https://books.google.co.uk/books?id=wtoKZODmoVsC&hl=en J. Foster, J. D. Nightingale, J. Foster, J. D. Nightingale; J. Foster, J. D. Nightingale, J. Foster, J. D. Nightingale (2006). A short course in general relativity (3 ed.). Springer Science & Business. p. 55. ISBN 0-387-26078-1. , Chapter 2, page 55