# Printing mistake in A First Course in General Relativity?

1. Apr 9, 2010

### Rasalhague

Is is a typo? The 1985 edition of Schutz: A First Course in General Relativity, p. 52, equation 2.37, which is supposed to express the idea that "photons have spatial momentum equal to their energy":

$$\vec{p} \cdot \vec{p} = -E^2 + E^2 = 0.$$

Shouldn't this be

$$\vec{p} \cdot \vec{p} = E^2$$

or

$$E^2 - \vec{p} \cdot \vec{p} = 0?$$

2. Apr 9, 2010

### Rasalhague

Oh, sorry, my mistake. I just realised--seconds after posting this!--that he's using the symbol $\vec{p}$ for the energy-momentum 4-vector, rather than for spatial momentum.