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> 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

by adekok 10y 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 10y 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 10y 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 10y 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
    - adekok 10y ago
      > But even disregarding this, there seems to be a need to propagate something through space. That's the problem with QM. So far as we can tell, nothing propagates through space. Even if something did propagate through space, it does so faster than the speed of light. And we know that information travelling faster than the speed of light is impossible. Because if it was possible, we would have causality violations. i.e. if faster than light travel was possible, the universe would necessarily look very different than it does. So we have multiple experiments, all of which are true, and which apparently disagree with each other in certain esoteric ways. Since the universe is consistent, we must conclude that the failure is in our understanding, not in the universe.
- adekok 10y ago
  > I am not sure that this is correct. <sigh> So you're smarter than every physicist alive. Well... no. > One can now argue whether that change should be called information ... No, you can't. You're using colloquial English to reason about physics. This is wrong. Physicists have a very specific definition for "information" (as I already said, and you ignored). And in this case, no information is exchanged. > That is pretty vague and might or might not be true. As the published nuclear physicist in the argument... yes, yes, it's true. > Do you mean every information exchanges between two spatially separated parties necessarily requires the exchange of (real) particles? Yes.
  - danbruc 10y ago
    <sigh> So you're smarter than every physicist alive. Well... no. The usual statement is that entanglement can not be used to exchange information between spatially separated observers, right? I totally agree with that. But what about the wave function? If the wave function is spatially extended, then the change of the wave function has to propagate through space if one assumes that the change is caused by a local measurement. And the wave function contains the information that fully describes the state of the system but one can not learn the wave function in general because some operators are not commutative. Nonetheless a change of the wave function means a change of the information describing the system whether an observer can detect that change or not. That are of course two different things, the information describing the state of the system and the information an observer can learn about the state of the system, but I see no conflict here. No, you can't. You're using colloquial English to reason about physics. This is wrong. Physicists have a very specific definition for "information" (as I already said, and you ignored). And in this case, no information is exchanged. If I measure a spin without prior knowledge of its state, I obtain one bit of information, I guess that is the sense in which you want to understand information, right? If I repeat the experiment with identical preparation over and over again, then I can learn the wave function, maybe 30 % up, 70 % down. If Alice and Bob do this with their entangled cards, then Bob can decide between the case that Alice saw a red card in which case he will always see his card blue and the case that Alice did not look at her card in which case he will see his card as randomly red or blue. When Alice looks at her card she must of course perform a post selection to get rid of the pairs where she saw her card as blue. I hope you notice that I am not trying to argue that you are wrong, I am just trying to understand what you say, especially why one would consider the result of a measurement as information but not consider the wave function as information.

by adekok 10y ago

danbruc 10y 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 10y 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 10y 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
  - adekok 10y ago
    > But even disregarding this, there seems to be a need to propagate something through space. That's the problem with QM. So far as we can tell, nothing propagates through space. Even if something did propagate through space, it does so faster than the speed of light. And we know that information travelling faster than the speed of light is impossible. Because if it was possible, we would have causality violations. i.e. if faster than light travel was possible, the universe would necessarily look very different than it does. So we have multiple experiments, all of which are true, and which apparently disagree with each other in certain esoteric ways. Since the universe is consistent, we must conclude that the failure is in our understanding, not in the universe.
adekok 10y ago
> I am not sure that this is correct. <sigh> So you're smarter than every physicist alive. Well... no. > One can now argue whether that change should be called information ... No, you can't. You're using colloquial English to reason about physics. This is wrong. Physicists have a very specific definition for "information" (as I already said, and you ignored). And in this case, no information is exchanged. > That is pretty vague and might or might not be true. As the published nuclear physicist in the argument... yes, yes, it's true. > Do you mean every information exchanges between two spatially separated parties necessarily requires the exchange of (real) particles? Yes.
- danbruc 10y ago
  <sigh> So you're smarter than every physicist alive. Well... no. The usual statement is that entanglement can not be used to exchange information between spatially separated observers, right? I totally agree with that. But what about the wave function? If the wave function is spatially extended, then the change of the wave function has to propagate through space if one assumes that the change is caused by a local measurement. And the wave function contains the information that fully describes the state of the system but one can not learn the wave function in general because some operators are not commutative. Nonetheless a change of the wave function means a change of the information describing the system whether an observer can detect that change or not. That are of course two different things, the information describing the state of the system and the information an observer can learn about the state of the system, but I see no conflict here. No, you can't. You're using colloquial English to reason about physics. This is wrong. Physicists have a very specific definition for "information" (as I already said, and you ignored). And in this case, no information is exchanged. If I measure a spin without prior knowledge of its state, I obtain one bit of information, I guess that is the sense in which you want to understand information, right? If I repeat the experiment with identical preparation over and over again, then I can learn the wave function, maybe 30 % up, 70 % down. If Alice and Bob do this with their entangled cards, then Bob can decide between the case that Alice saw a red card in which case he will always see his card blue and the case that Alice did not look at her card in which case he will see his card as randomly red or blue. When Alice looks at her card she must of course perform a post selection to get rid of the pairs where she saw her card as blue. I hope you notice that I am not trying to argue that you are wrong, I am just trying to understand what you say, especially why one would consider the result of a measurement as information but not consider the wave function as information.