 Orthonormal coframe in Painlevé–Gullstrand coordinates

ID: orthonormal-coframe-in-painleve-gullstrand-coordinates

An orthonormal coframe is

e^{0} = d t, e^{1} = d r + \frac{2 M}{r} d t, e^{2} = r d θ, e^{3} = r sin θ d ϕ .

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