Renowned cosmologist Professor Stephen Hawking passed away in the early hours of March 14, news that has saddened us all. His scientific output was truly incredible and his work in engaging the wider public in the complexities of the universe will continue to inspire.

His most famous formula, describing the entropy of a black hole, might even adorn his tombstone. He declared his wishes for this in 2002, during a workshop on the future of theoretical physics and cosmology held to mark his 60th birthday.

The formula is the centerpiece of our understanding of black holes and a crowning achievement for Hawking, who worked on it with his colleague Jacob Bekenstein. It connects important thermodynamical quantities such as entropy, represented by the capital S, to physical properties of the black hole, namely its area, A. The remaining letters are constants of the universe; k is the Boltzmann constant, c is the speed of light, h-bar is the reduced Planck constant, and G is the universal gravitation constant.

Entropy is described in school physics textbooks as a measure of disorder within a macroscopic system. But it can also be defined as the amount of information that you can pack into an object. And this is the crucial importance of the formula. The entropy of a black hole is proportional to its surface area, not its volume. The surface of the black hole is its event horizon, beyond which, nothing can escape.

Understanding the thermodynamics of black holes required the Cambridge physicist to apply quantum mechanics to these incredibly dense objects, and this led to the proposal of Hawking radiation. Black holes had entropy and a temperature.

Hawking himself extended this work to a more general and far-reaching interpretation. The whole universe could be seen as having a “cosmological event horizon” suggesting that the universe as a whole has an entropy value and a specific temperature. This idea was the base for the formulation of the holographic principle, suggesting that all the information encoded in the universe can be interpreted from the properties of a lower dimensional boundary.

There is also another interesting parallel that makes Professor Hawking's wish even more poignant. The first proposer of entropy was Austrian physicist Ludwig Boltzmann and his tombstone bears the inscription of his own entropy formula. It seems right that Hawking should have his own, too.

Hawking had just recorded a cameo for a new radio version of Douglas Adams' *Hitchhikers Guide to the Galaxy*, so as that other late, great visionary once wrote (sort of): So long, Professor Hawking, and thanks for all the fish.

[H/T: New York Times]