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A question about the speed of light puzzles me

  1. Aug 18, 2008 #1
    if there is a light source rotate with the earth (w), so there must be somewhere in the

    universe(r) the speed of the shadow of the light (v=r*w) would exceed the speed of light

    because the r could be very large. Is it right?

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  2. jcsd
  3. Aug 18, 2008 #2
    If I'm understanding you correctly, you are saying that the shadow that is produced from the pole blocking the light source, would travel greater than c.

    First of all, the shadow is just the absence of light surrounded by light, or it could just be defined as empty space. Further more, if you defined the shadow as an object, it would have 0 mass.
  4. Aug 18, 2008 #3


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    Shadows can certainly move faster than the speed of light. This does not break any laws of physics, because it cannot be used to communicate any information faster than the speed of light.

    - Warren
  5. Aug 18, 2008 #4
    Right, any change at the source would still need to travel at light speed before it could be seen by the viewer of the "shadow."

    Keep in mind that no physical object in your scenario is actually moving faster than light. The photons are still mvoing from the source to the distant viewers at light speed. The "shadow" is the absence of light, which is itself not a physical thing that's moving any more so than a dot on your computer monitor is physically moving across the screen.
  6. Aug 19, 2008 #5
    ok, I see.
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