Why does quantum entanglement not allow ftl communication

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  • #1
macd
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quite simply the above question.
Why does quantum entanglement not allow for faster than light communication?
Thanks
 

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  • #2
Ken G
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The way I would answer this is that the information encoded in entanglement is only extractable when you look at correlations between measurements on both the entangled systems. So to access that correlation information, you would need communication anyway, and that communication could not be FTL. If you only look at either system, but not the other, then you need no such communication, but you also can extract no information from the entanglement. This is actually a good thing, because much of science is done by ignoring entanglements, and the reason we get away with that is the information we are ignoring cannot interfere with our interpretation of the results of our experiment. In other words, aliens aren't talking to us in our laboratory experiments, because if they were, we could not disentangle their messages from the laws of physics themselves.
 
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  • #3
peter0302
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That is the practical answer and I believe the right one. There are more theoretical answers called "no signaling theorems" and similar names. I don't know how they are derived, but they do enforce the notion that you cannot send a signal using entanglement.

There are those (myself among them) who believe that there might be a way to use complementarity to send a signal by altering the behavior of entangled quanta depending on how they're measured. There have been experiments showing that an interference pattern in photons is created if and only if the entangled twins are detected with absolutely ambiguous position information. So far, these experiments all require correlations because only a subset of photons is able to be detected at a given time. However, if ALL of the photons can be detected ambiguosly, then, in theory, the entangled twins should create a visible interference pattern and, perhaps, allow for signaling. John Cramer, a physicst at Wash U. is working on such an experiment now.
 
  • #4
macd
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The way I would answer this is that the information encoded in entanglement is only extractable when you look at correlations between measurements on both the entangled systems. So to access that correlation information, you would need communication anyway, and that communication could not be FTL. If you only look at either system, but not the other, then you need no such communication, but you also can extract no information from the entanglement. This is actually a good thing, because much of science is done by ignoring entanglements, and the reason we get away with that is the information we are ignoring cannot interfere with our interpretation of the results of our experiment. In other words, aliens aren't talking to us in our laboratory experiments, because if they were, we could not disentangle their messages from the laws of physics themselves.

ok... so in other words, it's not something that someone has said is impossible because of relativity (specifically the impossibility of ftl travel).

I must say your aliens analogy is rather confusing, and hence I've ignored it... like any good quantum scientest ignores entanglements;)
 
  • #5
neu
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These explanations sound over complicated to me.

Say, Alice and Bob meet together and generate a 2 qubit entangled state, then Alice and Bob each take a qubit, separate and travel to distant locations.

Assuming each qubit is kept in absolute isolation then the measurement of, say Alice's qubit will be correlated with the corresponding measurement by Bob on his qubit. That is, Alice can tell what state Bob's qubit will be in if he measures in the same basis as her; thus by communicating to him this info classically, Bob can measure his qubit based on Alice's message to obtain a qubit in the state Alice predicted.

My understanding, therefore, is that the 2 qubits of the entangled state evolve in time such that the state of one implies the state of the other; they have a shared history (and future if isolated).

In this case the transfer of info is restricted by the speed of classical communications (the speed of light).


However, I'm also confused about this restriction. If Alice needs to communicate to Bob how to measure his qubit to get the same outcome as her, can't they agree when they generate the state to perform a measurement to get this result at a v.similar time when separated to transfer the info faster than light?

Showing my ignorance here perhaps.
 
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  • #6
Ken G
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ok... so in other words, it's not something that someone has said is impossible because of relativity (specifically the impossibility of ftl travel).
Relativity is a set of axioms. We should therefore never say something is impossible "because of relativity", we should say "if it is possible, then something in relativity is wrong". We have many tests of relativity, which gives us some confidence it is not wrong, but it does not mean it could not be wrong, i.e., it does not mean we can use relativity to make statements about how reality must behave.
I must say your aliens analogy is rather confusing, and hence I've ignored it... like any good quantum scientest ignores entanglements;)
It's not meant to be confusing-- the point is, we have to start our experiments somewhere, and we need to assume our particles are coming into our experiments with no hidden FTL entanglements that could mess up our experiment. If we could extract information from them in a acausal way (i.e., FTL), then we have lost control of our experiment, because it would require that two experiments prepared in identical ways could yield statistically different results (due to the acausal information arriving). So my point is simply, one man's "FTL communication" is another's "unreliable experiment". We can't have it both ways.
 
