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.