Simultaneity for entangled particles

[michelcolman]

Whenever you read about entangled particles, and how measuring one of them "immediately" changes the other, the problem of faster than light travel is usually dismissed with "o, but you can't use it to transmit information, so it doesn't contradict relativity".

However, I think there's a more fundamental problem with this: what's the meaning of the word "immediately" or "simultaneously"? After all, simultaneity is a very subjective thing. Two events that are simultaneous for one observer, will happen at very different times for someone else with a different frame of reference. In fact, there's no such thing as objective simultaneity for spatially separated events.

So, in which frame of reference are those waveform collapses "simultaneous"?

 

[bcrowell]

What you're saying seems to me to be a valid argument to the effect that anyone who says the wavefunction collapse of the entangled particles happens "immediately" is oversimplifying or misunderstanding.

 

[michelcolman]

What you're saying seems to me to be a valid argument to the effect that anyone who says the wavefunction collapse of the entangled particles happens "immediately" is oversimplifying or misunderstanding.

Well, I suppose that I must be one of those people who is misunderstanding. What exactly happens when someone measures (or not) one of the two particles?

Does the act of measuring one of the particles change the past so that both particles are retroactively put into a determined state from the time they were emitted? That's the only alternative to "something" happening at "some time" that I can come up with.

 

[Passionflower]

Whenever you read about entangled particles, and how measuring one of them "immediately" changes the other, the problem of faster than light travel is usually dismissed with "o, but you can't use it to transmit information, so it doesn't contradict relativity".

However, I think there's a more fundamental problem with this: what's the meaning of the word "immediately" or "simultaneously"? After all, simultaneity is a very subjective thing. Two events that are simultaneous for one observer, will happen at very different times for someone else with a different frame of reference. In fact, there's no such thing as objective simultaneity for spatially separated events.

So, in which frame of reference are those waveform collapses "simultaneous"?

Honestly I do not think anybody knows the answer to this question at this time.

What you're saying seems to me to be a valid argument to the effect that anyone who says the wavefunction collapse of the entangled particles happens "immediately" is oversimplifying or misunderstanding.

Ok, it seems you are implying that you one of those people who does understand it.
Care to share it or are you going to decline because we are perhaps not 'ready' for the answers?

 

[venton]

My understanding...Entangled particles are in a superposition, they are in two states at once, and only choose which state they are in when one is observed. It is proven that they are in two states at once due to interference, eg the double slit experiment.
Imagine an entangled particle is sent a light year away and we look at the 'our' particle, it will choose one of two states. Immediately, the other particle will adopt the same state. This could be proven by telling someone to move a light year away that in one light year + 1 minute to check the spin of the entangled particle and write it down. We wait a year, look at the state of our particle and write it down. We then travel to our friend and ask what they wrote down. This would prove that the infomation was transmitted instantly, although it takes a year to find out.

 

[kfx]

My guess is that the speed is relative to a static observer.

I did post a similar question today. https://www.physicsforums.com/showthread.php?t=445534

Although it got few good replies, I found out that in http://www.nature.com/nature/journal/v454/n7206/full/nature07121.html got it completely right)

To my understanding, the theoretical speed of "spooky action at distance" is indeed infinite. The experimentally measured speed is at least 10'000 times the speed of light.

 

[LBrandt]

My understanding...Entangled particles are in a superposition, they are in two states at once, and only choose which state they are in when one is observed. It is proven that they are in two states at once due to interference, eg the double slit experiment.
Imagine an entangled particle is sent a light year away and we look at the 'our' particle, it will choose one of two states. Immediately, the other particle will adopt the same state. This could be proven by telling someone to move a light year away that in one light year + 1 minute to check the spin of the entangled particle and write it down. We wait a year, look at the state of our particle and write it down. We then travel to our friend and ask what they wrote down. This would prove that the infomation was transmitted instantly, although it takes a year to find out.

I'm not a physicist, but my guess would be that both Einstein and Niels Bohr would find several things wrong with the above test. For one, they would probably say that due to the problem of relativity of simultaneity, you could not know with certainty that the second observer's measurement was simultaneous with the first observer's measurement. And secondly, the two observers would be using separate measuring instruments. One of the principles of QM is that the act of observation collapses the wave function, and the instrument and the observer are all part of the observation.

 

[venton]

And secondly, the two observers would be using separate measuring instruments. One of the principles of QM is that the act of observation collapses the wave function, and the instrument and the observer are all part of the observation.

