Black Hole Entropy

1983 paper on entanglement entropy: "On the Entropy of the Vacuum outside a Horizon"

We introduce the concept of *entanglement entropy* (as it is now called) and point out that it follows an *area law* which renders it a suitable source of black hole entropy.We also suggest to conceive the latter as residing on thehorizon at approximately one bit per"planckian plaquette''.