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Spherical symmetry implies that the metric has time-independent Schwarzschild geometry, even if a central mass is undergoing gravitational collapse (Misner et al. 1973; see [[Birkhoff's theorem (relativity)|Birkhoff's theorem]]). The [[Metric tensor (general relativity)|metric]] thus has form
:<math>ds^2 = - (1-2M/r)\, dt^2 + (1-2M/r)^{-1} \, dr^2 + r^2 \, d\Omega^2</math>
(using [[Geometrized unit system|geometrized units]], where <math>G=c=1</math>). For <math>r>R>0</math> (where <math>R</math> is the radius of some mass shell), mass acts as a [[delta function]] at the origin. For <math>r<R</math>, shells of mass may exist externally, but for the metric to be [[Singularity (mathematics)|non-singular]] at the origin, <math>M</math> must be zero in the metric. This reduces the metric to flat [[Minkowski space]]; thus external shells have no gravitational effect.
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