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B Information traveling faster than light

  1. Dec 18, 2016 #1
    I know there is a modern paradigm that states that nothing, including information, can travel faster than the speed of light. (With exception to quantum entanglement, but I'm not going to pretend like I know the exact details of that subject).

    Thought experiment.

    Imagine there are a series of gears in a row that span a much greater length than 3*10^8 meters. Now lets say you turn the gear at the beginning and it sets off a reaction to the other gears down line from the beginning. Could the last gear turn "instantaneously" or much under 1 second after the first gear is turned? Would this then be considered information being conveyed at faster than the speed of light? What are the limitations of such a device, and how much would friction or any other factors play a roll in the time delay from the first gear turning to the last.

    Now let's imagine a really long pole that is much longer that 3*10^8 meters. If you were to all of a sudden push that pole perfectly straight forward from one end, it seems like the other end would move at that exact moment because it is a rigid body. Would this be considered sending information faster than the speed of light, or even sending information instantaneously?

    Now, I'm sure I'm not clever enough to have created such an simple device that seems to defy the laws of physics, and to that end, I'd love for someone to tell me what I am missing.
  2. jcsd
  3. Dec 18, 2016 #2


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    Actions taken at one end of your gears or the pole travel at the speed of sound through the material, which is much slower than c.
  4. Dec 18, 2016 #3


    Staff: Mentor

    This is a pretty commonly asked question. Here is a good reference that discusses it


    The gears proposal would also be limited in the same way.
  5. Dec 18, 2016 #4
  6. Dec 18, 2016 #5


    Staff: Mentor

    Hmm, good question. Since movies play at like 22 or so frames per second, I would say that you could probably detect something at 100 ms for sure. The speed of sound in steel is about 6000 m/s, so you should be able to see it in a 600 m length.
  7. Dec 18, 2016 #6


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    The speed of sound in solid steel is somewhere around 6 kilometers/second, and the human eye can distinguish events separated by maybe a tenth of a second.... So you might think that under ideal conditions an alert person might just barely be able to perceive the time lag between pushing one end of a 600 meter steel rod and the other end moving.

    There's no relativity involved here, this is just about steel not being perfectly rigid. On the one hand, six km/sec is about one five-thousandth the speed of light, far too small for any relativistic effects to appear. On the other hand, the non-rigidity of a 600 meter steel rod is going to be very apparent; it will behave more like a steel cable, all stretchy and flexible, than a rigid bar.

    [edit: Dale beat me to it!]
  8. Dec 18, 2016 #7


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    Staff: Mentor

    Like these, for example...
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