Why can't we use quantum bit entanglement for FTL information?

[zeromodz]

I don't understand. Why can't we use quantum entanglement by the means of altering a state of an entangled electron so we can send an instantaneous bit of information to anywhere in the universe infinitely fast by the other electron changing its quantum state? We can can in theory transmit information faster than c.

Why exactly can't we do this?

 

[Academic]

Try this at home with a friend,

Consider a yes or no message to be sent. Then take a coin and flip it, hold what it lands on up to your eyes and the other side up to your friends. This is like an entangled state that collapses, if you see head your friend will see tails (or vise versa) and you each know what the other sees. Now using this coin alone see if you can transmit a yes or no message to your friend.

 

[zeromodz]

Try this at home with a friend,

Consider a yes or no message to be sent. Then take a coin and flip it, hold what it lands on up to your eyes and the other side up to your friends. This is like an entangled state that collapses, if you see head your friend will see tails (or vise versa) and you each know what the other sees. Now using this coin alone see if you can transmit a yes or no message to your friend.

Okay, I see the point. Is there anyway we could choose what state the electron goes into somehow so then we could transmit a bit?

 

[Academic]

Nope.

 

[zeromodz]

Nope.

Okay, what about sequencing when the electron has its wave function destroyed. It doesn't matter what the outcome is, it just depends on the timing of when the function is destroyed to transmit information indirectly. For example, collapsing 3 electron functions in 10 seconds means a specific bit say "yes" of information and collapsing 6 in ten seconds means "no". How can we not transmit information this way? Is this possible?

 

[Hurkyl]

How can we not transmit information this way?

By looking at my coin, how can I tell if you've looked at your coin yet to "destroy" the uncertainty?

 

[jambaugh]

I don't understand. Why can't we use quantum entanglement by the means of altering a state of an entangled electron so we can send an instantaneous bit of information to anywhere in the universe infinitely fast by the other electron changing its quantum state?
Why exactly can't we do this?

Because acting on one half of an entangled pair has no causal effect on the other half. There is no (reason to suppose a) FTL effect, not even one which is hidden.

The non-locality issues in Bell/EPR thought experiments are red herrings. The critical issue is that quantum systems do not have objective (classical) states of reality.

Entanglement is simply correlation. What is different in the quantum case is that the assertion of correlation (like any meaningful physical assertion) implies a measurement has been made (think of preparing the system as measuring a number of systems until one with the desired trait is found). This entangling measurement does not commute with the critical measurements of half the pair we take and thus prior to that second measurement each half does not have its own definite "state". Thus the correlation is meaningful in and of itself but can't be expressed in the classical way of:
"Either system A is in state x and so is system B, or"
"system A is in state y and so is system B, or "
"system A is in state z and so is system B,"

Each of these statements, when said along with "A and B are entangled so as to be perfectly correlated" is just as wrong as saying a particle has momentum p and has position x or has momentum p and position x'."

I think part of the problem is the use of the term "entanglement" which intuitively hints at a causal connection, i.e. some string connecting the two halves. Translate "entanglement" to "quantum correlation" and you'll be less likely to incorrectly introduce a classical bias.

Quantum systems are not just tiny classical systems.

 

[zeromodz]

Because acting on one half of an entangled pair has no causal effect on the other half. There is no (reason to suppose a) FTL effect, not even one which is hidden.

The non-locality issues in Bell/EPR thought experiments are red herrings. The critical issue is that quantum systems do not have objective (classical) states of reality.

Entanglement is simply correlation. What is different in the quantum case is that the assertion of correlation (like any meaningful physical assertion) implies a measurement has been made (think of preparing the system as measuring a number of systems until one with the desired trait is found). This entangling measurement does not commute with the critical measurements of half the pair we take and thus prior to that second measurement each half does not have its own definite "state". Thus the correlation is meaningful in and of itself but can't be expressed in the classical way of:
"Either system A is in state x and so is system B, or"
"system A is in state y and so is system B, or "
"system A is in state z and so is system B,"

Each of these statements, when said along with "A and B are entangled so as to be perfectly correlated" is just as wrong as saying a particle has momentum p and has position x or has momentum p and position x'."

I think part of the problem is the use of the term "entanglement" which intuitively hints at a causal connection, i.e. some string connecting the two halves. Translate "entanglement" to "quantum correlation" and you'll be less likely to incorrectly introduce a classical bias.

Quantum systems are not just tiny classical systems.

Okay, I understand what you mean by both particles still don't have definite states objectively, but wouldn't the particles show any objective sign whatsoever of the collapse of the function?

That's all we need, just some sign in change of measurement or momentum. Then we could use sequencing of particles to indirectly transmit information like I said above.

 

[zeromodz]

By looking at my coin, how can I tell if you've looked at your coin yet to "destroy" the uncertainty?

Wouldn't altering a quantum position change any sign of the position or momentum? One or the other more or less. We could use special measurement tools.

 

[Academic]

Im not sure what you mean. But like Hurkyl says, you cannot tell if the other person looked by looking yourself.

