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Faster-than-light communication

  1. Jul 17, 2009 #1
    Many seem to be skeptical whether faster-than-light communication is possible. Consider the experiments performed by Alain Aspect where the polarization of one of the entangeled photons force the other to collapse into the opposite axis.

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

    The Wikipedia article says that the experiment do not demonstrate faster than light communicaiton and I wonder why.

    If one of the polarization filters is switched from axis to axis, and the other filter kept steady, it should according to my logic, be possible to transfer information. What am I missing?
     
  2. jcsd
  3. Jul 17, 2009 #2
    Learning information is different from transmitting.
     
  4. Jul 17, 2009 #3

    Fredrik

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    How? Suppose e.g. that you flip a coin and want to send a 1-bit message (1="heads", 0="tails") to let someone at the other end know what the result was. How would you do this?
     
  5. Jul 17, 2009 #4
    The information is sent like this:

    Entangeled photons are created with a frequency that is known to the receiver. By keeping his filter steady he would have a "click" in his detector when a photon passes his filter, and a delay when the photon is stopped by the filter. If the experiment works like this it is possible to send a binary string of information. However, the problem seems to be that if the filters are adjusted to the same axis it will be random whether the photon is stopped by your filter, or the filter at the receiver. And I think it will be the same in a situation were you are located much closer to the photon transmitter (anhilation point) than the receiver.

    Am I correct?
     
  6. Jul 17, 2009 #5
    Ok, new idea:

    By carrying out a Bell test experiment we will observe the following:

    1) When the polarisers are oriented along opposite axis we will get clicks in both detectors

    2) When the polarisers are oriented along the same axis we have 50% chance of getting a hit in the detectors at both sites.

    Now we use 20 photons to transmit 1 bit of information. Data that gives us 100 % hits, versus data that gives us only 50 % hits.



    Is this faster-than-light communication or can somebody tell me where I go wrong?

    Best
    Albert
     
  7. Jul 18, 2009 #6

    DrChinese

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    There are always clicks at the detectors, assuming you put a polarizing beam splitter in front of a pair of detectors. Each photon is randomly polarized. They wouldn't be polarization entangled otherwise. As previously mentioned, entangled photons cannot be used to transmit information.
     
  8. Jul 18, 2009 #7
    You could try to create two entangled observers. A large quantum computer implements an observer. We take two such quantum computers and intitialize them in some mutually entangled state of the form

    Sum over_j A_j |j>|j>

    where |j> denotes some bitstring in a |0>, |1> basis which corresponds to some classical brain state of the observer and state of the virtual world the observer finds himself in. The quantum algorithm is such that bistrings in the |0>, |1> basis evolve deterministically. So, we assume that a classical determinisic algorithm can represent the mind of a real person.

    Suppose that the two entangled quantum computer pairs are separated by a large distance. Then if one observer makes what is in his mind a random choice for some numbers, he knows that his copy will have made exactly the same decision.
     
  9. Jul 19, 2009 #8
    Yes, this is also how I understand the system. I have taken this fact into account. Therefore one and one photon can not be used to transmit information. But let us start from the beginning with all my assumptions.

    1) A continuous source of photons emitted at an even frequence

    2) When the polarisers are oriented along opposite axis we will get clicks in both detectors (the receiver will therefor obtain clicks according to the given frequence. Let us say that 10 photons is enough data for him to conclude that the filter at the transmitter is oriented along the opposite axis)

    3) When the polarisers are oriented along the same axis we have 50% chance of getting a hit in the detectors at both sites. In this situation the receiver will have clicks but also delays regarding the agreed frequence. Statistically he will have 50% less clicks than in the former situation. 10 photons gives him enough data to conlude that the filter at the transmitter is oriented along the same axis.


    Sorry that I have to be spoonfed, but I need to know how these systems behave.

    Best
    Albert
     
  10. Jul 19, 2009 #9

    DrChinese

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    I think you may have a slight misunderstanding of what the actual setup produces in the way of results.

    There are 2 observers, Alice and Bob. They get a stream of photons arriving as you say, perhaps 10. They EACH have a polarizing beamsplitter and 2 detectors. When Alice gets a dectector hit on the first detector, she marks it a +. The other detector registers as a -. Ditto for Bob.

    Alice can vary her beamsplitter orientation so she can "send" a message. (Not really of course as we will find out.) Bob holds his beamsplitter fixed and steady so he can receive a "message". The beamsplitter separates the photons to one of the 2 detectors as mentioned, and he records a + or a - according to which of his 2 detectors fires.

    Now what does Bob see when he records his results? He see a random series of + and - results, something like this:

    ++-+-+---+

    Since the "message" is random, no information is received. It doesn't matter what Alice does, Bob's message is totally random by any measure. That is because the source of the entangled photons is random itself. The pattern that indicates there is entanglement is ONLY evident when Alice AND Bob's results are correlated.
     
  11. Jul 19, 2009 #10
    Thanks Dr. Chinese for spending time one these questions.

    I am a little confused by your beamsplitters because I am not familiar with how they work. I had a single channel Bell test in mind. And I use polarisers since they are more easy to understand.

    After anihilation of a positron and an electron, two photons are generated with opposite directions and planes. The planes are not determined, but will be after one of the photons passes a polariser. Opposite orientation of the polarisers will always allow BOTH the photons to pass and same orientation will only allow one of the photons to pass.

    Am I correct this far?
     

    Attached Files:

  12. Jul 19, 2009 #11

    DrChinese

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    Yes, for this setup, and notice that coincidence matching (CM) is necessary. That does not allow an opportunity for Alice to send a message to Bob. Alice must first send her results to be matched with Bob's. That occurs at light speed. So no FTL possible.
     
