5 ms·

But there's no information being passed between the cards. I am not sure that this is the correct or best way to describe it. That looking at the cards does no

by danbruc 11y ago

But there's no information being passed between the cards. I am not sure that this is the correct or best way to describe it. That looking at the cards does not allow exchanging any information between Alice and Bob does not necessarily imply that the process of determining the colors of the cards does not involve any information exchange. It seems almost necessary that an information exchange happens between the two cards, after all they have to acquire different colors and they have no definite color before looking at them. That this process can not be exploited by Alice and Bob to exchange any information is a wholly different issue.

adekok 11y ago
> I am not sure that this is the correct or best way to describe it. That's how physicists describe it. So it is the best way. The thing to remember is that information (in physics) means "particles". The screen gives your eyes information in the form of photons. In the case of QM entanglement, nothing is exchanged. So no information is exchanged. > It seems almost necessary that an information exchange happens between the two cards, after all they have to acquire different colors and they have no definite color before looking at them. That's the issue with QM. You would like to think that something happens. But as I said, whatever really happens is different than what you think.
- danbruc 11y ago
  That's how physicists describe it. So it is the best way. I am not sure that this is correct. I think the usual statement is that it does not allow Alice and Bob to exchange information, not that no information exchange is involved. When Alice looks at her card and sees that it is red something happens, i.e. red as a possible colors for Bob's card is eliminated. The wave function collapses if you want to use that picture. The state of Bob's cards changes instantly from maybe red or maybe blue to definitely blue. One can now argue whether that change should be called information or whether the term information should be reserved for things that can be exploited by Alice and Bob but that does not change the fact that a non-local instantaneous effect changed the state of Bob's card. The thing to remember is that information (in physics) means "particles". That is pretty vague and might or might not be true. Do you mean every information exchanges between two spatially separated parties necessarily requires the exchange of (real) particles? That's the issue with QM. You would like to think that something happens. Something happens, the wave function changes.
  - karmakaze 11y ago
    adekok's description was very clear to me and I was thinking exactly this. What happens is that the entangled state shares a wave function. Collapse one = both.
    - danbruc 11y ago
      The idea that two entangled particles share a wave function seems at least problematic. The ontological state of the wave function is an open problem but I think you can't get away with thinking of them as independent entities that can associate with and unassociate from particles. It is probably more like there is one very complex wave function for the entire universe and we can only sometimes get away with thinking of it as the product of the wave function of a system we are interested in and the wave function of all the rest, the environment. But even disregarding this, there seems to be a need to propagate something through space. The wave function of the entangled pair could be spread out in space and when Alice performs a measurement on her particle you have to notify the wave function at Bob's place that it should also change. You could also try to imagine that the entire wave function for the two entangled particles is not spread out in space and located next to Alice which makes it easier to imagine how the entire wave function could change at once. But now how does Bob's particle know that the far away wave function changed in order to behave appropriately? We still have to propagate that notification over to Bob. And what if Bob decides to perform the measurement instead of Alice but the wave function is sitting around at Alice's place? There is some inherent nonlocality in quantum mechanics and the effects of measurements have to propagate through space even when they can not be used to exchange classical information. One idea that I find really interesting is ER = EPR [1] by Susskind. The idea is vaguely that entanglement determines locality, i.e. two things are close together if they are entangled, especially two entangled and spatially separated particles are connected by a wormhole and therefore actually remain close together all the time. This would make spooky actions at a distance a lot less spooky because it would mean that you can never really separate entangled particles. It would probably also mean that the topology of space is actually extremely complex and only superficially looks like Euclidean three dimensional space. [1] https://en.wikipedia.org/wiki/ER%3DEPR https://en.wikipedia.org/wiki/ER%3DEPR

by danbruc 11y ago

adekok 11y ago

> I am not sure that this is the correct or best way to describe it. That's how physicists describe it. So it is the best way. The thing to remember is that information (in physics) means "particles". The screen gives your eyes information in the form of photons. In the case of QM entanglement, nothing is exchanged. So no information is exchanged. > It seems almost necessary that an information exchange happens between the two cards, after all they have to acquire different colors and they have no definite color before looking at them. That's the issue with QM. You would like to think that something happens. But as I said, whatever really happens is different than what you think.

danbruc 11y ago
That's how physicists describe it. So it is the best way. I am not sure that this is correct. I think the usual statement is that it does not allow Alice and Bob to exchange information, not that no information exchange is involved. When Alice looks at her card and sees that it is red something happens, i.e. red as a possible colors for Bob's card is eliminated. The wave function collapses if you want to use that picture. The state of Bob's cards changes instantly from maybe red or maybe blue to definitely blue. One can now argue whether that change should be called information or whether the term information should be reserved for things that can be exploited by Alice and Bob but that does not change the fact that a non-local instantaneous effect changed the state of Bob's card. The thing to remember is that information (in physics) means "particles". That is pretty vague and might or might not be true. Do you mean every information exchanges between two spatially separated parties necessarily requires the exchange of (real) particles? That's the issue with QM. You would like to think that something happens. Something happens, the wave function changes.
- karmakaze 11y ago
  adekok's description was very clear to me and I was thinking exactly this. What happens is that the entangled state shares a wave function. Collapse one = both.
  - danbruc 11y ago
    The idea that two entangled particles share a wave function seems at least problematic. The ontological state of the wave function is an open problem but I think you can't get away with thinking of them as independent entities that can associate with and unassociate from particles. It is probably more like there is one very complex wave function for the entire universe and we can only sometimes get away with thinking of it as the product of the wave function of a system we are interested in and the wave function of all the rest, the environment. But even disregarding this, there seems to be a need to propagate something through space. The wave function of the entangled pair could be spread out in space and when Alice performs a measurement on her particle you have to notify the wave function at Bob's place that it should also change. You could also try to imagine that the entire wave function for the two entangled particles is not spread out in space and located next to Alice which makes it easier to imagine how the entire wave function could change at once. But now how does Bob's particle know that the far away wave function changed in order to behave appropriately? We still have to propagate that notification over to Bob. And what if Bob decides to perform the measurement instead of Alice but the wave function is sitting around at Alice's place? There is some inherent nonlocality in quantum mechanics and the effects of measurements have to propagate through space even when they can not be used to exchange classical information. One idea that I find really interesting is ER = EPR [1] by Susskind. The idea is vaguely that entanglement determines locality, i.e. two things are close together if they are entangled, especially two entangled and spatially separated particles are connected by a wormhole and therefore actually remain close together all the time. This would make spooky actions at a distance a lot less spooky because it would mean that you can never really separate entangled particles. It would probably also mean that the topology of space is actually extremely complex and only superficially looks like Euclidean three dimensional space. [1] https://en.wikipedia.org/wiki/ER%3DEPR https://en.wikipedia.org/wiki/ER%3DEPR