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Transmitting Information faster than light?

  1. Aug 27, 2012 #1
    Hi everybody. I have always wondered something and have not been able to get an answer.
    Let us suppose we have a 300.000 km long straight rod. It is of graphene, or any very tough material, of absoulte stiffness, without any displacement or absorption between its molecules.
    Parallel to the rod I have a laser light. The laser is in the A end of the rod. In the B end of the rod there is a button, the rod is touching the button but not pushing it.
    Aside the button, there is a optical sensor that can detect the laser light.
    If I push the A end of the rod and simultaneously turn the laser on, the sensor will receive the siganl one second later, and the button in the B end of the rod... wil be pushed instantly?
  2. jcsd
  3. Aug 27, 2012 #2


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    Gold Member

    The button will be pushed by the rod a MUCH longer time since the "push" will traverse the rod at the speed of sound in graphene, whatever that is.
  4. Aug 27, 2012 #3


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    Trouble is, there are no infinitely stiff materials. Signal will propagate through the rod at the speed of sound for such a rod. Which can be as high as few kilometers per second, but that's still about 5 orders of magnitude slower than light.

    As an absolute limit on material stiffness, keep in mind that atoms within the material are still held together by electromagnetic forces. So even if we push the envelope of physically possible and look at an absolute upper limit, it's still the speed of light.

    There are ways to beat speed of light limit. They all have to do with the fact that it's a local limit, not a global one.
  5. Aug 27, 2012 #4
    Also a 300k Km long infinitely stiff rod will have mass that will require a set amount of energy to push it. Any mass that "can" be pushed at the speed of light will invariably compress the rear of the mass into a state of fusion as the front will remain static and will in all probability result in a fusion explosion. IMHO anyway!
  6. Aug 27, 2012 #5

    Doc Al

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    Staff: Mentor

    Please take a look at our FAQ entry on this topic: Can I send a signal faster than light by pushing a rigid rod?
  7. Aug 27, 2012 #6
    Ok, K2's reply is the best, I believe. I had taken into account the sound speed of the material, but not the fact that, ultimately, the electromagentic forces keep the molecules united. I didn´t know, either, it was a common question, as the link to the already explained subject shows.
    Thank you all very much.
  8. Aug 27, 2012 #7


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    You had assumed infinite stiffness, and therefore infinite sound speed of the material. But even graphene doesn't have infinite sound speed. (I'm not trying to sound condescending, just this is how I would explain what happens).
  9. Aug 28, 2012 #8
    Yes Bruce, you are right. Thanks to all for your time.
  10. Aug 28, 2012 #9
    I have wondered this myself. I have hypothesized that perhaps the molecules that make up the rod behave more like marbles than expected. Instead of them being fixed to each other, perhaps they push each other, and creates a domino effect, that does not transmit the information faster than light.

    That's just my hypothesis.
  11. Aug 28, 2012 #10


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    Staff Emeritus
    Science Advisor

    That's pretty much correct; the effect is more obvious in softer materials or liquids.
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