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I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the expla

I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?

sergiotapia 5y ago
Reminds me of the in-lore comms system in Mass Effect. I think the comms in the ship were two atoms that were entangled, allowing instant messages no matter how many lightyears away the ship was from Earth.
deleted 5y ago
[deleted]
misiti3780 5y ago
This book elucidates the concepts well: https://en.wikipedia.org/wiki/Something_Deeply_Hidden https://en.wikipedia.org/wiki/Something_Deeply_Hidden
drcode 5y ago
Imagine you have a pouch with a red and a blue marble in it, then take out a marble without looking at it and hand the pouch to a friend. Later, if you look at your marble, you instantly have information about the other marble at a speed greater than the speed of light... but you couldn't use that fact to send a message. The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend. (This is assuming the "multiple universes" interpretation- In the other interpretations there is "spooky action at a distance", but this action happens in EXACTLY THE RIGHT WAY to prevent you from transmitting a message to your friend)
- danbruc 5y ago
  The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend. I don't think it is helpful to talk about multiple universes, that makes a strong implication towards a many world interpretation. It is better to say that the difference is that in the classical case the decision who gets which marble happens when one of the marbles is taken out of the pouch while in the case of entanglement we do not really know when the decision happens but it does provably work differently than in the classical case. It might be that the decision is never truly made, that both outcomes happen in two parallel worlds, it might be that the decision is only made when one party inspects their marble, it might be that it happens at the same time as in the classical example, ...we don't know.
  - narrator 5y ago
    Or, in the transactional interpretation, the other guy's marble sends a signal from the future back to your marble to change its color when you look at it.
  - tbabb 5y ago
    > that makes a strong implication towards a many world interpretation You say that like it's a shortcoming. :) There are many who take the (very reasonable) position that the many worlds interpretation is the most epistemologically parsimonious one. Contrary to some misunderstandings of it, it doesn't "add" extra worlds; it removes the concept of "wave function collapse", and leaves all the other known laws of quantum mechanics completely unchanged. The "worlds" arise naturally as more and more particles in the environment become entangled with the measured system, and "wave function collapse" turns out to be the predicted observation of an observer who is themselves made out of quantum states. The only difference between many worlds and the "standard" Copenhagen interpretation is that Copenhagen adds that, at some point, the entanglement process stops, and a bunch of states in the wave function disappear. And it doesn't specify how, or why, or how to calculate when it will happen. Those that advocate for many worlds would point out that this extra epistemological burden is questionable, given that the correct prediction is made without it.
snissn 5y ago
Two balls are a box. Neither are spinning. The box gets “shaken up” and the balls hit each other. We know that one ball is spinning clockwise and the other is counter clockwise because angular momentum spin is conserved. The balls launch far away from each other. We know the spin is entangled in that one is clock wise the other is counter clockwise but we don’t know which is which until we measure. How do we use that to communicate?
- ende 5y ago
  By constraining all your communications to a game of interstellar rock paper scissors?
  - alisonkisk 5y ago
    that doesn't communicate across the distance. that communicates from the common starting point
stevenjgarner 5y ago
Or even better than instantaneously, let's get messages sent to us from the future using a Ronald Lawrence Mallett time machine based on a ring laser's properties, such that at sufficient energies, the circulating laser might produce not just frame-dragging but also closed timelike curves (CTC), allowing time travel into the past. I cannot believe that Ronald Mallett's biggest challenge is getting funding for a feasibility test. Isn't it the greatest venture capital opportunity of all time?
PavleMiha 5y ago
Can't say I have perfect intuition on it either, but the closes I've gotten was by reading this book: https://www.qisforquantum.org https://www.qisforquantum.org
wwarner 5y ago
Not a physicist, but my answer to you is that usually superliminal speeds is the price that physicists are willing to pay to explain what is observed in experiment. I get your objection to the rather convoluted argument that special relativity still applies to message transfer, but I accept it. John Preskill explains the information within entanglement with an analogy to a book. Normally with a book, you can read one page seperately from all the other pages. Further, if you unbound the book, and randomly distributed the pages to your friends, you could put your heads together and reconstruct the entire book. With a "quantum book", the information is encoded in the correlations between the observables, and you can only see the information when all the pages of the book are together and in the correct order. If you look at a single page of the quantum book, it's purely random gibberish, and you can't derive anything about the book by looking at a part of it.
