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Lightspeed consistency

  1. Dec 9, 2008 #1
    The concept of light speed being constant is puzzling to me. For instance, if a space vehicle is moving through space at near light speed (~c) and emits a light ray at speed c, does the light ray move away from the vehicle at speed c or is the separation speed (c - ~c) ? (Ignore constaints that would prevent the vehicle from achieving near light speed) Also, what would an observer see - maybe, the light ray and vehicle moving at almost the same speed c and ~c ?
  2. jcsd
  3. Dec 9, 2008 #2


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    Consider that the earth is a space vessel traveling at near light speed with respect to distant galaxies. We measure c to be exactly that which is predicted by Maxwells equations. Why would any other space ship measure it any differently?
  4. Dec 9, 2008 #3

    Claude Bile

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    From what frame of reference? From the point of view of an outsider, the separation would be c - ~c, while from the point of view of the ship, the separation would be c.

    Note that these two cases are completely consistent with each other by virtue of the fact that time flows differently for each observer.

  5. Dec 10, 2008 #4


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    Velocities do not add in the "usual" way …

    (i'm putting c = 1, to make the equations prettier :wink:)

    adding velocity u to velocity v (in the same direction) gives w = (u + v)/(1 + uv).

    Put u = 1 (ie, light-speed), then w = (1 + v)/(1 + v) = 1 also. :biggrin:

    So a speed of 1 is seen as 1 by every observer, and the only visual difference is the colour (wavelength) of the light.​
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