Black Holes and Entropy
Abstract
There are a number of similarities between blackhole physics and thermodynamics. Most striking is the similarity in the behaviors of blackhole area and of entropy: Both quantities tend to increase irreversibly. In this paper we make this similarity the basis of a thermodynamic approach to blackhole physics. After a brief review of the elements of the theory of information, we discuss blackhole physics from the point of view of information theory. We show that it is natural to introduce the concept of blackhole entropy as the measure of information about a blackhole interior which is inaccessible to an exterior observer. Considerations of simplicity and consistency, and dimensional arguments indicate that the blackhole entropy is equal to the ratio of the blackhole area to the square of the Planck length times a dimensionless constant of order unity. A different approach making use of the specific properties of Kerr black holes and of concepts from information theory leads to the same conclusion, and suggests a definite value for the constant. The physical content of the concept of blackhole entropy derives from the following generalized version of the second law: When common entropy goes down a black hole, the common entropy in the blackhole exterior plus the blackhole entropy never decreases. The validity of this version of the second law is supported by an argument from information theory as well as by several examples.
 Publication:

Physical Review D
 Pub Date:
 April 1973
 DOI:
 10.1103/PhysRevD.7.2333
 Bibcode:
 1973PhRvD...7.2333B