Why does quantum entanglement not allow ftl communication

  • Thread starter macd
  • Start date

JesseM

Science Advisor
8,492
12
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.
 

Ken G

Gold Member
4,437
330
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.
 

JesseM

Science Advisor
8,492
12
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.
 

JesseM

Science Advisor
8,492
12
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.
 
Last edited by a moderator:

Ken G

Gold Member
4,437
330
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".
 

JesseM

Science Advisor
8,492
12
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.
 

Ken G

Gold Member
4,437
330
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.
 
Last edited:

JesseM

Science Advisor
8,492
12
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.
 

Ken G

Gold Member
4,437
330
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.
 
Last edited:

JesseM

Science Advisor
8,492
12
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.
 

Ken G

Gold Member
4,437
330
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.
Note I did some editing of my last post, as we're exchanging in real time! But even in the CI, the act of measuring does not destroy previous entanglements (as usual, such entanglements only show up in correlations with other measurements, never on measurements of the same system). The CI is simply more honest that you have made a choice not to track them, so the CI makes no claims that the wave function is a complete description of the reality-- even for a maximal set of commuting observations. You are right that the MWI does try to retain that "reality" property, but it still fails unless you include the whole universe in the wave function. That's the problem with MWI in the first place, there's no evidence that such a wave function exists, and we certainly know we can never use it for anything. So with MWI, all I'd have to say is "any wavefunction that any physicist could ever actually use for anything cannot be the wavefunction of that system without losing some of the reality of the situation", it's still always going to reflect a choice of some kind in MWI or CI, or Bohm.
 
Last edited:

JesseM

Science Advisor
8,492
12
Note I did some editing of my last post, as we're exchanging in real time! But even in the CI, the act of measuring does not destroy previous entanglements (as usual, such entanglements only show up in correlations with other measurements, never on measurements of the same system).
Why do you say it does not destroy previous entanglements? If you measure a particle that's entangled with others, the results of that measurement may be correlated with measurements on the other particles, but then won't subsequent measurements of the same particle give results that are completely uncorrelated with the other particles in the system?
 

Ken G

Gold Member
4,437
330
Why do you say it does not destroy previous entanglements? If you measure a particle that's entangled with others, the results of that measurement may be correlated with measurements on the other particles, but then won't subsequent measurements of the same particle give results that are completely uncorrelated with the other particles in the system?
I'm not sure I understand the question, measurements you do on one system will not destroy correlations with another system, it will just determine those correlations. If you go back and do the measurement again, you'll get the same correlations again, so the correlations are not destroyed (in any interpretation). If you are using CI you are probably doing measurements on individual particles to generate single-particle eigenfunctions, and once you've done that, all the correlations are already actualized in each trial so they are already embedded in the whole ensemble if you choose to track them. If you are doing MWI, you have a lot more work to do, because you have to include what didn't happen as well as what did. That's pretty much why MWI is not used in practice, it seems to me.
 
Last edited:

JesseM

Science Advisor
8,492
12
I'm not sure I understand the question, measurements you do on one system will not destroy correlations with another system, it will just determine those correlations. If you go back and do the measurement again, you'll get the same correlations again, so the correlations are not destroyed (in any interpretation).
I should have been more specific, I meant measurements using a measurement operator that doesn't commute with the measurement operator(s) that were used in the initial measurement(s) that were found to be correlated with the other, distant particle due to entanglement.
 

Ken G

Gold Member
4,437
330
I should have been more specific, I meant measurements using a measurement operator that doesn't commute with the measurement operator(s) that were used in the initial measurement(s) that were found to be correlated with the other, distant particle due to entanglement.
Correlations can still be preserved even by measurements like that. And in cases where no correlation appears, my money says there would have been no correlation in the original wavefunction either, so it wasn't "destroyed" by the measurement. It's an interesting question if measurements (in CI) destroy correlations like they destroy phase coherence. I'm not sure that they do, and if I'm right, that's the fundamental reason that the CI is a complete description.
 