  • #7
Ken G
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However, I'm also confused about this restriction. If Alice needs to communicate to Bob how to measure his qubit to get the same outcome as her, can't they agree when they generate the state to perform a measurement to get this result at a v.similar time when separated to transfer the info faster than light?
Yes, they could for example agree on the axis of a spin measurement. Then each would know, as soon as they measure their own qubit, what the other got/will get. So they gain information about distant places instantly, but no information is going FTL, it only travels around in their brains, in a causal way (presumably).
 
  • #8
Hans de Vries
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quite simply the above question.
Why does quantum entanglement not allow for faster than light communication?
Thanks


In the essence it is this :

Say you have two random number generators, which always produce equal random
numbers even though they are 100 light years apart. They'll give you random numbers
but no way to communicate.


Now, say that the reason that the numbers are the same is because of some super-
luminal communication between the two. The often heard claim is then this:

"Special Relativity is not violated because no information is send"

This however goes straightly against Shannon's information theory which says that pure
random data has the highest "Information Entropy" This simply means that the number
of bits you need to represent the data is maximal. You can not compress a file with
random data.

http://en.wikipedia.org/wiki/Information_entropy


Therefor, the amount of information data bits which needs to be send at superluminal
speed is maximal instead of zero as suggested by the quoted claim above.


Regards, Hans.
 
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  • #9
Ken G
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I'm afraid I don't understand the point you are making there. Are you saying that information is sent and special relativity is violated, that information is sent but special relativity is not violated, or that information is not sent?
 
  • #10
Hans de Vries
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I'm afraid I don't understand the point you are making there. Are you saying that information is sent and special relativity is violated, that information is sent but special relativity is not violated, or that information is not sent?



What I (or rather Shannon's information theory) says is that:

Random-data-send may not be confused with No-information-send

Information = number of bits, and random data actually needs the maximum
amount of data bits to transmit from one place from another. It can not
be compressed.



If the random outcome of an entanglement experiment is communicated at
superluminal speed then the claim is:

"Special Relativity is not violated because no information is send"

It is erroneously assumed that random data does not contain information and
therefor nothing physically relevant is transmitted at superluminal speed and
so special relativity is not violated.


For example: One of the outputs of the detector may be configured to trigger the
"mother of all bombs" which "blows up earth". One can hardly say that the random
bit transmitted at superluminal speed has "no information", is not physically relevant,
and therefor doesn't violate special relativity...


Regards, Hans
 
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  • #11
JesseM
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There are those (myself among them) who believe that there might be a way to use complementarity to send a signal by altering the behavior of entangled quanta depending on how they're measured. There have been experiments showing that an interference pattern in photons is created if and only if the entangled twins are detected with absolutely ambiguous position information. So far, these experiments all require correlations because only a subset of photons is able to be detected at a given time. However, if ALL of the photons can be detected ambiguosly, then, in theory, the entangled twins should create a visible interference pattern and, perhaps, allow for signaling. John Cramer, a physicst at Wash U. is working on such an experiment now.
I think it's been definitively proven that if the accepted equations of quantum theory are correct, then no possible experiment can be used for FTL communication--this source says it's ruled out by "Eberhard's theorem" in section 2.3, and this article by Cramer says:
At the AQRTP Workshop we considered the question of whether quantum nonlocality was a possible medium for FTL communication. In the context of standard quantum mechanics there is good reason for believing that it is not. Eberhard has proved a theorem demonstrating that the outcomes of separated measurements of the same quantum system, correlated by nonlocality though they are, cannot be used for FTL observer-to-observer communication. A possible loophole in Eberhard's theorem could arise if, following the work of Nobel Laureate Steven Weinberg, one modifies conventional quantum mechanics by introducing a small non-linear element into the standard QM formalism. It has been shown that in slightly non-linear quantum mechanics, the observable nonlinear effects that would arise would make possible FTL communication through nonlocality.
So, it seems Cramer is pinning his hopes on a nonlinear modification to the accepted equations of QM. I think this is the paper by Eberhard they're referring to, by the way.
 
