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Relativistic velocity

  1. Jan 25, 2006 #1
    I would have another question. If I travel at the speed of light (or 99.9999999999....% of the speed of light) in a spaceship and i begin to run from the back of the spaceship to the front in direction of the spaceship movement, will I be moving faster than the speed of light?
  2. jcsd
  3. Jan 25, 2006 #2


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    As measured from the Earth frame (in which the spacecraft has the velocity indicated), you'd be measured to have a velocity strictly below c.
  4. Jan 25, 2006 #3
    You cannot just add velocities to get total velocity. Use:
    w = (u + v)/ (1 + uv/c2)
    If you define all the velocities in terms of light speed by dividing each one by “c” the formula is simpler and becomes:

    w = (u + v)/ (1 + uv)

    You’re more accustomed to living and working with speeds less than 0.0005
    Where doubling that speed gives you
    .001 / 1.00000025

    You’ve just never needed the accuracy of dividing by such a small number at those small speeds. And just used the .001 part or (u + v).

    But as one of the speeds becomes high, say above .25
    Then using the whole formula and dividing becomes important.
  5. Jan 25, 2006 #4
  6. Jan 25, 2006 #5


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    The .... means the 9's go on forever? That means you are traveling exactly the speed of light. This is impossible, unless, like a photon, you have zero mass.
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