Trouble with a question in gravitation of MTW

[abcdefgzasd]

Homework Statement

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

Homework Equations

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

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.

 

[vela]

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.