  • #12
Ken G
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If the random outcome of an entanglement experiment is communicated at
superluminal speed then the claim is:

"Special Relativity is not violated because no information is send"
But the reason no information is sent is not because the information looks random, it is because the information is not sent.
For example: One of the outputs of the detector may be configured to trigger the
"mother of all bombs" which "blows up earth". One can hardly say that the random
bit transmitted at superluminal speed has "no information", is not physically relevant,
and therefor doesn't violate special relativity...
But you still haven't explained why you think that information is ever sent in an entanglement experiment. I don't see it going anywhere.
 
  • #13
Ken G
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So, it seems Cramer is pinning his hopes on a nonlinear modification to the accepted equations of QM.
Yes, it seems that. My "horse sense" tells me that if quantum mechanics with Weinberg's nonlinearity worked in the real world, there would also appear some other aspect of the correction that still makes FTL communication impossible. In other words, there does not seem to be any principle of a corrected quantum mechanics that would be more fundamental than the principle of causality. Of course, experiment may prove me wrong, I'm just saying where'd I'd put my money if anyone gave me the chance.
 
  • #14
Hans de Vries
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But the reason no information is sent is not because the information looks random, it is because the information is not sent. But you still haven't explained why you think that information is ever sent in an entanglement experiment. I don't see it going anywhere.


The process goes by the name "quantum teleportation".

This literally says that something (the quantum state) is being transported over far.


Regards, Hans
 
  • #15
cesiumfrog
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The process goes by the name "quantum teleportation". This literally says that something (the quantum state) is being transported over far.
The "teleportation" process requires waiting for information to be transported across classical channels. (Are you unfamiliar with this?)
 
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  • #16
Hans de Vries
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The "teleportation" process requires waiting for information to be transported across classical channels. (Are you unfamiliar with this?)

Read more carefully.

You seem to have the idea that I said that superluminal communication is
possible. I never did.

I am pointing out the distinct difference between "random information" and
"no information" according to Shannon's information theory. I was criticizing
the idea that superluminal propagation of random data (random quantum states)
can be reconciled with SR.


Regards, Hans
 
  • #17
Ken G
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The process goes by the name "quantum teleportation".
Quantum teleportation is not FTL. As I said, I agree that transporting "random" information FTL would still violate SR, the point is, that's the incorrect reason that entanglement doesn't produce FTL communication. The correct reason is simply that it doesn't "transport" anything, random or otherwise, FTL. Perhaps you are not disagreeing with that.
 
  • #18
Hans de Vries
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Quantum teleportation is not FTL. As I said, I agree that transporting "random" information FTL would still violate SR, the point is, that's the incorrect reason that entanglement doesn't produce FTL communication. The correct reason is simply that it doesn't "transport" anything, random or otherwise, FTL. Perhaps you are not disagreeing with that.

No, This is the reasoning of the EPR crowd:

1) The quantum state is teleported instantaneous.
2) We can not control the collapse of the wave-function.
3) Therefor we can not use it to communicate data.
4) Therefor no information is send (FTL)

Claim 4) violates Shannon's information theory. that's my point

Of coarse, the majority of the EPR crowd doesn't believe this to be fundamental.
I get the impression that many of them are really chasing their Science Fiction
dreams and that statements like: Special Relativity is not really violated are
more to appease peer reviewers than that they themself believe in it.


