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What you've quoted there is a good analogy of an entangled state. In reality there are several differences, and the biggest piece of misinformation is that meas

by dools 11y ago

What you've quoted there is a good analogy of an entangled state. In reality there are several differences, and the biggest piece of misinformation is that measurement of one system somehow "collapses a wave function" in the other system which isn't the case. The phenomenon of entanglement is that measurements of certain things are either perfectly correlated or anti-correlated. Eg. Measurement of spin along some axis on electron A will always be -1 when measurement of spin along the same axis of electron B is +1. The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird. But it's not classical physics. I wrote more about it here: https://news.ycombinator.com/item?id=10133674 https://news.ycombinator.com/item?id=10133674 Also check out the leonard susskind messenger lectures for a quick overview and his stanford continuing ed lectures on youtube for the long version :)

coldtea 11y ago
>The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird. If that was the case (the coin analogy) it would be perfectly understandable by classical physics. In fact it wouldn't even need physics to understand it, just common sense. If that was the case there wouldn't be any EPR paradox etc. The coin analogy is very misleading in this way -- and doesn't convey the problems physicists have had with explaining this phenomenon.
- dools 11y ago
  No what I'm describing is a way of understanding why entangled states aren't spooky. It's also a direct lift from Leonard Susskind so it's not even me doing it. The point is that the state is prepared in some way X, and the state is such that by knowing something about one thing, you also know something about the other thing. The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand. Same as I can't visualise a 5-sphere but I have mathematical tools to work with one. I can't "picture" the mechanism that allows measurements of quantum states to be anti-correlated, but I can use maths to express that behaviour and make predictions about how the universe behaves.
  - coldtea 11y ago
    >No what I'm describing is a way of understanding why entangled states aren't spooky. I think that if the explanation doesn't also convey the spookiness, it's not right. >The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand. That's just pushing the spookiness under the carpet.
    - dools 11y ago
      The original phrase is spooky action at a distance, which has been summarised as spookiness. Actual spookiness notwithstanding, the most important thing really is that there is no action at a distance.
jblow 11y ago
I don't see how you can claim that the idea of wave function collapse here is a misunderstanding. The mathematical meaning of the statement "two things are entangled" is that you cannot factor the states into the product of two separate quantities. So there is only one equation for the two particles. So once you "collapse" this for one particle (whether or not collapse is a physical action) it is by necessity collapsed for the other particle, because there is no other state that could not be collapsed.
- dools 11y ago
  Yes that's a good way of putting it. What I've seen around is talk that "measurement of one particle effects the other" and often people use the term "collapse the wave function of the other particle" to articulate this (because they'd heard the term). The fact that the entangled state of the 2 particles is described by the same wave function resolves this.
z92 11y ago
The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin What you have explained is "Quantum local hidden variable theory". Which has been proven wrong again and again. The article says this test has closed the last loopholes that made this theory a possibility. https://en.wikipedia.org/wiki/Local_hidden_variable_theory https://en.wikipedia.org/wiki/Local_hidden_variable_theory
- sago 11y ago
  You're misunderstanding his point, I think (or I'm being overly generous in interpreting him!). Though I admit he over-eggs it to a confusing degree. The two coins metaphor illustrates the anti-correlation of two particles and why that doesn't allow faster-than-light communication. By observing one, you can deduce the other. Of course it isn't a metaphor for the whole process, such that you can derive implications from the metaphor that will hold in the quantum case. But the point of the 'spooky action at a distance' is that there is no metaphor based in classical physics that can do so. So physicists, and lecturers (in my limited experience) use metaphors to explain certain parts of the behavior. It's like when a physicist talks about a quantum being a wave or a particle. Both are metaphors from classical physics. It is neither, and both metaphors only illuminate very specific properties. The approach of saying 'that metaphor has been proved wrong', just leaves us with no way of explaining stuff. It is worth keep reinforcing that all metaphors are wrong, but your response seemed overly-negative to me.
  - dools 11y ago
    Yes that was my point.
ikeboy 11y ago
The entire point of Bell's theorem is that there can't be something corresponding to "the state was prepared before we separated", because no local hidden variable theory can predict the same as QM predicts.
- dools 11y ago
  This is not local hidden variable theory. The measurement of one half of an entangled pair doesn't modify the probability distribution of the other half of an entangled pair. There is no modification, just a phenomenon unexplainable by classical physics whereby certain measurements on either system are either perfectly correlated or anti-correlated.
  - ikeboy 11y ago
    How are you saying "the state was prepared before we separated" without meaning a local hidden variable theory? The only way for that to make sort of sense would be superdeterminism, or a nonlocal HV theory, which is definitely not the consensus in QM. >The measurement of one half of an entangled pair doesn't modify the probability distribution of the other half of an entangled pair. That's just saying it's local. Not relevant.
    - dools 11y ago
      The only way for that to make sort of sense would be superdeterminism, or a nonlocal HV theory, which is definitely not the consensus in QM. The only for it to make sense classically. It doesn't need to make sense, it just is. It's how QM works, you can't understand it by some classical mechanism. Basically this: https://en.wikipedia.org/wiki/Hidden_variable_theory#Declaration_of_completeness_of_quantum_mechanics https://en.wikipedia.org/wiki/Hidden_variable_theory#Declara...
      - ikeboy 11y ago
        You chose to use the phrase. If it doesn't make sense, don't use it. Saying "indeterminism" is fine.

