It can be expressed as
Null Tetrad
See also
TetradExplore with Wolfram|Alpha
References
d'Inverno, R. Introducing Einstein's Relativity. Oxford, England: Oxford University Press, pp. 248-249, 1992.Referenced on Wolfram|Alpha
Null TetradCite this as:
Weisstein, Eric W. "Null Tetrad." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/NullTetrad.html