Black Holes as Landauer-Saturating Erasure Channels: Horizon Diagnostics, Area Quantization, and the Measurement Complement

We develop a quantitative framework in which black holes function as erasure channels for exterior classical records at the horizon-side Landauer bound. At asymptotic infinity, greybody scattering turns the full radiation channel into a dissipative filter whose entropy efficiency η_∞ ≡ |dS_BH|/dS_rad is field-content dependent; for the spin-2 (graviton) channel in Schwarzschild evaporation, η_∞ ≈ 0.74 (Page 2005). Starting from the Cortês–Liddle result that Hawking evaporation saturates the Landauer principle, we make three contributions. First, we define a Landauer saturation ratio R_L as a bookkeeping diagnostic for horizon thermodynamics: Schwarzschild black holes yield R_L = 1 exactly, while the cosmological apparent horizon yields R_L = 1/2 in Trivedi's quasi-local energy accounting. Second, we show that within the standard Bekenstein–Hawking area law and the discrete transition model of Bagchi, Ghosh, and Sen, one-step Landauer-saturating area transitions select the Bekenstein–Mukhanov spacing ΔA = 4 ln 2 l_P², this discrete compatibility result complements, rather than derives, the continuous holographic scaling S ∝ M². Third, we argue that the black hole scrambling time t_* ~ (ℏ/2πk_BT_H) ln(S_BH/k_B) provides a partial gravitational analogue of the reversibility time τ_c in quantum measurement: for an old black hole it sets the delay after which newly injected information can begin to reappear in Hawking radiation. We formalize the horizon as an effective coarse-grained erasure channel within fixed-charge sectors via a semiclassical proposition that combines a strict exterior coarse-graining definition with GSL-compatible entropy bookkeeping and horizon-side first-law accounting. We check the supporting identities numerically across the Schwarzschild, Kerr, and Reissner–Nordström parameter spaces, and analyze robustness to the memory burden effect. The framework positions black holes as the thermodynamic complement to quantum measurement: measurement creates classical records by paying Landauer costs; horizons erase exterior access to those records at the quasi-static Landauer limit, while the asymptotic Hawking channel is greybody-dissipative.