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I am reading Smolin's Three Roads to Quantum Gravity, and have just re-encountered an old idea, Beckenstein's Law, which states: "With every [event] horizon that forms a boundary separating an observer from a region which is hidden from them, there is associated an entropy which measures the amount of information which is hidden behind it. This entropy is always proportional to the area of the horizon." (pages 86-7). Smolin goes on later in the book to relate this idea to the holographic principle, which states that the horizon can be thought of as a computer that represents the state of the object enclosed by the horizon.

Here's my question. Does this principle work in both directions, ie from either side of the event horizon? IOW, can you imagine an observer on the *inside* of the event horizon (or imagine that the black hole *is* the observer), and associate the entropy of the horizon with the entropy of the *outside* universe? If so, does that mean that the entropy inside a black hole is equal to the entropy of the remaining universe (which seems difficult to accept)? Alternatively, does the area of the event horizon look different depending on which side of it you are on?

David

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# Beckenstein's law

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