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.
> 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.
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.
<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?
<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.
Looking at the wave function changes nothing. It's all the same physics, and all the same concepts. You can't get different behavior from the physical system by "looking at the wave function" instead of looking at the particles.
> If the wave function is spatially extended, then the change of the wave function has to propagate through space
... via real particles. Which can't go faster than the speed of light.
> 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.
That... doesn't make any sense from a physics point of view.
You're trying to understand which is good. But you're mangling the concepts.
> 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
No. You're learning the probability distribution of the wave function. i.e the statistical distribution of the wave function, taken over many measurements.
> If Alice and Bob do this with their entangled cards,
... they learn that 50% of the cards are blue, and 50% of the cards are red. Which they already knew.
> ... 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
I'm not even sure what that means. It's based on a misunderstanding of the underlying concepts, so the sentence doesn't really make sense to me.
> 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.
I never said that.
The wave function is a probability distribution.
Information is exchanged via real particles.
You can learn new information through measurements... but not when the underlying measurements are random. i.e. with entangled particles, all you measure is that (in this case) half of the cards are red, and half are blue. Which you already knew.
Since you already knew that half of the cards are red and half are blue, the measurements give you no new information.
The concepts are really quite simple, once you throw away your "common sense" understanding of what is going on.
Looking at the wave function changes nothing. It's all the same physics, and all the same concepts. You can't get different behavior from the physical system by "looking at the wave function" instead of looking at the particles.
Alice hands Bob a particle that is either spin up or a superposition of spin up and spin down. Bob can not distinguish those two cases with a measurement. But if Bob would know the wave function he could tell the difference and Bob could learn the wave function be repeating the experiment many times. So there is a difference between looking at the outcome of a measurement and looking at the wave function.
... via real particles. Which can't go faster than the speed of light.
If Alice measures her particle of an entangled pair the wave function of Bobs particle will change without any particle transporting any information from Alice to Bob. Bob is unable to detect that change with a local measurement but the wave function changed and no particle has been exchanged.
That... doesn't make any sense from a physics point of view. You're trying to understand which is good. But you're mangling the concepts.
What is wrong with that? The wave function fully describes the state of a system but you can not learn that state in general because you would have to perform several different measurements and those measurements do not commute in general and change the state to an eigenstate of the measured observable. Only in an experimental setup with repeated measurements on identically prepared systems can you learn the wave function of the system.
No. You're learning the probability distribution of the wave function. i.e the statistical distribution of the wave function, taken over many measurements.
No, I said identically prepared systems, so the wave functions are identical in every run of the experiment. I then obtain a distribution of the eigenstates of the operator by repeating the experiment and that determines the wave function up to the phase.
I'm not even sure what that means. It's based on a misunderstanding of the underlying concepts, so the sentence doesn't really make sense to me.
Alice prepares a set of entangled particle pairs. On two thirds of the sets she measures her particle and sorts them by outcome. Now Alice has three sets, unmeasured, measured up and measured down. Bob can determine which set is which by measuring his particles, he gets 50/50 up and down, 100 % down and 100 % up respectively.
I never said that.
You did or at least I understood it that way. You say that information is what an observer can learn about the state of a system by performing measurements but there is also the information describing the state of the system, the wave function, which is not necessarily accessible to an observer.
The wave function is a probability distribution.
It is not, it is a description of the state of the system. You can obtain a probability distribution of eigenstates by repeatedly performing measurements on identical wave functions and that distribution is determined by the magnitude of the amplitude of the wave function but the wave function itself is not a probability distribution.
You can learn new information through measurements... but not when the underlying measurements are random. i.e. with entangled particles, all you measure is that (in this case) half of the cards are red, and half are blue. Which you already knew.
Since you already knew that half of the cards are red and half are blue, the measurements give you no new information.
I did not dispute that, observers obtain information about the state of a system by performing measurements, but that is not the same as the information describing the state of the system. The later is the truth, the former is what an observer knows about the truth.
You're using colloquial English to reason about physics. This is wrong. Physicists have very specific definitions of the terms they use. Which are sort of similar to the common ones, but differ in key points. Those key points are what you're getting hung up on. And it's difficult to explain the differences without explaining all pf physics.
On top of that, you've disagreed with the common definitions used by physicists "I'm not sure that's correct". Well, it is. If you think that's wrong, you either think you're smarter than all physicists alive, or you're wrong. Pick one.
> ... But if Bob would know the wave function he could tell the difference ...
No. No. A thousand times no.
It just doesn't work like that.
> If Alice measures her particle of an entangled pair the wave function of Bobs particle will change without any particle transporting any information from Alice to Bob. Bob is unable to detect that change with a local measurement but the wave function changed and no particle has been exchanged.
That is a bunch of pseudo-physics words put together in a sentence.
>> You're trying to understand which is good. But you're mangling the concepts.
> What is wrong with that?