simonh 5y ago
It can't be used to send a message because all you can do is measure your particle. Even if doing so changes the state of the other particle far away (which isn't really what's happening, but that doesn't matter), all the other person at the end can do is measure their particle. Neither of you can choose what the state of either particle is. You have no control, so there's no way to transmit information. What you can do is agree in advance that you will both take certain actions based on the measured state of the particles. There's no way to be sure the person at the other end actually does so though.
- ende 5y ago
  I now have to imagine that Quantum Game Theory is a thing that exists.
  - drdeca 5y ago
    AIUI, yes, it has been found that, e.g. there are games where two players who can’t communicate but who do have a pair of entangled qubits and can each make a measurement, would have a strategy which is better for both players than if they didn’t have this resource. (where the games in the version where they don’t have the resource of the entangled pair of qubits, are fairly simple games where each player receives some information and then has a choice between two options)
superposeur 5y ago
> how you can be sure about the state of the other particle, what if someone already measured it and then did something to it? Indeed, you are only sure about the state of the other particle in the instant just after they measured it. Whoever measures first instantly destroys the entanglement link, so if they chose to manipulate the particle after measurement, you will have no knowledge of these manipulations. More generally, note that in quantum mechanics "reading" the state of a particle (i.e. performing a measurement) is drastically different than "writing" information by manipulating a particle. Most entanglement-related weirdness hinges on this fundamental asymmetry between "read" and "write" operations for quantum information.
axelf4 5y ago
Maybe this video by Veritasium could help: https://www.youtube.com/watch?v=kTXTPe3wahc https://www.youtube.com/watch?v=kTXTPe3wahc
abetusk 5y ago
Let's say particles have a 'direction angle' that we can measure with a detector that only gives 'up' or 'down' relative to a direction angle measurement. We can change this direction angle measurement with a knob to set what the measured 'up' and 'down' answers are relative to the detector's direction angle. Further let's say particles can be quantum entangled so that when when two detectors are placed very far apart, many light years apart, say, and measure a quantum entangled pair of particles. When the two detectors are set to the same, but arbitrary, angle, the detectors give the same answer. This is normal correlation. Quantum correlation says that as one dial moves away from the other reference point, the correlation falls off as a sine wave, not a linear decrease as would be expected by classic probability. To see how bonkers this is, do the following experiment: Set detector X to be at angle 0 and detector Y to give a 1% error rate. Call that 1% angle 'a'. So a sample experiment run might be: X(0): 0001000101001110...111010 Y(a): 0011000101001110...111010 In the above, 1 could be an 'up' and 0 could be a 'down' detection, say. For concreteness, let's just say A and B ran 100 detections and there was one difference between them (giving 1% error), represented by the third differing bit in the above. Now let let's change both X and Y by the same angle so the relative error rate between them is still 1%, this might give something like: X(a): 0011000001001110...111010 Y(2a): 0011000101001110...111010 X and Y still have one difference in the above, but now with the 8th position changed. So far this is nothing unexpected from classical probability. Now, we know that from X(0) to Y(a) there's one change, from X(a) to Y(2a) there's one change. Classic probability says that there can be at most two flipped bits from X(0) to Y(2a). Quantum mechanics predicts three. To convince yourself, try making a list of bits such that there's one difference between X(0) and Y(a), one difference between X(a) and Y(2a) but three differences from X(0) to Y(2a). It's impossible and this is the heart of Bell's theorem. Bell's theorem is a classical probability statement, generalized from my above statement that if |X(0)-Y(a)|=1, |X(a)-Y(2a)|=1 then |X(0)-Y(2a)|<=2. Quantum entanglement violates Bell's inequality. The 0 reference point has to be arbitrary (in the above it should really be X(ref_angle + a), Y(ref_angle + 2a), etc.) and you have to assume no faster than light communication (that is, independence) to get the contradiction. There are some further subtleties with the above argument but hopefully that's intuitive enough to follow why quantum entanglement is so counter intuitive. EDIT: corrected X(a) bit string
