12 ms·

ELI5 how we should be talking about it?

by rco8786 4y ago

amelius 4y ago
The main issue is that the explanation should not imply faster-than-light communication.
- orlp 4y ago
  That isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes. This still doesn't allow you to use this to transfer information from one point to another faster than light due to the way nature lets you observe and manipulate things (e.g. the no-cloning theorem crucially prevents you from by-passing the permanence of quantum collapse). But the effect is very much real. Much more real than you might think. You can't use it to communicate faster than light, but you absolutely can use it to coordinate faster than light. It's called quantum pseudo-telepathy. See for example the Mermin–Peres magic square game. Two players with two pairs of entangled particles can win this game with 100% probability even if they're lightyears apart, whereas players without this resource can't win 100% of the time if they're separated (e.g. by adding sufficient distance and a time limit to the game).
  - slabity 4y ago
    > This isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes. If you were to use this explanation to anyone that isn't already deeply familiar with how quantum entanglement works, the "instantaneous" part of it would absolutely imply either FTL communication or at the very least the idea of a hidden variable behind the particles at the point of entanglement. And that's assuming they even understand what an "eigenstate" even is.
    - jquery 4y ago
      Non local hidden variables aren’t ruled out. It absolutely is a FTL effect, even if we can’t use it to communicate classical information.
  - donkarma 4y ago
  - Strilanc 4y ago
    Saying it happens "instantly" implies a preferred frame, but the process is invariant under Lorentz boosts. It also assumes a specific way of representing the quantum state (eg a state vector over all parts) whereas other representations like Feynman paths have no corresponding notion of an instant change.
    - jquery 4y ago
      Instantly implies FTL. How is that incorrect? The effect is very much FTL. https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy
      - Strilanc 4y ago
        What I'm trying to convey is that the statement "entanglement is instant" is implementation dependent. You can meet the specifications of quantum mechanics using a simulator where entanglement is instantaneous, but you can also meet those specifications using a simulator where entanglement is not instantaneous or where that concept doesn't even type check. What we can objectively say is that entanglement doesn't care about the distinction between time-like separation and space-like separation. If two non-communicating parties are entangled and they perform an experiment, there is no observable distinction between the cases where one of them goes first or the other goes first or in whether there is time for light to travel between them in the delay between their two parts. But that is much looser than requiring the effect to be instantaneous in some unspecified frame, because only requiring that there's no observable distinction is a much looser constraint than specifying that it must be instantaneous. And the only thing that we can experimentally verify is the looser constraint.
  - Dylan16807 4y ago
    > That isn't implied. I disagree. Ask 10 random people to read the phrase "links them so that changes in one affect the other", without any other disclaimers attached. I bet some of them will think that means you can change A and see the change on B. It's not good wording.
    - jquery 4y ago
      The wording is fine. Making people realize that a FTL effect is happening is more important than telling them that due to the no-cloning theorem we can’t directly observe it.
      - Dylan16807 4y ago
        1. Observing the event or trying to clone entire states isn't what anyone was worried about. It's the idea that you could set even one bit to measure later (which is trivial to do with a normal particle). 2. There's no tradeoff here. "more important" doesn't matter. It's easy to make a sentence that states an FTL effect occurs and that data can't be sent.
        jquery 4y ago
        But a type of non-classical data can be sent. See quantum pseudo telepathy.
  - bergenty 4y ago
    When one collapses does it’s entangled counter part collapse to a particular state immediately?
sbf501 4y ago
Here's a SO discussion: https://physics.stackexchange.com/questions/15282/quantum-entanglement-faster-than-speed-of-light https://physics.stackexchange.com/questions/15282/quantum-en... "Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, and once you make a measurement you lose entanglement. You can infer the state of B anywhere in the universe by noting that it must be complementary to A. The no-cloning theorem stops you from employing any sneaky tricks like making a bunch of copies of B and checking if they all have the same state or a mix of states, which would otherwise allow you to send information faster than light by choosing to collapse the entangled state or not." Basically you know a that since they are complements, you know the value of the other after you measure one. No information was transmitted at all. IANAQP. At all. Not one bit.
- _8j50 4y ago
  Is there some known property that is keeping the changes random? Or is it pure observation? Because if it is the latter then is it not correct to say that it is not known if FTL transmission can work?
  - Retric 4y ago
    QM says unmeasured quantities are uncertain, which shouldn’t seem odd on the surface. Entanglement means measuring one thing is effectively equivalent to measuring a property of the other. It’s vaguely like randomly cutting a board in half and by measuring how long one board is you can figure out how long the other one is. The odd bit isn’t really measurement it’s all the oddity around what happens before something is measured.
- rubicks 4y ago
  "...making a measurement on some entangled property of A will give you a random result and B will have the complementary result." "...once you make a measurement you lose entanglement." One entangled pair can transmit exactly one bit of information exactly once. The measurement necessarily destroys the pair. Do I understand correctly?
