5 ms·

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable without collapsing the quantum state. However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed. When the particles are separated to great distance and one is observed to collapse to k, the other must be observed to collapse to ~k because it was always a ~k particle. Clearly, there's some aspect of the experiments that I don't understand that invalidates this simple interpretation, but I do not know what it is.

Osmium 14y ago
"However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed." This is incorrect, both particles have a superposition of k and ~k (to use your terminology). It is not the case that when the wavefunction collapses, we just suddenly find out the "internal state" of the particle. This "internal state" simply does not exist until the wavefunction collapse has occurred. The "no faster than light" doctrine refers specifically to the transfer of information. And no information, in the strictest sense, has been transferred. Think of it like this: try to imagine how you could communicate with someone using quantum entanglement. If you can communicate something, then information has been transferred. But you can't. All you know is that if your particle is k, then theirs is ~k. Now, you could make up a rule book that says "if my particle is k, then I'll do x, and you'll do y" but that rule book will have to have been shared between both parties before hand, at less than the speed of light. Edit: should just mention I'm not an expert here, and articles like this are always interesting. Finding loopholes in our conventional understanding. But it's important to know just /what/ our conventional understanding is first to realise why things like this are important.
- iliis 14y ago
  Even with such a preshared rule book you wouldn't get any FTL communication. You could crate a pair of random number generators this way which will always give the same result. Certainly useful for cryptographic purposes but sadly not direct communication.
  - Osmium 14y ago
    Agreed; that's what I was trying to say. Perhaps it's disingenuous to frame the problem in this way (that is, talking about rule books and the like), but I was just trying to make it more relatable to normal human experience.
- IvoDankolov 14y ago
  But when you "collapse the wavefunction" and "create" the internal state of the particle, do you also create the internal state of the other one that is entangled to it? Do you create it instantaneously? You see, the problem is not whether you can transfer information in human readable form (though if you could that would certainly be a huge problem with relativity!), but whether any effect that propagates faster than the speed of light exists. You'll have a hard time explaining that in the frame of wavefunction collapse, I think.
- spuz 14y ago
  > both particles have a superposition of k and ~k (to use your terminology). ... This "internal state" simply does not exist until the wavefunction collapse has occurred. What would the implications be if particles did have an internal state?
  - Osmium 14y ago
    It would be a violation of Bell's theorem as we currently understand it. In other words, it can't happen, unless our understanding is wrong. (For more info, the wiki on Bell's theorem has been linked to elsewhere in this thread.)
beothorn 14y ago
http://en.wikipedia.org/wiki/Bells_Theorem http://en.wikipedia.org/wiki/Bells_Theorem ?
NegativeK 14y ago
If you're willing to read through it, there's a Wikipedia article on Bell's theorem: http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem The summary: If you assume simple locality (your k and ~k,) you get different results than what quantum entanglement predicts. If you want to get rid of the spooky action at a distance, then you need much, much more complicated hidden variables.
IvoDankolov 14y ago
The way I've usually seen this one presented involves precisely the observation, in that you gain knowledge of the other particle. Of course, saying "because it was always a ~k particle" does not a good explanation make, because that would imply that the resolution of the measurement was somehow predetermined, which is a fancy way of saying that there's a hidden variable. Not that interpreting all of this to mean that physics is non-local "spooky-action-at-a-distance" is the only viable route, mind. Consider this: why, exactly, do you believe that when you measure the particle you somehow force it to enter one particular state and therefore the entangled one that's sitting X miles away suddenly enters the opposite one? Are we, humans, sitting outside of quantum mechanics and looking down upon it - and then what we observe is the one true way the world is? Why would you not, instead, when you measure the particle, entangle the measuring device, and yourself, with the state of the particle? You are, after all, only another part of quantum mechanics the same as anything else.
- dllthomas 14y ago
  > [...] which is a fancy way of saying that there's a hidden variable. I'd say it's a less fancy (and more specific) way of saying "hidden variable." I read the parent as saying, "I don't understand why there can't be a hidden variable" which is quite a different thing than saying "this can work without a hidden variable: [thing involving hidden variable]".
mrestko 14y ago
I believe you're talking about "hidden variables" and the argument against it is Bell's Theorem http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem I'm not a physicist and I don't really understand it fully, but I believe this is what you're talking about.
hasenj 14y ago
I agree with you. And further more, wouldn't entanglement itself be some sort of evidence that the particle does have an internal state even before being observed? For instance, I could phrase things like this: - Let particles A and B get into an entangled state - Measure the state of particle A, see that it has property k. - Predict that B has property ~k even though it's not being measured yet. - Measure B. If B has property ~k, then we've proven that B had an internal state before we performed the measurement. From all the "popular" description of "quantum entanglement", I don't see anything in it that's significantly different from things like, say, the preservation of momentum in Newtonian physics. Of course, I must be missing something, but I don't know what is that thing yet.
- Osmium 14y ago
  "If B has property ~k, then we've proven that B had an internal state before we performed the measurement." This part is the flaw in the logic. That doesn't prove that B has an internal state. All it proves is that, in some sense, B's state is related to A (assuming you repeat the experiment until you reach statistical significance). Entanglement is nothing more than saying one particle is connected, in some sense, to another. Like a see-saw: if one side goes up, the other must go down. It's only "confusing" in the sense that the particles are physically separated, and the link between them is not mediated by a force as we understand it. Instead, they're linked by their very nature. It's almost as if you have just one particle, that's somehow been split into two and spatially separated. That's the conceptually difficult part.
