# You will be missed, Stephen Hawking

Physicists are born in two ways–either lured in by the mysteries of particle physics or the wonder of the vast cosmos. For myself and many others, Stephen Hawking offered us one of our first glimpses of both worlds.

My very first exposure to physics was Isaac Asimov’s *The Collapsing Universe: The Story of the Black Holes*. This quickly led me to Stephen Hawking’s *A Brief History of Time*.

And from there I was hooked.

Hawking was my gateway drug, leading to Michio Kaku’s *Hyperspace*, Leon Lederman’s *The God Particle*, on and on down the rabbit hole.

Hawking’s tombstone will apparently bear his most famous equation:

$$\large S_{BH} = \frac{kc^3}{4\hbar G}A$$

This is the equation for the entropy, \(S_{BH}\), of a black hole. Beautifully, it draws in important physical constants from thermodynamics (the Boltzmann constant \(k\)), relativity (the speed of light \(c\)) and gravitation (the gravitational constant \(G\)), and quantum mechanics (Planck’s constant \(\hbar\)). Importantly, the total entropy only depends on the surface area \(A\) of the black hole, with its surface defined by its event horizon. A black hole’s event horizon is the distance at which not even light can escape its gravitational pull.

Hawking extended his insight into black holes to the entire universe: the entropy of the universe can be defined based on its so-called “cosmological event horizon”. In an accelerating universe, there is a distance at which an object will never be able to reach you. Unlike in a black hole, this “event horizon” is two-way: you will also never be able to reach that object.

Defining a universal entropy is interesting because entropy is a way to describe the amount of information in a system. Hawking’s equation implies that the information in a 3D object like a black hole or the universe itself is encoded by its 2D surface. This idea forms the basis of the holographic principle.