9
1
Quantum Temporal Paradox

There are indications that despite the Grandfather Paradox and Eberhard's proof to the contrary, quantum nonlocality may in fact support FTL. The lines of evidence are as follows;

1. Teleportation does in fact transmit information, since the state of the particle cannot be reconstructed w/o both the classical and the nonlocal channel. This is not FTL only because the classical channel is required.
2. Gisin's 2001 experiment in Geneva disproving Scarani and Suarez's conjecture that the correlations between EPR pairs in which both measurements occurred prior to the other in the local frames of reference of the actual measurements would disappear, means physicists have no causal explanation of quantum nonlocality. Global timelike causality is eliminated because the measurements are spacelike separated, common timelike cause is eliminated by Bell's theorem, and Gisin's null result eliminates local timelike causality. Unless there is yet some other kind of causality (Gisin argues this should be considered), the only other option is spacelike causality. Thus, this opens the door to considering spacelike causality despite the conceptual hurdles.
3. Conventional quantum mechanics (CQM) suffers from 5 anomalies, fundamental unsolved problems that according to Kuhn should have been solved in due course as the field matured. They are the measurement problem, interpretation problem, collapse problem, supercedence problem, and the nonlocality problem. There is therefore reason to believe that progress has been impeded by a paradigm barrier and that on the other side of this barrier lies new physics waiting to be discovered.
4. The Grandfather Paradox (and its twin sister argument against FTL, the Shakespeare Indeterminacy) are examples of self-reference. Mathematicians and logicians do not have a good track record in dealing with self-reference. A few who have made progress in this area are G. Spencer Brown, "Laws of Form" who first introduced the idea of imaginary truthvalues as a way to make sense of logical paradox, Hellerstein, "Diamond Logic," Kaufman, Shoup, and Goff have also contributed to our understanding of nonlinear logics. The best know popular account is "Gödel, Escher, Bach" by Hofstadter. These advances suggest that self-reference might be fundamental to quantum mechanics, the measurement process in particular, and to a censor mechanism that would permit spacelike causality while prohibiting temporal paradox.
5. For an example of an abstract quantum system (AQS) where self-reference is central to the measurement process, backwards-in-time causality, and a censor mechanism preventing temporal paradox, see Quantum Tic-Tac-Toe at ParadigmPuzzles.
6. Impossibility proofs, such as Eberhard's, that are eventually overturned, almost always reveal not a technical flaw but a lack of imagination. In the 40's, a respected scientist showed that going to the moon was impossible. He thoroughly understood the astrodynamics and the expected advances in technology including H2/LOX. He showed that a vehicle that could travel to the moon and return to earth would have to carry 200 times its weight in propellant; clearly impossible. We went to the moon anyway. Why? Because we left bits and pieces of the spacecraft all along the way, there, and all along the way back. The lack of imagination was to envision a throw-away design. Eberhard's proof may suffer similarly for it assumes a linear architecture for the nonlocality with an observer-dependent measurement on each end. A pair of entanglements that extends from sender to receiver in a folded pattern and can be self collapsed in either of two ways by local actions on only one end, can in principle exceed mere teleportation achieving true FTL.
7. The theoretical framework that integrates these ideas into a conceptual whole is quantum temporal paradox (QTP). A key piece of this framework is the idea of symmetric spacetime intervals (SSI) along which collapse of the wave function can occur in a relativistically consistent way. A paper that derives symmetric intervals from the concept of world ribbons (generalizations of the world lines of relativity applicable to the uncertainty of quantum objects) is in review at the Foundations of Physics Journal. If this paper is accepted for publication (it is classic speculative physics, so publication hinges on the eccentricities of the reviewers) then we are a step closer to allowing spacelike causality in quantum mechanics and thus discussions of FTL and even time travel become a tad bit more respectable. Symmetric intervals counter the relativity and causality arguments against spacelike causality.
8. Self-reference introduces nonlinearity into QM in a natural way, not in the ad hoc way being explored by adding various nonlinear terms to the Schrödinger equation. Self-reference also shows how to overcome the Grandfather Paradox and Shakespeare Indeterminacy which are the strongest arguments against spacelike causality.
9. An alternative nonlinear operator may be hiding in the normalization process associated with indistinguishable particles. The reduction in the dimensionality of the Hilbert space when indistinguishable particles become entangled cannot be reduced to a linear operator. This disputable fact is hidden by the typically casual way physicists perform the mathematical trick of renormalization.
10. The mathematics of QM may be a red herring, playing the role of extra information not strictly needed for a solution, that by its very presence makes finding the solution much more difficult. The vector which is supposed to represent a state contains more information than is physically significant. The phase of a state is physically irrelevant unless interference is expected, and then only the relative phase is physically significant. There is reason to believe therefore, that an objective measurement system might exist, no pesky observers required, if only the mathematics could be reduced to have a better impedance match with the actual physics.
11. A metaphor might help. In classical physics, the present is envisioned as an infinitely thin dividing line between the past and the future. If QTP is correct, then in quantum physics it is possible to entangle the near future with the recent past so that the "present" has a temporal width. Within this entanglement, the concepts of past, present, and future become ambiguous, the present becomes a window in time. From the quantum perspective, causality is maintained and clear even with the statistical nature of the outcomes, but from the classical perspective, the explanation of cause and effect looks an awful lot like time travel. No real "traveling" occurred, but what this window in time allows is the selection, at the very last moment, of which pair of histories we are going to find ourselves in, versus which histories became contradictory, pruned out of existence because of paradox. The essence of time travel is childlike wish fulfillment; make it didn't happen. One of the surprises of Quantum Tic-Tac-Toe is the recognition that to play it at the highest strategic level requires one to realize that the present move is changing the past. The implications for basic physics and technology are exciting, and potentially troubling.