 

[zeromodz]

Im not sure what you mean. But like Hurkyl says, you cannot tell if the other person looked by looking yourself.

You don't need to check. I just want to know if you entangle 2 electrons and separate them across the universe, could you break one and the other will act instantly by the means of any measurement. It doesn't matter what it does, just as long as it does something. Once this happens you can take more electrons and use a pattern of when the electrons behave in a certain way to transmit information indirectly.

 

[Vanadium 50]

Zeromodz, what would it take to convince you that the answer is "no"?

 

[unusualname]

jambaugh just complicated the issue by spamming the thread with an unnecessary personal interpretation of QM.

The original answer by Academic is sufficient: ie there is no known way of deterministically selecting a quantum state so there is no known way of transmitting information via entanglement.

And you can't tell whether a measurement was made on an entangled partner by the result of measuring the other one, so zeromodz's other suggestion of 'just as long as it does something' won't work either.

 

[JDługosz]

It doesn't matter what it does, just as long as it does something. Once this happens you can take more electrons and use a pattern of when the electrons behave in a certain way to transmit information indirectly.

It doesn't play an alarm or raise a flag or anything. It just carries on being an electron. Nothing comes of it until you make a measurement. When you do, you get a random result. Can you tell if I also obtained a result on my half?

Worse, it doesn't matter when each side makes a measurement. There is no "first". They just agree. If they are far apart and the measurements made near the same time, there is no universal agreement over the ordering! It depends on the observer, and there is no time ordering between them.

If you still can't understand, it might do to work through a more concrete example. E.g. you are on a star ship, in deep space. There is an electron sitting in a trap in the safe. Has it "done something"? No, and it won't. At some point you decide to pass it through a polarizer and measure the result. What can you tell from that?

 

[Born2bwire]

It doesn't play an alarm or raise a flag or anything. It just carries on being an electron. Nothing comes of it until you make a measurement. When you do, you get a random result. Can you tell if I also obtained a result on my half?

Worse, it doesn't matter when each side makes a measurement. There is no "first". They just agree. If they are far apart and the measurements made near the same time, there is no universal agreement over the ordering! It depends on the observer, and there is no time ordering between them.

If you still can't understand, it might do to work through a more concrete example. E.g. you are on a star ship, in deep space. There is an electron sitting in a trap in the safe. Has it "done something"? No, and it won't. At some point you decide to pass it through a polarizer and measure the result. What can you tell from that?

Special relativity challenges the classical ideas of simultaneity. Depending upon the conditions, we could easily have observers that could say that group A measured before group B and another set of observers that say group B was first. Even with these conflicting views, they would all have to be correct. So as JDługosz states, there is no "first."

Not only that, but you have to measure the system and the simple act of doing this collapses the state. You cannot monitor the system passively on your end and tell when the other group measured the system themselves.

 

[jambaugh]

jambaugh just complicated the issue by spamming the thread with an unnecessary personal interpretation of QM...

Firstly my "personal" interpretation is the orthodox one. It just doesn't get as much Hollywood airtime since it doesn't predict such ridiculous effects as FTL signals or infinite parallel dimensions that makes for good SciFi plot elements or fantastic article titles in the popular press.

Secondly the quickest way to prove a theory (with multiple interpretations) doesn't in anyway predict an effect is to demonstrate that a valid interpretation of that theory excludes that effect. (Similar to demonstrating that say the parallel postulate is not a result of the other postulates by demonstrating a valid model satisfying the other postulates but definitely invalidating the parallel postulate).

Thirdly and finally, I am not initiating the invocation of interpretations here. The whole FTL business comes from a ("mis")interpretation of the QM. I'm just piping in with classic CI for equal time.

To zeromodz,

Okay, I understand what you mean by both particles still don't have definite states objectively, but wouldn't the particles show any objective sign whatsoever of the collapse of the function?

To make it clear, "not having an objective state" implies no observation of an objective state. Without such observation there is no possibility of observing a change. "any objective sign" as I understand your meaning must mean an observable sign.

Also The wave-function is initially (in the sequence of definition) a representation of our knowledge about the system. It takes an act of (mis-)interpretation to also give it meaning as a representation of a state of reality. In the first case the collapse is just a representation of the change in our knowledge about the system. Only if you add the "reality" do you assert that this collapse is actually occurring "out there" in the same instantaneous way that it is occurring on paper (and thus get instantaneous (meta)physical effects instead of just instantaneous updates in predictions.)

Without that ("mis")interpretation one can only say that the wave-function collapse is expressing the same sort of discontinuity as we see in a classical probability distribution collapse once one invokes new information. One is making the transition to a conditional probability. "Give we observed X we no longer look at Pr(A) but Pr(A|X)." e.g. "Once we know the number 4 is drawn in the lotto, all the tickets without a 4 instantaneously become worthless."

Now you can choose an alternative to the minimal interpretation I describe provide it consistent as far as empirical predictions goes. But given it is consistent with QM and given it doesn't predict beyond the predictions of QM it cannot predict phenomena inconsistent with the orthodox Copenhagen interpretation. Since that interpretation is consistent with relativistic causal locality and with QM there is
a.) no way QM explicitly predicts an observable FTL causal event (of which a FTL signaling device would certainly be an example) and
b.) no way an FTL causal event is consistent with both QM and relativity.