Regards, Hans
 
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  • #19
JesseM
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No, This is the reasoning of the EPR crowd:

1) The quantum state is teleported instantaneous.
2) We can not control the collapse of the wave-function.
3) Therefor we can not use it to communicate data.
4) Therefor no information is send.
Are you sure about the claim that the "quantum state is teleported instantaneously"? Do you have a reference? It seems to me that if that were the case, one could still gain probabilistic information about the original, distant state that was teleported by looking at the outcome when the teleported state was measured.
 
  • #20
Ken G
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No, This is the reasoning of the EPR crowd:

1) The quantum state is teleported instantaneous.
There is no meaning to "instantaneous" except for the person doing the original experiment. Nothing is "teleported" unless there is classical slower-than-light communication, so it is not instantaneous.
3) Therefor we can not use it to communicate data.
That's not the correct reason why we can't communicate instantaneously, the correct reason is that nothing is transported instantaneously in the first place. I cannot speak for whoever you mean by "the EPR crowd"-- I agree that argument would be spurious, but it's not the right argument anyway.
Claim 4) violates Shannon's information theorem. that's my point
But it's all a strawman, that's my point. If the "EPR crowd" think they require that explanation, they don't understand information theory, but since a lot of people do, I don't see that as likely. There may be a difficulty in finding people interested in philosophy who are also versed in physics.
Of coarse, the majority of the EPR crowd doesn't believe this to be fundamental.
I get the impression that many of them are really chasing their Science Fiction
dreams and that statements like: Special Relativity is not really violated are
more to appease peer reviewers than that they themself believe in it.
I don't know what they believe, but I don't think personal beliefs are terribly relevant either.
 
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  • #21
Hans de Vries
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There is no meaning to "instantaneous" except for the person doing the original experiment. Nothing is "teleported" unless there is classical slower-than-light communication, so it is not instantaneous.


Of coarse the whole concept of instantaneous propagation doesn't make any sense
at all in the first place if not defined with respect to a "preferred" reference frame.


That's not the correct reason why we can't communicate instantaneously, the correct reason is that nothing is transported instantaneously in the first place. I cannot speak for whoever you mean by "the EPR crowd"-- I agree that argument would be spurious, but it's not the right argument anyway.


Ok, but you are using the expression "correct reason" in the sense of 2)
while I am using it in the sense of 1)

1) The reason which correctly describes the arguments used by the EPR people.
2) Something what you think (or something what I think) is the correct physics.


Regards, Hans
 
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  • #22
Hans de Vries
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Are you sure about the claim that the "quantum state is teleported instantaneously"? Do you have a reference?

For instance in Zeilinger's popular article in the Scientific American (April 2000)
he claims:

Zeilinger said:
By "spooky action at a distance", the measurement also instantly alters the
the quantum state of the faraway counter matter.

The article must be online somewhere.


Regards, Hans
 
  • #23
JesseM
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Of coarse the whole concept of instantaneous propagation doesn't make any sense
at all in the first place if not defined with respect to a "preferred" reference frame.
It does make sense in the context of conventional nonrelativistic QM, though. Are you claiming that according to the equations of this theory, the quantum state of the teleported system goes instantaneously from one location to another, before the classical signal has had time to travel between the locations? If so I would like to see a reference for this.

edit: I see you reference a popular article above, but I'd like to see something more technical as the precise meaning of statements in popular articles is often unclear--Zeilinger may have only meant that the quantum state is instantly teleported after the classical signal has gone from one location to another and the information in that signal is used to prepare the far away system.
 
  • #24
Ken G
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Of coarse the whole concept of instantaneous propagation doesn't make any sense
at all in the first place if not defined with respect to a "preferred" reference frame.
Right, and all the evidence we have is that there is no such frame.

Ok, but you are using the expression "correct reason" in the sense of 2)
while I am using it in the sense of 1)

1) The reason which correctly describes the arguments used by the EPR people.
2) Something what you think (or something what I think) is the correct physics.
I don't dispute that because I haven't seen the arguments of the "EPR people", and I agree that would be a wrong argument. I just didn't want to leave the impression that this represented the best argument against FTL communication.
 