- 5y ago
ethn 5y ago
Look up how entanglement is done experimentally. It will always involve a technology which can be used to classically transmit information at a distance. What happens in entanglement is that the two entangled objects receive say an entangled photon, it is at this point where the two objects are entangled. Entanglement is a dance of the statistical limits and position of a particle/object given a specific space/energy configuration (initial condition). From this we know the probability of where it can be, what states it can assume, and the limits of both—given the energy it takes to traverse space and assume those states at once. They are entangled because once information of the states of one of the entangled objects is measured (mainly by analyzing the exiting photon), we can apodictically discern the state of the other.
hintymad 5y ago
It's as if the universe is a simulation in a gigantic computer. We get entanglement because particles are aliases of the same pointer. edit: I didn't mean it as an explanation of entanglement. Just thought it was a convenient joke.
blueplanet200 5y ago
You can measure a particle's spin to be up or down. But you can't choose to measure it to be up. It's random and up to nature. This is exactly why it can't be used to send information.
725686 5y ago
Funny, I never understood how you could possibly send a message using entanglement. Try to explain how would you do it, and either you will understand why it can't be done... or earn a Nobel prize. Win-Win.
codezero 5y ago
It's easy to understand (I'm being a bit hyperbolic) if you can believe that space and time are emergent properties of matter and not required for the underlying physics.
guyomes 5y ago
I find that this article [0] from Conway and Kochen is helpful. The authors do not really explain the paradoxes of quantum mechanics. Instead they reduce them to minimal fundamental axioms that have been tested and observed experimentally, even though they are arguably highly counter-intuitive (notably SPIN and TWIN). Based on those axioms, the authors show that you cannot send a message through entanglement. More precisely, they show that a particle has a free will, in the sense that the result of a measurement on it "is not a function of properties of that part of the universe that is earlier than this response". [0]: https://www.ams.org/notices/200902/rtx090200226p.pdf https://www.ams.org/notices/200902/rtx090200226p.pdf
naasking 5y ago
> I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. Say you and Bob share a bunch of entangled particles. Bob wants to send you a message using those particles, so he takes one particle at a time and encodes his information. How would you know he did so? At the very least, Bob would still have to send you a classical signal to say he did something. There are more subtle arguments why this doesn't work even at the particle level, but that at least should give you an idea why superluminal communication won't work.
crystalmeph 5y ago
Total layman, I’m sure I’ll get corrected if I’m wrong here: Bell's Theorem is just a model showing how two sets of measurements of certain properties of entangled particles would differ in a “Quantum Physics” regime where there is spooky action at a distance changing the measured properties versus a “common sense” regime where both particles leave the entanglement site with those properties already set. [0] is a fairly easy to understand graph of the correlation of the measurements at the two sites in both regimes, even if the math that generates those charts is beyond me. Given that, just knowing what measurements you got only gives you an idea of what the other guy across the universe would see were he to measure your particle’s entangled partner - he has no more control over what those measurements actually are than you do, you just know that there’s a certain correlation between what you saw and what he saw. That’s why you can’t use Bell’s Theorem to communicate, neither one of you is actually controlling the measured property, there’s just a certain correlation between the measurements both of you got. It seems to this layman that in order to communicate FTL using QM, you’d need a way to determine that the property being measured has already been “collapsed” (if that’s the right word) by spooky action at a distance, e.g if the other guy had already measured it. Bell’s Theorem gives us no way to determine if the other guy has actually made the measurements, it assumes that both you and he have both made those measurements. [0]: https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bell.svg https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bel...
- seph-reed 5y ago
  Why can't you control the outcome on your end? Or can we just not do it yet?
  - crystalmeph 5y ago
    You’re measuring the value, not setting it. Just like looking at a ball that was wrapped in a bag, seeing that it is red means your partner must therefore have picked the blue ball, but there’s no way you can control whether your partner had the red or blue ball, so the two of you can’t use that information to communicate FTL.