This does not preclude an alternative theory: "QM plus a a relativity violating theory" which allows FTL signals. But my point is that adding QM in no way adds to our expectation of FTL effects. Speculating about "hyperwave radios" and "warp drives" are independent of the assertions of QM excepting that they should be consistent with QM if you believe in QM. But it is simply speculating both classically or quantum mechanically that relativity is wrong and not inferring anything at all from QM itself.

Ask if there is any way we might sent an FTL signal. Why invoke QM per se?

P.S. Actually, technically Relativity doesn't preclude FTL causality as such. It simply asserts that FTL causal signals as seen by one observer are backward in time signals as seen by another. You cannot invoke FTL without invoking Time Travel and remain consistent with relativity. It is sufficient to resolve the causal loop paradoxes one generates by asserting backward-in-time communication to retain both relativity and the hoped for FTL causation. But without some empirical examples such belief is an act of faith and such speculation is just "out there".

 

[unusualname]

Firstly my "personal" interpretation is the orthodox one. It just doesn't get as much Hollywood airtime since it doesn't predict such ridiculous effects as FTL signals or infinite parallel dimensions that makes for good SciFi plot elements or fantastic article titles in the popular press.

Secondly the quickest way to prove a theory (with multiple interpretations) doesn't in anyway predict an effect is to demonstrate that a valid interpretation of that theory excludes that effect. (Similar to demonstrating that say the parallel postulate is not a result of the other postulates by demonstrating a valid model satisfying the other postulates but definitely invalidating the parallel postulate).

Thirdly and finally, I am not initiating the invocation of interpretations here. The whole FTL business comes from a ("mis")interpretation of the QM. I'm just piping in with classic CI for equal time.

By "orthodox one" I think you mean "the oldest one" (it's hard to tell because you just posted vague stuff about correlations), I don't think there's anything "orthodox" about it, it was accepted mostly because the mainstream scientists actually producing results weren't really bothered by the interpretation problem since it doesn't add anything useful to the work they were developing.

This is still the case really, since the QM + unification project doesn't immediately require a resolution to the interpretation problem.

In fact the interpretation issue will probably fall out rather easily once the really difficult work of building a mathematical model of microscopic reality and cosmology is complete. Unfortunately this is taking humankind a long time and in the the interim we have to put up with unhelpful would-be philosophers espousing their underwhelming ideas on what it might be like (yawn).

You have no idea how FTL causality might be operating in QM, it may be just an apparent FTL effect due to holographic projection into the 3 dimensions we observe, or there may be a tachyon field generating a bohm type pilot wave, or there may be other dimensional spaces in which signalling can take place etc etc. The uncertainty principle may fall out of a simple resolution issue due to finite 'pixels' on a remote holographic screen, quantum randomness may occur at singularities in the microscopic dynamics or at dimensional boundaries where new degrees of freedom suddenly become available etc etc

Of course it may just be a case of matched correlations due to some unknown mechanism or model of reality, but you certainly don't know that.

You see there is little point answering a question about FTL signalling by appealing to "orthodox" interpretations or similar, the answer to the question is, that as far as we know there is no way of deterministically selecting a quantum state, so we can't transfer FTL information. This is a scientific observation which explains the state of our current knowledge.

 

[ACPower]

A personal thought experiment on classical entanglement:
Consider a sock draw with N pairs of different colored socks, with each pair clipped together. Alice closes her eyes, takes out a pair, unclips the socks, and puts one in each of two waiting brown lunch bags. She then hands one bag to Bob, who carries it across town on the bus. They are now poised for an EPR type experiment. Can useful information be transmitted instantaneously in this arrangement?
I think it should be obvious that the answer is no.

 

[jambaugh]

By "orthodox one" I think you mean "the oldest one" (it's hard to tell because you just posted vague stuff about correlations), I don't think there's anything "orthodox" about it, it was accepted mostly because the mainstream scientists actually producing results weren't really bothered by the interpretation problem since it doesn't add anything useful to the work they were developing.

I mean "orthodox" one because it is the most widely accepted one. This is the (orthodox) definition of "orthodox" whether anyone person thinks it should be or not. http://wordnetweb.princeton.edu/perl/webwn?s=orthodoxy"

This is still the case really, since the QM + unification project doesn't immediately require a resolution to the interpretation problem.

I see no problem...except for those demanding a particular metaphysical interpretation to satisfy their own need to retain an old classical mindset. The operational interpretation is already unambiguously given in the mapping from formal mathematics to physical operations in the laboratory. CI simply makes that explicit which is why it has become the orthodox interp.

In fact the interpretation issue will probably fall out rather easily once the really difficult work of building a mathematical model of microscopic reality and cosmology is complete. Unfortunately this is taking humankind a long time and in the the interim we have to put up with unhelpful would-be philosophers espousing their underwhelming ideas on what it might be like (yawn).

