FTL Communication-Best Candidate?

  1. 1st Question:
    Just how firmly established is the idea that, not only can't anything with a non-zero mass be accelerated to C+ speeds, but that *information*, of *any* kind, can not be sent from one point to another in less time than it would take a beam of light to cover the distance in a total vacuum?

    I can understand why anything with a non-zero mass would fall under that restriction, but I'm at a loss to understand how information would also fall under that same restriction.

    2nd Question:
    Working on the (possibly bad) assumption that the answer to the above question isn't by any means set in stone, and is therefore still up for debate between people with a sufficient background in the necessary areas of expertise, what, at the moment, are the best candidates for at least some semblance of a possibility of FTL communication?
  2. jcsd
  3. selfAdjoint

    selfAdjoint 7,521
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    Your first question: In relativity not only can you not accelerate a massive body to c, but any massless body travels at exactly c. So c is the fastest that anything with real or zero mass can travel.

    Now the caveats. If there were a particle with imaginary mass, that is m^2 < 0, then it not only could but would have to travel faster than c. It could no more travel below c than a particle with real mass can travel over c.

    Such a hypothetical particle is called a tachyon (from Greek for fast), Tachyons do come up in various theories, such as bosonic string theory. No tachyon has ever been observed in nature. If a tachyon could be made to interact with us bradyons, we could send messages back in time. But anyone who claims to have done that had better display their work!
  4. LURCH

    LURCH 2,507
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    I would say that Quantum entanglement the so-called "spooky behavior" associated with it constituted our best chance at FTL communication.

    Quantum tunneling has been used to send music across the room, so I suppose it could be said to have already accomplished the goal. However, I have my doubts as to whether this method could send information any further than crossing the room.
  5. Quantum entanglement a dead end 4 comm?

    My understanding (which could very easily be very wrong) is that quantum entanglement is pretty much a dead end as a candidate for FTL communications.
    The fundamental problem with this one is that the way you "send a message" is by recording what you see going on at the "transmitter" end as a response to something you did to the entangled particles there, while at the same time recording the reaction of the entangled particles at the "receiver" end to what you did to the entangled particles at the "transmitter" end, and then the "message" is deciphered by comparing the differences between the reactions of the entangled particles at the "transmitter" end to the reaction of the particles at the "receiver" end.
    The problem with that is, that although the "message" makes the trip between the "transmitter" and the "receiver" instantly, no one at the "receiving" end can decipher the "message" sent until they receive what essentially boils down to the key to deciphering the message (the reaction of the entangled particles at the "transmitter" end) from the whoever sent the "message" at the other end of the line.

    To get a clearer view of the problem imagine this...
    It's the 1800's, but in this imaginary world, telegraphs are ONLY capable of sending *encrypted* messages that absolutely positively can not be decrypted without a completely unique decryption key that is generated at random by the telegraph process itself each time a message is sent, and although telegraphs can send the encrypted messages, in this imaginary world, telegraphs are physically incapable of sending the unique decryption keys required to read the messages they send.
    So now you send the *encrypted* message via telegraph from New York to Dodge City, and wonder of wonders, that *encrypted* message goes from New York to Dodge City almost instantly.
    But it doesn't do anyone any good, because the people in Dodge City can't decrypt the telegram they receive until they get the decryption key from New York, and since telegraphs can't send decryption keys, the only way to get that decryption key from New York to Dodge City is still by Pony Express.
    In the end the speed of communication between New York and Dodge City hasn't improved one iota in any practical terms.
    Sure, the message in its encrypted form went from New York to Dodge City all but instantly, but the people in Dodge STILL have to wait for somebody on horseback, carrying the golden decryption key (that will only decode that one particular message and no others) to make the long and arduous trip from New York to Dodge.
    So at that point you have to ask - what's the point of the telegraph?
    You might as well just send the message by Pony Express, since you're going to have to wait just as long for the decryption key to arrive by that means before you can read the message that was sent with such amazing speed over the telegraph wires.

    So it seems to me that quantum entanglement just isn't a viable candidate for practical FTL communications.
    Although the reactions of the entangled particles are linked, so that anything done at one end produces a reaction at the other instantly, the need for people at the receiving end to use information unique to the transmitting end to decode any message sent, makes it just as impractical as the imaginary telegraph above as a means of accomplishing FTL communication.
  6. chroot

    chroot 10,351
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    You are correct. Quantum entanglement cannot be used to transmit information superluminally.

    - Warren
  7. What about tiny wormholes?

    1) Just how small can a wormhole be?