  13. Jul 19, 2009 #12
    Hence, when the polarisers are oriented along opposite axis, Bob gets a message like this; ++++++++++, but when the polarisers are oriented at the same axis Bob will get a message like this; +delay++delay+delaydelay++ (where + signifies clicks at the preagreed frequence of photons)?

    Am I also correct this far?
     
    Last edited by a moderator: Jul 20, 2009
  14. Jul 19, 2009 #13

    DrChinese

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    No, Bob ALWAYS gets a "message" (in your terms) like:

    +delay++delay+delaydelay++

    Meanwhile, Alice gets a somewhat similar "message" depending on her polarizer orientation. At some orientations it will be the same message, at some it will be opposite, and at others a mixture.
     
    Last edited by a moderator: Jul 20, 2009
  15. Jul 19, 2009 #14

    DrChinese

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    Keep in mind that Alice is located one place, and Bob is located at another. Each sees one side (one half) of the stream of entangled photon pairs. Assuming they were equidistant from the source, they would receive photons going in to their respective polarizer simultaneously (within a narrow window). Since those photon pairs have random (but opposite) polarization, both Alice and Bob see random streams. When the streams are compared at a later time, and you know the respective polarizer orientations, you can detect that the source pairs were (or were not) entangled.
     
  16. Jul 19, 2009 #15

    jtbell

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    Two key points affect the results of an attempt to send a signal via entangled photons:

    1. The setting on Alice's polarizer does not affect the result that Bob gets, for any individual photon. And vice versa, swapping Alice and Bob.

    2. Also, neither Alice nor Bob can control the sequence of states in which the photons are produced. They can only control their polarizers.

    First suppose that Alice and Bob both keep their polarizers horizontal. Each column indicates one photon pair. H and V indicate the polarizer settings, + and - indicate whether a photon got through that polarizer. They might get something like this. I generated Alice's sequence of + and - by flipping a coin thirty times.

    Code (Text):

    Alice: HHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
           ++-++++-+-++-+-+-----++-+---++
    Bob:   HHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
           ++-++++-+-++-+-+-----++-+---++
     
    Now suppose that instead, Alice had kept her polarizer vertical, and Bob had kept his horizontal. For the same photons they would have gotten the following results

    Code (Text):

    Alice: VVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
           --+----+-+--+-+-+++++--+-+++--
    Bob:   HHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
           ++-++++-+-++-+-+-----++-+---++
     
    If Alice had alternated between horizontal and vertical polarizations, in "chunks" of five photons at a time, with Bob keeping his horizontal the whole time:

    Code (Text):

    Alice: HHHHHVVVVVHHHHHVVVVVHHHHHVVVVV
           --+----+-+--+-+-+++++--+-+++--
    Bob:   HHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
           ++-++++-+-++-+-+-----++-+---++
     
    How does Bob extract Alice's "signal" (HHHHHVVVVVHHHHHVVVVVHHHHHVVVVV) from his data, when they cannot actually repeat the experiment with the same sequence of photons? Each time they repeat the experiment, they get a different sequence of photon polarization states, at random, like me flipping my coin another thirty times.
     
    Last edited: Jul 19, 2009
  17. Jul 19, 2009 #16
    DrChinese & jtbell,

    Isn't this thread an example of what you just told me in another thread was prohibited? It clearly is opposed to mainsteam theory. From your guidlines:

    "It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion."

    And yet you made no objections and participated in it. Why?
     
  18. Jul 20, 2009 #17
    Thank you both for being patient with me

    This is the point where I don't follow


    I said:
    Opposite orientation of the polarisers will always allow BOTH photons to pass and same orientation will only allow one of the photons to pass.

    DrChinese said:
    Yes

    If Bob keeps his filter horizontal and Alice keeps her filter vertical, why does not the frequency of photons passing the polarizer go up compared to the situation where Bob and Alice both keep their polarizers at a, let's say, vertical position?

    Albert
     
  19. Jul 20, 2009 #18
    I see now that these systems did not behave as I had understood. I thought that the wave-function collapsed into a certain plane when the photons hit a polarizer, well perhaps it does, but the plane can still be different from the orientation of the polarizer.

    The measurer is unable to feed information into the system.

    But what is the probability of passing photons. There are many more planes than horisontal and vertical.

    I don't see why Einstein thought this was spooky. The planes of the photons are opposite, and thats it.

    Thanks for helping me
     
    Last edited: Jul 20, 2009
  20. Jul 20, 2009 #19

    jtbell

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    Indeed, and the cases where the polarizers are not at right angles to each other, are the really "spooky" ones! In these cases you cannot explain the experimentally observed probabilities of photons passing through the polarizers (more precisely the correlations between pairs of entangled photons) by using a "natural" simple model that explains the case where the polarizers are at right angles.

    Dr. Chinese's Web site has a lot of useful information and links to original papers related to entanglement and Bell's Theorem. For simplified examples that illustrate the "spooky" correlations see particularly

    Bell's Theorem with Easy Math (Dr. Chinese's own example)

    http://www.iafe.uba.ar/e2e/phys230/history/moon.pdf [Broken] from Physics Today magazine (April 1985).
     
    Last edited by a moderator: May 4, 2017
  21. Jul 20, 2009 #20

    DrChinese

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    Yes, it is true that when you talk about opposite or parallel orientations, then there does not seem to be anything weird happening. Einstein essentially said that in such cases, the results must in fact be predetermined. This is often referred to as the Bertlmann's socks anaology (you can look it up if you like).

    But that is not the tricky situation. Bell discovered that at other specific angles, the quantum statistics were incompatible with the idea of predetermination. Experiments have since shown that quantum statistics are correct, and the idea of predetermination cannot be.

    So in the end, and to use Einstein's words: either there is spooky action at a distance or the moon is not there when noone is looking.
     
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