Time travel is one of those scifi concepts that ought to stay firmly in the genre, and not poke its disturbing head into actual reality. Yet, if we are ever to travel to the stars, the speed of light has to be overcome, and since FTL and time travel are two sides of the same coin, perhaps developments in this area are to be hoped for, looked for, and pursued with due scientific rigor.
 
Last edited:
4,222
1
There are indications that despite the Grandfather Paradox and Eberhard's proof to the contrary, quantum nonlocality may in fact support FTL. The lines of evidence are as follows;

1. Teleportation does in fact transmit information, since the state of the particle cannot be reconstructed w/o both the classical and the nonlocal channel. This is not FTL only because the classical channel is required.
2. Gisin's 2001 experiment in Geneva disproving Scarani and Suarez's conjecture that the correlations between EPR pairs in which both measurements occurred prior to the other in the local frames of reference of the actual measurements would disappear, means physicists have no causal explanation of quantum nonlocality. Global timelike causality is eliminated because the measurements are spacelike separated, common timelike cause is eliminated by Bell's theorem, and Gisin's null result eliminates local timelike causality. Unless there is yet some other kind of causality (Gisin argues this should be considered), the only other option is spacelike causality. Thus, this opens the door to considering spacelike causality despite the conceptual hurdles.
3. Conventional quantum mechanics (CQM) suffers from 5 anomalies, fundamental unsolved problems that according to Kuhn should have been solved in due course as the field matured. They are the measurement problem, interpretation problem, collapse problem, supercedence problem, and the nonlocality problem. There is therefore reason to believe that progress has been impeded by a paradigm barrier and that on the other side of this barrier lies new physics waiting to be discovered.
4. The Grandfather Paradox (and its twin sister argument against FTL, the Shakespeare Indeterminacy) are examples of self-reference. Mathematicians and logicians do not have a good track record in dealing with self-reference. A few who have made progress in this area are G. Spencer Brown, "Laws of Form" who first introduced the idea of imaginary truthvalues as a way to make sense of logical paradox, Hellerstein, "Diamond Logic," Kaufman, Shoup, and Goff have also contributed to our understanding of nonlinear logics. The best know popular account is "Gödel, Escher, Bach" by Hofstadter. These advances suggest that self-reference might be fundamental to quantum mechanics, the measurement process in particular, and to a censor mechanism that would permit spacelike causality while prohibiting temporal paradox.
5. For an example of an abstract quantum system (AQS) where self-reference is central to the measurement process, backwards-in-time causality, and a censor mechanism preventing temporal paradox, see Quantum Tic-Tac-Toe at ParadigmPuzzles.
6. Impossibility proofs, such as Eberhard's, that are eventually overturned, almost always reveal not a technical flaw but a lack of imagination. In the 40's, a respected scientist showed that going to the moon was impossible. He thoroughly understood the astrodynamics and the expected advances in technology including H2/LOX. He showed that a vehicle that could travel to the moon and return to earth would have to carry 200 times its weight in propellant; clearly impossible. We went to the moon anyway. Why? Because we left bits and pieces of the spacecraft all along the way, there, and all along the way back. The lack of imagination was to envision a throw-away design. Eberhard's proof may suffer similarly for it assumes a linear architecture for the nonlocality with an observer-dependent measurement on each end. A pair of entanglements that extends from sender to receiver in a folded pattern and can be self collapsed in either of two ways by local actions on only one end, can in principle exceed mere teleportation achieving true FTL.
