in respect to detection outcome at the other end) has to come either exclusively from past light cone of detection event OR some FTL communication/interaction is involved too.
So you see, the first option we can say in two different ways:
- detection event is exclusively determined by past light cone of detection event;
- there is nothing outside past light cone that affects outcome of detection event or in other words there is no FTL communication/influence (and no retrocausality of course).
So the second statement is just negation of opposite statement of the first statement.

P.S. I equated "local points in space-time" with "past light cone". I had to state this as you might object to that.
 
  • #40
Simon Phoenix
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Is there any known process / experiment that can create an entangled pair where by they are not created at the same moment?

Yes. Here's a simple one.

Take a high-Q optical cavity with a vacuum field. Fire an excited 2-level atom through it such that its interaction time with the cavity results in a 50% chance of leaving the cavity in its ground state. Now fire a second 2-level atom in its ground state through the cavity. This time tailor the interaction time so that if the cavity contains a photon there is a 100% probability of absorption by the atom.

Hey presto - the two atoms are now entangled.
 
  • #41
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Now if you look at the formula you will notice somethingly profoundly weird about it, something that doesn't happen with the gloves: if Alice changes the angle at which she chooses to measure, it will change the probability of Bob getting a given result even though he hasn't changed anything in his setup. Alice can even change her setup while the two particles are in flight and Bob and Bob's particle are light-years away, with Bob's particle just centimeters away from his detector - and Bob's probabilities will change. That's what makes entanglement interesting.

It is possible to prove (google for "Bell's theorem", and check out the web site maintained by our own DrChinese) that if QM is correct Bob's results cannot be determined just from the setting of his detector and the properties the two particles had when they were created. One way or another, you have to include Alice's setting as well.

Are you sure you mean this Nugatory?

The probability of Bob measuring any specific result is always 1/2 (assuming the usual spin-1/2 singlet state set-up) - and that's quite independent of any change of detector setting by Alice.

In fact if there were a change in Bob's probabilities when Alice changed her measurement settings then this would enable us to construct a FTL communication scheme.

The formula you quote is the probability of agreement between Alice's and Bob's results - and that certainly does depend upon both measurement settings (being a function of the relative angle).
 
  • #42
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Take a high-Q optical cavity with a vacuum field. Fire an excited 2-level atom through it such that its interaction time with the cavity results in a 50% chance of leaving the cavity in its ground state. Now fire a second 2-level atom in its ground state through the cavity. This time tailor the interaction time so that if the cavity contains a photon there is a 100% probability of absorption by the atom.

Hey presto - the two atoms are now entangled.

Thanks for the reply. I am new to QM field so patience please if I blunder. I just like to check my understanding.

In your process when atom 1 leaves cavity it is photon to atom #1 entangled correct? Then Atom 2 comes and absorbs photon and bam …. Atom 2 entangled to atom 1.
We know have “Entanglement swapping” I like it.

Have you read the Delft loop hole free test, if I read it right. Corrections in how I interpreted it and state it are greatly appreciated.

2 distant locations, 2 electrons trapped in 2 diamonds. When they get a spin flip in both locations the emitted photons make a fiber optic trip to central location and if everything hits just right (photon to photon absorption ??) they have distant entangled electrons.

Page 3 right side from top is the basic

Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
http://arxiv.org/pdf/1508.05949v1.pdf
 
  • #43
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Are you sure you mean this Nugatory?

The probability of Bob measuring any specific result is always 1/2 (assuming the usual spin-1/2 singlet state set-up) - and that's quite independent of any change of detector setting by Alice.

In fact if there were a change in Bob's probabilities when Alice changed her measurement settings then this would enable us to construct a FTL communication scheme.

The formula you quote is the probability of agreement between Alice's and Bob's results - and that certainly does depend upon both measurement settings (being a function of the relative angle).
I don't think that FTL expectation is right. Alice has no way of predicting up or down, and has a 50% chance of measuring either. But if Alice measures UP, and Bob is set to measure 120-degrees off of Alice, then for THAT entangled particle, the measurement will be DOWN 75% of the time and UP 25% of the time. If Alice measures DOWN, and Bob is set to measure 120-degrees off of Alice, then for THAT entangled particle, the measurement will be UP 75% of the time and DOWN 25% of the time. There would be the possibility of a FTL only if Alice could control for UP or DOWN (or could communicate her result FTL).

Since Alice is getting (independently) random UP and DOWN results, the results are also (dependently) random at Bob, and there will be a 25% equal measurements at Alice and Bob (for those 120-degree separated measurements, if they get together later).
 
  • #44
Simon Phoenix
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Hi Voting,

The point is that the probability of up/down measured by Bob, for a specific spin direction, cannot be changed by anything Alice does!

There is no experiment Bob can do on his particle alone that will be able to distinguish between it being
(1) a partner particle from an entangled source
(2) completely unentangled (i.e. just a single particle) but prepared in up/down uniformly at random

If Bob's probabilities did indeed change - then this is measureable is it not? If something Alice does causes a measurable effect at Bob then information can be transferred by that. So why wouldn't we be able to construct a FTL scheme here?