deleted 4y ago
[deleted]
armchairhacker 4y ago
My understanding: 2 entangled particles are 2 particles whose states are unknown, except that they are linked: e.g. 2 electrons which have opposite spins, or 2 photons which have opposite polarization. So when you measure one particle, you instantly know the state of the other even if it's light-years away (this is not FTL communication as you can't communicate this information FTL). It's sort of like putting an "up-spin" and "down-spin" electron in a box, having someone shift around the boxes behind your back, putting the boxes far away from each other, and then opening one: before you open the box you have no idea what either box contains, after you open one you know both including the far away one. Except it's not the same as putting an "up-spin" and "down-spin" electron in a box, because of Bell's Theorem (https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem). Electrons' spin is actually determined when you measure them: you can measure up/down, but also left/right or any other 3D direction. And if you measure both electrons at a slightly different angle, their measurements probabilistically correlate in a way which means the electrons never really had any true internal "spin" (hidden variable) until you measured one of them. The fact that measuring one electron changes the others' "spin" faster than light seemingly violates the principle of locality. The part which confuses me in the article is that, once you measure an entangled particle, they are no longer entangled. So I figure in order for this clock to be feasible you would either need an unbelievable amount of entangled atoms, you'd need to instantaneously re-entangle, or you'd need to use a "standard" atomic clock and only break entanglement after a certain number of ticks, to re-synchronize.
- nomel 4y ago
  I'm a dummy. Why does this make them "linked"? If I have two legos, always put them in opposite orientations, then walk away with a random one, being careful to not change its orientation, are the two legos "linked"? When I look down at the one in my hand, I will know the other ones orientation. How is the different with entangled particles? Is "entangled" just a way to say "still related because it's undisturbed"?
  - drexlspivey 4y ago
    Using your analogy, you don’t put the legos in opposite orientation and walk away with one, it’s more like the two legos keep spinning and you walk away with one and you stop it from spinning and instantaneously the other one stops in the opposite direction.
frutiger 4y ago
GP and all sibling comments are unfortunately incorrect. The measurement of one _does_ instantaneously affect the other. No useful information can be transmitted, since the outcomes are random, but that does not falsify the original statement. In fact "affect the other" has no implication of "information is transferred" - someone would need to be an expert on classical information theory and simultaneously a novice of quantum information theory to draw that conclusion.
- tauwauwau 4y ago
  Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?
  - MAMAMassakali 4y ago
    Yeah, I too want to know this. If we know that the collapse has happened, then it can be used to infer that someone has tried to measure the other entangled particle. Can this be be leveraged or my understanding is wrong?
  - frutiger 4y ago
    You don’t know the collapse happened. Locally you can just perform a measurement and you will concretely see one of the possible outcomes.
    - spiddy 4y ago
      I had an understanding that quantum entanglement can be used to for encryption and guaranty there are no man-in-the-middle attacks by making sure the entanglement is not broken. if we can’t know the collapse happened how can we make such claim (mitm can meassure stolen briefcase put it back and no one would know)
      - fsh 4y ago
        This is incorrect. Quantum key distribution ("quantum cryptography" is a misnomer) is vulnerable to man-in-the-middle attacks. Security relies on having a pre-shared key which makes it essentially useless in practice (just use AES if you have a PSK).
wyager 4y ago
There are any number of models that are consistent with the observed behavior. Only some of them require things to happen "instantaneously". The accurate way to describe entanglement is probably that the entangled systems will never be observed to be in an incompatible state.
- tadfisher 4y ago
  My favorite head-canon is that measurement of one particle determines the other's state at the time of entanglement. As in, the measurement changes the past, or sets the universe on a worldline compatible with the entangled pair's state.
bergenty 4y ago
If you have the left sock you know the other one is the right sock. I guess my question here is when you measure one entangled bit, does it always collapse to the same state or can it be either of the two states? Based on my understanding it’s the former right?
- denton-scratch 4y ago
  > If you have the left sock you know the other one is the right sock. Possibly a poor analogy; none of my socks are chiral.
abarker 4y ago
In quantum information theory the ability of Alice to instantaneously change Bob's quantum state (and hence measurement probabilities) is referred to as quantum steering. No information can be sent, but the influencing effect can be defined and observed. Steering is distinct from both Bell-nonlocality and nonseparability. See, e.g., [1] and later papers. [1] https://arxiv.org/abs/quant-ph/0612147 https://arxiv.org/abs/quant-ph/0612147
- da-x 4y ago
  Do you mean that it was proven that observation affects the other end immediately, or is it only just a theory? If so, how it was proven, without it also being useful for sending information?
  - abarker 4y ago
    It is a theory in the sense that it is epistemic. Bob's quantum state changes depending on Alice's measurement, essentially collapsing the probability distribution to the conditional probability distribution. As science it cannot be proven, but the empirical results match the model. The paper I cited defines precise protocols to define terms like "steerable states." This is standard quantum theory, so the no-communication theorem still applies. Is the quantum state also ontic, and in what sense? Those are open questions of interpretation.