7. The theoretical framework that integrates these ideas into a conceptual whole is quantum temporal paradox (QTP). A key piece of this framework is the idea of symmetric spacetime intervals (SSI) along which collapse of the wave function can occur in a relativistically consistent way. A paper that derives symmetric intervals from the concept of world ribbons (generalizations of the world lines of relativity applicable to the uncertainty of quantum objects) is in review at the Foundations of Physics Journal. If this paper is accepted for publication (it is classic speculative physics, so publication hinges on the eccentricities of the reviewers) then we are a step closer to allowing spacelike causality in quantum mechanics and thus discussions of FTL and even time travel become a tad bit more respectable. Symmetric intervals counter the relativity and causality arguments against spacelike causality.
8. Self-reference introduces nonlinearity into QM in a natural way, not in the ad hoc way being explored by adding various nonlinear terms to the Schrödinger equation. Self-reference also shows how to overcome the Grandfather Paradox and Shakespeare Indeterminacy which are the strongest arguments against spacelike causality.
9. An alternative nonlinear operator may be hiding in the normalization process associated with indistinguishable particles. The reduction in the dimensionality of the Hilbert space when indistinguishable particles become entangled cannot be reduced to a linear operator. This disputable fact is hidden by the typically casual way physicists perform the mathematical trick of renormalization.
10. The mathematics of QM may be a red herring, playing the role of extra information not strictly needed for a solution, that by its very presence makes finding the solution much more difficult. The vector which is supposed to represent a state contains more information than is physically significant. The phase of a state is physically irrelevant unless interference is expected, and then only the relative phase is physically significant. There is reason to believe therefore, that an objective measurement system might exist, no pesky observers required, if only the mathematics could be reduced to have a better impedance match with the actual physics.
11. A metaphor might help. In classical physics, the present is envisioned as an infinitely thin dividing line between the past and the future. If QTP is correct, then in quantum physics it is possible to entangle the near future with the recent past so that the "present" has a temporal width. Within this entanglement, the concepts of past, present, and future become ambiguous, the present becomes a window in time. From the quantum perspective, causality is maintained and clear even with the statistical nature of the outcomes, but from the classical perspective, the explanation of cause and effect looks an awful lot like time travel. No real "traveling" occurred, but what this window in time allows is the selection, at the very last moment, of which pair of histories we are going to find ourselves in, versus which histories became contradictory, pruned out of existence because of paradox. The essence of time travel is childlike wish fulfillment; make it didn't happen. One of the surprises of Quantum Tic-Tac-Toe is the recognition that to play it at the highest strategic level requires one to realize that the present move is changing the past. The implications for basic physics and technology are exciting, and potentially troubling.

