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FTL communication: might QM entanglement trump relativity?

  1. Dec 26, 2008 #1
    FTL communication: might QM entanglement "trump" relativity?

    As we presently understand entanglement, it can't be used for FTL communication, since some speed-of-light exchage of information between Bob and Alice is required. So c apparently still remains the communication speed limit.

    But is it crystal clear that entanglement does not somehow permit FTL communication? My thinking is as follows:The speed of light limit on signaling times is predicted by a "classical" physics theory, i.e., relativity. But the apparent instantaneity (>10,000xc according to a recent experiment) of "effects" between entangled particles is a quantum mechanical phenomenon. At this point, the two theories are of course incompatible and thus incomplete, since no testable unified theory has yet been developed. But when experiments examined Bell's inequality, and thus whether (classical) locality/separability or QM entanglement is correct, QM won out. This type of result implies to me that the current QM theory may represent a significantly more basic description of the universe than does the current relativity theory. If so, perhaps the final unified theory will in fact permit FTL signaling, utilizing some aspect of entanglement not yet envisaged.

    Comments on this admittedly intuitive (and possibly wrong-headed) argument will be appreciated.
     
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  3. Dec 26, 2008 #2

    Vanadium 50

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    Re: FTL communication: might QM entanglement "trump" relativity?

    Before I answer this, I have to know - do you really think you were the first person to think of this? And that somehow Einstein, Pauli, Dirac, Fermi and everybody else working in the past century somehow missed this?

    There is no way to signal faster than light using entanglement. Every measurement to date - and there have been billions - is consistent with both SR and QM. If you want to argue SR is wrong, you will have to explain why it agrees with measurement after measurement.
     
  4. Dec 26, 2008 #3

    turbo

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    Re: FTL communication: might QM entanglement "trump" relativity?

    It's not a bad question, Vanadium 50. Einstein himself worried about how quantum theory might invalidate field theory. Please read chapter 1 of "The Philosophy of Vacuum" by Saunders and Brown. That chapter is a translation of Einstein's "On the Ether" (1924) by Simon Saunders and it is under copyright so I will not transcribe it here.
     
  5. Dec 26, 2008 #4

    mgb_phys

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    Re: FTL communication: might QM entanglement "trump" relativity?

    EPR doesn't violate causality because you can't choose which state the particle will collapse into - so you can't use it to send data. You are allowed to transmit random numbers faster than light.

    Some instantaneous symmetry breaking functions can be used to send data faster than light.
    The famous philosopher Pratchett suggested that since when a king dies the oldest prince instantly becomes king - you can use this to send information faster than light if your are able to kill enough kings and detect the resulting state of kinglyness in the heir.
     
  6. Dec 26, 2008 #5

    Vanadium 50

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    Re: FTL communication: might QM entanglement "trump" relativity?

    But it's not a new question either. Hence my point.
     
  7. Dec 26, 2008 #6

    turbo

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    Re: FTL communication: might QM entanglement "trump" relativity?

    I realize that, and it is a valid complaint. If you will refer to Einstein's memoriam on the death of Ernst Mach, you will see that he placed an extremely high value on epistemology - the questioning of previously-held beliefs and opinions to re-affirm or falsify current theory. Einstein bemoaned the fact that epistemology was ignored by modern science ( in his life-time) as a critical tool. He was ignored then, and he is being ignored today. Without epistemology, science heads down blind alleys for decades (or longer) without a clue.
     
  8. Dec 26, 2008 #7
    Re: FTL communication: might QM entanglement "trump" relativity?

    ?? Is this the renowned T. Pratchett of infinite (turtle) regression fame?
     
  9. Dec 26, 2008 #8

    turbo

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    Re: FTL communication: might QM entanglement "trump" relativity?

    It's turtles all the way down????
     
  10. Dec 26, 2008 #9

    Hurkyl

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    Re: FTL communication: might QM entanglement "trump" relativity?

    When presented in a form like this, it is crystal clear: local time evolution is completely determined by local 'components' of quantum state. (Disclaimer: I believe QFT has not actually been shown to satisfy such axioms)

    Another strike against is that an experiment involving only one half of an entangled pair only has access to the relative state of that particle -- thus losing all information about the other half -- and so such an experiment cannot possibly be influenced by interactions involving the other half.

    Then it's no longer the phenomenon of entanglement that QM describes, but instead some phenomenon described by some other scientific theory. If you want to speculate about the latter, it's quite misleading to speak as if you're talking about the former. (Even if you expect them to approximate each other)
     
    Last edited: Dec 26, 2008
  11. Dec 26, 2008 #10
    Re: FTL communication: might QM entanglement "trump" relativity?

    Don't be silly. The world rests upon the backs of three great Elephants astride a Tortise (no turtles here), on his way to Andromida. But this could be erroniously construed as off topic.
     
  12. Dec 27, 2008 #11

    ZapperZ

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    Re: FTL communication: might QM entanglement "trump" relativity?

    I don't see any incompatibility here between QM and SR. Here's the reason.

    Did anyone detect ANY signal that traveled between the entangled pair when a measurement is made? Has this ever been published in any of the EPR/Bell-type experiments at all? I'll answer that for you : NO.

    In other words, there is nothing that "traveled" between one location in space to another. What QM does say is that there is an intrinsic connection between the entangled pair that does not depend on things moving between the two. This is what is meant by "non-local", something that is very common in physics, believe it or not (look at the wavefunction of a free particle).

