Weyl scalar
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The Weyl scalars form a set of five scalar quantities,
,
describing the curvature of a four-dimensional spacetime. They are part of the Newman-Penrose Formalism for general relativity.
A particularly important case is the Weyl scalar Ψ4, which is defined as
It can be shown to describe outgoing gravitational radiation (in an asymptotically flat spacetime) as
Here, h + and 
are the "plus" and "cross" polarizations of gravitational radiation, and the double dots represent double time-differentiation.
For more details, see the article on the Newman-Penrose Formalism.
