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It is widely believed that exact global symmetries do not exist in theoriesthat admit quantum black holes. Here we propose a way to quantify the degree ofglobal symmetry violation in the Hawking radiation of a black hole by usingcertain relative entropies. While the violations of global symmetry that weconsider are non-perturbative effects, they nevertheless give $\mathcal{O}(1)$contributions to the relative entropy after the Page time. Furthermore, using"island" formulas, these relative entropies can be computed withinsemi-classical gravity, which we demonstrate with explicit examples. Theseformulas give a rather precise operational sense to the statement that a globalcharge thrown into an old black hole will be lost after a scrambling time.  The relative entropies considered here may also be computed using a replicatrick. At integer replica index, the global symmetry violating effects manifestthemselves as charge flowing through the replica wormhole.

Signatures of global symmetry violation in relative entropies and replica wormholes