sergiotapia 5y ago

Reminds me of the in-lore comms system in Mass Effect. I think the comms in the ship were two atoms that were entangled, allowing instant messages no matter how many lightyears away the ship was from Earth.

deleted 5y ago

[deleted]

misiti3780 5y ago

This book elucidates the concepts well: https://en.wikipedia.org/wiki/Something_Deeply_Hidden https://en.wikipedia.org/wiki/Something_Deeply_Hidden

drcode 5y ago

Imagine you have a pouch with a red and a blue marble in it, then take out a marble without looking at it and hand the pouch to a friend. Later, if you look at your marble, you instantly have information about the other marble at a speed greater than the speed of light... but you couldn't use that fact to send a message. The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend. (This is assuming the "multiple universes" interpretation- In the other interpretations there is "spooky action at a distance", but this action happens in EXACTLY THE RIGHT WAY to prevent you from transmitting a message to your friend)

danbruc 5y ago
The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend. I don't think it is helpful to talk about multiple universes, that makes a strong implication towards a many world interpretation. It is better to say that the difference is that in the classical case the decision who gets which marble happens when one of the marbles is taken out of the pouch while in the case of entanglement we do not really know when the decision happens but it does provably work differently than in the classical case. It might be that the decision is never truly made, that both outcomes happen in two parallel worlds, it might be that the decision is only made when one party inspects their marble, it might be that it happens at the same time as in the classical example, ...we don't know.
- narrator 5y ago
  Or, in the transactional interpretation, the other guy's marble sends a signal from the future back to your marble to change its color when you look at it.
- tbabb 5y ago
  > that makes a strong implication towards a many world interpretation You say that like it's a shortcoming. :) There are many who take the (very reasonable) position that the many worlds interpretation is the most epistemologically parsimonious one. Contrary to some misunderstandings of it, it doesn't "add" extra worlds; it removes the concept of "wave function collapse", and leaves all the other known laws of quantum mechanics completely unchanged. The "worlds" arise naturally as more and more particles in the environment become entangled with the measured system, and "wave function collapse" turns out to be the predicted observation of an observer who is themselves made out of quantum states. The only difference between many worlds and the "standard" Copenhagen interpretation is that Copenhagen adds that, at some point, the entanglement process stops, and a bunch of states in the wave function disappear. And it doesn't specify how, or why, or how to calculate when it will happen. Those that advocate for many worlds would point out that this extra epistemological burden is questionable, given that the correct prediction is made without it.

snissn 5y ago

Two balls are a box. Neither are spinning. The box gets “shaken up” and the balls hit each other. We know that one ball is spinning clockwise and the other is counter clockwise because angular momentum spin is conserved. The balls launch far away from each other. We know the spin is entangled in that one is clock wise the other is counter clockwise but we don’t know which is which until we measure. How do we use that to communicate?

ende 5y ago
By constraining all your communications to a game of interstellar rock paper scissors?
- alisonkisk 5y ago
  that doesn't communicate across the distance. that communicates from the common starting point

stevenjgarner 5y ago

Or even better than instantaneously, let's get messages sent to us from the future using a Ronald Lawrence Mallett time machine based on a ring laser's properties, such that at sufficient energies, the circulating laser might produce not just frame-dragging but also closed timelike curves (CTC), allowing time travel into the past. I cannot believe that Ronald Mallett's biggest challenge is getting funding for a feasibility test. Isn't it the greatest venture capital opportunity of all time?