  - hasenj 14y ago
    Why not choose the mundane and least astonishing explanation? If an object A is moving (with a known momentum) and hits a stationary object B, we can measure the momentum of either A or B and instantly know the momentum of the other object without having to measure it. The two objects are somehow "connected"; their state (momentum) is "entangled". Why is quantum entanglement different?
    - Osmium 14y ago
fixermark 14y ago
(Replying to my own comment as basically every response raised Bell's Theorem) This is what I like about Hacker News; when I ask a deeply technical question in a field removed from my own, there are plenty of people who can point me in an interesting direction. Bell's Theorem was the aspect of observational measurement that I was missing, and it's absolutely fascinating. Even moreso in that it was envisioned mathematically before we had the technology to prod at its claims. Thank you everyone for showing it to me.
morpher 14y ago
This article[1] by David Mermin does a good job explaining why the second particle couldn't have always been a ~k particle. [1] http://www.phy.duke.edu/undergraduate/physics-articles/mermin-is-the-moon-there-when-nobody-looks-physics-today-1985.pdf http://www.phy.duke.edu/undergraduate/physics-articles/mermi...
- derleth 14y ago
  PDF
deleted 14y ago
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zellyn 14y ago
Thank you for asking this so well. I have been keeping the page hanging around in a tab, waiting until the answers showed up!
jessriedel 14y ago
Physicist here. NegativeK is correct. Bell's theorem is the key to understanding why your explanation doesn't work. Bell's theorem, and the associated experimental demonstration, has been called the most profound discover of science. It's well worth a few hours of your time to try to understand it. You don't need to know any quantum mechanics or advanced math to do so. Incidentally, I'll mention that your argument is essentially what Einstein et al were arguing for in their famous EPR paper. The fact that it's even possible to rule out local hidden variable theories is highly surprising, and is what makes Bell's discovery (30 years after EPR framed the problem) so monumental.
- GregBuchholz 14y ago
  Non-physicst here. What do you think of Caroline Thompson's work, like: "Chaotic Ball" model,local realism and the Bell test loopholes http://arxiv.org/abs/quant-ph/0210150 http://arxiv.org/abs/quant-ph/0210150 ...I've always wondered if much of the Einstein-was-wrong business is really a category error. I personally liked: Clearing up Mysteries - The Original Goal http://bayes.wustl.edu/etj/articles/cmystery.pdf http://bayes.wustl.edu/etj/articles/cmystery.pdf " ...we must keep in mind that Einstein's thinking is always on the ontological level; the purpose of the EPR argument was to show that the QM state vector cannot be a representation of the "real physical situation" of a system. Bohr had never claimed that it was, although his strange way of expressing himself often led others to think that he was claiming this. From his reply to EPR, we find that Bohr's position was like this: "You may decide, of your own free will, which experiment to do. If you do experiment E1 you will get result R1. If you do E2 you will get R2. Since it is fundamentally impossible to do both on the same system, and the present theory correctly predicts the results of either, how can you say that the theory is incomplete? What more can one ask of a theory?" While it is easy to understand and agree with this on the epistemological level, the answer that I and many others would give is that we expect a physical theory to do more than merely predict experimental results in the manner of an empirical equation; we want to come down to Einstein's ontological level and understand what is happening when an atom emits light, when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having no answer to any question of the form: "What is really happening when - - - ?", forbids us to ask such questions and tries to persuade us that it is philosophically naive to want to know what is happening. But I do want to know, and I do not think this is naive; and so for me QM is not a physical theory at all, only an empty mathematical shell in which a future theory may, perhaps, be built."
zellyn 14y ago
Nobody seems to like my theory that the programmers of the simulation we're all living in were deferring procedurally generating that level of detail until it was necessary because we'd actually examined it closely. They're probably still cursing our Hubble telescope, not to mention computers with their decreasing semiconductor scales. This comment probably just triggered their simulation-awareness detector. :-) Actually, I think there's a silly but fascinating SciFi short story waiting to be written about assembling a group of people to try to privilege-escalate out of the simulation without triggering the simulation-awareness detectors. "I need you to think about this, but not all at the same time, or in the same place. Oh, and try to think about it only in vague terms."
- cpeterso 14y ago
  Your theory reminds me of Fassbinder's 1973 film Welt am Draht (World on a Wire). The technical director of a simulated world project slowly realizes he lives within a larger simulation. https://en.wikipedia.org/wiki/Welt_am_Draht https://en.wikipedia.org/wiki/Welt_am_Draht
  - cpeterso 14y ago
    Your theory reminds me of Fassbinder's 1973 film Welt am Draht ("World on a Wire"). The technical director of a simulated world project slowly realizes he lives within a larger simulation. He tries to outwit the simulation's programmers, as they deactivate or reprogram his friends and co-workers to stop him. https://en.wikipedia.org/wiki/Welt_am_Draht https://en.wikipedia.org/wiki/Welt_am_Draht
    - cpeterso 14y ago
      doh! I replied to my own post instead of editing it in place. This is getting pretty meta. <:)
- krsunny 14y ago
  That's because the simulation is telling everyone to reject your theory and ridicule you for presenting it.
lnanek2 14y ago
In the usual experiment used, if the two electron spin detectors for the two entagled particles are tilted the same, they'll get opposite results always. But if you tilt them differently, however, you no longer get results that are exactly opposite each other all the time. Therefore the electrons are in a superposition until they hit the detectors. It isn't decided yet what they will be. Keep in mind we're working with quantized values. Spin is always completely up or down, no matter what angle you measure it from. Measuring it at that angle locks it down, but tilting another detector in the chain after means you go back to a non-linear probability calculated by quantum mechanics (cosine of the angle in this case) to if it is up or down again at the new tilt and locks it down to that new tilt. There isn't any pre-determined value the electrons can be set to beforehand that matches the observed number that deflect in the detectors up or down at all measurement angles.

