GHP formalism

The GHP formalism (or Geroch–Held–Penrose formalism), also known as the compacted spin-coefficient formalism, is a technique used in the mathematics of general relativity that involves singling out a pair of null directions at each point of spacetime. It is a rewriting of the Newman–Penrose formalism which respects the covariance of Lorentz transformations preserving two null directions. This is desirable for Petrov Type D spacetimes, including black holes in general relativity, where there is a preferred pair of degenerate principal null directions but no natural additional structure to fully fix a preferred Newman–Penrose (NP) frame.

Covariance

The GHP formalism notices that given a spin-frame $(o^A,{\iota }^A)$ with $o_A{\iota }^A=1,$ the complex rescaling $(o^A,{\iota }^A)\to (\lambda o^A,{\lambda }^{-1}{\iota }^A)$ does not change normalization. The magnitude of this transformation is a boost, and the phase tells one how much to rotate. A quantity of weight $(p,q)$ is one that transforms like $\eta \to {\lambda }^p{\overline{\lambda }}^q\eta .$ One then defines derivative operators which take tensors under these transformations to tensors. This simplifies many NP equations, and allows one to define scalars on 2-surfaces in a natural way.

See also

• General relativity
• NP formalism

References

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