In the first chapters of Schutz's "A first course in general relativity", the energy of a particle is defined to be E = p0 (e.g. in 2.20). But in the chapter 7.4 discussing "conserved quantities" in curved spacetimes, the energy is defined to be E = - p0 = - g0upu. After reading 7.4 it is not clear for me why p0 is used for the definition of E instead of p0, as done in the first chapters of the book. Any help would be appreciated.