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But onto the main reason for my post: In the same notation as above, given a timelike curve parametrized by ##\tau##, the tangent vector ##u^a## is defined as the ##4-##velocity of the curve. Wald says that it follows directly from the definition of ##\tau## that the ##4-##velocity has unit length: ##\eta(u, u) = -1##. I don't see how this is true?