PavleMiha 5y ago

Can't say I have perfect intuition on it either, but the closes I've gotten was by reading this book: https://www.qisforquantum.org https://www.qisforquantum.org

wwarner 5y ago

Not a physicist, but my answer to you is that usually superliminal speeds is the price that physicists are willing to pay to explain what is observed in experiment. I get your objection to the rather convoluted argument that special relativity still applies to message transfer, but I accept it. John Preskill explains the information within entanglement with an analogy to a book. Normally with a book, you can read one page seperately from all the other pages. Further, if you unbound the book, and randomly distributed the pages to your friends, you could put your heads together and reconstruct the entire book. With a "quantum book", the information is encoded in the correlations between the observables, and you can only see the information when all the pages of the book are together and in the correct order. If you look at a single page of the quantum book, it's purely random gibberish, and you can't derive anything about the book by looking at a part of it.

simonh 5y ago

It can't be used to send a message because all you can do is measure your particle. Even if doing so changes the state of the other particle far away (which isn't really what's happening, but that doesn't matter), all the other person at the end can do is measure their particle. Neither of you can choose what the state of either particle is. You have no control, so there's no way to transmit information. What you can do is agree in advance that you will both take certain actions based on the measured state of the particles. There's no way to be sure the person at the other end actually does so though.

ende 5y ago
I now have to imagine that Quantum Game Theory is a thing that exists.
- drdeca 5y ago
  AIUI, yes, it has been found that, e.g. there are games where two players who can’t communicate but who do have a pair of entangled qubits and can each make a measurement, would have a strategy which is better for both players than if they didn’t have this resource. (where the games in the version where they don’t have the resource of the entangled pair of qubits, are fairly simple games where each player receives some information and then has a choice between two options)

superposeur 5y ago

> how you can be sure about the state of the other particle, what if someone already measured it and then did something to it? Indeed, you are only sure about the state of the other particle in the instant just after they measured it. Whoever measures first instantly destroys the entanglement link, so if they chose to manipulate the particle after measurement, you will have no knowledge of these manipulations. More generally, note that in quantum mechanics "reading" the state of a particle (i.e. performing a measurement) is drastically different than "writing" information by manipulating a particle. Most entanglement-related weirdness hinges on this fundamental asymmetry between "read" and "write" operations for quantum information.

axelf4 5y ago

Maybe this video by Veritasium could help: https://www.youtube.com/watch?v=kTXTPe3wahc https://www.youtube.com/watch?v=kTXTPe3wahc

abetusk 5y ago

Let's say particles have a 'direction angle' that we can measure with a detector that only gives 'up' or 'down' relative to a direction angle measurement. We can change this direction angle measurement with a knob to set what the measured 'up' and 'down' answers are relative to the detector's direction angle. Further let's say particles can be quantum entangled so that when when two detectors are placed very far apart, many light years apart, say, and measure a quantum entangled pair of particles. When the two detectors are set to the same, but arbitrary, angle, the detectors give the same answer. This is normal correlation. Quantum correlation says that as one dial moves away from the other reference point, the correlation falls off as a sine wave, not a linear decrease as would be expected by classic probability. To see how bonkers this is, do the following experiment: Set detector X to be at angle 0 and detector Y to give a 1% error rate. Call that 1% angle 'a'. So a sample experiment run might be: X(0): 0001000101001110...111010 Y(a): 0011000101001110...111010 In the above, 1 could be an 'up' and 0 could be a 'down' detection, say. For concreteness, let's just say A and B ran 100 detections and there was one difference between them (giving 1% error), represented by the third differing bit in the above. Now let let's change both X and Y by the same angle so the relative error rate between them is still 1%, this might give something like: X(a): 0011000001001110...111010 Y(2a): 0011000101001110...111010 X and Y still have one difference in the above, but now with the 8th position changed. So far this is nothing unexpected from classical probability. Now, we know that from X(0) to Y(a) there's one change, from X(a) to Y(2a) there's one change. Classic probability says that there can be at most two flipped bits from X(0) to Y(2a). Quantum mechanics predicts three. To convince yourself, try making a list of bits such that there's one difference between X(0) and Y(a), one difference between X(a) and Y(2a) but three differences from X(0) to Y(2a). It's impossible and this is the heart of Bell's theorem. Bell's theorem is a classical probability statement, generalized from my above statement that if |X(0)-Y(a)|=1, |X(a)-Y(2a)|=1 then |X(0)-Y(2a)|<=2. Quantum entanglement violates Bell's inequality. The 0 reference point has to be arbitrary (in the above it should really be X(ref_angle + a), Y(ref_angle + 2a), etc.) and you have to assume no faster than light communication (that is, independence) to get the contradiction. There are some further subtleties with the above argument but hopefully that's intuitive enough to follow why quantum entanglement is so counter intuitive. EDIT: corrected X(a) bit string

