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The Future is written?

  1. Aug 27, 2006 #1
    Lets say we mastered near lightspeed travel and instantaneous communication using quantum teleportation. Then we send the spaceship off to someplace 1 lightyear away. It turns around and for a brief moment travels twords earth at near lightspeed. During this moment the spaceship asks earth how things are goin and earth responds. The space ship reduces it velocity to 0 relative to earth now the spaceship has information about earth from 1 year in the future, because of the change in the alignment of its inertial frame, and has the ability to tell earth all about it.

    Can any1 see a flaw in that logic? Otherwise it seems that everythin is predestined.:confused:
  2. jcsd
  3. Aug 27, 2006 #2
    The first and foremost flaw is that you your conclusion begs the question. If and only if we ever master near lightspeed travel and quantum teleportation (these things may very well be impossible). I'm sure there may be other flaws but I'm too lazy to think about it (and I probably won't find them). But that is the biggest problem with your situation.
  4. Aug 27, 2006 #3


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    Fair enough, but this is only a difficulty in scale and possibility, not a flaw in logic.

    With what we know today, is there any fdlaw in the logic, and thus any reason it cannot be done in theory?
  5. Aug 27, 2006 #4


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    Quantum teleportation does not allow for FTL communication, even in theory--in fact I'm pretty sure it's been proven that quantum effects can never be used this way. So your thought-experiment is contradicting known laws of physics, although you're right that relativity says that if FTL communication were possible somehow, and it worked the same way in all inertial frames, then it would also be possible to send information into your own past.
  6. Aug 28, 2006 #5
    In 1935; Einstein, Rosen and Podolsky (EPR) presented a famous paper its title was “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” They considered what Einstein called the "spooky action-at a- distance communication” that seemed to be part of Quantum Mechanics, and concluded that the theory must be incomplete if not outright wrong. Bell's Theorem also seems to indicate the probability of faster than light (instantaneous) communication, I think it is too early to write off any of these concepts.

    The only problem with our spaceship is that if it travelled at very close to light speed for a distance of say fifty light years the communication that it would receive from Earth would be the 1956 Olympics from Melbourne, Australia.
  7. Aug 28, 2006 #6


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    Bell's theorem and the EPR results indicate that if there are hidden variables there must be a sort of FTL coordination (or backwards-in-time coordination, perhaps) between entangled particles, but most interpretations of QM do not feature hidden variables. If you use something like the many-worlds interpretation of QM, for example, than the results of the EPR experiment can be explained without the need for anything to travel faster than light. In any case, even if the particles themselves are somehow exchanging hidden signals faster than light, I'm pretty sure it's been shown that experimenters can never actually use these effects to send their own signals faster than light, in much the same way that even if you believe in hidden variables and the idea that particles actually do have well-defined positions and momentums at all times, the uncertainty principle shows that we can never measure them simultaneously to arbitrary accuracy unless the theory of quantum mechanics is totally wrong. For example, see quant-ph/9801014 for a possible proof that the no-cloning theorem forbids superluminal signalling, although this is disputed in quant-ph/980303 and I don't know enough about the subject to evaluate the arguments.
    But if you could send a signal that was FTL in the earth's frame, in the ship's frame the ship might recieve the signal at an earlier time than the signal was sent from the earth. And if the ship immediately sent a reply that was FTL in the ship's frame, in the earth's frame this reply might arrive at earth at an earlier time than the ship sent it, with it being possible that the earth would recieve the reply before it sent out the original signal. Of course this is assuming that FTL would work the same way in every reference frame, if there were a preferred frame for FTL signals (a frame in which no FTL signal can ever arrive before it is sent) then you could have FTL without violating causality, at the expense of destroying Lorentz-symmetry.
    Last edited: Aug 28, 2006
  8. Aug 28, 2006 #7


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    *slaps forehead* I knew that. :blushing:
  9. Aug 28, 2006 #8


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    What you have done is rediscover the theorem in SR that if you have FTL motion or communication - in your case instantaneous - then some observers will see you moving or communicating into your own past. It's a perfectly correct theorem, and when I say "see" I mean you really can get news from the future this way.

    That's SR. It has not yet been proven in QM or anywhere else that nature truly forbids FTL. In my opinion the QM conclusions are too broad for the narrow experimental result. Consider the delayed choice quantum eraser, on which there is a long thread in the Quantum Mechanics forum. Does it or doesn't it show the electron "knowing" what the future choice in the experiment will be?
    Last edited: Aug 28, 2006
  10. Aug 28, 2006 #9
    I know SR forbids ftl travel for an object with mass. Iv only just come to grips with SR so im really just speculating but i cant see how one would deduce that information also cant b sent ftl or even instantaneous.

    My understanding of QM is limited to the concepts and the basic 1s at that. From what iv read im quantum computation, information is stored in the state of the qubits.

    This site talks about Quantum Teleportation:


    So if like this the state of an atom can be teleported 2 the atoms (qubits) of the spaceship computer. couldnt that be used as instananeous communication. Im not 100% on the instantaneous bit.

    I think the practicality of near light speed travel is irrelevant, its possible
  11. Aug 28, 2006 #10


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    No, because quantum teleportation always requires a "classical channel" where information about the system being teleported is transported by normal classical means like a light signal--without this information the people at the other end won't know what measurements to perform to create the teleported quantum state. In the diagram on the page you linked to, the classical channel is represented by the white line titled "send data". They also refer to it indirectly in their description of the process (I put the part about the classical channel in bold):
    And the more detailed wikipedia article on quantum teleportation says:
    And the article on the "no communication theorem" linked to there is relevant to the subject of instantaneous communication too...at the end it says:
  12. Aug 29, 2006 #11
    I remember the "by applying to C a treatment depending on the scanned-out information" bit now!

    Does that mean their talk about a Quantum Internet is a load of BS? Considerin the isp guys would hav 2 phone ya up and tell you how 2 treat each qubit.

    (This has become more about QM than SR now!)
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