* {{cite book | last=Rindler | first=Wolfgang | title=Relativity: special, general, and cosmological | publisher=Oxford University Press | publication-place=Oxford ; New York | date=2001 | isbn=0-19-850836-0}}
<ref name="y903">{{cite book | last=Rindler | first=Wolfgang | title=Relativity: special, general, and cosmological | publisher=Oxford University Press | publication-place=Oxford ; New York | date=2001 | isbn=0-19-850836-0 | page=}}</ref>