Yes they would use separate instruments but I don't think that matters. They would find that the two particles, whereas before they were in a superposition, would adopt the related spin of the other...somehow the first one must have communicated it's state to the other. Provided the other is measured just after the other, but the separation is further than the speed of light, I would assume that the communication was faster than the speed of light. But again, we cannot know the results immediately, we would have to travel to the other observer and ask them what it was they recorded at the 'same time'. Agreed, recording at the 'same' time is a tricky one..but I would think it would be possible.

 

[atyy]

My guess is that entanglement of spatially separated particles must itself be defined using a frame. So the instantaneous collapse is relative to that frame. I'm not entirely sure about the first point, as there seems to be some counterexamples at the end of http://arxiv.org/abs/quant-ph/0610030

 

[venton]

I found out that in http://www.nature.com/nature/journal/v454/n7206/full/nature07121.html got it completely right)

To my understanding, the theoretical speed of "spooky action at distance" is indeed infinite. The experimentally measured speed is at least 10'000 times the speed of light.

I'm not a physicist either but it seems to me that the experiments you mention have proven that spooky action at a distance does indeed exist, end of. Does anyone disagree?

 

[JesseM]

I'm not a physicist either but it seems to me that the experiments you mention have proven that spooky action at a distance does indeed exist, end of. Does anyone disagree?

Experiments have supported quantum-mechanical predictions about entanglement which are incompatible with "local realism", see [post=2195880]this post[/post] for a simple explanation of why QM statistics contradict local realism. However that doesn't necessarily mean reality is nonlocal and FTL influences exist, there is also the possibility of dropping some of the assumptions that fall into the category of "realism". For example, the many-worlds interpretation drops the assumption that each measurement must have a unique outcome, and many of its supporters argue that it is compatible with locality--see the references I gave in [post=1647627]this post[/post], along with a simply "toy model" of how we might picture a local MWI-like model that agrees with QM's statistical predictions in [post=1557143]this post[/post].

 

[bcrowell]

What you're saying seems to me to be a valid argument to the effect that anyone who says the wavefunction collapse of the entangled particles happens "immediately" is oversimplifying or misunderstanding.

Well, I suppose that I must be one of those people who is misunderstanding. What exactly happens when someone measures (or not) one of the two particles?

Does the act of measuring one of the particles change the past so that both particles are retroactively put into a determined state from the time they were emitted? That's the only alternative to "something" happening at "some time" that I can come up with.

It seems to me that questions like "what exactly happens?" and "Does the act of measuring one of the particles change the past so that both particles are retroactively put into a determined state from the time they were emitted?" are questions that don't have well-defined answers. Science can only answer questions that boil down to observable quantities. If two entangled particles are prepared at spacetime event A, and observed later at events B and C, the only observables are ones like "an observer at B measures X" and "an observer at C measures Y."

 

[michelcolman]

My guess is that the speed is relative to a static observer.

I did post a similar question today. https://www.physicsforums.com/showthread.php?t=445534

Although it got few good replies, I found out that in http://www.nature.com/nature/journal/v454/n7206/full/nature07121.html got it completely right)

To my understanding, the theoretical speed of "spooky action at distance" is indeed infinite. The experimentally measured speed is at least 10'000 times the speed of light.

The problem is that there's no such thing as a "stationary observer".

Take the example of the train that's hit by two bolts of lightning, one at the front and one at the back of the train. To an observer standing next to the track, the two strikes happened simultaneously. To someone on board the train, the one at the front happened first. Both are right. You may say that the observer standing next to the train is "stationary", but what about a train that's going from east to west over the equator at 1667 km/h? I'm sure we'll agree that the train is stationary while the Earth is spinning underneath it, right? Unless you take into account the movement of the Earth around the sun, the sun around the center of our galaxy, etc...

The whole point of relativity is that ANY observer may consider himself to be stationary.

If two events happen at the same time for one observer, they will happen at different times for someone else. If something travels "faster than light" or even "infinitely quickly" for one observer, you will easily find a different observer who will say the events did not happen at the same time, the information traveled at a snail-like pace, or even traveled backward (the second particle choosing its state before the first one was measured).

And you can't say "this observer is right, and the other is wrong, because the first one was stationary". Again, that's the whole point of relativity.

