## How Stephen Hawking’s Greatest Discovery Revolutionized Black Holes

“Increasing entropy, over time, should be okay, but decreasing it should be forbidden. The only way to ensure that would be by forcing an increase in the black hole’s mass to cause entropy to go up by at least the largest amount you can imagine.

The way that people working on that problem – including Hawking – assigned an answer was to make entropy proportional to the surface area of a black hole. The more quantum bits of information you can fit on a black hole, the greater its entropy was. But that brought up a new problem: if you have entropy, then that means you have a temperature. And if you have a temperature, you have to radiate energy away. Originally called “black” because nothing, not even light, can escape, now it became clear it had to emit something after all. All of a sudden, a black hole isn’t a static system anymore; it’s one that changes over time.”

Stephen Hawking may be most famous, today, for his popular accounts of astrophysics and cosmology, for his inspirational personal story, and for his overcoming of adversity and disability to achieve all that he has. But he was truly a tremendous physicist, and his contributions to our understand of black holes was truly transformative. His greatest discovery was that they weren’t merely static objects in space, but that they were active and evolving. They didn’t just absorb mass, but had an entropy, a temperature, and radiated energy away. How this occurred was far from intuitive, and it took a decade of research for Hawking to arrive at his incredible 1974 result, where he derived the existence of Hawking radiation. All of a sudden, black holes had a finite lifetime, and would eventually evaporate away entirely.