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Attaining speed of light

  1. Mar 3, 2010 #1
    Every one know that the speed of light "c" is unattainable but IS half the speed of life is attainable?
    i.e 1/2(c)
    If we get two bodies of 1/2(c) speed then by moving these bodies in opposite direction the spees of light is attainable?
    Is it possible?
  2. jcsd
  3. Mar 3, 2010 #2


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    A speed of 0.5c is attainable, but velocities don't add up like that in special relativity. Instead of w=u+v, the rule is


    With u=-0.5c and v=0.5c, you get w=c/(1+0.25)=0.8c.
  4. Mar 3, 2010 #3
    I dont know about the details of relativity but know about it,s results.
    Is from this equation the velocity of light is attainable or not by the motion of two bodies having speed less than C
  5. Mar 3, 2010 #4


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    It's not. If u<c and v<c, then w<c.
  6. Mar 3, 2010 #5
    Since this is the Relativity section, think of the situation in terms of Special Relativity.
    The 2nd postulate says that all observers measure the speed of light to be the same c.
    So it doesn't matter to me how 'fast' any other object is traveling, I always measure c as c. I could accelerate for a long time, making me go a lot faster than other objects, doesn't change anything - we will each continue to measure c.
    A consequence of this idea is that, as stated above, the (relative) speeds of individual moving objects does not add algebraically.
  7. Mar 4, 2010 #6
    yes. Fredriks answer is precisely correct as observed from either u or v,

    but if these are the speeds of spaceships relative to earth, an observer will see the ships moving apart at speed c....but if each ship emitted light, the light from each would be observed at speed c among all observers....
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