Go back to the train example again, only now someone measures a particle at the back of the train while the entangled particle is at the front. If the two took their definite state at exactly the same time for someone on the train (hence, the information traveled infinitely quickly for them), someone standing next to the train would say (correctly!) that the particle in front took its state maybe one second later than the measurement at the back, and therefore the information traveled at a very finite speed equal to the length of the train divided by one second.

The only "preferred" reference frame I can imagine, is one determined by the source emitting the two particles. You can't even take the reference frames of the observers, because then you can get into a race condition with two observers with different speeds each claiming quite rightfully to be the first to make the measurement.

The other option, of course, is that you cannot define a time "when" the decoherence occurred. But I'll get into that in a separate reply to someone else's post.

 

[venton]

Imagine this experiment.
Two people meet and they have very accurate time pieces which can measure time to a billionth of a second and are synchronised. They agree that at 08:00 the next day one will measure the spin of an entangled particle. And at 08:00 + 1 billionth of a second the other will measure the spin of the other entangled particle.
The other gets on his bike and cycles 20 kilometers away, with his entangled particle in a box ;)
The next day at 08:00 observer one notes the spin of his particle, and at 08:00+1 billionth of a second the second observer notes the spin of his particle.
He cycles back to the first observer. They compare watches and they are still synchronised.
They compare notes on the particles' spin and realize that observer 2's particle 'knew' of particle 1's state within 1 billionth of a second. Quicker than the time it would have taken a light beam to reach him. There are no moving trains, they are both moving on the Earth at the same speed. Can we not conclude that spooky distant at an action has occurred?
I do apologise if the above have already answered this, but I don't understand some of the terms.

 

[michelcolman]

It seems to me that questions like "what exactly happens?" and "Does the act of measuring one of the particles change the past so that both particles are retroactively put into a determined state from the time they were emitted?" are questions that don't have well-defined answers. Science can only answer questions that boil down to observable quantities. If two entangled particles are prepared at spacetime event A, and observed later at events B and C, the only observables are ones like "an observer at B measures X" and "an observer at C measures Y."

That makes sense, but... if I am not mistaken, it is possible for someone to determine whether or not a particle is in a determined state, right? How else can scientists claim to have created entangled particles that only chose their state when they were measured? They might have been in a determined state all the time, but they claim to have done experiments where this was definitely not the case. How do they know?

I suppose you can use interference effects like the double slit experiment or something similar to find out whether particles really made the entire trip while being in an undetermined state.

The experiment around Geneva in 2008 that someone mentioned, even set a lower limit for the speed at which "quantum information" would have had to have traveled, which to me seems to indicate they have a pretty good idea when the decoherence happened. Clearly, they claim the photons were still very entangled and undetermined when they left the source and traveled all the way through the fibers, they even took care to make the fibers the same length up to a cm. They seem to make a pretty definite claim "the decoherence happened at THAT precise time" (simultaneously in their reference frame, with a lower limit of 10000c if there was any time lag).

Or is there the non-disproven possibility that, each time they decided to measure a photon, the photons turned out to have traveled in a determined untangled state during the whole trip? That's the only other explanation that appears to make sense to me right now.

 

[venton]

Or is there the non-disproven possibility that, each time they decided to measure a photon, the photons turned out to have traveled in a determined untangled state during the whole trip? That's the only other explanation that appears to make sense to me right now.

I think that we have to accept that the entangled particles are in a superposition of states before measurement, this is key to quantum theory isn't it?

 

[Q-reeus]

I'm not a physicist either but it seems to me that the experiments you mention have proven that spooky action at a distance does indeed exist, end of. Does anyone disagree?

Ditto - not a physicist but strongly dislike 'mystical-magical' notions. It supposedly has been settled that the 'real' interpretation is not one of instantaneous communication, but that reality only exists after a measurement has been made (at either end) http://www.arxiv.org/pdf/quant-ph/0106119". There are respected theorists who claim to prove entanglement is a myth. Two I can think of are:
Athanasios Prikas: http://arxiv.org/find/physics/1/au:+Prikas_A/0/1/0/all/0/1"
Joy Christian: http://arxiv.org/find/quant-ph/1/au:+Christian_J/0/1/0/all/0/1"
He uses Clifford Algebra to attack the basis of the Bell Inequality. Makes the strong assertion that ALL the predictions of QM can be met by a purely local, causal, and realistic theory. I don't pretend to understand the math, but it is very appealing, and he proposes an experiment to prove it.

