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> But what about the wave function? Looking at the wave function changes nothing. It's all the same physics, and all the same concepts. You can't get differe

by adekok 10y ago

> But what about the wave function? 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.

danbruc 10y ago
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.

by adekok 10y ago

danbruc 10y ago

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.