Entanglement entropy of the black hole horizon

We examine the possibility that, when a black hole is formed, the information on the collapsed star is stored as the entanglement entropy between the outside and the thin region (of the order of the Planck length) of the inside of the horizon. For this reason, we call this the entanglement entropy of the black hole ``horizon.'' We construct two models: one is in Minkowski spacetime and the other is in the Rindler wedge. To calculate the entropy explicitly, we assume that the thin regions of the order of the Planck length of the outside and the inside of the horizon are completely entangled by quantum effects. We also use the property of the entanglement entropy that it is symmetric under an interchange of the observed and unobserved subsystems. Our setting and this symmetric property substantially reduce the needed numerical calculations. As a result of our analysis, we can explain the Bekenstein-Hawking entropy itself (rather than its correction by matter fields) in the context of entanglement entropy.