 

[michelcolman]

I think that we have to accept that the entangled particles are in a superposition of states before measurement, this is key to quantum theory isn't it?

I don't know. When you measure a particle, how do you know whether or not it was still in a superposition of states one microsecond earlier?

All you know is that, if you don't measure it, it remains in a superposition of states (causing interference patterns etc.) And if you do measure it, it turns out to be in a definite state. But for all you know, it may have been in a definite state all along, knowing in advance that you were going to measure it. How can you tell?

 

[venton]

I don't know. When you measure a particle, how do you know whether or not it was still in a superposition of states one microsecond earlier?

All you know is that, if you don't measure it, it remains in a superposition of states (causing interference patterns etc.) And if you do measure it, it turns out to be in a definite state. But for all you know, it may have been in a definite state all along, knowing in advance that you were going to measure it. How can you tell?

I believe that the act of observation collapses the wavefunction, I don't believe it can know that you are going to measure it before you do, but I have no proof.

 

[michelcolman]

And there's another problem with these entanglement experiments that has been bugging me:

Suppose you send a whole series of pairwise entangled photons to two destinations far away from each other. As long as nobody does any measurements, the photons remain in a superposition of states. This can be shown by letting them go through one of two slits depending on some property, so that the undetermined photons go through both slits and cause an interference pattern.

Interference pattern -> nobody messed with the photons
No pattern -> somebody has been measuring them.

Now suppose the photons travel to two scientists, Alice and Bob, that are 100 light years apart. Bob does not make any measurements on the photons, he just looks at the interference patterns. Alice has a message to send to Bob: if she wants to send a "1", she measures incoming photons for one millisecond. This destroys the interference pattern at Bob's end during that time. Mission accomplished, the information was sent "instantly", "faster than light". And of course they can agree on a protocol to send information back and forth, creating instant faster than light communication.

Of course this must be wrong, but... why? What assumption did I make that was incorrect?

(I imagine there will be no interference pattern no matter what Alice does because physics decided not to be tricked by us humans...)

 

[venton]

michelcolman I think you should put a patent on that!

 

[michelcolman]

Imagine this experiment.
Two people meet and they have very accurate time pieces which can measure time to a billionth of a second and are synchronised. They agree that at 08:00 the next day one will measure the spin of an entangled particle. And at 08:00 + 1 billionth of a second the other will measure the spin of the other entangled particle.
The other gets on his bike and cycles 20 kilometers away, with his entangled particle in a box ;)
The next day at 08:00 observer one notes the spin of his particle, and at 08:00+1 billionth of a second the second observer notes the spin of his particle.
He cycles back to the first observer. They compare watches and they are still synchronised.
They compare notes on the particles' spin and realize that observer 2's particle 'knew' of particle 1's state within 1 billionth of a second. Quicker than the time it would have taken a light beam to reach him. There are no moving trains, they are both moving on the Earth at the same speed. Can we not conclude that spooky distant at an action has occurred?
I do apologise if the above have already answered this, but I don't understand some of the terms.

Suppose now that each puts up his hand at the precise moment of 08:00. Then someone that happens to be riding past on a fast train will swear (and he won't even be lying) that one put up his hand a little bit earlier than the other. Depending on the speed and direction the train is traveling in, the second observer may have measured his spin before the first one. So whose measurement influenced whom?

Of course things are a lot simpler if you don't include moving trains, but that's the whole point. Everything needs to remain consistent no matter how many trains you add.

In this case it does not really matter which measurement came first, but I seem to have understood that there are ways of knowing whether or not decoherence has taken place, in which the order of measurements has some importance.

 

[michelcolman]

michelcolman I think you should put a patent on that!

I'm not sure I wish to be remembered as "the guy that invented FTL information transfer, enabling people to transfer messages into the past, bringing the world's financial system down and resulting in world wide chaos destroying civilisation as we know it".

I would prefer to be wrong, and I'm pretty sure I am ;)

 

[JesseM]

And there's another problem with these entanglement experiments that has been bugging me:

Suppose you send a whole series of pairwise entangled photons to two destinations far away from each other. As long as nobody does any measurements, the photons remain in a superposition of states. This can be shown by letting them go through one of two slits depending on some property, so that the undetermined photons go through both slits and cause an interference pattern.

Interference pattern -> nobody messed with the photons
No pattern -> somebody has been measuring them.