    2) What would the minimum size of a wormhole have to be in order to send a radio wave or laser through it?

    3) What would be involved (in broad strokes) in creating a wormhole? Would creating even the tiniest wormhole require something silly like the combined energy of an entire galaxy to produce?

    4) Is there even a theoretical basis for determining where (not to mention when) a wormhole would go even if you assume you could make one?

    5) How long would such a wormhole last? Long enough to send a short burst transmission? Forever once it's made?

    What I'm wondering here is if you did make the trip say from Sol to Tau Ceti in a "generation ship" or whatever, could you then on some date certain two thousand years later (or however long it took to make the trip) then create and use a wormhole to open a permanent communications conduit between Earth and a colony on Tau Ceti?
    Or even MORE useful...
    Could you create a tiny wormhole between the generation ship and a home base here on Earth before even leaving, and then take one end of the wormhole with you on the trip so that you could stay in touch for the entire trip, and then continue to use it once you arrive?
  8. Could someone point out a paradox or a thought experiment or whatnot that would show why ftl comm/travel is impossible?
  9. We know so little about wormhole that they can pretty much do or be anything you want and no one could really support you or disprove you.
  10. LURCH

    LURCH 2,507
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    Yes, I should have pointed out that this is the standard position stated by many experts in the field. I simply disagree, and am fairly certain that this will be proven incorrect eventually. Certainly, it cannot be stated dogmatically. IMHO, QE will be used to transmit information superluminally.
  11. selfAdjoint

    selfAdjoint 7,521
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    Now explain to me how your "humble" opinion, which AFAIK you haven't backed up with math or any other checkable development, is worth more than that of the professional physicists who have good technical reasons for saying what they say.
  12. I agree with Lurch

    Quantum entanglement seems like a good possibility. Now, here's what I don't understand about the Pony Express criticism offered by MFTI. If you consider the quantum eraser experiment, where you mark the photons going through the double slit test with opposite polarizations, collapsing the wave interference of the light; the wave interference is restored to this portion of the experiment by inserting a polarizer into the entangled stream of light (i.e. NOT the stream headed toward the double slits). Thus, the person examining the light pattern coming through the double slits knows whether an operator has inserted a polarizer into the other entangled stream or not. This is a piece of binary information...yes/no information. Let's say the person at station 1 (the double slit station) and the person at station 2 (the entangled-stream-that-may-or-may-not-have-a-polarizer-inserted-into-it station) have pre-agreed to consider light patterns formed by the accretion of photon strikes against the detector plate over periods of 20 seconds duration as units (or whatever minimum time interval allows a stream of photons to begin to form a recognizable interference pattern). Station 2 will either insert the polarizer into the entangled stream for a whole time unit, or he will not. Station 1 will either see an interference pattern, revealing that Station 2 inserted the polarizer, or they will see two crests of light that fade off to each side, revealing that Station 1 did not insert the polarizer. These packets of yes/no information can be assembled into a prearranged binary language by which instantaneous communication can be effected, the same way your home computer turns a button press of the "A" key into a binary code that is different from the "B" key. The reassembly of the binary code into language is the slow part, but given that this experiment could theoretically be conducted over millions of miles, it would seem like very fast communication indeed. The problem with the Pony Express criticism is that it assumes the receiver of the medium is trying to communicate to the transmitter of the medium, or vice versa, New York to Dodge City. There are actually two transmissions needed in FTL via QE. There is the transmission of the medium, let's say entangled photon streams like we've been using. This requires there to be two "detection" stations and a light transmitter at a third location, somewhere more or less equidistant from them both. The second transmission is the transmission of the binary message...the yes/no proposition of inserting or not inserting the polarizer into the second stream of entangled photons. Granted this system would require a total of three "stations" (1. a light transmitter, 2. a message sender who is also a light receiver, and 3. a message receiver who is also a light receiver) but it seems that it WOULD permit packets of information to be transmitted instantaneously at a distance. So its not just New York and Dodge City. It's more like a light emitter in Chicago sends a stream of entangled photons to both Los Angeles and New York (pretend the Earth isn't curved as that would really complicate the travel of light from Chicago to either of those places, or go ahead and let it be curved but route the light through some sort of fiber optic cable). Los Angeles routes their light through a double slit test with a clockwise polarizer in front of one slit and a counterclockwise polarizer in front of the other. The fact that the slits mark the photons collapses their probability wave function and the light only appears as two crests on a detector plate. But the experiment seems to hold that as long as the streams of photons consist entirely of entangled photons, unadulterated by any other kind of non-entangled photons, then the insertion of the right polarizing screen in New York will have a corresponding effect on the stream in Los Angeles which will make it impossible to determine which slit a given photon passed through, thereby unmarking them and restoring the interference pattern indicating a probability wave interference. It seems too good to be true, but I can't find the flaw. Is it impossible to generate two streams of "unadulterated entangled photons" (i.e. not adulterated by run-of-the-mill non-entangled photons)? If so, why? And wouldn't a small degree of adulteration be essentially harmless to the experiment?
  13. vanesch

    vanesch 6,189
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    In standard quantum mechanics, it can't be done. There is a mathematical proof of that, which I spelled out in this post:

