Irreducible mass
= Irreducible mass
{title2=$M_{\mathrm{irr}}=\sqrt{A/(16\pi)}$}
The <irreducible mass> of a four-dimensional uncharged <Kerr black hole> is $M_{\mathrm{irr}}=\sqrt{A/(16\pi)}$. Its <mass> satisfies $M^2=M_{\mathrm{irr}}^2+J^2/(4M_{\mathrm{irr}}^2)$. <Hawking's area theorem> prevents a classical process from decreasing $M_{\mathrm{irr}}$ under the theorem's energy and global hypotheses.