Don't hold your breath. Your supposed need for a "mathematical model of microscopic reality..." is itself interpretationially driven and represents a pre-QM bias towards classical worldviews. What we DO need is a synthesis of the operational, predictive principles of both QM and GR. Recent attempts (Field Theories and String/Brane theories and even Loop QG) have been motivated by this same classical bias. [I'll happily elaborate in another thread or private message if you like] Their lack of success (in incorporating GR) is thus not unexpected (in hindsight).

You have no idea how FTL causality might be operating in QM...

See below [Reparse my ...]

Of course it may just be a case of matched correlations due to some unknown mechanism or model of reality, but you certainly don't know that.

It IS due to matched correlations due to a known mechanism, the prior interactions of the two half-systems. There is no mystery until you try to reconcile the QM prediction with a realist's interpretation.

You see there is little point answering a question about FTL signalling by appealing to "orthodox" interpretations...

Re-parse my last post more carefully. I demonstrated that there IS an answer to the question. It just isn't the answer you want. Given a valid interpretation exists which denies FTL signaling then this demonstrates quite clearly that there is no prediction of FTL signaling within QM itself. As I said QM doesn't exclude the possibility but neither does it suggest it. The question itself doesn't come up unless you wish to --without any empirical or theoretical support-- posit that FTL signaling occurs in nature. Without such an assertion there is no problem to resolve and no need to reconcile FTL effects with QM, anymore than there is need to reconcile QM with unicorn levitation.

 

[unusualname]

I can't work out if you're differentiation between FTL signalling and FTL causality, the latter may exist without the former since you may have conservation laws requiring FTL causality (conservation of spin with entangled photons) but have no way of using that for signalling (since neither spin state can be deterministically selected)

 

[jambaugh]

I can't work out if you're differentiation between FTL signalling and FTL causality, the latter may exist without the former since you may have conservation laws requiring FTL causality (conservation of spin with entangled photons) but have no way of using that for signalling (since neither spin state can be deterministically selected)

If FTL causality is observable then it can be used to send a signal. If it isn't observable then it isn't operationally meaningful and may (like the aether) be excised from the theory.

Contrawise, of course an FTL signal is an example of FTL causality. In so far as what theories predict, the two are synonymous. But this distinction is a good one to red flag the fact that one is speaking about unobservable realities and thus going beyond physical predictions and into ontological speculations (again like the aether).

A local relativistic conservation law cannot require FTL causality it rather enforces relativistic locality since physical effects must propagate via some gauge charge flux be it energy, spin, or whatever.

Typically conservation laws dictate the correlations in the EPR experiments, they do not interpret them as real FTL signaling between the separated acts of observation. That's happening in the metaphysical speculator's mind.

 

[Caelus]

well if you had two entangled coins in there original state (when they where entangled) and one chainges to a random state, then you know that the other one is not in it's original state. (does what somthing isent count as information)

 

[unusualname]

If FTL causality is observable then it can be used to send a signal. If it isn't observable then it isn't operationally meaningful and may (like the aether) be excised from the theory.

Contrawise, of course an FTL signal is an example of FTL causality. In so far as what theories predict, the two are synonymous. But this distinction is a good one to red flag the fact that one is speaking about unobservable realities and thus going beyond physical predictions and into ontological speculations (again like the aether).

A local relativistic conservation law cannot require FTL causality it rather enforces relativistic locality since physical effects must propagate via some gauge charge flux be it energy, spin, or whatever.

Typically conservation laws dictate the correlations in the EPR experiments, they do not interpret them as real FTL signaling between the separated acts of observation. That's happening in the metaphysical speculator's mind.

Gee, and to think so much fuss was made of the experimental results of Aspect et al, pity you weren't around to set them straight and explain it all so simply, maybe you should write to the 2010 Wolf Prize panel and point out that all those experiments haven't really demonstrated anything worth knowing.

Actually you should go post in the ~100 page thread on EPR/Bell in the QM forum, you'd have a ball.

 

[jambaugh]

well if you had two entangled coins in there original state (when they where entangled) ...

That original "state" is a composite state of the pair namely that they were entangled. Presupposing individual states for each goes beyond the QM. You at best can ascribe to each a "statistical state" i.e. assign each a non-trivial density operator. And so...

and one chainges to a random state, then you know that the other one is not in it's original state. (does what somthing isent count as information)

I don't know what you mean by one or the other changing to a random state. The presupposed entanglement dictates that the measurement of one correlates with the measurement of the other. But that is the first measurement of each individual half of the pair so there's no sense in stating anything about the individual pieces changed...or didn't change. It is exactly the same issue as speaking about e.g. the momentum of a particle with a known position. The entangling action doesn't commute with the individual measurements.

 

[jambaugh]

Gee, and to think so much fuss was made of the experimental results of Aspect et al,...

Wonderful confirmations of the predictions of quantum mechanics, they were.

... pity you weren't around to set them straight ...maybe you should...Actually you should...

And this tirade evades the issue and doesn't deter me in the least. I've stated the clear point that QM w/ CI does not imply FTL effect so thus QM does not imply FTL effect.

