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Speed of light

  1. Jun 21, 2006 #1
    say your traveling in a space craft at relatavistic speeds, about .700c and you shoot a laser out of the space craft (3.00 x 10^8 m/s foward). How was does the light travel. If you do the same thing but shoot it out the abck the oppossite way how was does the light travel.

    This was a question on my exam this morning
    the light out of the front would travel at 3.00 x 10 ^ 8 m/s correct? becuase the speed of light cannot be broken.
  2. jcsd
  3. Jun 22, 2006 #2


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    Strictly speaking, the question is not well posed since it doesn't say whether the speed is to be taken relative to you or the person with relative to whom you are moving at 0.700c. It really dosn't matter, the light will, of course, travel at c, about 3 x 10^8 m/s, whether it is directed forward or backward, whether relative to you in the space craft or relative to the person who sees you moving at 0.700c. If you want to do it the hard way, remember that the formula for adding relativistic speeds is
    [tex]\frac{u+ v}{1+ \frac{uv}{c^2}}[/tex]
    so that if u= c, the speed is
    [tex]\frac{v+ c}{1+ \frac{cv}{c^2}}= \frac{v+c}{1+ \frac{v}{c}}= \frac{(v+c)c}{v+c}= c[/tex]
  4. Jul 3, 2006 #3
    Well in my understanding of the matter we always travel at 0c.
    The speed of light is always c, we cannot catch up or approach it.

    The exam question was worded poorly, if should have said something like travel at 0.700c relative to a particular object in space. You cannot measure speed relative to the speed of light since Einstein's theory proved that the speed of light remains constant.
  5. Jul 3, 2006 #4


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    I think the question was probably worded that way on purpose. Only because lightspeed is constant can the question be asked that way.
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