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Velocity of laser light pt. on moon > c ?

  1. Oct 29, 2008 #1


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    Gold Member

    The earth moon distance = 3.8 * 10^8 meters
    Time for light to reach moon = 1.266 sec.
    A laser at the origin on earth pointed at moon rotates .001 radian
    with angular speed w = 10 radians/sec.
    The arc length distance described on moon ,s= (radians) (radius) =3.8*10^5 meters
    The velocity of the point on the moon described by the change in direction of the laser=
    v=(w)(radius) =3.8 *10^9 meters/sec
    So the time it would take this point to travel on the moon = distance/rate = 1/10^4 sec as well as .001 radian/10 radians/sec at the origin, being the rotation of the laser.
    Since the light takes 1.266 second to travel to moon then it does not seem possible for laser light point on moon to exceed c.
    For example in this case it took 1/10^4 seconds for direction of laser point to describe the path on the moon but when the laser is in this final position after rotating .001 radian it will take the light 1.266 seconds to travel to moon.
    To restate it seems that the actual velocity of the directed laser light point on the moon would not exceed c because of the time lag
    from earth to moon in light travel time .
    Last edited: Oct 29, 2008
  2. jcsd
  3. Oct 29, 2008 #2

    Doc Al

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    Staff: Mentor

    Sure, it took the light 1.266 seconds to travel to the moon, but the light that hit the "right" edge of moon arrived just 1/10^4 seconds after the light that hit the "left" edge (assuming you swept the laser from left to right). The travel time to the moon has nothing to do with the speed of the laser "spot" across the moon.
  4. Oct 29, 2008 #3


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    Science Advisor

    There is a fairly good discussion of this in the thread;

    "Lighthouse paradox revisited"

    It is three pages long, but most of the answers you seem to be looking for are on the first page.
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