The fact that some QM w/ Reified wave-function interpretation does require FTL causality isn't a supporting argument for FTL causality but rather a blow against the interpretation.

You can't separate these FTL questions from the issues of interpretation because that is the source.

My posts are directly on point with the OP so kindly address them and save your sarcastic jibes for PM.

 

[unusualname]

Wonderful confirmations of the predictions of quantum mechanics, they were.

And this tirade evades the issue and doesn't deter me in the least. I've stated the clear point that QM w/ CI does not imply FTL effect so thus QM does not imply FTL effect.

The fact that some QM w/ Reified wave-function interpretation does require FTL causality isn't a supporting argument for FTL causality but rather a blow against the interpretation.

You can't separate these FTL questions from the issues of interpretation because that is the source.

My posts are directly on point with the OP so kindly address them and save your sarcastic jibes for PM.

No, what you've done is use this thread as an oppurtunity to promote your preferred interpretation of QM, which, as I pointed out, has unnecessarily complicated the issue of FTL information.

There is no need for an interpretation here, this is a science forum so you should stick to scientific results where possible. In this case it is an experimentally demonstrated fact that we can not deterministically select a quantum state. We don't know the mechanism which underlies entanglement correlations but we don't need to speculate here. Whatever the mechanism, it doesn't allow us to transmit information.

That may or may not change in the future, but neither you nor I know that.

If want to think FTL influences are ruled out then that's up to you, I hope it makes you happy, you may even be right, but don't post misinformation suggesting it is a settled issue.

 

[JDługosz]

well if you had two entangled coins in there [their?] original state (when they where entangled) and one chainges [changes] to a random state, then you know that the other one is not in it's [its] original state. (does what somthing [something] isent [isn't ?] count as information)

Wow, the density of errors in your prose is amazing. "there" is a word which means something, and it's painful to figure out after reading on that you meant "they are" (or "their"). Likewise, "not in it is original state" is jarring to read. If you are not paying attention to what you're writing, one wonders if you are really taking in what we are posting, too.

Now PAY ATTENTION! When you choose to look at your particle, you get a random result. Suppose mine is still in the vault back home, and has not been disturbed, and won't be for some time. When you look at yours, you get a random result.

Now suppose instead that I took mine out of the vault and made my observation, early on, while you were brushing your teeth. Then, you go make your observation: when you look, you get a random answer.

How are those two cases different? Can you tell when your electron, sitting in its trap undisturbed, underwent a collapse of some kind? You don't know anything until you look at it. When you look at it, you note what you find. Summarize the cases:

A) I already examined mine. - your result is random.
B) I have not examined mine yet. - your result is random.
C) I'm not going to look at mine at all. - your result is random.
D) I gave you the wrong electron. - your result is random (to you anyway).

Based on your observation, which case is it? Tell me.

 

[jambaugh]

Pardon the delay but our semester starts early and I've been terribly busy.

No, what you've done is use this thread as an oppurtunity to promote your preferred interpretation of QM, which, as I pointed out, has unnecessarily complicated the issue of FTL information.

The issue of FTL information arises as a complication of issues of interpretation. Without an ontological interpretation one never raises the issue of FTL communication between entangled pairs.

There is no need for an interpretation here,

I agree but there is the need to point out the influence of interpretations. CI if you read it carefully is the absence of interpretation of QM beyond its necessary operational interpretation (what the symbols mean w.r.t. laboratory operations).

this is a science forum so you should stick to scientific results where possible. In this case it is an experimentally demonstrated fact that we can not deterministically select a quantum state.

Nope, it is not (experimentally demonstrated). It is a theoretical prediction of QM. (Specifically that sharp descriptions are maximal.) That you assert --experimental proof of the absence of a possibility-- should have made that glaringly obvious to you.

We don't know the mechanism which underlies entanglement correlations...

Of course we do. Two systems become entangled (correlated) by virtue of prior interactions. No mystery, no issues. ... and so... yes

... we don't need to speculate here.

If want to think FTL influences are ruled out then that's up to you, I hope it makes you happy, you may even be right, but don't post misinformation suggesting it is a settled issue.

Not so much a "settled issue" alternative theories may change ones expectation, but with respect to QM it is a non-issue, which is something else all together.

In particular, in parsing the assumptions of Bell's inequality derivation in order to identify the correct RAA hypothesis (since QM and experiment violate this ineq.) Those unable to drop the implicit hypothesis inherent in their ontological interpretation are left only with the local causality hypothesis. I've pointed out in other posts that this isn't really a necessary hypothesis for the derivation. Locality is a red herring w.r.t. Bell's derivation and Einstein's thought experiment. Reality and hence interpretation is the crux of the issue and you can't avoid that "reality".


Look at the OP. The original question shows the OP'er's belief that entanglement involved causal interaction at a distance between the pairs. (This in part stems from the unfortunate choice of term "entanglement" which would better have been simply called quantum correlation.) This assumption historically emerged from attempts to interpret QM phenomena in terms of some underlying ontological structure. You cannot divorce the question of interpretation from this subject since it is the starting point. To educate the OP as to the fallacy in his assumption it is best IMNSHO to point out the root cause of that fallacy.