ELI5 how we should be talking about it?

amelius 4y ago

The main issue is that the explanation should not imply faster-than-light communication.

orlp 4y ago
That isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes. This still doesn't allow you to use this to transfer information from one point to another faster than light due to the way nature lets you observe and manipulate things (e.g. the no-cloning theorem crucially prevents you from by-passing the permanence of quantum collapse). But the effect is very much real. Much more real than you might think. You can't use it to communicate faster than light, but you absolutely can use it to coordinate faster than light. It's called quantum pseudo-telepathy. See for example the Mermin–Peres magic square game. Two players with two pairs of entangled particles can win this game with 100% probability even if they're lightyears apart, whereas players without this resource can't win 100% of the time if they're separated (e.g. by adding sufficient distance and a time limit to the game).
- slabity 4y ago
  > This isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes. If you were to use this explanation to anyone that isn't already deeply familiar with how quantum entanglement works, the "instantaneous" part of it would absolutely imply either FTL communication or at the very least the idea of a hidden variable behind the particles at the point of entanglement. And that's assuming they even understand what an "eigenstate" even is.
  - jquery 4y ago
    Non local hidden variables aren’t ruled out. It absolutely is a FTL effect, even if we can’t use it to communicate classical information.
- donkarma 4y ago
- Strilanc 4y ago
  Saying it happens "instantly" implies a preferred frame, but the process is invariant under Lorentz boosts. It also assumes a specific way of representing the quantum state (eg a state vector over all parts) whereas other representations like Feynman paths have no corresponding notion of an instant change.
  - jquery 4y ago
    Instantly implies FTL. How is that incorrect? The effect is very much FTL. https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy
    - Strilanc 4y ago
      What I'm trying to convey is that the statement "entanglement is instant" is implementation dependent. You can meet the specifications of quantum mechanics using a simulator where entanglement is instantaneous, but you can also meet those specifications using a simulator where entanglement is not instantaneous or where that concept doesn't even type check. What we can objectively say is that entanglement doesn't care about the distinction between time-like separation and space-like separation. If two non-communicating parties are entangled and they perform an experiment, there is no observable distinction between the cases where one of them goes first or the other goes first or in whether there is time for light to travel between them in the delay between their two parts. But that is much looser than requiring the effect to be instantaneous in some unspecified frame, because only requiring that there's no observable distinction is a much looser constraint than specifying that it must be instantaneous. And the only thing that we can experimentally verify is the looser constraint.
- Dylan16807 4y ago
  > That isn't implied. I disagree. Ask 10 random people to read the phrase "links them so that changes in one affect the other", without any other disclaimers attached. I bet some of them will think that means you can change A and see the change on B. It's not good wording.
  - jquery 4y ago
    The wording is fine. Making people realize that a FTL effect is happening is more important than telling them that due to the no-cloning theorem we can’t directly observe it.
    - Dylan16807 4y ago
      1. Observing the event or trying to clone entire states isn't what anyone was worried about. It's the idea that you could set even one bit to measure later (which is trivial to do with a normal particle). 2. There's no tradeoff here. "more important" doesn't matter. It's easy to make a sentence that states an FTL effect occurs and that data can't be sent.
      - jquery 4y ago
        But a type of non-classical data can be sent. See quantum pseudo telepathy.
- bergenty 4y ago
  When one collapses does it’s entangled counter part collapse to a particular state immediately?

