Does quantum entanglement allow information to travel faster than light?

[byron178]

Could you elaborate a bit? What exactly are you saying?

i thought entangled particles were proven to communicate instantly?

 

[Drakkith]

i thought entangled particles were proven to communicate instantly?

Have you actually read anything that has been posted? There isn't any proof beyond reasonable doubt that proves anything yet.

 

[bobc2]

It's not about entangled particles communicating with each other. There is a system wave function for the two particles. In QM you only consider particles after the wave function collapses. The Schroedinger wave evolves with time over extended space. So, don't think about one point in the wave causing anything to happen at another point (and certainly not one particle causing something to happen with another particle--it's all about a traveling wave).

But, when a measurement is performed (Bohr Copenhagen interpretation) the wave function collapses. Now, here is the problem. QM has no physical description of what exactly happens in transforming from a probability amplitude wave to a particle showing up in some state at a particular point. There's no communication between particles. The resulting states of two particles showing up at distant locations is strictly a result of the Schroedinger probability amplitude wave and its collapse.

Of course different physicists have had different interpretations of the mysterious wave collapse. Roger Penrose seems to think the wave function should be regarded as the objective reality. Hawking is probably more of a Copenhagen guy. Then there is Hugh Everett who thought there had to be a physical explanation for the wave collapse and came up with the many worlds concept.

In any case there is no faster-than-light information transfer from one point to another.

 

[edpell]

I have read this whole thread and am lost. Let me give an example. We have a process that emits two photon in opposite directions with opposite spins. One travels to the left and on to the right.

1) The observers on the left measure the spin in the up/down direction and finds half the time it is spin up and half the time it is spin down. Like for spin in the direction 90 degree from up/down. OK this poses no surprise no paradox.
2) Much later the observer on the right makes the same measurements and get the same results. No surprise no paradox.
3) Now we have both observers make up/down spins measures at the same time (or at times separated by less than distance between them divided by the speed of light). Each measures spin up 50% of the time no surprise no paradox. When we bring the list of results to one central location and compare them we see that they are always opposite. Again no surprise no paradox.

The thing that upsets people is that the observer on the left know the result that the observer on the right is getting before light can travel between the two observers. Why is this upsetting?

Lets image a different experiment a central person has a supply of tiles labeled with A and a supply labeled with B and a supply of boxes. They place at random a tile in a box that will be sent to the left and the other type tile in the box that will be sent to the right. When both boxes arrive both observers open their box at the same time. The observer on the left knows the result that the observer on the right is getting even before light can travel between the two. This is no surprise and no paradox. So why is the photon experiment upsetting?

 

[SpectraCat]

I have read this whole thread and am lost. Let me give an example. We have a process that emits two photon in opposite directions with opposite spins. One travels to the left and on to the right.

1) The observers on the left measure the spin in the up/down direction and finds half the time it is spin up and half the time it is spin down. Like for spin in the direction 90 degree from up/down. OK this poses no surprise no paradox.
2) Much later the observer on the right makes the same measurements and get the same results. No surprise no paradox.
3) Now we have both observers make up/down spins measures at the same time (or at times separated by less than distance between them divided by the speed of light). Each measures spin up 50% of the time no surprise no paradox. When we bring the list of results to one central location and compare them we see that they are always opposite. Again no surprise no paradox.

The thing that upsets people is that the observer on the left know the result that the observer on the right is getting before light can travel between the two observers. Why is this upsetting?

Lets image a different experiment a central person has a supply of tiles labeled with A and a supply labeled with B and a supply of boxes. They place at random a tile in a box that will be sent to the left and the other type tile in the box that will be sent to the right. When both boxes arrive both observers open their box at the same time. The observer on the left knows the result that the observer on the right is getting even before light can travel between the two. This is no surprise and no paradox. So why is the photon experiment upsetting?

You are correctly expressing the relationships between the information gained by the humans making the measurements. The question that vexes people who ask this sort of question (I know because I was once one of them) is not so much about the information the humans are getting, but whether or not the collapse of the entangled state itself somehow involves FTL information transfer. Before either person has made a measurement, the entangled wavefunction extends over a huge distance. At the instant one person makes a measurement, the wavefunction collapses, and *somehow*, the two results end up being properly correlated (or anti-correlated) 100% of the time (according to predictions of QM, and the idealized versions of the experiments that have been done to test this). What people are discussing is how that can possibly happen without some FTL information transfer between the particles.