  • #25
Ken G
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For instance in Zeilinger's popular article in the Scientific American (April 2000)
he claims:
"By "spooky action at a distance", the measurement also instantly alters the
the quantum state of the faraway counter matter."
Sure, but that says nothing about anything being transported, not even random data. I don't know if he means this or not, but in my view the statement is perfectly correct, insofar as the "quantum state" is interpreted as "the way the observer doing the measurement would characterize the state of the faraway system". Personally, I don't know of any other meaningful definition of that phrase, but I agree that a lot of people seem to think there is one.
 
  • #26
JesseM
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I don't know if he means this or not, but in my view the statement is perfectly correct, insofar as the "quantum state" is interpreted as "the way the observer doing the measurement would characterize the state of the faraway system". .
I would interpret the quantum state to refer to the set of probability amplitudes for different outcomes when you measure the system. If you look at the schematic diagram on this page, I think the idea is that at the moment "C" turns green, it now has the same amplitudes that "A" had when it was green, up until the moment it was disrupted by becoming entangled with "B". The diagram suggests that C's state only becomes identical to A's original state (before being disrupted) after the classical data has been transmitted from the location of A to the location of C.
 
  • #27
Ken G
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I would interpret the quantum state to refer to the set of probability amplitudes for different outcomes when you measure the system.
So would I, but the question is, for whom? The problem with probabilities is many people treat them like absolutes, and ask questions like "what is the probability of...". But implicit in those kinds of questions is a host of information that is assumed to be known, along with a host of information that is assumed to not be known. Without those assumptions, probabilities are meaningless-- and those assumptions are often different for different people (witness a poker game).
If you look at the schematic diagram on this page, I think the idea is that at the moment "C" turns green, it now has the same amplitudes that "A" had when it was green, up until the moment it was disrupted by becoming entangled with "B".
Absolutely, and the information to do that was transported classically. Thus there is no issue all all when C "turned green", as it is a purely local event. What the observer back at A uses for the wave function of the entangled pair is irrelevant to making C "turn green", at least until the classical information arrives.
The diagram suggests that C's state only becomes identical to A's original state (before being disrupted) after the classical data has been transmitted from the location of A to the location of C.
Exactly.
 
  • #28
JesseM
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So would I, but the question is, for whom? The problem with probabilities is many people treat them like absolutes, and ask questions like "what is the probability of...". But implicit in those kinds of questions is a host of information that is assumed to be known, along with a host of information that is assumed to not be known. Without those assumptions, probabilities are meaningless-- and those assumptions are often different for different people (witness a poker game).
I'm not sure what you mean by "treat them like absolutes", but in theoretical QM every possible outcome for a measurement on a system is given an unambiguous probability amplitude, that set of amplitudes is essentially what the wavefunction for a system is.
 
  • #29
JesseM
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For instance in Zeilinger's popular article in the Scientific American (April 2000)
he claims:
By "spooky action at a distance", the measurement also instantly alters the
the quantum state of the faraway counter matter.
The article must be online somewhere.
Found the article http://tqd1.physik.uni-freiburg.de/~walter/lehre/quinfoSS03/zeilinger.pdf [Broken]. If you look on p. 54, where he discusses the experiment and again uses the word "instantaneous", in this context what he means is that if you have two entangled particles A and B, and you perform a certain type of "joint measurement" on A and another particle X, this will "instantaneously" create a 3-particle entangled system which also involves B. But at this point B's state is not actually identical to X's before the measurement, it only becomes identical when you interact with B in a certain way, making use of classical information about the outcome of the joint measurement on on A and X.
 
