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Faster than the speed of light paradox

  1. Apr 12, 2011 #1
    Ok, so I'm not a physicist by any means, and I don't actually understand the first thing about physics. So, I'm guessing that somebody's probably realized this before, and I just want to know the name of this "paradox" (i'm not even sure it's the right word) so I can read up on it.

    Here's the thing: light would take 1 whole year to travel the distance of 1 light-year. So even if we had communication that could travel at the speed of light, it would take 1 year to cover that distance.
    So we got this epiphany, where if you built a (hypothetical) stick that's 1 light-year long, and you tapped it on that planet that's a light year away, you could communicate much faster than the speed of light. Right? :confused:

    So what's this called, and what other information can you give me about it? Thanks for the info guys!
  2. jcsd
  3. Apr 12, 2011 #2


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    Wrong. The tap would travel through the stick at the speed of sound for the material that the stick is made from, and the speed of sound through any material will always be less than the speed of light.
  4. Apr 12, 2011 #3
    I think he is saying something more to the effect of having the end of the stick dipped in ink and put next to a piece of paper one light year away. Then when you push the stick it should also be pushed against the piece of paper, and you could write morse code or something else.
    I think this is a quite good thought by the way. I have never thought of it. However you may think that the stick would instantaneously move as you push it, but I am thinking that your pushing will cause a compression of the stick which will travel at some speed... actually perhaps the speed of sound. For small sticks we of course would never notice such a thing, but i'm thinking that something along those lines would explain it. But perhaps i am wrong.
  5. Apr 12, 2011 #4
    Alex, that's exactly what i meant. i mean physically pushing the stick, not tapping it so the vibration travels THROUGH the stick. jesus, i've always been bad at physics but not THAT bad.

    so if you pushed the base of the stick (from earth), wouldn't the end of the stick (1 light-year away) move instantly? even if you, say, swung it, it would end up like the classic "2 points on an LP" thing. the end of the same object would still move as quickly as it was pushed on earth....i think.
  6. Apr 12, 2011 #5

    Doc Al

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    Push it or 'tap it', it still takes time for the mechanical pulse to travel from one end of the stick to the other.

  7. Apr 12, 2011 #6
    Well in our experience, when we push something, the object seems to move all at once. But on big scales, the fact that the object is made of atoms will come into play. When we push the stick, this compresses the atoms that are on the end of the stick that we pushed. I am thinking that explanation lies in the idea that the other end of the stick will not move until the compression wave that you have caused reaches the other end, and that the compression wave will travel at some velocity lower than light.
  8. Apr 12, 2011 #7

    Doc Al

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    Exactly. Typically it will travel at the speed of sound in the material, much lower than the speed of light. (As Janus has stated.)
  9. Apr 12, 2011 #8
    wow guys, that's fascinating. thanks for the replies, it was worth a shot huh?
  10. Apr 12, 2011 #9
    also, if that last post came off as sarcastic for any reason, it wasn't. that really is fascinating.
  11. Apr 12, 2011 #10


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    Yep. In addition, a push should arrive much faster than a side to side motion, since the velocity of a compression wave will be substantially higher than a lateral wave. So, pushing the stick would result in a faster transmission than swinging the stick would, but both would be substantially slower than light.
  12. Nov 2, 2012 #11
    sry for bumping an old thread but i have 2 questions that relate to the OPs original questions...

    i understand perfectly why pushing a very long stick or rod extended across a large distance would not result in faster than light communication....but i was wondering what about rotating/spinning a rigid rod? would spinning a cylindric rod also rely on a compression wave? hope this isnt such a dumb question. thanks guys
  13. Nov 2, 2012 #12


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    It is a different type of wave, but it is transmitted in the same way, so it would have the same speed (speed of sound).
  14. Nov 3, 2012 #13
    Why is it the speed of sound that it is transmitted at has anyone done an experiment to show that this is conclusuve. A stick is made of matter and gravity travells at the speed of light why the delay.
  15. Nov 3, 2012 #14

    D H

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    There is no such thing as a rigid body. Treating something as a rigid body is a simplifying assumption that makes the math much simpler.

    In reality, the stresses that would build up inside the rod would cause the rod to tear itself into pieces long before any part of the rod was moving even close to the speed of light. So what if we make the rod out of unobtanium? First off, there is no such substance as unobtanium, so we're verging into the "what do the laws of physics say would happen if we violate the laws of physics" territory. That said, even unobtanium has it's limits. Read up on Born rigidity.
  16. Nov 3, 2012 #15

    What about a rod made of neutronium? :devil:
  17. Nov 3, 2012 #16


    Staff: Mentor

    We have a FAQ specifically on this topic
    https://www.physicsforums.com/showthread.php?t=536289 [Broken]
    Last edited by a moderator: May 6, 2017
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