Trouble with a question in gravitation of MTW

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SUMMARY

The discussion focuses on a problem from the gravitation textbook by Misner, Thorne, and Wheeler (MTW), specifically exercise 2.5 on page 65, which involves a particle of rest mass m and its 4-momentum as observed by an observer with 4-velocity u. The key conclusion is that the energy E of the particle can be expressed as E = -p * u under the -1, 1, 1, 1 metric. The misunderstanding arises from incorrectly equating the four-velocity of the particle with that of the observer, leading to confusion regarding the gamma factor in relativistic equations.

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Homework Statement


It is a problem on gravitation of MTW pg 65 exercise 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.
 
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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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