sbf501 4y ago

Here's a SO discussion: https://physics.stackexchange.com/questions/15282/quantum-entanglement-faster-than-speed-of-light https://physics.stackexchange.com/questions/15282/quantum-en... "Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, and once you make a measurement you lose entanglement. You can infer the state of B anywhere in the universe by noting that it must be complementary to A. The no-cloning theorem stops you from employing any sneaky tricks like making a bunch of copies of B and checking if they all have the same state or a mix of states, which would otherwise allow you to send information faster than light by choosing to collapse the entangled state or not." Basically you know a that since they are complements, you know the value of the other after you measure one. No information was transmitted at all. IANAQP. At all. Not one bit.

_8j50 4y ago
Is there some known property that is keeping the changes random? Or is it pure observation? Because if it is the latter then is it not correct to say that it is not known if FTL transmission can work?
- Retric 4y ago
  QM says unmeasured quantities are uncertain, which shouldn’t seem odd on the surface. Entanglement means measuring one thing is effectively equivalent to measuring a property of the other. It’s vaguely like randomly cutting a board in half and by measuring how long one board is you can figure out how long the other one is. The odd bit isn’t really measurement it’s all the oddity around what happens before something is measured.
rubicks 4y ago
"...making a measurement on some entangled property of A will give you a random result and B will have the complementary result." "...once you make a measurement you lose entanglement." One entangled pair can transmit exactly one bit of information exactly once. The measurement necessarily destroys the pair. Do I understand correctly?

deleted 4y ago

[deleted]

armchairhacker 4y ago

My understanding: 2 entangled particles are 2 particles whose states are unknown, except that they are linked: e.g. 2 electrons which have opposite spins, or 2 photons which have opposite polarization. So when you measure one particle, you instantly know the state of the other even if it's light-years away (this is not FTL communication as you can't communicate this information FTL). It's sort of like putting an "up-spin" and "down-spin" electron in a box, having someone shift around the boxes behind your back, putting the boxes far away from each other, and then opening one: before you open the box you have no idea what either box contains, after you open one you know both including the far away one. Except it's not the same as putting an "up-spin" and "down-spin" electron in a box, because of Bell's Theorem (https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem). Electrons' spin is actually determined when you measure them: you can measure up/down, but also left/right or any other 3D direction. And if you measure both electrons at a slightly different angle, their measurements probabilistically correlate in a way which means the electrons never really had any true internal "spin" (hidden variable) until you measured one of them. The fact that measuring one electron changes the others' "spin" faster than light seemingly violates the principle of locality. The part which confuses me in the article is that, once you measure an entangled particle, they are no longer entangled. So I figure in order for this clock to be feasible you would either need an unbelievable amount of entangled atoms, you'd need to instantaneously re-entangle, or you'd need to use a "standard" atomic clock and only break entanglement after a certain number of ticks, to re-synchronize.