Osmium 14y ago

"However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed." This is incorrect, both particles have a superposition of k and ~k (to use your terminology). It is not the case that when the wavefunction collapses, we just suddenly find out the "internal state" of the particle. This "internal state" simply does not exist until the wavefunction collapse has occurred. The "no faster than light" doctrine refers specifically to the transfer of information. And no information, in the strictest sense, has been transferred. Think of it like this: try to imagine how you could communicate with someone using quantum entanglement. If you can communicate something, then information has been transferred. But you can't. All you know is that if your particle is k, then theirs is ~k. Now, you could make up a rule book that says "if my particle is k, then I'll do x, and you'll do y" but that rule book will have to have been shared between both parties before hand, at less than the speed of light. Edit: should just mention I'm not an expert here, and articles like this are always interesting. Finding loopholes in our conventional understanding. But it's important to know just /what/ our conventional understanding is first to realise why things like this are important.

iliis 14y ago
Even with such a preshared rule book you wouldn't get any FTL communication. You could crate a pair of random number generators this way which will always give the same result. Certainly useful for cryptographic purposes but sadly not direct communication.
- Osmium 14y ago
  Agreed; that's what I was trying to say. Perhaps it's disingenuous to frame the problem in this way (that is, talking about rule books and the like), but I was just trying to make it more relatable to normal human experience.
IvoDankolov 14y ago
But when you "collapse the wavefunction" and "create" the internal state of the particle, do you also create the internal state of the other one that is entangled to it? Do you create it instantaneously? You see, the problem is not whether you can transfer information in human readable form (though if you could that would certainly be a huge problem with relativity!), but whether any effect that propagates faster than the speed of light exists. You'll have a hard time explaining that in the frame of wavefunction collapse, I think.
spuz 14y ago
> both particles have a superposition of k and ~k (to use your terminology). ... This "internal state" simply does not exist until the wavefunction collapse has occurred. What would the implications be if particles did have an internal state?
- Osmium 14y ago
  It would be a violation of Bell's theorem as we currently understand it. In other words, it can't happen, unless our understanding is wrong. (For more info, the wiki on Bell's theorem has been linked to elsewhere in this thread.)

beothorn 14y ago

http://en.wikipedia.org/wiki/Bells_Theorem http://en.wikipedia.org/wiki/Bells_Theorem ?