Now suppose the photons travel to two scientists, Alice and Bob, that are 100 light years apart. Bob does not make any measurements on the photons, he just looks at the interference patterns. Alice has a message to send to Bob: if she wants to send a "1", she measures incoming photons for one millisecond. This destroys the interference pattern at Bob's end during that time. Mission accomplished, the information was sent "instantly", "faster than light". And of course they can agree on a protocol to send information back and forth, creating instant faster than light communication.

Of course this must be wrong, but... why? What assumption did I make that was incorrect?

(I imagine there will be no interference pattern no matter what Alice does because physics decided not to be tricked by us humans...)

As you suggest at the end, no interference pattern will be seen in the total pattern of photons seen by Bob, although if Bob later learns which of Alice's detectors registered each entangled twin, he may see an interference pattern in the subset of photons on his screen which registered at a certain detector of Alice's (specifically, a detector of Alice's whose position makes it impossible to deduce which slit the entangled twin went through, as opposed to a detector in a different position which does allow one to deduce this). See my discussion of the "delayed choice quantum eraser", which is quite similar to what you're imagining, in post #4 [post=2908070]here[/post].

 

[michelcolman]

As you suggest at the end, no interference pattern will be seen in the total pattern of photons seen by Bob, although if Bob later learns which of Alice's detectors registered each entangled twin, he may see an interference pattern in the subset of photons on his screen which registered at a certain detector of Alice's (specifically, a detector of Alice's whose position makes it impossible to deduce which slit the entangled twin went through, as opposed to a detector in a different position which does allow one to deduce this). See my discussion of the "delayed choice quantum eraser", which is quite similar to what you're imagining, in post #4 [post=2908070]here[/post].

I think I'm missing something here.

Suppose both Alice and Bob have a double-slit system set up for the incoming photons.

- If Alice does not measure any photons, Bob will see an interference pattern
- If Alice measures ALL of the photons (determining with certainly whether they went through the first or the second slit), Bob will not see any interference pattern.
- Alice can decide on a whim whether or not to measure the photons, with an "instant" result on Bob's side 100 light years away.

What "subset" of measurements are you talking about? Either Alice measures her photons, or she doesn't.

 

[JesseM]

There are respected theorists who claim to prove entanglement is a myth. Two I can think of are:
Athanasios Prikas: http://arxiv.org/find/physics/1/au:+Prikas_A/0/1/0/all/0/1"
Joy Christian: http://arxiv.org/find/quant-ph/1/au:+Christian_J/0/1/0/all/0/1"
He uses Clifford Algebra to attack the basis of the Bell Inequality. Makes the strong assertion that ALL the predictions of QM can be met by a purely local, causal, and realistic theory. I don't pretend to understand the math, but it is very appealing, and he proposes an experiment to prove it.

I don't think either of these theorists are very widely "respected", and I'd be curious if any of their papers claiming to disprove Bell's theorem (which shows QM is incompatible with all possible local realist theories) have been published in any peer-reviewed journals, as far as I can see it's all arxiv preprints. When nearly all physicists agree Bell's theorem is mathematically airtight and only a tiny minority disagree, is it really wise to trust that small handful just because it fits with your philosophical preconceptions?

 

[JesseM]

I think I'm missing something here.

Suppose both Alice and Bob have a double-slit system set up for the incoming photons.

- If Alice does not measure any photons, Bob will see an interference pattern

No, it doesn't work that way. The total pattern of photons seen by Bob never shows an interference pattern regardless of what happens to the entangled photons, entangled photons behave differently than unentangled ones in this sense. However, if Alice measures the photons in a particular way that ensures the "which-path" information is "erased" (so her measurements give no information about which slit each of Bob's photons went through), then if Bob and Alice get together and compare notes, an interference pattern will be seen in the subset of Bob's photons whose entangled twins went to either of Alice's two detectors. I really recommend looking over the [post=2908070]example[/post] involving the delayed choice quantum eraser if you want to get a better understanding.

 

[Q-reeus]

I don't think either of these theorists are very widely "respected", and I'd be curious if any of their papers claiming to disprove Bell's theorem (which shows QM is incompatible with all possible local realist theories) have been published in any peer-reviewed journals, as far as I can see it's all arxiv preprints. When nearly all physicists agree Bell's theorem is mathematically airtight and only a tiny minority disagree, is it really wise to trust that small handful just because it fits with your philosophical preconceptions?

