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Can information travel faster than light?

  1. Jun 5, 2008 #1
    Let's say theres a stick, and two persons are holding the extremities:

    Person A ----------stick---------- Person B

    Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

    Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

    I know this can't be true, but I fail to find the answer.
    Thanks in advance.
  2. jcsd
  3. Jun 5, 2008 #2


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    The rate of information about the pull or lack of it usually propagtes at the speed of sound in the medium involved. Think of the stick as being a very dense slinky.

    I seem to recall some sort of instant "communication" between pairs of electrons, such that some obverved effect on one of the electrons causes an immediate repsonse by the other (maybe it was the spin)? This couldn't be used to transfer information, but it was a "faster than light speed" response between two particles, although I don't remember the details. Perhaps someone could respond with a correct explanation?
    Last edited: Jun 5, 2008
  4. Jun 5, 2008 #3
    And you must remember that at small distances the speed of sound might as well be the speed of light when compared to the sensitivity of human perception. I.e. synaptic operations peak around 120 m/s.
  5. Jun 6, 2008 #4
    absolute rigidity

    I have seen the same problem in the context of absolute rigidity of the stick a concept not in accordance with SR
  6. Jun 6, 2008 #5
    Hello Mafarazzo.


    ----Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.----

    I think this may not be true but my knowledge is insufficient to say for certain. It needs some consideration.

  7. Jun 6, 2008 #6


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    It only seems to be instant to human perception. If you think of a spring instead of a stick it's quite easy to see the information about the 'release' travels at the speed of a wave in the spring. With a stick the information travels at approximately the speed of sound (sound wave) in the stick. It's far, far slower than c.
  8. Jun 6, 2008 #7
    Consider what a stick is made of. Several atoms pushing and pulling against each other. When person B lets go, A does NOT observe it instantly. Rather, he observes it in the time it takes for the "information" to travel across all of the atoms. While this might seem instantaneous, it actually happens at the speed of light, via photons.

    Of course, one could rewrite the problem to be with say, an electron, and to that I don't have an actual answer. It might be, simply, that motion travels through a completely rigid body at the speed of light.
  9. Jun 6, 2008 #8
    Last edited: Jun 6, 2008
  10. Jun 6, 2008 #9


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    No (to both parts!). I have no idea what would lead you to think such a thing.
    I'm not sure why you would think that either. The wavelength of "light" used by a microwave oven is on the order of 12 cm. The effect of the screen across the door is quite similar to trying to kick a soccer ball through a chain link fence.
  11. Jun 6, 2008 #10
    Quote from http://en.wikipedia.org/wiki/Speed_of_light#Faster-than-light_observations_and_experiments:
    "It is generally considered that it is impossible for any information or matter to travel faster than c. The equations of relativity show that, for an object travelling faster than c, some physical quantities would be not represented by real numbers."

    Correct me if I'm wrong, but I don't think there is such a thing as a completely rigid object. The wave would propagate through the stick at the speed of sound of that medium, not at c.

    I think this is quantum entanglement. See this: http://en.wikipedia.org/wiki/Quantum_entanglement
    Last edited: Jun 6, 2008
  12. Jun 6, 2008 #11
    If two entangled particles will change when the other does and we have the instruments to detect this change then it is information. Think of the particles two states as on and off 1 and 0. there is no reason that information could not be transfered like this.. Also if we detect the change then that is information in itself because we now know the state of its entangled partner has been changed
  13. Jun 6, 2008 #12
    GRB, are fundamental particles not completely rigid? If they aren't, how so?

    Neh, entanglement cannot be used to transfer information. Say Alice and Bob share a pair of entangled photons. If Alice makes a measurement on her photon, that means that Bob will always find a certain thing when he measures his. However, he cannot tell without communication with Alice whether or not she did anything.
  14. Jun 6, 2008 #13
    Is this not similar to:

    - Telling Alice and Bob that they will each receive a sealed box, with one containing a blue ball and one containing a red ball. After walking 1000 paces away from each other, they are allowed to open their box and determine their ball's colour. Alice opens her box and sees that it's red, and so Bob's ball must be blue. That's not instantaneous information transfer over 2000 paces worth of space, but plain old mutual exclusivity. The owner's ball colour was determined at the event of entanglement, not at the event where the box was opened. My assumption here lies in the fact that quantum states cannot be shared due to the Pauli exclusion principle, hence mutual exclusivity by default.

    My understanding of quantum mechanics is pretty primitive, so I must repeat that this is a question, not a statement.
    Last edited: Jun 6, 2008
  15. Jun 6, 2008 #14


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    No, it's definitely not like that shalyaka. If we assume that it is, we can use that assumption to prove mathematical relationships called Bell inequalities. Experiments have been performed to test them, and the results are consistent with quantum mechanics and contradict the Bell inequalities.
  16. Jun 6, 2008 #15


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    There are no "completely rigid" objects of non-zero size in SR. It's fairly easy to see this by trying to figure out how such an object would look in two different inertial frames before and after a boost.

    A classical point particle would be rigid (because its size is exactly zero in every frame), but the interactions between them can still only propagate at maximum speed c.
  17. Jun 6, 2008 #16
    So particles being rigid is only permissible in SR because they have zero size?
  18. Jun 7, 2008 #17


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    I'd rather say that the concept of "rigid" doesn't make sense for objects of zero size.

    Also, note that I was talking about classical point particles. The concept of "size" is tricky when we're talking about quantum particles. The closest thing to a "size" they can have is the width of a certain function, and the shape of that function can't be rigid. (The argument is the same as for any object).
  19. Jun 16, 2008 #18
    Nothing Moves Faster than "C"

    Wrong this transfer of information was at the speed of light, not faster than it. It happens whens photons (and possibly electrons) are ejected from the same atom and are sent across (in this case it was done at 6.2 miles apart) and yet they feel each other's energy states.

    However this wasn't done faster than "c". Believe, it would have made tremendous headlines had it did.
  20. Jun 17, 2008 #19
    Although information can not be trasmitted faster then the speed of light I believe it can be recived from distances further then 300,000,000 m in less then 1 second. (You can not break the laws of physics, but you can work around them.)
  21. Jun 17, 2008 #20
    I recall a paper by Einstein on this subject. The idea is that there are no rigid bodies in existance. In your description above it appears that you're thinking of the rod as a rigid body.

    Regarding information traveling faster than light (ftl). There have been some articles in the physics literature about superluminal signals which is accomplished through the use of entagled states (a quantum phenomena). I can find some references to these articles if you'd like?

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