by dools 11y ago

coldtea 11y ago

>The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird. If that was the case (the coin analogy) it would be perfectly understandable by classical physics. In fact it wouldn't even need physics to understand it, just common sense. If that was the case there wouldn't be any EPR paradox etc. The coin analogy is very misleading in this way -- and doesn't convey the problems physicists have had with explaining this phenomenon.

dools 11y ago
No what I'm describing is a way of understanding why entangled states aren't spooky. It's also a direct lift from Leonard Susskind so it's not even me doing it. The point is that the state is prepared in some way X, and the state is such that by knowing something about one thing, you also know something about the other thing. The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand. Same as I can't visualise a 5-sphere but I have mathematical tools to work with one. I can't "picture" the mechanism that allows measurements of quantum states to be anti-correlated, but I can use maths to express that behaviour and make predictions about how the universe behaves.
- coldtea 11y ago
  >No what I'm describing is a way of understanding why entangled states aren't spooky. I think that if the explanation doesn't also convey the spookiness, it's not right. >The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand. That's just pushing the spookiness under the carpet.
  - dools 11y ago
    The original phrase is spooky action at a distance, which has been summarised as spookiness. Actual spookiness notwithstanding, the most important thing really is that there is no action at a distance.

jblow 11y ago

I don't see how you can claim that the idea of wave function collapse here is a misunderstanding. The mathematical meaning of the statement "two things are entangled" is that you cannot factor the states into the product of two separate quantities. So there is only one equation for the two particles. So once you "collapse" this for one particle (whether or not collapse is a physical action) it is by necessity collapsed for the other particle, because there is no other state that could not be collapsed.

dools 11y ago
Yes that's a good way of putting it. What I've seen around is talk that "measurement of one particle effects the other" and often people use the term "collapse the wave function of the other particle" to articulate this (because they'd heard the term). The fact that the entangled state of the 2 particles is described by the same wave function resolves this.

z92 11y ago

The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin What you have explained is "Quantum local hidden variable theory". Which has been proven wrong again and again. The article says this test has closed the last loopholes that made this theory a possibility. https://en.wikipedia.org/wiki/Local_hidden_variable_theory https://en.wikipedia.org/wiki/Local_hidden_variable_theory

sago 11y ago
You're misunderstanding his point, I think (or I'm being overly generous in interpreting him!). Though I admit he over-eggs it to a confusing degree. The two coins metaphor illustrates the anti-correlation of two particles and why that doesn't allow faster-than-light communication. By observing one, you can deduce the other. Of course it isn't a metaphor for the whole process, such that you can derive implications from the metaphor that will hold in the quantum case. But the point of the 'spooky action at a distance' is that there is no metaphor based in classical physics that can do so. So physicists, and lecturers (in my limited experience) use metaphors to explain certain parts of the behavior. It's like when a physicist talks about a quantum being a wave or a particle. Both are metaphors from classical physics. It is neither, and both metaphors only illuminate very specific properties. The approach of saying 'that metaphor has been proved wrong', just leaves us with no way of explaining stuff. It is worth keep reinforcing that all metaphors are wrong, but your response seemed overly-negative to me.
- dools 11y ago
  Yes that was my point.

ikeboy 11y ago

The entire point of Bell's theorem is that there can't be something corresponding to "the state was prepared before we separated", because no local hidden variable theory can predict the same as QM predicts.

dools 11y ago
This is not local hidden variable theory. The measurement of one half of an entangled pair doesn't modify the probability distribution of the other half of an entangled pair. There is no modification, just a phenomenon unexplainable by classical physics whereby certain measurements on either system are either perfectly correlated or anti-correlated.
- ikeboy 11y ago
  How are you saying "the state was prepared before we separated" without meaning a local hidden variable theory? The only way for that to make sort of sense would be superdeterminism, or a nonlocal HV theory, which is definitely not the consensus in QM. >The measurement of one half of an entangled pair doesn't modify the probability distribution of the other half of an entangled pair. That's just saying it's local. Not relevant.
  - dools 11y ago
    The only way for that to make sort of sense would be superdeterminism, or a nonlocal HV theory, which is definitely not the consensus in QM. The only for it to make sense classically. It doesn't need to make sense, it just is. It's how QM works, you can't understand it by some classical mechanism. Basically this: https://en.wikipedia.org/wiki/Hidden_variable_theory#Declaration_of_completeness_of_quantum_mechanics https://en.wikipedia.org/wiki/Hidden_variable_theory#Declara...
    - ikeboy 11y ago
      You chose to use the phrase. If it doesn't make sense, don't use it. Saying "indeterminism" is fine.