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  • #30
Ken G
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I'm not sure what you mean by "treat them like absolutes", but in theoretical QM every possible outcome for a measurement on a system is given an unambiguous probability amplitude, that set of amplitudes is essentially what the wavefunction for a system is.
But that's just what I'm talking about-- there is no need to treat the wave function like it is unique, and in fact it is not. It is perfectly possible to imagine an experiment where two different participating physicists arrive at two different wave functions for the same system, based on different information about that system, and have "quantum mechanics work" perfectly well for both physicists. Indeed, that is precisely what can happen with entanglement. You might say "one of them has the complete wave function, and the other has an incomplete one" but there's no prescription in quantum mechanics for identifying a "complete" wave function-- we "go with the wave function we have". Indeed, Bohmians seem to feel we never are using the complete wave function. So my point is, just as "the probability" in poker is a completely relative concept, so is the "probability amplitude" of a wave function. This is annoying for people who like to think of the wave function as something real, but personally I cannot see the least bit of evidence to support that viewpoint, and it leads to all kinds of bizarre problems like "spooky action at a distance".
 
  • #31
JesseM
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But that's just what I'm talking about-- there is no need to treat the wave function like it is unique, and in fact it is not. It is perfectly possible to imagine an experiment where two different participating physicists arrive at two different wave functions for the same system, based on different information about that system, and have "quantum mechanics work" perfectly well for both physicists.
Can you give a specific example of what you mean?
Ken G said:
Indeed, that is precisely what can happen with entanglement. You might say "one of them has the complete wave function, and the other has an incomplete one" but there's no prescription in quantum mechanics for identifying a "complete" wave function
Sure there is--if you measure a maximal set of communting operators for the system, that determines a unique wavefunction.
Ken G said:
we "go with the wave function we have". Indeed, Bohmians seem to feel we never are using the complete wave function.
No they don't--I think you're confusing "wave function" with "complete state of the system, including hidden variables". Just because we know the complete wave function, that doesn't mean we're ruling out the possibility that there may be other hidden variables not accounted for by the wave function.
Ken G said:
This is annoying for people who like to think of the wave function as something real, but personally I cannot see the least bit of evidence to support that viewpoint, and it leads to all kinds of bizarre problems like "spooky action at a distance".
Well, local realistic theories can be proved incompatible with QM just based on the statistics of measured outcomes predicted in QM, saying nothing about the wave function.
 
  • #32
Ken G
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Can you give a specific example of what you mean?
Sure, the standard 1/2-spin entangled pair with zero total angular momentum. If I do a measurement on one and get a spin of +1/2 in the "z direction", I will instantly make the wave function of your particle -1/2 in the z direction, and use that to predict the outcome of any experiment you do. You, on the other hand, will stick with a mixed-state wavefunction for your particle, with 50% up and 50% down. You will use that to predict the outcome of any experiment you can do, and you will do just fine. We both will, even though we make different predictions on that particular trial, because on an ensemble our predictions will be indistinguishable without looking at correlations (which would require slower-than-light communication to do).
Sure there is--if you measure a maximal set of communting operators for the system, that determines a unique wavefunction.
Not so. What if you do that on an entangled particle? You have no idea what correlations exist between your measurements and some other set of measurements, so your description is incomplete. What you have done is to assume that you have a single-particle wave function, but reality doesn't hand you that-- if you think a wavefunction is real, it must include everything your particle is entangled with. That's why even the wavefunction you describe is not "the complete wavefunction" that involves that particle, it is merely the most complete description you can find within the confines of a single-particle wavefunction (note also that the universe is full of identical particles and you are simply ignoring the exchange terms in the hope that they don't matter). I would say that a wave function is just a model, and hence reflects a choice by a physicist-- not a reality.
No they don't--I think you're confusing "wave function" with "complete state of the system, including hidden variables". Just because we know the complete wave function, that doesn't mean we're ruling out the possibility that there may be other hidden variables not accounted for by the wave function.
True, but if the wave function does not include that information, then it is not "the reality". That dovetails with my claim that a wave function is simply a reflection of the information we are choosing to use. That must have something to do with the reality or it would not be so useful, but "the reality" has to include more information than we are using. (Indeed, even if the Bohmian approach is a good model, I would say it still isn't going to be "the reality" because even if you have all the information, information is still reality passed through a filter, not reality itself-- but that gets philosophical).
Well, local realistic theories can be proved incompatible with QM just based on the statistics of measured outcomes predicted in QM, saying nothing about the wave function.
Correct, but a wave function is not based on local realism, so most seem to hold that wave functions are real, that there is such a thing as "the wave function" of a particle, or more correctly, a universe. Why they believe that is pretty much a mystery to me.
 
