# Problem with notation

1. Oct 30, 2012

### Federico

Hi everyone!

I have a stupid question. It's about the notation used sometimes in the Covariant Derivative.

For example, if somebody tells me:

"If you have a Killing Vector Field $\xi$ with timelike orbits, you can define a time coordinate satisfying $\nabla_{\xi}t=1$"

what does this last equation means?

I suppose that it has something to be with the normalisation (using +--- signature) , but how do i get the usual $\xi_{a}\xi^{a}=1$ notation from it?\$