 

[Drakkith]

I THINK it is because in an experiment which I can't remember the name of, they measured the value of a particular property or direction, and found that when they measure that one AND another one together, the chance that it was in a particular value or state was NOT what it was supposed to be for pure chance.

 

[bobc2]

My previous post seems to have not been communicated clearly. I think you guys are missing the point--it's about the wave function. Don't look to some magical communication from one point to another for the answer as to why the two particles always have a particular combination of states after the wave function collapses, i.e, maintains an Up and a Down for example. Again, there is no communication between the individual particles.

It's the wave function, and you must understand the wave function for a system of two particles in this case. The wave function represents the entangled combination of two particles. It's the combination of particles making up a global system that produces a system wave function. Thus, when the wave function collapses, it produces a specific allowed combination of particles, which again satisfy a global system state. No communication from point to point. Just a system wave function collapsing into an allowed system state, such as an Up at one location and a Down at the other location.

Again, the physics of Quantum Mechanics has absolutely no description of the mechanism of the collapse itself. Before collapse you have a wave function--after collapse you have a system of particles.

 

[Drakkith]

I understand you bob. That makes sense.

 

[SpectraCat]

My previous post seems to have not been communicated clearly. I think you guys are missing the point--it's about the wave function. Don't look to some magical communication from one point to another for the answer as to why the two particles always have a particular combination of states after the wave function collapses, i.e, maintains an Up and a Down for example. Again, there is no communication between the individual particles.

It's the wave function, and you must understand the wave function for a system of two particles in this case. The wave function represents the entangled combination of two particles. It's the combination of particles making up a global system that produces a system wave function. Thus, when the wave function collapses, it produces a specific allowed combination of particles, which again satisfy a global system state. No communication from point to point. Just a system wave function collapsing into an allowed system state, such as an Up at one location and a Down at the other location.

Again, the physics of Quantum Mechanics has absolutely no description of the mechanism of the collapse itself. Before collapse you have a wave function--after collapse you have a system of particles.

Sorry, but that's not an answer to the problem ... at least not any more than, "it just happens that way" or "it's MAGIC" is an answer. The fact is that we simply don't know the details right now .. all we can do is describe the predictions of QM, and the experimental results, which seem to agree based on the experiments that have been done in this area by Aspect, Zeilinger and others.

You are correct that QM has no description of how the collapse occurs, but that's just avoiding the question. The question of how the collapse happens is a legitimate one ... because it certainly seems right now that it might have to happen in a way that violates relativity. The other answer that is commonly given is, "QM is non-local", but again, that is not really an answer of how, or even whether or not, the breaking of entanglement avoids requiring FTL transfer of information.

 

[Drakkith]

Spectra, do you know the experiment(s) where they showed the effect I mentioned up a few posts ago? Something to do with Bells theorem or something?

 

[DevilsAvocado]

The thing that upsets people is that the observer on the left know the result that the observer on the right is getting before light can travel between the two observers. Why is this upsetting?

Lets image a different experiment a central person has a supply of tiles labeled with A and a supply labeled with B and a supply of boxes. They place at random a tile in a box that will be sent to the left and the other type tile in the box that will be sent to the right. When both boxes arrive both observers open their box at the same time. The observer on the left knows the result that the observer on the right is getting even before light can travel between the two. This is no surprise and no paradox. So why is the photon experiment upsetting?

That’s a good question. The 'setup' you describe is the one that was discussed by Niels Bohr and Albert Einstein for many years, starting with the 1935 EPR paper.

In 1964 John Bell formulated Bell's inequality, which shows that there are more 'subtle' values to gain from the EPR experiment, i.e. not only so called perfect correlations, i.e. 100% up/down (or A/B). There are several cases where the 'correlation rate' is spread over the full 'probability spectrum', i.e. 0-100%.

Please have a look at https://www.physicsforums.com/showpost.php?p=2833234&postcount=1241" for a fairly easy explanation, the example by Nick Herbert starts halfway down the post.

 

[Drakkith]

Devils, do you have a good link to experiments that show this by chance?

 

[DevilsAvocado]

ehh, do you mean my specific example or EPR-Bell test experiments in general (the most 'famous')?

 

[Drakkith]

ehh, do you mean my specific example or EPR-Bell test experiments in general (the most 'famous')?

One that shows the discordance discussed in the post you linked.

