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A thought Experiment on velocity greater than C

  1. Aug 25, 2009 #1
    Supoose at the center of A huge cylinder of radious say 1km we place a high speed motor of say angular speed w > 3E5 . And in the motor we attach a laser directed towards the walls of the cylinder. When we rotate the motor with the laser lighted, won't the laser point move along the cylinder walls at speed greater than the speed of light?

    (I have attaced a drawing)

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  3. Aug 25, 2009 #2
    Yes. "Points" are free to move at any speed. Of course each photon travels at c, and no photons (or masses) are moving "along the cylinder wall".
  4. Aug 25, 2009 #3
    THEN this means "something" can travel faster than light as opposed to the famaous quote
  5. Aug 25, 2009 #4


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    Correct, that famaous quote is wrong. It is an oversimplification.
  6. Aug 25, 2009 #5
    That famous quote is simply not true in SR or GR. It's just a faulty paraphrase.

    The speed limit of c is for particles relative to inertial reference frames only. In your example, no particle exceeds the speed of light.
  7. Aug 26, 2009 #6
    Sorry I didn't understand the experiment quite clearly. But acc. to my knowledge optimum speed is the speed of light.
  8. Aug 26, 2009 #7


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    In common language "optimum" means "best"! In any case everyone here is agreeing that no physical object (as opposed to a spot of light on a wall) can move faster than light.

    If I aim a very, very bright laser at the moon, and then move it through a small angle, the spot of light it makes on the moon will appear to move across the face of the moon faster than the speed of light. That's the basic concept of the initial post. But neither the laser nor the photons making up the light has moved faster than the speed of light.
  9. Aug 26, 2009 #8
    What is the "something" that is traveling faster than c? The points at which the photons strike the cylinder? That's not really a "something".
  10. Aug 26, 2009 #9
    Wait a minute. If we consider the spot of light as some sort of information,
    Doesn't this also implies that information has traveled faster than light ?
  11. Aug 26, 2009 #10


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    No, the information in the spot has traveled from the laser to the wall at the speed of light. No information traveled from one point on the wall to another.
  12. Aug 27, 2009 #11
    These types of reasoning are largely referred to a gorgian arguments. It's like saying "If you are a father, then you must be the father of everything, because you cannot not be a father." It's semantic.
  13. Aug 27, 2009 #12


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    Think of it this way. One have person standing where the spot of light starts and one that is standing where it moves to. There is no way for the person at the first position to affect the spot in any way that would allow him to use it to send a signal to the second person.
  14. Aug 30, 2009 #13
    Yeah, I have now understood this. Thanks Janus.
  15. Aug 31, 2009 #14
    What about if there weren't two people, but 1guy moving the laser and on the other side some sort of a digital sensor-platform for example, that would recognise the movement of the laser from one point to the other as one bit of information?
    Last edited: Aug 31, 2009
  16. Aug 31, 2009 #15


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    So? What about it?
  17. Aug 31, 2009 #16
    Anyway one thats not transferring information faster than light. No one on the cylinder wall can control the laser.
  18. Sep 1, 2009 #17
    Sorry, I didn't read the whole thread. I read this:

    and I thought that if you'd have a laser point fixated on the moon, then even a slow movement of the laser on Earth would make the point move very fast(possible FTL) on the moon. And if something would recognise just the smallest movement of the laser in one direction as information, then it(info) would travell FTL. But rereading what HallsofIvy said, the laser only appears to be moving FTL but that doesn't acctualy happen on the face of the moon? Could somebody clear this up for me?
  19. Sep 1, 2009 #18
    Do one thing. Arrange two bulbs at a separation of 3e8m. Light one bulb and then turning this off, light another bulb at t=0.5 sec.
    You are now reasoning that the information from one bulb has traveled faster than light to the other bulb.
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