nomel 4y ago
I'm a dummy. Why does this make them "linked"? If I have two legos, always put them in opposite orientations, then walk away with a random one, being careful to not change its orientation, are the two legos "linked"? When I look down at the one in my hand, I will know the other ones orientation. How is the different with entangled particles? Is "entangled" just a way to say "still related because it's undisturbed"?
- drexlspivey 4y ago
  Using your analogy, you don’t put the legos in opposite orientation and walk away with one, it’s more like the two legos keep spinning and you walk away with one and you stop it from spinning and instantaneously the other one stops in the opposite direction.

frutiger 4y ago

GP and all sibling comments are unfortunately incorrect. The measurement of one _does_ instantaneously affect the other. No useful information can be transmitted, since the outcomes are random, but that does not falsify the original statement. In fact "affect the other" has no implication of "information is transferred" - someone would need to be an expert on classical information theory and simultaneously a novice of quantum information theory to draw that conclusion.

tauwauwau 4y ago
Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?
- MAMAMassakali 4y ago
  Yeah, I too want to know this. If we know that the collapse has happened, then it can be used to infer that someone has tried to measure the other entangled particle. Can this be be leveraged or my understanding is wrong?
- frutiger 4y ago
  You don’t know the collapse happened. Locally you can just perform a measurement and you will concretely see one of the possible outcomes.
  - spiddy 4y ago
    I had an understanding that quantum entanglement can be used to for encryption and guaranty there are no man-in-the-middle attacks by making sure the entanglement is not broken. if we can’t know the collapse happened how can we make such claim (mitm can meassure stolen briefcase put it back and no one would know)
    - fsh 4y ago
      This is incorrect. Quantum key distribution ("quantum cryptography" is a misnomer) is vulnerable to man-in-the-middle attacks. Security relies on having a pre-shared key which makes it essentially useless in practice (just use AES if you have a PSK).

wyager 4y ago

There are any number of models that are consistent with the observed behavior. Only some of them require things to happen "instantaneously". The accurate way to describe entanglement is probably that the entangled systems will never be observed to be in an incompatible state.

tadfisher 4y ago
My favorite head-canon is that measurement of one particle determines the other's state at the time of entanglement. As in, the measurement changes the past, or sets the universe on a worldline compatible with the entangled pair's state.

bergenty 4y ago

If you have the left sock you know the other one is the right sock. I guess my question here is when you measure one entangled bit, does it always collapse to the same state or can it be either of the two states? Based on my understanding it’s the former right?

denton-scratch 4y ago
> If you have the left sock you know the other one is the right sock. Possibly a poor analogy; none of my socks are chiral.

abarker 4y ago

In quantum information theory the ability of Alice to instantaneously change Bob's quantum state (and hence measurement probabilities) is referred to as quantum steering. No information can be sent, but the influencing effect can be defined and observed. Steering is distinct from both Bell-nonlocality and nonseparability. See, e.g., [1] and later papers. [1] https://arxiv.org/abs/quant-ph/0612147 https://arxiv.org/abs/quant-ph/0612147

da-x 4y ago
Do you mean that it was proven that observation affects the other end immediately, or is it only just a theory? If so, how it was proven, without it also being useful for sending information?
- abarker 4y ago
  It is a theory in the sense that it is epistemic. Bob's quantum state changes depending on Alice's measurement, essentially collapsing the probability distribution to the conditional probability distribution. As science it cannot be proven, but the empirical results match the model. The paper I cited defines precise protocols to define terms like "steerable states." This is standard quantum theory, so the no-communication theorem still applies. Is the quantum state also ontic, and in what sense? Those are open questions of interpretation.