Time travel is one of those scifi concepts that ought to stay firmly in the genre, and not poke its disturbing head into actual reality. Yet, if we are ever to travel to the stars, the speed of light has to be overcome, and since FTL and time travel are two sides of the same coin, perhaps developments in this area are to be hoped for, looked for, and pursued with due scientific rigor.
That certainly clears things up.
 

Ken G

Gold Member
4,437
330
There are indications that despite the Grandfather Paradox and Eberhard's proof to the contrary, quantum nonlocality may in fact support FTL. The lines of evidence are as follows;
No doubt these issues are at the forefront of our understanding, but I don't see any fundamental problems here. This is how I would react to each of these, for what it's worth:
1. Teleportation does in fact transmit information, since the state of the particle cannot be reconstructed w/o both the classical and the nonlocal channel. This is not FTL only because the classical channel is required.
Eliminating the word "only" makes this a non-problem.
2. ...Unless there is yet some other kind of causality (Gisin argues this should be considered), the only other option is spacelike causality.
Or, we simply haven't yet found a versatile enough meaning for "causality". When a concept reaches the limit of its service to us, need we torture it further?
3. Conventional quantum mechanics (CQM) suffers from 5 anomalies, fundamental unsolved problems that according to Kuhn should have been solved in due course as the field matured. They are the measurement problem, interpretation problem, collapse problem, supercedence problem, and the nonlocality problem.
I don't see any inconsistency in "the measurement problem", I would call it "the science problem" and liken it to how following Polaris is a good way to go north but a lousy way to go to Polaris. There's nothing to "solve" there. The interpretation problem is also not a problem, because relativity already taught us not to expect the existence of unique intepretations. Collapse is not a problem either, it is like the measurement "problem" and simply stems from the way we choose to do science-- there's no need to solve that either. I don't know what the supercedence problem is, but it sounds like something about quantum erasure and the only problem I see there is in our own unwillingness to let go of ideas that reach the limit of their usefulness, like causality. The nonlocality problem is also nothing that needs solving-- physical systems are indeed nonlocal because they are linked by their history to the rest of the universe, and not in a way that is "stored" locally in the elements of the system.
There is therefore reason to believe that progress has been impeded by a paradigm barrier and that on the other side of this barrier lies new physics waiting to be discovered.
I don't see it in that light, to me this is just how reality works, why would we start telling it that it has "problems"? We are like out-of-work psychiatrists trying to convince a perfectly healthy patient that they need our services.
4. ...These advances suggest that self-reference might be fundamental to quantum mechanics, the measurement process in particular, and to a censor mechanism that would permit spacelike causality while prohibiting temporal paradox.
There is no harm in speculating, but the shooting percentage of speculation is even worse than in dealing with self-referential paradoxes.
5. For an example of an abstract quantum system (AQS) where self-reference is central to the measurement process, backwards-in-time causality, and a censor mechanism preventing temporal paradox, see Quantum Tic-Tac-Toe at ParadigmPuzzles.
Can you give a link and a summary? That's always helpful, it sounds interesting.
6. Impossibility proofs, such as Eberhard's, that are eventually overturned, almost always reveal not a technical flaw but a lack of imagination.
Yes, I would say that "impossibility proofs" are a misnomer, for they don't say what result is impossible, they actually point to the hurdles that need to be overcome to make something possible. They should really be called "why you can't get there this way" proofs.

