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 Hawkings, and thanks for all the fish.
RIP to a man who expanded our minds .

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