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How to certify that the proper legth of light is zero

  1. Mar 20, 2017 #1
    1. The problem statement, all variables and given/known data
    I'm a new one on general relativity
    there are two prolems:
    first how to certify the proper length of light is zero
    second ,how to certify proper length of light ,do not change in all inertial frame

    2. Relevant equations
    for second question, probally Lorentz transform,

    3. The attempt at a solution
    for the first
    ds2=-c2dt2+dl2
    dl/dt = 0
    ds=0
    I have no idea on Lorentz transform of delta s
     
  2. jcsd
  3. Mar 21, 2017 #2

    phinds

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    Gold Member
    2016 Award

    Proper length is the length of an object in its rest frame. Light doesn't HAVE a rest frame so doesn't have a proper length, unless you're using some different definition.
     
  4. Mar 21, 2017 #3
    Yeah, from the moment a photon is created, it is instantly and always moving at c.
     
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