Hermiticity is usually treated as a foundational axiom of quantum mechanics, guaranteeing real spectra and unitary time evolution. In this work we argue that Hermiticity is more naturally understood as a symmetry law as a consequence of the global conservation of an inner-product current. We show that in spacetimes admitting complete Cauchy surfaces without boundary flux, this conservation reduces to the familiar condition $H^{†}=H$ in the canonical inner product. However, in the presence of causal horizons—most strikingly in black hole geometries—this conservation law is obstructed for restricted observers. Tracing over inaccessible degrees of freedom inevitably yields completely positive trace-preserving dynamics with an effective non-Hermitian generator. Using quantum thermodynamics and the monotonicity of relative entropy, we demonstrate that the generalized second law may be reinterpreted as an entropy balance compensating precisely for the flux of inner-product charge through the horizon. The structure of Einstein’s equations, via the Bianchi identity and the Raychaudhuri focusing equation, provides the geometric mechanism underlying this balance. We also show that black hole ringdown could be a realistic observational probe for this idea, with it also being an avenue to provide quantitative upper bounds on the strength of horizon-induced inner-product flux. In this way, gravity, entropy production and effective non-Hermiticity are unified under a single structural principle, with Hermiticity emerging as the special case of globally conserved inner-product symmetry.

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• Alternative gravity theories
• Quantum aspects of black holes
• Quantum gravity