    This is not what SR describes, where a signal or information has to traveled through both space and time. Even when we have the apparent FTL group velocity in the NEC experiment from several years ago, when something has to make such a travel, all the postulates of SR are still obeyed.

    As Sean Carroll said,

    We don't know yet how they connect with each other, or even if there is a way to know. What we DO know is that they don't pass information between each other via sending a signal that requires motion from one location to another over time. This is the part that does not violate Relativity.

    Zz.
     
  13. Dec 27, 2008 #12
    Re: FTL communication: might QM entanglement "trump" relativity?

    To answer the OP...

    Entanglement is not some mathematically mysterious process that can't be analyzed. In fact, it's fairly easy to convince yourself that FTL communication via entanglement is not possible in the simplest case, when two qubits are entangled.

    If (as you say) Alice and Bob share an entangled pair [tex]|\phi\rangle = |00\rangle + |11\rangle[/tex], you might hope that Bob could selectively increase the amplitude only the "11" part of the qubits through only local operations on his qubit, but (assuming the Schroedinger equation is correct and only unitary transformations are physically possible) any unitary operation U on his qubit will simply transform the system into [tex]|0\hat 0\rangle + |1\hat 1\rangle[/tex], where [tex]\{ |\hat 0\rangle, |\hat 1\rangle\}[/tex] is another orthonormal basis.

    Taking a partial trace reveals that the density operator of Alice's qubit by itself is the maximally-mixed [tex]I/2 = 0.5|0\rangle\langle0| + 0.5|1\rangle\langle1|[/tex], exactly the same as if Bob did nothing first, and thus revealing nothing.

    On the other hand, if we suppose that non-unitary operations ARE possible, then FTL communication IS possible. Then you would be able to "pry apart" an orthonormal basis and increase the amplitude of only the "11" or "00" part of the entangled system. But this would mean abandoning the Schroedinger equation, the cornerstone of quantum theory, when all the available evidence points to it being correct.
     
  14. Dec 27, 2008 #13
    Re: FTL communication: might QM entanglement "trump" relativity?

    It seems to me a similar "exception" to SR takes place when photons are emitted (by a simple candle, for example).

    Even though SR says something with mass <> 0 can't be accelerated to the speed of light, in the "creation" of photons their constituting energy "jumps" to the speed of light.

    BTW, I've recently read that 'causality' is not really a requirement of SR, even though it is often presented as if it were. Comments on this are welcome :).
     
  15. Dec 27, 2008 #14

    Vanadium 50

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    Re: FTL communication: might QM entanglement "trump" relativity?

    Photons are massless, and they are not accelerated. It's not like the candle wick has a store of stopped photons and starts tossing them out when lit.
     
  16. Dec 27, 2008 #15

    ZapperZ

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    Re: FTL communication: might QM entanglement "trump" relativity?

    This statement makes very little sense to me. What does it mean that "...their constituting energy "jumps" to the speed of light..."? And what's so contradictory to SR is that?

    In this forum, whenever one says "I've recently read...", we require exact citation of the source. If not, we have no way to know whether you read some crackpottery, or if you've misinterpreted what you read. Till then, it is pointless to write any comments on something like this.

    Zz.
     
  17. Dec 27, 2008 #16
    Re: FTL communication: might QM entanglement "trump" relativity?

    Sorry, I thought the meaning would be quite obvious: The photons are "created" from energy which the valence electrons release. The valence electrons, as far as I know, do not move with the speed of light, nor are they massless. The energy of the emitted photons contributed to the mass of the system before they were emitted. This mass has moved at less than the speed of light, and SR says (in general) that a mass cannot be accelerated to the speed of light. However, when the photon is emitted, a certain amount of mass in the valence electrons is gone, and instead we have a photon moving at the speed of light. Of course, this is not an "acceleration" in the usual sense, and that is why I see a similarity to the text which I responded to.

    Sure, if wikipedia may be quoted here: http://en.wikipedia.org/wiki/Faster...t_.28Casimir_vacuum_and_quantum_tunnelling.29

    "Causality is not required by special or general relativity, but is nonetheless generally considered a basic property of the universe that should not be abandoned."
     
    Last edited: Dec 27, 2008
  18. Dec 27, 2008 #17

    ZapperZ

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    Re: FTL communication: might QM entanglement "trump" relativity?

    You do know that this is not the only way a photon can be created, don't you?

    I'm not the person you want to use a wikipedia as a reference. You shouldn't pay that much emphasis on exact quotes out of Wikipedia. I could easily go in and edit that away, and now what will you be left with?

    You need a better source than wikipedia to back your claim. Please note that for the physics forum, our main preferences and what we highly recommend would be peer-reviewed publications as valid sources. The use of other sources is done at your own risk.

    Zz.
     
  19. Dec 27, 2008 #18
    Re: FTL communication: might QM entanglement "trump" relativity?

    By a candle? No, I thought in a candle that would be the only way.

    It's not a claim at all, just something I've read. Which is why I was wondering what the response on this forum would be, as I was expecting that this could be controversial. However I do have suspicions that FTL necessarily implies time-travel and is therefore a priori impossible, but that's just a personal opinion since I don't think that SR/causality will be the first theory which won't be superseded by another one.
     
  20. Dec 27, 2008 #19

    ZapperZ

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    Re: FTL communication: might QM entanglement "trump" relativity?

    Please do a search on how EM radiation is created by your incandescent light bulb, and at all those synchrotron research centers.

    Zz.
     
  21. Dec 27, 2008 #20
    Re: FTL communication: might QM entanglement "trump" relativity?

    It seems that SR can't handle any of those cases either... :)
     
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