 

[DevilsAvocado]

One that shows the discordance discussed in the post you linked.

Aha! That’s what I suspected... This is of course a thought experiment, and I’m sorry to say I can’t give you a link to an experimental setup exactly like this... The original text by Nick Herbert is here:

A SIMPLE PROOF OF BELL'S THEOREM
http://quantumtantra.com/bell2.html

The closest I can think of when it comes to experiments, is Alain Aspect’s famous 1982 experiment (link to DrC):

A. Aspect, Dalibard, G. Roger: "Experimental test of Bell's inequalities using time-varying analyzers" Physical Review Letters 49 #25, 1804 (20 Dec 1982)
http://www.drchinese.com/David/Aspect.pdf

DrChinese has an excellent site with a lot of info:
http://www.drchinese.com/David/EPR_Bell_Aspect.htm

If I should add something, maybe this:

Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory
http://arxiv.org/abs/quant-ph/0205171

QuantumLab
http://www.didaktik.physik.uni-erlangen.de/quantumlab/english/

Experimental demonstration of quantum correlations over more than 10 km
http://arxiv.org/abs/quant-ph/9707042

Testing spooky action at a distance
http://arxiv.org/abs/0808.3316

Bell test experiments
http://en.wikipedia.org/wiki/Bell_test_experiments

Quantum Entanglement Experiment

https://www.youtube.com/watch?v=c8J0SNAOXBg

 

[Drakkith]

Whew, I'll have to take a bit and look at all this. Thanks Devils!

Edit: Ugh, can't look at half of that from work here. I'll have to look at home I guess.

 

[DevilsAvocado]

Welcome!

 

[bobc2]

Sorry, but that's not an answer to the problem ... at least not any more than, "it just happens that way" or "it's MAGIC" is an answer. The fact is that we simply don't know the details right now .. all we can do is describe the predictions of QM, and the experimental results, which seem to agree based on the experiments that have been done in this area by Aspect, Zeilinger and others.

You seem to keep missing the point. After you made the comment about the wave function spreading out over space in an earlier post, I thought you would understand that the wave function provides the state information for the particles at the instant of collapse, which answers the problem of Faster-Than-Light communication from particle to particle. Just because QM does not provide a detailed description of the collapse mechanism does not take away from the physics of the global wavefunction.

Our problem is still fundamentally the mystery of the two slit experiment. The entanglement issues all spring from that fundamental phenomena. After all of these decades (going all the way back to Planck and Einstein's papers around the turn of the 20th century) physics still does not answer the fundamental questions about the dual particle and wave properties of photons, electrons, muons, taus, Up quarks, Down quarks, Strange quarks, Charmed quarks, Top quarks and Bottom quarks--much less the entangled coherent combinations of these elementary particles forming complex systems.

I am certainly not the one who will unravel this fundamental mystery and was not trying to in my post, and I don't expect to see it unraveled on this forum--but I'll be cheering on any honest efforts.

But I still maintain that you are missing the point of the global character of the wave function that produces a system of coherent particles at the instant of collapse (in other words, don't look for Faster-Than-Light communication between the particles--the wave function already had that taken care of that--even if we don't understand the mechanism).

 

[DevilsAvocado]

... because it certainly seems right now that it might have to happen in a way that violates relativity.

Spectra, you do mean Relativity of Simultaneity (RoS), right?


EDIT:

... What people are discussing is how that can possibly happen without some FTL information transfer between the particles.

... but again, that is not really an answer of how, or even whether or not, the breaking of entanglement avoids requiring FTL transfer of information.

IMHO, it’s maybe a little bit 'risky' to use the words "FTL" and "information"... some reader might get the wrong impression...

It’s completely impossible to send any information from Alice to Bob, or vice versa, utilizing QM Entanglement. All we’ll ever get is random noise, in both 'ends'. There is not even a way to know if a measurement has been performed in the other 'end', or not. Everything that is 'weird' about entanglement is established after the measurements, at or below the speed of light.

Hence, FTL is completely out of the question.

I agree though, that the shared (global) wavefunction (between Alice and Bob) seems to collapse instantaneously, and it has been proven that the speed of this "spooky influence" would have to exceed that of light by at least 4 orders of magnitude. Still, this is not the 'usual' FTL we are talking about...

 

[SpectraCat]

Spectra, you do mean Relativity of Simultaneity (RoS), right?


