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

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 thermodynamically dissipative filter (in the sense of excess entropy production, not energy loss) whose entropy efficiency \( \eta_\infty \equiv |dS_{BH}|/dS_{\mathrm{rad}} \) is field-content dependent; for the spin-2 (graviton) channel in Schwarzschild evaporation, \( \eta_\infty \approx 0.74 \) (quoted here as the closest-to-saturation channel, not the dominant total-flux channel; Page [1]). Starting from the Cortês-Liddle result that Hawking evaporation saturates the Landauer principle, we make three contributions. First, we define a Landauer saturation ratio \( \mathcal{R}_L \) as a bookkeeping diagnostic for horizon thermodynamics, using exact-first-law black holes as the saturating benchmark and non-unity cases as the main diagnostic payoff: Schwarzschild black holes yield \( \mathcal{R}_L = 1 \) exactly, while the cosmological apparent horizon yields \( \mathcal{R}_L = 1/2 \) 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 \( \Delta A = 4\ln 2\, l_P^2 \); this discrete compatibility result complements, rather than derives, the continuous holographic scaling \( S \propto M^2 \). Third, we argue that the black hole scrambling time \( t_* \sim (\hbar/2\pi k_B T_H)\ln (S_{BH}/k_B) \) provides a partial gravitational analogue of the reversibility time \( \tau_c \) 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. Within a fixed-charge sector, the Bekenstein-Hawking entropy increase supplies the erased-record capacity \( \Delta S_{BH}/(k_B \ln 2) \), with exact Landauer saturation when that capacity is filled. 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 counterpart 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. The manuscript is intentionally synthetic and classificatory: rather than proposing a new gravitational field equation or a single isolated theorem, it organizes several horizon regimes within one Landauer-based bookkeeping framework.