A pair of entanglements that extends from sender to receiver in a folded pattern and can be self collapsed in either of two ways by local actions on only one end, can in principle exceed mere teleportation achieving true FTL.
If this is truly a prediction of existing physics, it should be easy enough to set up a gedankenexperiment that shows it. If it requires other physics, it is no different from any other magical means of FTL, because the new physics first has to be demonstrated.
7. ...If this paper is accepted for publication (it is classic speculative physics, so publication hinges on the eccentricities of the reviewers) then we are a step closer to allowing spacelike causality in quantum mechanics and thus discussions of FTL and even time travel become a tad bit more respectable.
Hang on, how does the capriciousness of "eccentric reviewers" bring us closer to allowing spacelike causality? It will take experiment to do that, not reviewers. It's kind of a "pet peeve" of mine when people use theory, and now reviewers of theory, to tell reality what to do. The real goal of this work should be to motivate the right experiment.
8. Self-reference introduces nonlinearity into QM in a natural way, not in the ad hoc way being explored by adding various nonlinear terms to the Schrödinger equation. Self-reference also shows how to overcome the Grandfather Paradox and Shakespeare Indeterminacy which are the strongest arguments against spacelike causality.
Again with the theory telling reality what to do. None of it means a thing until there is experimental justification. That doesn't make it worthless, it makes it worthless unless it is used to motivate experiment.
9. An alternative nonlinear operator may be hiding in the normalization process associated with indistinguishable particles. The reduction in the dimensionality of the Hilbert space when indistinguishable particles become entangled cannot be reduced to a linear operator.
I'm a bit confused what this means, I thought the Hilbert space was a space of linear operators. Note that "nonlinear terms" in an operator do not stop it from being a linear operator-- the operator formalism is itself linear, at least as far as I have seen.
10...The phase of a state is physically irrelevant unless interference is expected, and then only the relative phase is physically significant.
That's not a significant problem, it just means the wave function is not explicitly respecting a symmetry that is present. This redundancy is eliminated in the "Heisenberg picture" as it never appeared in the matrix elements of the wave function anyway.
There is reason to believe therefore, that an objective measurement system might exist, no pesky observers required, if only the mathematics could be reduced to have a better impedance match with the actual physics.
I can't really see how that follows. That sounds like saying that because of some relatively trivial redundancy in the Schroedinger picture, we should do science totally differently.
11. ...If QTP is correct, then in quantum physics it is possible to entangle the near future with the recent past so that the "present" has a temporal width. Within this entanglement, the concepts of past, present, and future become ambiguous, the present becomes a window in time.
This sounds like a perfectly reasonable hypothesis, entirely analogous to the Heisenberg uncertainty principle applied to our knowledge of when events occur. But such is hardly suitable for using as a window into FTL travel of anything but a tiny particle whose relation to time has always been quite a bit different from the irreversible macroscopic version. To me it merely sounds like a nice way to travel femtoseconds into the past, and not even be able to establish that you did.

No real "traveling" occurred, but what this window in time allows is the selection, at the very last moment, of which pair of histories we are going to find ourselves in, versus which histories became contradictory, pruned out of existence because of paradox.
I agree with the start of this-- we generally find that such "selection" ends up being like trying to change the weather by blowing at clouds. You will indeed change the weather that way, but only in the meaningless way that you can change a dice roll by yelling at the person releasing the dice.
Time travel is one of those scifi concepts that ought to stay firmly in the genre, and not poke its disturbing head into actual reality.
It is certainly fascinating to think about, and good fodder for sci fi.
Yet, if we are ever to travel to the stars, the speed of light has to be overcome, and since FTL and time travel are two sides of the same coin, perhaps developments in this area are to be hoped for, looked for, and pursued with due scientific rigor.
But why do we need to overcome the speed of light to go to the stars? It would suffice to be able to reach very close to that speed. A daunting task, I admit, but I don't see much evidence that time travel is any less daunting.
 
Last edited:

JesseM

Science Advisor
8,492
12
Correlations can still be preserved even by measurements like that.
Do you have any specific examples of problems where they would be preserved with these kinds of measurements? I haven't studies such problems in detail, but consider a situation where if we find one particle in an eigenstate of some measurement operator; if we keep measuring with the same operator it'll stay in that eigenstate forever (if there's no time dependence), but then if we stick a measurement with a noncommuting operator in between, then when we return to the original operator the system may no longer be in the same eigenstate. But there can't be any way this change in eigenstate can be reflected in the other, entangled particle, because if it was this would allow for the possibility of FTL communication. So this is one intuitive reason for thinking you won't necessarily see correlations preserved after you've made multiple measurements on entangled particles, where the later measurements don't commute with the initial measurement.
 
