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Trouble with a question in gravitation of MTW

  1. Apr 7, 2013 #1
    1. The problem statement, all variables and given/known data
    It is a problem on gravitation of MTW pg 65 excercise 2.5.
    A particle of rest mass m and 4-momentum is examined by an observer with 4 velocity u.
    Show E=-p*u under -1,1,1,1 metric

    2. Relevant equations
    Treating c=1, we have u*u=-1 and p=m*u.

    3. The attempt at a solution
    u*u=-1, we have p*u=m*u*u=-m which does not make any sense as I cannot see gamma factor below.
  2. jcsd
  3. Apr 7, 2013 #2


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    The mistake you're making is that you're taking the four-velocity of the particle to be the same as the four-velocity of the observer, so the particle is at rest with respect to the observer. It makes sense then that its energy is simply its rest energy.
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