# Question:Working with metrics called from Stephani et al.

## Question:Working with metrics called from Stephani et al.

Maple

If I call for the metric (27.27) in Stephani et al. in the Physics package, I expected the null tetrad employed to compute say the Weyl scalars would be the one given in conjunction with (27.27) (equation (27.22) in Stephani et al.). But, Weyl[scalars] returned long expressions for each of the five scalars, whereas with respect to the null tetrad in Stephani et al one expects the first two (or last two) scalars to be zero since the null vector k in their tetrad is the multiple pnd of Weyl. e_[nullvectors] and Setup(tetradmetric=null) followed by e_[ ] seem to output the same null tetrad, which does not appear to be that of Stephani et al. I assume this null tetrad is the null tetrad associated to the orthonomral tetrad that e_[ ]  returns if one hasn't used Setup(tetradmetric=null). How does Maple select this default orthonormal tetrad? What is the best way to set the null tetrad of Stephani et al as the null tetrad to compute Weyl[scalars]?

Here is a simpler example. Calling [27, 37, 1]. Stephani et al give the null tetrad in terms of the spacetime coordinates along with the metric in their equaiton (27.37). After using Setup(tetradmetic=null), e_[ ] returns a tetrad that might, I suppose, be that of Stephani et al. in disguise. Specifically, the tetrad vector defining the null congruence should have only a component with respect to the coordinate r, yet the Maple output gives expressions for the components with respct to the u and r coordinates for both the k and l elements of the null tetrad (while the expressions for the complex element m is exactly what one would expect). As in the previous example, all Weyl scalars are given by nontrivial expressions, even though two of them should be zero (since k is a multiple pnd). So is it the case that the experssions in the Maple output where one expects zero are in fact zero in disguise? The experssions are complicated enough that it is not obvious.  I have uploaded the Maple document for these calculations. SKMHH27_37.mw ﻿