You disagree? I don't care.

 

[unusualname]

Pardon the delay but our semester starts early and I've been terribly busy.

The issue of FTL information arises as a complication of issues of interpretation. Without an ontological interpretation one never raises the issue of FTL communication between entangled pairs.

I agree but there is the need to point out the influence of interpretations. CI if you read it carefully is the absence of interpretation of QM beyond its necessary operational interpretation (what the symbols mean w.r.t. laboratory operations).

By trying to argue FTL signalling is forbidden by claryfying the interpretation you get dubious circular arguments eg see http://www.jstor.org/pss/188551.

Which is why the question can't really be answered by appealing to a "correct" interpretation

Nope, it is not (experimentally demonstrated). It is a theoretical prediction of QM. (Specifically that sharp descriptions are maximal.) That you assert --experimental proof of the absence of a possibility-- should have made that glaringly obvious to you.

There's no such thing as an "experimental proof", my sloppy use of the term "experimentally demonstrated fact" may have confused you, but I have used the phrases "as far as we know" and "this may change" to emphasize that quantum randomness is experimentally observed and no counter-observations have been reported

Of course we do. Two systems become entangled (correlated) by virtue of prior interactions. No mystery, no issues. ... and so... yes

well you can argue that the entire universe is entangled by that argument so it doesn't really help much here.

Look at the OP. The original question shows the OP'er's belief that entanglement involved causal interaction at a distance between the pairs. (This in part stems from the unfortunate choice of term "entanglement" which would better have been simply called quantum correlation.) This assumption historically emerged from attempts to interpret QM phenomena in terms of some underlying ontological structure. You cannot divorce the question of interpretation from this subject since it is the starting point. To educate the OP as to the fallacy in his assumption it is best IMNSHO to point out the root cause of that fallacy.

You disagree? I don't care.

You're entitled to your opinion. I believe that no-signalling is ruled out by the (experimentally supported) fundamental randomness of QM, you believe it is ruled out by particular interpretation of correlations.

It seems likely that the topology of reality is quite complex and locality does not have a simple description in a holographic reconstruction of bulk space for example, so I wouldn't appeal to theoretical interpretations until we understand reality better.

Best to stick to what experiments demonstrate, and experiments can't differentiate between interpretations (yet) but they do show us that QM seems random.

 

[jambaugh]

By trying to argue FTL signalling is forbidden by claryfying the interpretation you get dubious circular arguments...

Read my posts more carefully. The only argument I've put forth that FTL signaling is forbidden has been the classic SR argument. My principle point, repeatedly, is that FTL signaling has never been a part of QM or necessary to explain QM except with regard to certain (in my opinion bad) interpretations.

This can be seen by looking at a "correct" interpretation.

well you can argue that the entire universe is entangled by that argument so it doesn't really help much here.

Yes, an excellent way to look at entropy is as the entanglement of a given system with "the rest of the universe".

You're entitled to your opinion. I believe that no-signalling is ruled out by the (experimentally supported) fundamental randomness of QM, you believe it is ruled out by particular interpretation of correlations.

It is rather like the understanding that such signaling is on par with the lumineferous aether...SR didn't "rule it out" but showed it an unnecessary component. There is no reason to believe there is a channel with which one might otherwise be able to signal should one be able to access beyond QM randomness.

If the OP had rather asked...
"Why can't we use quantum bit entanglement to ______________?"
where the blank was one of...
"talk to God"
"create a wormhole."
"pick up trashy women in bars"

Would you not then rather ask that person to re-examine their assumptions rather than point out that they can't control the assumed effect?

It seems likely that the topology of reality is quite complex and locality does not have a simple description in a holographic reconstruction of bulk space for example, so I wouldn't appeal to theoretical interpretations until we understand reality better.

There you go implicitly invoking an interpretation again. Why must you invoke a topological reality? or holographic reconstruction? Things happen. QM describes how, or what we can know about how they happen.

At the classical level we can then model the happenings in terms of an objective reality. That indeed is what defines and distinguishes the classical limit. That "reality" is an idealization. Speculate what forms you can give it, consistent with experiments, but don't speculate about how it "really is" since its is your and my invention.

 

[ThomasT]

jambaugh, thanks for your clarifications. It's clearly your primary antagonist, unusualname, that has more or less hijacked the thread for personal reasons (he's one of those would be metaphysicians who seems to like to think of reality in 'holographic' terms).

Anyway, the OP's question has been answered.

 

[huge10]

transmit information indirectly.

But basically you're not transmitting any information.
The effects of entanglement are not due to any transfer of information (which would violate Einstein's special relativity).

The information is already contained in the whole system since the beginning.

 

[least_action]

How does one state the original question mathematically? (As one might do before attempting to prove it)

You use the tensor product of dirackets to entangle states but how can you say the information cannot travel faster than light?

 

[unusualname]

How does one state the original question mathematically? (As one might do before attempting to prove it)

You use the tensor product of dirackets to entangle states but how can you say the information cannot travel faster than light?

You essentially argue that a measurement at either end doesn't alter the probability distribution of any observable at the other end, but as mentioned above the argument might be considered tautological.