9
1
To Ken G.
Thank you for taking the time to comment. It is a little late here, so I'll respond in full to selected comments tomorrow. This blog prevents posting URL's until at least 15 posts have been made, a rule I presume exists to keep spam to a minimum, but you should have no trouble finding quantum tic-tac-toe with a quick google search. Today's post was partially intended to capture the "forest," answering your questions and responding to your points will help me articulate each "tree." Like you, I find this area irresistibly interesting. I'm looking forward to a lively exchange.

P.S. How do you get the quotes before your responses? Thanks.
 

Ken G

Gold Member
4,437
330
Do you have any specific examples of problems where they would be preserved with these kinds of measurements?
One example would be a spin measurement tilted at some angle other than 90 degrees. That will maintain some correlation, yet not commute. Still, the case of 90 degrees does seem to destroy the correlation, but even that leads to some subtleties-- if you can still tell what the outcome of the previous experiment was even after you do the new one, then the correlation is not destroyed. You have to "erase" the information of the first measurement-- but then it will be as if that measurement never happened and your new one will establish the correlation we are talking about. So I don't think you can ever really "destroy" a correlation.
 
Last edited:

Ken G

Gold Member
4,437
330
P.S. How do you get the quotes before your responses? Thanks.
Click on the box that says "quote" under this line, and I think it will become clear how to get that.
 
9
1
QTP - The Anomalies in CQM

I don't see any inconsistency in "the measurement problem", I would call it "the science problem" and liken it to how following Polaris is a good way to go north but a lousy way to go to Polaris. There's nothing to "solve" there. The interpretation problem is also not a problem, because relativity already taught us not to expect the existence of unique intepretations. Collapse is not a problem either, it is like the measurement "problem" and simply stems from the way we choose to do science-- there's no need to solve that either. I don't know what the supercedence problem is, but it sounds like something about quantum erasure and the only problem I see there is in our own unwillingness to let go of ideas that reach the limit of their usefulness, like causality. The nonlocality problem is also nothing that needs solving-- physical systems are indeed nonlocal because they are linked by their history to the rest of the universe, and not in a way that is "stored" locally in the elements of the system.
In my first post I presented several lines of evidence that spacelike causality may be allowed in physics. Each was presented, briefly, and without justification. Ken G took issue with item number 3, the 5 anomalies of classical quantum mechanics (CQM), so I'll respond just to this item.

The relevant background information for the claims in this item is the concept of paradigm, as articulated by Thomas Kuhn in his seminal work, "The Structure of Scientific Revolutions." Indeed it is this work which provides the modern meaning of the term "paradigm", derived from the Greek word for pattern. For those with an interest in science, and in particular how physics might change in the future, this is a must read. Kuhn presents a model of scientific advancement at odds with the model we were all presented with in grade school. In his model any field is dominated by an existing paradigm, it defines the problems of interest, how they are to be attacked, and what a successful solution will look like in general. In this view, scientific problems are seen as puzzles, problems with guaranteed but unknown solutions. The incremental advance of science occurs as each puzzle is solved. In the course of this process, however, some problems resist solution, even by the greats in the field. If they remain unsolved even when the field has by other measures matured, then they take on the status of anomalies, problems which are not puzzles. There is no longer a guarantee that solutions exist. In the history of physics, such problems have been the leading clues for the next paradigm shift. Because they are a professional embarrassment, the typical establishment response is to declare them non problems by fiat. This has the unfortunate effect of killing research in the area because astute careerists will select other problems to work on. This is part of the reason that paradigm shifts are often achieved by outsiders.

What I'd like to do in the rest of this post is explain why these five unsolved problems deserve the label anomaly.