Added later :

Of course Bob's probabilities conditioned upon Alice's measurement results can, and do, change depending on Alice's setting for her measurement. It is these probabilities that Nugatory was probably referring to. But these conditional probabilities are not measureable by Bob alone - he needs Alice's data in order to construct them.
 
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  • #45
Simon Phoenix
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In your process when atom 1 leaves cavity it is photon to atom #1 entangled correct? Then Atom 2 comes and absorbs photon and bam …. Atom 2 entangled to atom 1.
We know have “Entanglement swapping” I like it.

Yes, that's essentially correct - although strictly speaking 'entanglement swapping' is a name given to another process (entanglement swapping can be seen as the teleportation of an entangled state from this perspective). The simple set-up I outlined is not entanglement swapping in this sense.

Entangled atoms have been created in the lab this way using micromaser cavities and Rydberg atoms.
 
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  • #46
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So we say that this certainty of detection outcome at one end (in respect to detection outcome at the other end) has to come either exclusively from past light cone of detection event OR some FTL communication/interaction is involved too.
I think what determines the outcomes is neither inside the light crone or FTL, its outside of spacetime. My view is that there is no causality between those measurements, and quantum information exists outside of spacetime.
For simplicity, let's consider perfect anticorelation scenario. The first observer measure the particle, but he doesn't decide the outcome, thus the first measurement and the first outcome have no causal relations, and the second observer's measurement result is determined by the outcome of the first measurement. So, although the two outcomes have causal relations, the second outcome have no causal relations with the first measurement. No information can be passed through the system of two EPR particles to the other outcome by either observers. Thus, phenomelogically, there is no causality between the two measurement events, only correlation between the outcomes.
Think about this, if we define what we perceive as reality(as opposed to quantum universe where what we see as a reality is just one of many parallel universes) and we think of spacetime as a closed system that contains information, then measurement of a particle delivers a result that cannot be predicted by existing information in the spacetime. Where does the new bit of information come from? I think measurement of a particle can be perceived as acquiring information from outside the spacetime, and the correlation in EPR paradox doesn't necessarily mean communication between the two measurement events, it could be perceived as that both measurement results are delivered by the same quantum information source outside of spacetime.
 
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Hi Voting,

The point is that the probability of up/down measured by Bob, for a specific spin direction, cannot be changed by anything Alice does!

There is no experiment Bob can do on his particle alone that will be able to distinguish between it being
(1) a partner particle from an entangled source
(2) completely unentangled (i.e. just a single particle) but prepared in up/down uniformly at random

If Bob's probabilities did indeed change - then this is measureable is it not? If something Alice does causes a measurable effect at Bob then information can be transferred by that. So why wouldn't we be able to construct a FTL scheme here?

Added later :

Of course Bob's probabilities conditioned upon Alice's measurement results can, and do, change depending on Alice's setting for her measurement. It is these probabilities that Nugatory was probably referring to. But these conditional probabilities are not measureable by Bob alone - he needs Alice's data in order to construct them.
The"Bob's probabilities conditioned upon Alice's measurement results" change. But the raw probabilities are unchanged simply because Alice sees half up and half down.

Another thing to consider is that if Alice is set to measure at the same angle as Bob, and measures UP, then Bob measures DOWN. The particles are entangled. Alice measuring UP or DOWN is a random thing, but the opposite measurement will happen for Bob.

So if Alice knows the agreed upon settings for Bob, she can know the results for Bob before him. But that doesn't seem to allow FTL from Alice to Bob.

But in one sense, the probability for Bob, as seen by Alice is now no longer 50%. Alice knows. If they are measuring n the same lab and Bob sees an experimental time delay, Bob would know also. Alice could tell him: "your next particle will measure UP, after she gets DOWN (but do physicists REALLY get down?).
 
  • #48
Simon Phoenix
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Hi Voting,

I think you misunderstand me. I'm saying that IF something Alice did affected the probabilities measured by Bob - THEN we'd be able to construct a FTL communication scheme.

Of course, no such scheme is possible because there is nothing Alice can do to affect Bob's probabilities.
 
  • #49
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Hi Voting,

I think you misunderstand me. I'm saying that IF something Alice did affected the probabilities measured by Bob - THEN we'd be able to construct a FTL communication scheme.

Of course, no such scheme is possible because there is nothing Alice can do to affect Bob's probabilities.


You can. Even though the outcomes are completely random(philosophy), if you have many different entangled states, you can choose when to measure("collapse") each pair and depending on the timing between 'measurements', use it as a morse code to send messages(presumably) to the other part of the galaxy. The difficulty is mostly technical to preserve the entanglement intact long enough.
 
  • #50
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You can. Even though the outcomes are completely random(philosophy), if you have many different entangled states, you can choose when to measure("collapse") each pair and depending on the timing between 'measurements', use it as a morse code to send messages(presumably) to the other part of the galaxy. The difficulty is mostly technical to preserve the entanglement intact long enough.
No, this will not work. There is no way Bob can tell if Alice has measured her particle or not.
 
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