I can't get tex to work but there should be a derivation on the wikipedia page (if it's not ruined by an edit war)
http://en.wikipedia.org/wiki/No-communication_theorem

For a deeper argument see Eberhard & Ross' 1989 paper Quantum field theory cannot provide faster-than-light communication

 

[mrspeedybob]

But basically you're not transmitting any information.
The effects of entanglement are not due to any transfer of information (which would violate Einstein's special relativity).

The information is already contained in the whole system since the beginning.

How can that be? Suppose two particles are entangled. If I precisely measure the position of one and the momentum of the other then I should know precisly the position and momentum of both. This is impossible.

 

[jambaugh]

How does one state the original question mathematically? (As one might do before attempting to prove it)

You use the tensor product of dirackets to entangle states but how can you say the information cannot travel faster than light?

To begin with, considering your two observers Alice and Bob of two halves of an entangled pair, let us consider whether Alice's actions can affect Bob's observations.

Define the density operator of the entangled pair (or any pair): \rho.

Now consider that Alice's available action is to choose what variable to observe. As unusualname pointed out she can't pick what outcome she is to observe only what variable to look at. To keep it simple let Alice's observation be boolean with a 0 or 1 outcome (eigen-values). Let X be Alice's local observable. (Note the negation is 1-X).

Now consider any variable Bob might look at, call it Y and let it be likewise boolean. You can later generalize these to many variable cases.

A signal could be passed only if the outcome of Bob's measurement depended in some way on the choice of measurement Alice made regardless of the outcome. So we consider the conditional probability:

P(Y=1| X=1 or X = 0) = P(Y=1 and [X = 1 or X = 0]) /P(X = 1 or X = 0).
(The probability Y=1 given either X =1 or X =0, i.e. the probability Y=1 given X was measured.)

Note the denominator is 1, Alice will either see 1 or 0.
P(Y=1 and [X=1 or X= 0]) = P(Y=1 and X=1)+P(Y=1 and X = 0).

Work through the calculations with the density operators and see that P(Y=1 and X = 1) + P(Y=1 and X=0) = P(Y=1). Bob's observation is independent of Alice's choice to measure X though it may correlate with what outcome Alice sees.


P(Y=1 \cap X=1) = Tr( Y\otimes X \circ \rho )
P(Y=1 \cap X=0) = Tr(Y \otimes (1-X)\circ \rho)
Tr(Y\otimes X \circ \rho) +\Tr(Y\otimes (1-X)\circ \rho) = Tr(Y\otimes X\circ \rho + Y\otimes(1-X)\circ \rho)
=Tr([Y\otimes X +Y\otimes(1-X)]\circ\rho) = Tr(Y\otimes 1\circ \rho)
=P(Y=1)

Note that any general observation is a linear combination of boolean observations (the multipliers being the eigen-values and the boolean observations being projectors onto the corresponding eigen-space). Thus this derivation generalizes to any acts of observations.
Alice can't send Bob a message by choosing what to observe.

 

[jambaugh]

How can that be? Suppose two particles are entangled. If I precisely measure the position of one and the momentum of the other then I should know precisly the position and momentum of both. This is impossible.

Not quite and "that's QM". Measuring the momentum of one means you know what you would have measured if you had measured the momentum of the other. Not what it is but what you would have seen. Likewise with the positions. But just as you cannot simultaneously measure the momentum and position of one you cannot simultaneously measure the momentum of one and measure both momentum and position of the other.

Part of the problem is that we speak of "the one" and "the other" as if they were objects but part of how you determine "the one" and "the other" is to label them by position.

Consider an entangled pair first as a whole. Specifically consider one anti-correlated in position and momentum. You can then subdivide the pair by momentum (the one with the +x momentum and the one with the -x momentum) or you can subdivide the pair by position (the one into the left and the one to the right) but these are distinct factorizations of the Hilbert space because the implicit projectors do not commute.

It is easier to see with spin. Consider two spin 1/2 particles and speak of one being "the spin z +1/2" and the other being "the spin z -1/2" one. Or alternatively speak of one being "the spin x +1/2" and the other being "the spin x -1/2". But you can't do both at the same time because Sx and Sz do not commute.

This is what happens when you for example consider the case where "the left one had spin z = +1/2". You have by selection correlated position (usually here treated classically) with spin z (which here is non-classical) and thus you can no longer treat the position of the particle (whether Alice or Bob has it) as a classical variable.

Our implicit instinct to treat the system as classical objects with classical properties leads us to ruin in trying to parse these EPR experiments. You must retrain this instinct and trust the formal language of QM.

 

[DrChinese]

How can that be? Suppose two particles are entangled. If I precisely measure the position of one and the momentum of the other then I should know precisly the position and momentum of both. This is impossible.

jambaugh has provided an excellent response, I just wanted to add a quick comment.

Your last statement is "This is impossible". Not sure which part you are referring to, but of course it IS impossible to know both P and Q. So that means that measuring P of Alice and Q of Bob does NOT yield simultaneous P and Q for either. So that is the source of the contradiction: Once Alice is observed, Bob is no longer entangled with Alice on that basis.