    It indicates that there do not exist measurements, performed on one side of an entangled system only (so, at a distance), that can reveal anything about whatever is done on the other side of the system: all one-side measurable quantities (which are all expectation values of some operator) are independent of whatever measurement or action one undertakes on the other side.
  14. Cthugha

    Cthugha 1,717
    Science Advisor

    You are completely misunderstanding the experiment. Interference is only visible in coincidence count data. Station 1 will never see an interference by itself. Only a subset of the counted photons will show an interference pattern and to know, which photons belong to this subset, information from station 2 is needed. Therefore there is no ftl information transfer.

    However, is it really necessary to use an 4 year old topic for this? This is some kind of forum necromancy. ;)

    edit: vanesch was faster.
  15. Patrick - does your proof take into account the possibility of interference exhibited in one entangled particle, depending on the type of measurement performed upon the other, i.e., the Dopfer / Zelinger experiment?
  16. vanesch

    vanesch 6,189
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    Of course, that's exactly what it does. ANY measurable quantity (such as "there's an interference pattern") is - in quantum theory - always, there's no exception - an expectation value of an operator.

    Well, it turns out (it's the proof) that the expectation value of an operator which corresponds to a measurement on only one branch of an entangled pair is independent of what is measured or even done to the other branch, simply because this expectation value is the trace of the product of said operator and the "reduced density matrix", which itself is independent of any measurement or unitary transformation of the opposite branch.
  17. Though it's widely believed that instantaneous communication is not possible (because of causal issues), there has been specific research done that may prove the contrary. For those that can read German, there was thesis written by Birgit Dopfer which focused on communicating instantaneously using momentum entangled states via down conversion. Unlike polarization entanglement, where destroying the state at one end of the signal cannot convey any information to the other side, a destroyed momentum state will cause an instant collapse of the wave function at the other end (i.e. a photon will no longer interfere with itself). I will mention however that, skepticism has been directed at the measurement devices that ensure the two entangled photon arrived at the two detectors at the same time and further research needs to be done to remove any doubt. If you could produce many momentum entangeled photons, you could send a signal instantaneously by choosing to mesure the momentum state at your end using a simple Heisenberg lens. For binary communication 0 and 1 would be no interference and no intereference.
  18. Cthugha

    Cthugha 1,717
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    But one should add that the Dopfer thesis actually stresses (page 44-47), that this setup can't be used to achieve ftl communication (fortunately I can read German). In fact the interferences, which can be destroyed are found only in coincidence measurements, which cannot carry ftl information anyway. In this case the single photon at the double slit will never show any interference phenomena as it is as incoherent as thermal light and the down conversion nonlinear crystal is located too close to the double slit to have the photons in one arm show interference.

    However you can have single photon interference by just increasing the distance of the crystal to the double slit. Now you have photon self interference, but the large distance decreases the number of states in k-space (roughly speaking the number of possible directions a photon can have, when it goes through the double slit). This destroys the interference pattern in the coincidence count data. So it is your choice: single photon interference or interference in coincidence counting. Both at once is not possible and therefore there will be no instantaneous information transfer.
  19. What do you mean the coincident measurments cannot carry ftl information. This is due to the experiemental setup of the crystal location correct, and not because it's not possible?
  20. Cthugha

    Cthugha 1,717
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    No, coincidence counting means you have to compare data from two spatially separated detectors. So you have to send the data from A to B to get some information. This data cannot be transferred faster than light, therefore there is no possibility for ftl information transfer by means of experiments, which require coincidence counting.
  21. If you have two momentum entangled photons, one heading down axis "a" and the other down axis "b" and you destroy all the axis "a" photons by measuring their position, all the axis "b" photons should not interefere with themsselves because you have forced them into a postion definite state. Setting up a double slit on axis b and observing no intereference would be a good indication the other photons are being measured given most of the photons are being down converted and are entangled.
    Last edited: Jun 9, 2008
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