Everything. If you're not using the correct terms in the correct way, you might as well be putting random words together in a sentence.
>> No. You're learning the probability distribution of the wave function. i.e the statistical distribution of the wave function, taken over many measurements.
> No, I said identically prepared systems, so the wave functions are identical in every run of the experiment.
So... you know better than the nuclear physicist.
This should set off alarm bells that you either don't understand the topic, or you don't care to understand it.
> You did or at least I understood it that way. You say that information is what an observer can learn about the state of a system by performing measurements but there is also the information describing the state of the system, the wave function, which is not necessarily accessible to an observer.
<sigh> The observer can do multiple measurements to determine the wave function.
>> The wave function is a probability distribution.
> It is not, it is a description of the state of the system.
Again, you're arguing with the nuclear physicist.
The probability distribution is a description of the system.
>> Since you already knew that half of the cards are red and half are blue, the measurements give you no new information.
> I did not dispute that, observers obtain information about the state of a system by performing measurements,
Yes.
> but that is not the same as the information describing the state of the system. The later is the truth, the former is what an observer knows about the truth.
<sigh> I'm trying to educate you on physics, and you're arguing pseudo-philosophical metaphysics.
Please stop. Your understanding of the terms is largely wrong. As a result, your arguments are based on falsities, and therefore also wrong.
Please go read a popular book about QM before discussing this with anyone.
And stop arguing with the nuclear physicist. This is one situation where I can appeal to authority without it being a logical fallacy. I've explained repeatedly why you're wrong. You don't seem to understand.
Unfortunately I have not enough time for a full reply right now, but I will come back over the weekend.
Wave function and probability distribution. You repeatedly insisted on precise language, that is why I objected your statement that the wave function is a probability distribution. And I still do, nuclear physicist or not. A wave function is complex-valued, a probability distribution is usually defined by a real-valued probability density function or a real-valued cumulative distribution function. The state 1/sqrt(2)(|0> + |1>) has probability amplitudes of magnitude 1/sqrt(2) for both states, the probability distribution you obtain when you repeatedly measure particles in that state has probability 1/2 for both states, the square of the magnitude of the amplitude. The wave function certainly defines a probability distribution but it is not itself one.
<sigh> I'm trying to educate you on physics, and you're arguing pseudo-philosophical metaphysics. Please stop. Your understanding of the terms is largely wrong. As a result, your arguments are based on falsities, and therefore also wrong. Please go read a popular book about QM before discussing this with anyone. And stop arguing with the nuclear physicist. This is one situation where I can appeal to authority without it being a logical fallacy. I've explained repeatedly why you're wrong. You don't seem to understand.
I haven't read everything again but most of your responses, especially in your last comment, simply state that I am wrong without pointing out why or providing a supposedly correct point of view. It is of course not you job to teach me physics, but simply stating that I am wrong is only of limited help to overcome misunderstandings. Besides that we are discussing a topic that has not been definitely settled and where there is disagreement even among professional physicists, so maybe we should not be too surprised about a certain amount of disagreement.
I would very much appreciate if we could continue this discussion a bit once I had time for a full reply - and I agree, we have to back up quite a bit, the discussion got fragmented pretty quickly - but I won't blame you if you declare me a lost case.
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.
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.
A wave function as I visualize it, doesn't exist at points it time, it exists over all points in time. Until the box is opened the cat has been alive and been alive the whole time. After collapse only one of those states is left which applies the whole time.
So if it we're possible for entangled particles to have one wave function, collapsing it makes the observed result the one that applies all the time, back to when it got entangled in a pair. I don't see this a communicating anything over distance. It's more like 'communicating' backward in time, but I don't really think that's communicating any more than opening the box communicates back in time to effect an outcome.
Quite right. It is increasingly clear that holographic theories imply 'space' has at least one dimension (spatial degree of freedom) less than we think. The Holographic Principle says a volume only has a possible number of independent states proportional to its surface area (r^2), not its internal volume (r^3). This means that what we think of as a 3D volume, is really a plate of spaghetti, with the number of threads proportional to r^2, but each thread actually corresponding to the same 'place', with instant state changes along its whole non 'length', appearing as non-local correlations to our dimensionally ambitious eyes.
The limitation of the number of threads to r^2 might be because they all have to puncture the surface to create the appearance of a volume in the first place. The theory of Loop Quantum Gravity can even generate a discrete spectrum of area values in response to various meshed (graph) 'edges' puncturing a topological surface. Geometry is quantized.
QM is obviously an incomplete theory, and there is no point repeating the philosophical arguments of the 20th century, ad nauseam. In the 21st century, physicists are beginning to put together new theories of quanta, space, time and gravity that will be a lot more coherent and satisfying, even though we will lose the cherished notion of locality.
The Dining Philosophers should stop arguing over the forks and start to discuss the spaghetti!
P.S. Douglas Adams was right, it's The Great Spaghetti Monster after all.
> 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.
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.