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B Relative speed for two beam lights

  1. Mar 2, 2017 #1
    here "c" is the speed of light ,

    i have a question in my mind and i have no answer for it ,
    if two light beams are moving in opposite directions then their relative speed is simply c using the law of velocities addition in case of special relativity
    but what if those two light beams are moving in the same direction will their relative speed be also c or zero
    it seems to be zero using the law of velocities addition but my friend claims that it should be c , is that true and how can it be c logically

    thanks :)
  2. jcsd
  3. Mar 2, 2017 #2
    Are you asking how fast a beam of light travels from the perspective of another beam of light?

    The answer is that there is no answer, because light doesn't have a "perspective."

    In order to speak of how fast something is traveling relative to an observer, it must be possible (in principle) to measure the speed of that something when you're at rest relative to the observer. But we can never be at rest relative to anything that travels at ##c##. Light simply cannot be an "observer."
  4. Mar 2, 2017 #3


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    Also note that the velocity addition formula yields 0/0 for the relative velocity of two light beams in the same direction, telling you there is a problem.
  5. Mar 2, 2017 #4
    what if i am riding one of the light beams ?
    in other words if i am riding one of the beams that moves in the same direction of another beam what will be the speed of the other beam ?
  6. Mar 2, 2017 #5


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    You can't travel at the speed of light unless you're a massless object , in which case you can only travel at the speed of light.
  7. Mar 2, 2017 #6


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    You can't. If you were "riding a beam of light", then you would be moving at the same speed as that beam of light so its speed relative to you would be zero - but that's impossible because the speed of light is always ##c## for all observers.
  8. Mar 2, 2017 #7

    Mister T

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    No, it isn't. That law relates the speed in one frame to the speed in another, given the relative speeds of the two frames. That relative speed cannot be ##c##. The conditions under which the law is derived forbid it. In other words, if it were indeed possible for one frame to have a speed ##c## relative to another that law wouldn't be valid.
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