Black-hole thermodynamics

To the physicist casually interested in gravitation, a black hole is a passive object that swallows anything near it and cannot be made to disgorge it; it absorbs but cannot emit. At the close of the last decade the experts shared this view. Recently, however, this simple picture has changed entirely. Perhaps no single development highlighted more the new views about black holes than the quantum argument presented by Stephen Hawking of Cambridge University in 1974 that a black hole must radiate spontaneously with a thermal spectrum. The importance of this phenomenon is not so much in possible practical applications, not even in its astrophysical implications, but rather in that it has confirmed earlier suspicions that gravitation, thermodynamics and the quantum world are deeply interconnected. This connection, which might be symbolized by the thermodynamic engine shown in figure 1, engenders hope that we may achieve a synthesis of these three branches of physics in our time and bears witness to the profound unity of physics, a unity too often veiled in an age of increasing specialization.