NegativeK 14y ago

If you're willing to read through it, there's a Wikipedia article on Bell's theorem: http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem The summary: If you assume simple locality (your k and ~k,) you get different results than what quantum entanglement predicts. If you want to get rid of the spooky action at a distance, then you need much, much more complicated hidden variables.

IvoDankolov 14y ago

The way I've usually seen this one presented involves precisely the observation, in that you gain knowledge of the other particle. Of course, saying "because it was always a ~k particle" does not a good explanation make, because that would imply that the resolution of the measurement was somehow predetermined, which is a fancy way of saying that there's a hidden variable. Not that interpreting all of this to mean that physics is non-local "spooky-action-at-a-distance" is the only viable route, mind. Consider this: why, exactly, do you believe that when you measure the particle you somehow force it to enter one particular state and therefore the entangled one that's sitting X miles away suddenly enters the opposite one? Are we, humans, sitting outside of quantum mechanics and looking down upon it - and then what we observe is the one true way the world is? Why would you not, instead, when you measure the particle, entangle the measuring device, and yourself, with the state of the particle? You are, after all, only another part of quantum mechanics the same as anything else.

dllthomas 14y ago
> [...] which is a fancy way of saying that there's a hidden variable. I'd say it's a less fancy (and more specific) way of saying "hidden variable." I read the parent as saying, "I don't understand why there can't be a hidden variable" which is quite a different thing than saying "this can work without a hidden variable: [thing involving hidden variable]".

mrestko 14y ago

I believe you're talking about "hidden variables" and the argument against it is Bell's Theorem http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem I'm not a physicist and I don't really understand it fully, but I believe this is what you're talking about.

hasenj 14y ago

I agree with you. And further more, wouldn't entanglement itself be some sort of evidence that the particle does have an internal state even before being observed? For instance, I could phrase things like this: - Let particles A and B get into an entangled state - Measure the state of particle A, see that it has property k. - Predict that B has property ~k even though it's not being measured yet. - Measure B. If B has property ~k, then we've proven that B had an internal state before we performed the measurement. From all the "popular" description of "quantum entanglement", I don't see anything in it that's significantly different from things like, say, the preservation of momentum in Newtonian physics. Of course, I must be missing something, but I don't know what is that thing yet.

Osmium 14y ago
"If B has property ~k, then we've proven that B had an internal state before we performed the measurement." This part is the flaw in the logic. That doesn't prove that B has an internal state. All it proves is that, in some sense, B's state is related to A (assuming you repeat the experiment until you reach statistical significance). Entanglement is nothing more than saying one particle is connected, in some sense, to another. Like a see-saw: if one side goes up, the other must go down. It's only "confusing" in the sense that the particles are physically separated, and the link between them is not mediated by a force as we understand it. Instead, they're linked by their very nature. It's almost as if you have just one particle, that's somehow been split into two and spatially separated. That's the conceptually difficult part.
- hasenj 14y ago
  Why not choose the mundane and least astonishing explanation? If an object A is moving (with a known momentum) and hits a stationary object B, we can measure the momentum of either A or B and instantly know the momentum of the other object without having to measure it. The two objects are somehow "connected"; their state (momentum) is "entangled". Why is quantum entanglement different?
  - Osmium 14y ago

fixermark 14y ago

(Replying to my own comment as basically every response raised Bell's Theorem) This is what I like about Hacker News; when I ask a deeply technical question in a field removed from my own, there are plenty of people who can point me in an interesting direction. Bell's Theorem was the aspect of observational measurement that I was missing, and it's absolutely fascinating. Even moreso in that it was envisioned mathematically before we had the technology to prod at its claims. Thank you everyone for showing it to me.

morpher 14y ago

This article[1] by David Mermin does a good job explaining why the second particle couldn't have always been a ~k particle. [1] http://www.phy.duke.edu/undergraduate/physics-articles/mermin-is-the-moon-there-when-nobody-looks-physics-today-1985.pdf http://www.phy.duke.edu/undergraduate/physics-articles/mermi...

derleth 14y ago
PDF

deleted 14y ago

[deleted]

zellyn 14y ago

Thank you for asking this so well. I have been keeping the page hanging around in a tab, waiting until the answers showed up!

jessriedel 14y ago

Physicist here. NegativeK is correct. Bell's theorem is the key to understanding why your explanation doesn't work. Bell's theorem, and the associated experimental demonstration, has been called the most profound discover of science. It's well worth a few hours of your time to try to understand it. You don't need to know any quantum mechanics or advanced math to do so. Incidentally, I'll mention that your argument is essentially what Einstein et al were arguing for in their famous EPR paper. The fact that it's even possible to rule out local hidden variable theories is highly surprising, and is what makes Bell's discovery (30 years after EPR framed the problem) so monumental.