EDIT:




IMHO, it’s maybe a little bit 'risky' to use the words "FTL" and "information"... some reader might get the wrong impression...

It’s completely impossible to send any information from Alice to Bob, or vice versa, utilizing QM Entanglement. All we’ll ever get is random noise, in both 'ends'. There is not even a way to know if a measurement has been performed in the other 'end', or not. Everything that is 'weird' about entanglement is established after the measurements, at or below the speed of light.

Hence, FTL is completely out of the question.

I agree though, that the shared (global) wavefunction (between Alice and Bob) seems to collapse instantaneously, and it has been proven that the speed of this "spooky influence" would have to exceed that of light by at least 4 orders of magnitude. Still, this is not the 'usual' FTL we are talking about...

Yes .. I should not have used "information" at all, even though I was careful to qualify that I was talking about "information" between the quantum particles, not anything that is accessible to Alice or Bob. It's better scientifically to just say that the nature of the collapse is not understood and leave it at that ... but the problem is that the real significance of that kind of phrasing is rather subtle, and you need to have more than a lay-persons familiarity with quantum mechanics to understand what is being said.

 

[DevilsAvocado]

Okidoki

 

[SpectraCat]

You seem to keep missing the point. After you made the comment about the wave function spreading out over space in an earlier post, I thought you would understand that the wave function provides the state information for the particles at the instant of collapse, which answers the problem of Faster-Than-Light communication from particle to particle. Just because QM does not provide a detailed description of the collapse mechanism does not take away from the physics of the global wavefunction.

Our problem is still fundamentally the mystery of the two slit experiment. The entanglement issues all spring from that fundamental phenomena. After all of these decades (going all the way back to Planck and Einstein's papers around the turn of the 20th century) physics still does not answer the fundamental questions about the dual particle and wave properties of photons, electrons, muons, taus, Up quarks, Down quarks, Strange quarks, Charmed quarks, Top quarks and Bottom quarks--much less the entangled coherent combinations of these elementary particles forming complex systems.

I am certainly not the one who will unravel this fundamental mystery and was not trying to in my post, and I don't expect to see it unraveled on this forum--but I'll be cheering on any honest efforts.

But I still maintain that you are missing the point of the global character of the wave function that produces a system of coherent particles at the instant of collapse (in other words, don't look for Faster-Than-Light communication between the particles--the wave function already had that taken care of that--even if we don't understand the mechanism).

I am not missing the point at all ... in fact you are repeating some of the same points I have made in my posts, but seem to be attaching different significance to them. QM tells us what the entangled wavefunction looks like before measurement, and it tells us about the results after the measurement is complete. This seems clear for both of us. However, QM does not tell us anything about the nature of the transition between the two states (i.e. what is conventionally called "collapse"), or the timescale on which the transition between the two states occurs ... THAT is the point.

The phrase you used in your post, "the instant of collapse", is in fact not well-defined, but it does serve to emphasize the point I have been making, namely that there is a conflict between this colloquial description and relativity, which says that events separated by a space-like interval will not be found to be simultaneous in all reference frames. Thus from the point of view of relativity, it is problem that QM tells us that a local interaction of the entangled wavefunction with a measuring device ends up determining the state of an unentangled particle (here I refer to the post-measurement state) at an arbitrary separation from the site of the measurement, in a way such that the initiation of the measurement and the "appearance" of the unentangled particle appear simultaneous to all observers in all reference frames.

The existence of the conflict is not just hypothetical, because the collapse DOES seem to be instantaneous, or at least superluminal, based on experimental evidence. The experiments that have been done to test this put a lower limit of ~10,000 times the speed of light on the "speed of collapse". Thus, as I said, it is a completely legitimate scientific question to ask, "how can that be consistent with relativity"? This is what bothered Einstein so many years ago, and as far as I can tell, that question have never been answered.

Let me be clear that a NO point have I stated, nor intended to imply, that whatever is happening when the entangled wavefunction collapses allows information to be transmitted between human observers.

One thing we definitely do agree on is that the fundamental mystery at issue is the apparent non-locality of quantum mechanical wavefunctions, which lies at the root of both entanglement and the double-slit experiment.

 

[bobc2]

...It's better scientifically to just say that the nature of the collapse is not understood and leave it at that ...

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles. Thus, no Faster-Than-Light communication between particles is implied.

 

[SpectraCat]

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles.

