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Homework Help: Speed of light pulse if you have a velocity of c

  1. Feb 9, 2010 #1
    1. The problem statement, all variables and given/known data

    A light pulse is emitted in an inertial frame. You are moving at a constant velocity of c AWAY from the pulse, and initially 10m away from the pulse. Will the pulse reach you, and what velocity will you measure for c?

    2. Relevant equations

    3. The attempt at a solution

    Well I know that c is invariant in inertial frames so I'm going to say yes the pulse will reach me and it'll be travelling at velocity c, but surely this is the most illogical answer ever? My classical brain wants to say, No the pulse will not reach me and will remain 10m away. Any thoughts? Thanks
  2. jcsd
  3. Feb 9, 2010 #2


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    Who wrote this?? :confused:
    This makes no sense.

    Nothing happens in a frame.

    A frame is what you observe things from. The same thing can be observed from every possible frame (both inertial and non-inertial). This light pulse, however it was created, can be observed in every possible frame.
    This is the opposite of making no sense. This is so inevtiable true that it contains no information at all. Your speed will always be c relative to light. And if you're moving away from the light, then it's moving away from you.
  4. Feb 9, 2010 #3
    But how is it possible for the light pulse to catch up to you if you're 10m ahead of it and both travelling a c?
  5. Feb 9, 2010 #4
    Just to clarify, by light pulse, I mean for example a bulb is switched on and off sending out a sphere of light.
  6. Feb 9, 2010 #5


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    You can't travel at c relative to the bulb.

    The light travels at c relative to everyone. It inevitably travels at c relative to you, and you inevitably travel at c relative to it, even if you're just standing there.

    Assuming the bulb emits light in only one direction, it will either miss you or hit you, and it will only hit you if you're in the way! :rolleyes:
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