GregBuchholz 14y ago
Non-physicst here. What do you think of Caroline Thompson's work, like: "Chaotic Ball" model,local realism and the Bell test loopholes http://arxiv.org/abs/quant-ph/0210150 http://arxiv.org/abs/quant-ph/0210150 ...I've always wondered if much of the Einstein-was-wrong business is really a category error. I personally liked: Clearing up Mysteries - The Original Goal http://bayes.wustl.edu/etj/articles/cmystery.pdf http://bayes.wustl.edu/etj/articles/cmystery.pdf " ...we must keep in mind that Einstein's thinking is always on the ontological level; the purpose of the EPR argument was to show that the QM state vector cannot be a representation of the "real physical situation" of a system. Bohr had never claimed that it was, although his strange way of expressing himself often led others to think that he was claiming this. From his reply to EPR, we find that Bohr's position was like this: "You may decide, of your own free will, which experiment to do. If you do experiment E1 you will get result R1. If you do E2 you will get R2. Since it is fundamentally impossible to do both on the same system, and the present theory correctly predicts the results of either, how can you say that the theory is incomplete? What more can one ask of a theory?" While it is easy to understand and agree with this on the epistemological level, the answer that I and many others would give is that we expect a physical theory to do more than merely predict experimental results in the manner of an empirical equation; we want to come down to Einstein's ontological level and understand what is happening when an atom emits light, when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having no answer to any question of the form: "What is really happening when - - - ?", forbids us to ask such questions and tries to persuade us that it is philosophically naive to want to know what is happening. But I do want to know, and I do not think this is naive; and so for me QM is not a physical theory at all, only an empty mathematical shell in which a future theory may, perhaps, be built."

zellyn 14y ago

Nobody seems to like my theory that the programmers of the simulation we're all living in were deferring procedurally generating that level of detail until it was necessary because we'd actually examined it closely. They're probably still cursing our Hubble telescope, not to mention computers with their decreasing semiconductor scales. This comment probably just triggered their simulation-awareness detector. :-) Actually, I think there's a silly but fascinating SciFi short story waiting to be written about assembling a group of people to try to privilege-escalate out of the simulation without triggering the simulation-awareness detectors. "I need you to think about this, but not all at the same time, or in the same place. Oh, and try to think about it only in vague terms."

cpeterso 14y ago
Your theory reminds me of Fassbinder's 1973 film Welt am Draht (World on a Wire). The technical director of a simulated world project slowly realizes he lives within a larger simulation. https://en.wikipedia.org/wiki/Welt_am_Draht https://en.wikipedia.org/wiki/Welt_am_Draht
- cpeterso 14y ago
  Your theory reminds me of Fassbinder's 1973 film Welt am Draht ("World on a Wire"). The technical director of a simulated world project slowly realizes he lives within a larger simulation. He tries to outwit the simulation's programmers, as they deactivate or reprogram his friends and co-workers to stop him. https://en.wikipedia.org/wiki/Welt_am_Draht https://en.wikipedia.org/wiki/Welt_am_Draht
  - cpeterso 14y ago
    doh! I replied to my own post instead of editing it in place. This is getting pretty meta. <:)
krsunny 14y ago
That's because the simulation is telling everyone to reject your theory and ridicule you for presenting it.

lnanek2 14y ago

In the usual experiment used, if the two electron spin detectors for the two entagled particles are tilted the same, they'll get opposite results always. But if you tilt them differently, however, you no longer get results that are exactly opposite each other all the time. Therefore the electrons are in a superposition until they hit the detectors. It isn't decided yet what they will be. Keep in mind we're working with quantized values. Spin is always completely up or down, no matter what angle you measure it from. Measuring it at that angle locks it down, but tilting another detector in the chain after means you go back to a non-linear probability calculated by quantum mechanics (cosine of the angle in this case) to if it is up or down again at the new tilt and locks it down to that new tilt. There isn't any pre-determined value the electrons can be set to beforehand that matches the observed number that deflect in the detectors up or down at all measurement angles.