 

[NetMage]

Haha, I had a similar post not too long ago. Essentially you CAN technically have information sent via entanglement. I recently spoke with my math professor about this as well. Theoretically one could set up a system and have a computer understand the bit set up of the entangled particles...if that is one could figure out HOW to set up the bit, this would be meticulous in itself. Next let's say one can set up a bit according to thousands and thousands or whatever arbitrary number one would need to produce the binary for information being sent and understood by x distance. The problem becomes at this point...how would the computer or user know which particles to examine in order to process the information. If you sent the binary for the letter 'g': 01000111 this is the binary for g. But when the entangled particles that you set up the bit for 'g' may send 01010000 which is 'p'. The point being, there is no way for the user or cpu at x distance to know which bits or entangled particles to check to understand the information being sent, it would be all random. Now, this goes to say...that entanglement CAN be used for information security. One could call or text, or w/e method preferred, to let the other user know WHICH particles to check in order to decode a message via entanglement. Better yet, sending encrypted information to let a user know which bits or entangled particles to check...this is highly valuable...there is no way to intercept this information. :D

 

[NetMage]

Remember, entangled particles -- once one particle is observed, it only gives information about the other particle...information is not REALLY sent. Its just redundant information, which is why measurements of information sending via entanglement has really been reduced to limits of security. If you and your friend had a plan A - Z to let's say, rob a bank :D. At some distance you are calling the shots as to which plan shall be executed before the robbery takes place. The feds are hot on your trail, and you call your friend and tell him to check particles 001, 002, 006, and 145. Your friend who already knows what the message means, will then know which plan to execute. This may be an exotic example, but nonetheless, gets the point across as to how it could be used to secure information. The feds may have tapped your phone and heard the whole conversation..but they have no information as to what is about to happen according to your plan that was chosen via entanglement. Now, apply this to I suppose 'better' examples lol.

 

[Gigasoft]

Entanglement is of no consequence in your example. It is no different from any other method involving an one-time pad. It is possible for the police to intercept the particles, measure them, and pass them on, without your friend becoming any wiser.

Now, you might think, what if I tell my friend to measure each particle along a random basis of my choosing? Then, if someone has already measured the particles, my friend will know.

Well, not really. If they are able to measure the particles, they are just as able to store the particles, and replace them with some other particles that are entangled with their own particles. When you call your friend, the feds pick up the phone and pretend to be him. They decrypt the message using the particles they got from you, then they encrypt the message with their own particles, call your friend and pretend to be you.

So, entanglement can only help you if you already have a way of transmitting a message unaltered.

 

[NetMage]

lol, what are you talking about, you cannot 'intercept' the particles...

 

[NetMage]

Gigasoft, you do not seem to understand what the implications of entanglement are...I'm no expert in the field, but I have done enough research to know that what you are talking about is nonsense.

 

[NetMage]

haha, they cannot just randomly and willingly entangle particles from some x distance...lol. They could not entangle their particles with yours...

 

[Gigasoft]

Well, then you obviously can't read.

As I said, they would capture your particles, as you try to send them to your friend. They would then generate new entangled pairs of particles, and send one particle from each pair to your friend. When you think you're talking to your friend on the phone, they decrypt your message using the particles they received from you. They then encrypt the message again, using the particles they kept from the corresponding other pair. Now, they call your friend and deliver the newly encrypted message.

In your scenario, if your agreement is to measure each particle in a particular basis, and the police knows this, the police can just measure each particle in that basis and pass the particles on as they are. When your friend measures the particles, his measurements will be the same as they would have been otherwise, so they don't even have to intercept your phone call.

On the other hand, if you have some means of giving your friend a bunch of particles that can't be intercepted, then you could just as well give him a notebook full of random numbers which can't be intercepted, which would be much cheaper.

 

[NetMage]

capture your particles? you are assuming that they are able to find you in a hypothetical situation that i created...wow

 

[NetMage]

not to mention the police don't know what any of the particles mean...so you are still wrong..lmao only you and your friend know haha

 

[NetMage]

I refuse to engage in a battle of witts with an unarmed individual...I'm done having this conversation lol...wow

 

[Gigasoft]

Oh great, so Mr. junior undergraduate's "cryptography" basically consists of assuming that no one knows where you or your friend are, or how you're communicating. Congratulations on completely missing the point of quantum cryptography.

Good luck at university. You'll really impress your professor with your "lmao"'s and your "wow"'s, I'm sure.

 

[NetMage]

Well naturally I assume they don't know where we are...it is the principal of the argument rather than the realism as stated...and quantum cryptography typically relates the release of a photon assigned to a data packet that when disturved notifies the network security. Entanglement is not REALLY cryptography. Not to mention, EVEN IF the feds were to find out where you were, they could not just send 'information' to your friend...the feds do not know what any of the entangled particles mean...only you and your friend do...they would have no method of communication with him. You have no clue as to what it is you're talking about. I'm done hijacking this thread with your nonsense. Good day.

P.S. I am doing just fine with my studies thank you. More than you know.