That is a very vaguely phrased statement .. what precisely are you trying to say? What does "the information spanning the space of the wave function" mean? How is it "adequate to assure an allowed state for the particles"? Take the case of counter-propagating, polarization entangled photons as an example, since that is probably the case where the most experimental work has been done.

Thus, no Faster-Than-Light communication between particles is implied.

Right, and it's not ruled out either. QM simply doesn't have anything to say about the question, which is the point of this whole discussion.

One thing that may be worth pointing out is that the degrees of freedom involved in these collapse events are purely quantum mechanical in nature, since they are associated with the spins of the particles. Thus, since the particle spins are not associated with any spatial degrees of freedom (at least in non-relativistic QM), it may be that the collapse of the spin-entangled wavefunction doesn't need to propagate through space-time in the way that is assumed when we impose the restrictions of special relativity on the propagation of information. (That is just a personal speculation).

Since spin is a property that arises from the proper consideration of symmetry in the full relativistic description of QM, it may be that deeper answers lie within the framework of that theory. Sadly, I have not had the time to study relativistic QM in any depth yet, although it is on my to-do list.

 

[bobc2]

I've sketched a space time diagram for two entangled photons in an attempt to illustrate one of the issues arising with entanglement and special relativity.

A red guy moving along his X4'' world line at some relativistic speed (with respect to stationary black reference coordinate) while a blue guy moves in the opposite direction at the same relativistic speed as red. Red measures an UP state of the left moving photon at event A. And a short instant later red measures a DOWN state of the right moving photon at event B.

But the event at B occurs in the blue's instantaneous 3D cross-section of the 4-D universe at tB', whereas the original event A does not occur for Blue until tA', much later than event B. So, for the blue guy, event A happened after event B, whereas for the red guy event A was first followed an instant later by event B.

 

[xts]

bobc2: such experiment had been made over 10 years ago by N.Gisin and his group.

See: http://arxiv.org/abs/quant-ph/0002031
Journal of Physics A: 2001, Volume 34 Number 35

Experimental test of relativistic quantum state collapse with moving reference frames
H Zbinden, J Brendel, W Tittel and N GisinAn experimental test of relativistic wave-packet collapse is presented. The tested model assumes that the collapse takes place in the reference frame determined by the massive measuring detectors. Entangled photons are measured at 10 km distance within a time interval of less than 5 ps. The two apparatuses are in relative motion so that both detectors, each in its own inertial reference frame, are first to perform the measurement. The data always reproduces the quantum correlations and thus rule out a class of collapse models. The results also set a lower bound on the `speed of quantum information' to 2/3 ×10^7 and 3/2 ×10^4 times the speed of light in the Geneva and the background radiation reference frames, respectively. The very difficult and deep question of where the collapse takes place—if it takes place at all—is considered in a concrete experimental context.

 

[edpell]

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum. It is like they are two disjoint effects.

I think it would be worthwhile pushing the experimental lower limit on the speed of the QM collapse.

 

[xts]

I think it would be worthwhile pushing the experimental lower limit on the speed of the QM collapse.

It had been done already! See the article I quoted few posts above.

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum.

What even more important: no information is exchanged! The information is created (not exchanged) at the time of collapse and made available for both parties (but is not passed from one to another). It may seems to be weird (Einstein never could accept it), but that is what Bell->Aspect->Gishin had proved.

 

[edpell]

It had been done already!

Yes I see the lower limit of .6E7 times c for a separation of 10km. I am saying it would be good to do it for a separation of 100km and try to push the lower bound to .6E8 times c.

 

[bobc2]

bobc2: such experiment had been made over 10 years ago by N.Gisin and his group.

See: http://arxiv.org/abs/quant-ph/0002031
Journal of Physics A: 2001, Volume 34 Number 35

Experimental test of relativistic quantum state collapse with moving reference frames
H Zbinden, J Brendel, W Tittel and N Gisin

Great reference. Thanks.

An experimental test of relativistic wave-packet collapse is presented. The tested model assumes that the collapse takes place in the reference frame determined by the massive measuring detectors. Entangled photons are measured at 10 km distance within a time interval of less than 5 ps.

In which reference frame is the 5 ps measured?

The two apparatuses are in relative motion so that both detectors, each in its own inertial reference frame, are first to perform the measurement. The data always reproduces the quantum correlations and thus rule out a class of collapse models. The results also set a lower bound on the `speed of quantum information' to 2/3 ×10^7 and 3/2 ×10^4 times the speed of light in the Geneva and the background radiation reference frames, respectively. The very difficult and deep question of where the collapse takes place—if it takes place at all—is considered in a concrete experimental context.

