 Irreducible mass

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.