Well 'respected' is a relative label. Both might well argue they are victims of scientific 'consensus politics' - genuine openness to anything radically different is largely an ideal not reality. This is the cry of climate change sceptics - peer-reviewed journals refuse to publish their papers, then they are accused of being fringe scientists because they don't get papers published in peer-reviewed journals.

At any rate both Prikas and Christian do make quite strong arguments, from quite different perspectives. If you read the pretty non-technical intro in Prikas's paper, I find it hard to counter the logic - energy/momentum will 'magically' alter given a particular succession of measurements, assuming entanglement. He has also proposed experimental tests. Joy Christian seems to be particularly rigorous and beat off all critics to his earlier papers. I guess the one real acid test would be if his theory predicts an upper 'q-bit limit' for quantum computing that would fail, but I'm unaware of any such.

 

[venton]

Regarding Michlel's FTL communicator:-

I imagine a Cathode Ray Tube which either shows bands (interference) or spots (non interference) of photons. It is the changing of the banded pattern to spots of light and back which are used to communicate a 1 or a zero. The banded pattern or spot is controlled by Alice looking at which slit the photon goes through - causing a spot. She stops looking/detecting and it goes back to an interference pattern. Because Bob's photons are entangled he can sit back and watch his CRT go from bands to spots.
Are we saying that entangled photons cannot be made to behave this way, ie like double slit experiment?

 

[darkhorror]

If you measure the entangled particle in ether frame of reference aren't you going to get the results that you would normally get? Can you even tell which measurement happened "first"

 

[Dale]

Does the act of measuring one of the particles change the past so that both particles are retroactively put into a determined state from the time they were emitted?

Some people believe exactly that:
http://arxiv.org/abs/0706.1232

 

[michelcolman]

No, it doesn't work that way. The total pattern of photons seen by Bob never shows an interference pattern regardless of what happens to the entangled photons, entangled photons behave differently than unentangled ones in this sense. However, if Alice measures the photons in a particular way that ensures the "which-path" information is "erased" (so her measurements give no information about which slit each of Bob's photons went through), then if Bob and Alice get together and compare notes, an interference pattern will be seen in the subset of Bob's photons whose entangled twins went to either of Alice's two detectors. I really recommend looking over the [post=2908070]example[/post] involving the delayed choice quantum eraser if you want to get a better understanding.

I have heard about the "erasing" experiment, haven't quite understood the fine parts of it (for example, the addition would not appear to add up to a uniform picture at first sight), but anyway, it's a different experiment.

Mine is a lot simpler. But why can't entangled photons behave like non-entangled photons in the double slit experiment? They're just photons in an indeterminate state that happen to have a twin somewhere. Or did nature somehow understand our little trick so it decided to consider the whole setup a measurement that causes decoherence? I know QM is full of this sort of unintuitive results, so I wouldn't even be totally surprised.

 

[michelcolman]

Some people believe exactly that:
http://arxiv.org/abs/0706.1232

At least that would solve the simultaneity problem. Measuring one particle causes the pair to have been decoherent from the beginning, so there's no fighting over whose reference system is to be used.

But don't the past experiments already include some component to verify that decoherence has not taken place yet before the measurement?

 

[t_barlow]

Are we saying that entangled photons cannot be made to behave this way, ie like double slit experiment?

No they can't. Entangled particles always look like spots, in particular Bob's will always look like spots regardless of what Alice does to hers. The reason is, each particle when considered in isolation appears to be in a mixed state, which means it resembles a statistical mixture of up/down, which never show interference, unlike a coherent superposition of up/down.

The way I see it is, if you measure an entangled particle, you do not change the state of its entangled pair. What changes is your information about the other particle, which is stored in your physical state (e.g. as a dark spot on a screen), so it is local to you. That information tells you what you will get if you go and measure the other particle, but it doesn't tell you what someone else found when they measured it. The only way to meaningfully compare what two people measured on spatially separated systems is to bring those two observers together into causal contact and see what they say. Fortunately quantum mechanics guarantees us consistency, which is nice.

 

[michelcolman]

If you measure the entangled particle in ether frame of reference aren't you going to get the results that you would normally get? Can you even tell which measurement happened "first"

It doesn't matter much except if you are trying to determine when the decoherence happened. If it is possible for an observer to receive a photon and say for sure "this photon was in an indeterminate state before I looked at it, and now it took state x", then you can have a problem with two observers who can both be considered to be "first" but only the real first will have received an initially indeterminate photon.