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  • #33
JesseM
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Not so. What if you do that on an entangled particle? You have no idea what correlations exist between your measurements and some other set of measurements, so your description is incomplete.
What I said was that "if you measure a maximal set of commuting operators for the system, that determines a unique wavefunction"--it may not have been sufficiently clear, but what I meant was that for any entangled multiparticle system, you would have to measure a maximal set of commuting operators for all parts of the system to construct a wavefunction, not just a single particle.
Ken G said:
True, but if the wave function does not include that information, then it is not "the reality". That dovetails with my claim that a wave function is simply a reflection of the information we are choosing to use.
I never said the wave function was "the reality", just that all the probability amplitudes can be uniquely determined with the right kind of measurements--if you've made these measurements, you don't have any "choice" of what the wavefunction should look like, even if there could be other realities to the system that aren't specified by the wavefunction.
 
  • #34
Ken G
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What I said was that "if you measure a maximal set of commuting operators for the system, that determines a unique wavefunction"--it may not have been sufficiently clear, but what I meant was that for any entangled multiparticle system, you would have to measure a maximal set of commuting operators for all parts of the system to construct a wavefunction, not just a single particle.
How do you know what the entangled system is? You still have to specify the system, you have to decide what entanglements you want to track, so you are still making a choice. The only system the universe hands you is the whole universe, so the only "complete" wavefunction of a system is a maximal set of all commuting operators for the whole universe. That's impossible, because an "operator" is an observable, which implies you have to do the observation from outside the system, i.e., outside the universe (there's a self-referential problem, I mean). So in reality you will consider a subsystem, but any subsystem you specify will still suffer from the incompleteness problem, because you cannot trace the entanglements and so will still be losing information about potential correlations. Completeness is impossible, so why do we pretend it isn't? Because we can achieve effective completeness in our chosen model-- but hey, it isn't the reality, which is all I'm saying.
I never said the wave function was "the reality", just that all the probability amplitudes can be uniquely determined with the right kind of measurements--if you've made these measurements, you don't have any "choice" of what the wavefunction should look like, even if there could be other realities to the system that aren't specified by the wavefunction.
The issue I was addressing is if there was a unique wavefunction that includes everything that is real about a system, or if it is merely a way for us to encode whatever information we have about the system. In other words, when we say we know "the wavefunction" in some absolute way, can we address not just questions like "what is the probabilty I'll measure X", but also questions like, "what is the probability I'll measure X given that some other entangled system gave result Y"? The answer is no, even with what you are calling the complete wavefunction for that system, we cannot answer the latter questions.

So the price for defining what you mean by a "complete wavefunction" is to rule out that you can address everything that is real about it. That is the choice-- you have chosen a weak form of completeness that cannot encompass all that is real about your system. That suffices to establish that a wavefunction is an expression of a choice we have made about modeling systems, not a complete description of the reality. That's all I'm claiming-- every physicist with different information about a system will successfully use a different wave function to describe it, and moreover, none of them will be using a complete description of all that is real about that system, so none have a claim to knowing "the wave function" of the system.
 
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JesseM
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But the point is, you still have to specify the system, so you are still making a choice. The only system the universe hands you is the whole universe, so the only "complete" wavefunction of a system is a maximal set of all commuting operators for the whole universe.
Only in the MWI, where measurements are themselves just new entanglements, is this really true. In Copenhagen QM, the act of measuring a particle can destroy previous entanglements it may have had up until that measurement (though it won't always, it depends on what measurement you perform)--subsequent measurements on this particle won't show any correlations with other particles it was entangled with prior to the first measurement.
 

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