1. The Measurement Problem. The concept of a measurement is central to the mathematical and conceptual structure of CQM. It is the process by which the state of quantum systems, in general in a superposition of possibilities, is reduced to a single classical value. The only problem is that we have no frigg'n clue what causes a measurement. The problem is so severe, and so unexpected, that Penrose calls it the measurement paradox, a misuse of the term, but indicative of how serious this gap is for the foundations of quantum mechanics. Physicists find themselves in the uncomfortable position of having to admit that a measurement is like good art, "I know one when I see one." In an effort to solve this, (I believe it was Von Neuman) showed that one could draw the line of measurement anywhere. If beta decay is to be measured, is it the tracks in the bubble chamber that form the measurement? Or the photo of the bubbles? Or when the tech develops the film? Or when the grad student looks at the film? Or when the professor reviews the grad student's work? The infinite regress is hard to avoid. Von Neuman argued that this process could be continued until encountering a conscious observer, and then we didn't know enough to take the process further. This has lead some to conclude that measurements require a conscious observer, a dubious conclusion.

In contrast, in the abstract quantum systems we have studied, such as quantum tic-tac-toe, there is an objective measurement process. An entanglement that becomes cyclic is typically the trigger for a measurement, no outside macro system, much less a conscious observer, needs to be invoked. While such systems are abstractions and do not represent real physical systems, they do show that it is plausible that an objective measurement system is the real case in quantum physics. It becomes reasonable therefore to seek one, and this provides a fresh attack on the measurement problem.

Since this has become a long reply, I'll return to the other anomalies at a later time.
 

Ken G

Gold Member
4,437
330
...If they remain unsolved even when the field has by other measures matured, then they take on the status of anomalies, problems which are not puzzles. There is no longer a guarantee that solutions exist. In the history of physics, such problems have been the leading clues for the next paradigm shift.
I don't think this is by any means a statement of how most scientific advancements occur. Far more often than "long-standing or lingering problems", the advances come from stunning new observations that were entirely unexpected. When the unexpected result is found, everyone knows a new theory is needed, and it is then just a matter of coming up with it-- and that rarely requires more than a few decades to half a century. As long as we recognize our theories are just models, and do not have important philosophical implications, we face no difficulties.

A classic example of what I mean is "action at a distance" in Newtonian mechanics. No one was more philosophically bothered by that than the theory's own creator, but there were no observations that created any difficulties at the time. Some went so far as to read in philosophical implications, such as that all of reality was deterministic by virtue of being described completely by Newton's laws. That was a foolish extrapolation, so we are not surprised when "action at a distance" models are found wanting in later more precise observations. Should we say that the philosophical "problem" of action at a distance was evidence all along that we needed a new theory? It's not very meaningful to take that stance, because the "problem" was not sufficient to motivate a successful new theory, observations were needed for that, and furthermore, it is always silly to think that we need "evidence" that some new theory might be better than the one we have now-- we can just accept that as given, without reference to any specific "problems".

Because they are a professional embarrassment, the typical establishment response is to declare them non problems by fiat.
That very rarely happens, it's basically a complete myth. What significant event in the history of science can you point to that suggests such "dismissal by fiat" of challenging observations?
This has the unfortunate effect of killing research in the area because astute careerists will select other problems to work on. This is part of the reason that paradigm shifts are often achieved by outsiders.
More myths. It is every scientists dream to replace an old paradigm with a new one. The difficulty is not in finding the motivation to do it, or even the support-- it is figuring out how to do it. Wild speculation and protestations of "suppression" generally don't lead there.

More on the other stuff after I've had a chance to see the quantum tic tac toe.
 
9
1
The Structure of Scientific Revolutions

I don't think this is by any means a statement of how most scientific advancements occur. Far more often than "long-standing or lingering problems", the advances come from stunning new observations that were entirely unexpected. When the unexpected result is found, everyone knows a new theory is needed, and it is then just a matter of coming up with it-- and that rarely requires more than a few decades to half a century. As long as we recognize our theories are just models, and do not have important philosophical implications, we face no difficulties.
Read Kuhn.
 

Related Threads for: Why does quantum entanglement not allow ftl communication

Replies
1
Views
730
Replies
63
Views
6K
Replies
3
Views
3K
Replies
4
Views
899
Replies
6
Views
625
  • Posted
2
Replies
25
Views
5K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top