Notice that in the space diagram representation in my previous post that you always get the quantum correlation, regarless of which instantaneous 3-D slices of the 4-D universe that you choose. So, there is no conflict with the experimental results in your reference.

But the locations of the particle pair is ambiguous when you consider the red observer's 3-D space compared to the blue observer's 3-D space.

 

[xts]

In which reference frame is the 5 ps measured?

In Swiss Alps frame, but it doesn't really matter, as the relative speed of the detectors were pretty nonrelativistic: 100 m/s - "Ferrari may do it!" (Gisin's comment)

One more great reading (quite easy, even for non-physicists!) on this thread's topic:
David Mermin, "Is the moon there when nobody looks? Reality and the quantum theory", PHYSICS TODAY / APRIL 1985 PAG. 38-47,
http://www-f1.ijs.si/~ramsak/km1/mermin.moon.pdf“Anybody who’s not bothered by Bell’s theorem has to have rocks in his head.”
To this moderate point of view I would only add the observation that contemporary physicists come in two varieties.
Type 1 physicists are bothered by EPR and Bell’s theorem.
Type 2 (the majority) are not, but one has to distinguish two subvarieties.
Type 2a physicists explain why they are not bothered. Their explanations tend either to miss the point entirely (like Born’s to Einstein) or to contain physical assertions that can be shown to be false.
Type 2b are not bothered and refuse to explain why. Their position is unassailable. (There is a variant of type 2b who say that Bohr straightened out the whole business, but refuse to explain how.)If you don't want to be of type 2b - you must read it!

 

[DevilsAvocado]

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles. Thus, no Faster-Than-Light communication between particles is implied.

I’m walking on thin line here... since I’ve been 'nagging' about "NO FTL!"... which is true.

But I have to ask you; you’re not saying that "the information the wave function carries" is all we need, right?

Because that is not true, the polarizer’s ('rotating' randomly at very high speed) are outside each other’s light-cone, and the total time for the final setting of the polarizer + the measurement registration is only a few nanoseconds, and at a distance of 10 or 18 km there is no way for this information (the final polarizer setting) to reach the other photon/apparatus.

We need TWO 'things': The final setting of the polarizer’s + the shared (global) wavefunction.

Only the information the wave function carries is not enough for EPR-Bell experiments.

 

[DevilsAvocado]

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum. It is like they are two disjoint effects.

The contradiction becomes clear for Relativity of Simultaneity. In one frame of reference Alice will perform the measurement first. In another frame of reference Bob will perform the measurement first.

Problem: There is only ONE shared wavefunction, which can only collapse ONCE, hence you get in conflict with good old Albert...

 

[xts]

Problem: There is only ONE shared wavefunction, which can only collapse ONCE, hence you get in conflict with good old Albert...

You may try to see that in the following way: shared wavefunction collapses not when Alice or Bob measure their parts, but at the moment when they meet to check correlations between their observations.

Such approach is valid from any of: Alice's, Bob's and their dad's points ov view.

 

[bobc2]

In Swiss Alps frame, but it doesn't really matter, as the relative speed of the detectors were pretty nonrelativistic: 100 m/s - "Ferrari may do it!" (Gisin's comment)

One more great reading (quite easy, even for non-physicists!) on this thread's topic:
David Mermin, "Is the moon there when nobody looks? Reality and the quantum theory", PHYSICS TODAY / APRIL 1985 PAG. 38-47,
http://www-f1.ijs.si/~ramsak/km1/mermin.moon.pdf


“Anybody who’s not bothered by Bell’s theorem has to have rocks in his head.”
To this moderate point of view I would only add the observation that contemporary physicists come in two varieties.
Type 1 physicists are bothered by EPR and Bell’s theorem.
Type 2 (the majority) are not, but one has to distinguish two subvarieties.
Type 2a physicists explain why they are not bothered. Their explanations tend either to miss the point entirely (like Born’s to Einstein) or to contain physical assertions that can be shown to be false.
Type 2b are not bothered and refuse to explain why. Their position is unassailable. (There is a variant of type 2b who say that Bohr straightened out the whole business, but refuse to explain how.)


If you don't want to be of type 2b - you must read it!

Nice post! Thanks.