I am a bit confused at "orthonormal tetrad" in General Relativity...
I think orthonormal tetrad should be a set of vectors like
[FONT="Georgia"]e0= (1,0,0,0)
e1= (0,1,0,0)
e2= (0,0,1,0)
e3= (0,0,0,1)
However, in my book, it is written as
[FONT="Georgia"]e0= (-1,0,0,0)
e1=...