5y ago

ethn 5y ago

Look up how entanglement is done experimentally. It will always involve a technology which can be used to classically transmit information at a distance. What happens in entanglement is that the two entangled objects receive say an entangled photon, it is at this point where the two objects are entangled. Entanglement is a dance of the statistical limits and position of a particle/object given a specific space/energy configuration (initial condition). From this we know the probability of where it can be, what states it can assume, and the limits of both—given the energy it takes to traverse space and assume those states at once. They are entangled because once information of the states of one of the entangled objects is measured (mainly by analyzing the exiting photon), we can apodictically discern the state of the other.

hintymad 5y ago

It's as if the universe is a simulation in a gigantic computer. We get entanglement because particles are aliases of the same pointer. edit: I didn't mean it as an explanation of entanglement. Just thought it was a convenient joke.

blueplanet200 5y ago

You can measure a particle's spin to be up or down. But you can't choose to measure it to be up. It's random and up to nature. This is exactly why it can't be used to send information.

725686 5y ago

Funny, I never understood how you could possibly send a message using entanglement. Try to explain how would you do it, and either you will understand why it can't be done... or earn a Nobel prize. Win-Win.

codezero 5y ago

It's easy to understand (I'm being a bit hyperbolic) if you can believe that space and time are emergent properties of matter and not required for the underlying physics.

guyomes 5y ago

I find that this article [0] from Conway and Kochen is helpful. The authors do not really explain the paradoxes of quantum mechanics. Instead they reduce them to minimal fundamental axioms that have been tested and observed experimentally, even though they are arguably highly counter-intuitive (notably SPIN and TWIN). Based on those axioms, the authors show that you cannot send a message through entanglement. More precisely, they show that a particle has a free will, in the sense that the result of a measurement on it "is not a function of properties of that part of the universe that is earlier than this response". [0]: https://www.ams.org/notices/200902/rtx090200226p.pdf https://www.ams.org/notices/200902/rtx090200226p.pdf

naasking 5y ago

> I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. Say you and Bob share a bunch of entangled particles. Bob wants to send you a message using those particles, so he takes one particle at a time and encodes his information. How would you know he did so? At the very least, Bob would still have to send you a classical signal to say he did something. There are more subtle arguments why this doesn't work even at the particle level, but that at least should give you an idea why superluminal communication won't work.

crystalmeph 5y ago

Total layman, I’m sure I’ll get corrected if I’m wrong here: Bell's Theorem is just a model showing how two sets of measurements of certain properties of entangled particles would differ in a “Quantum Physics” regime where there is spooky action at a distance changing the measured properties versus a “common sense” regime where both particles leave the entanglement site with those properties already set. [0] is a fairly easy to understand graph of the correlation of the measurements at the two sites in both regimes, even if the math that generates those charts is beyond me. Given that, just knowing what measurements you got only gives you an idea of what the other guy across the universe would see were he to measure your particle’s entangled partner - he has no more control over what those measurements actually are than you do, you just know that there’s a certain correlation between what you saw and what he saw. That’s why you can’t use Bell’s Theorem to communicate, neither one of you is actually controlling the measured property, there’s just a certain correlation between the measurements both of you got. It seems to this layman that in order to communicate FTL using QM, you’d need a way to determine that the property being measured has already been “collapsed” (if that’s the right word) by spooky action at a distance, e.g if the other guy had already measured it. Bell’s Theorem gives us no way to determine if the other guy has actually made the measurements, it assumes that both you and he have both made those measurements. [0]: https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bell.svg https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bel...

seph-reed 5y ago
Why can't you control the outcome on your end? Or can we just not do it yet?
- crystalmeph 5y ago
  You’re measuring the value, not setting it. Just like looking at a ball that was wrapped in a bag, seeing that it is red means your partner must therefore have picked the blue ball, but there’s no way you can control whether your partner had the red or blue ball, so the two of you can’t use that information to communicate FTL.