The discoveries that made Stephen Hawking famous

Stephen Hawking is probably the most famous genius of the modern age.

But what exactly is he famous for - apart from his astonishing resilience to an incapacitating disease, that instantly-recognizable retro-robotic voice, and his walk-on roles on The Simpsons and Star Trek?

Didn't he discover black holes? Or the Big Bang? Or tell us what time is, or something? No, no and no again. But it's hard to cut through the thickets of myth and get to the things that he really did discover. Hawking's own legend risks obscuring his real achievements.

Today, Hawking delivers this year's Reith Lecture: an appointment that signifies the speaker's status, not just as an expert in his or her discipline, but as a public intellectual. So now seems an opportune moment to put aside Hawking the icon and examine Hawking the physicist.

In several polls of the greatest physicists of the 20th century, or even of the top living physicists, Hawking is either absent or crawls in at the bottom of the list. Is he, then, not all he's cracked up to be?

On the contrary, he is a huge presence in modern physics. It's just that physics has a lot of astonishing minds, and Hawking is one among many.

Hawking's genius, which arguably deserves a Nobel Prize, is to have brought together several different but equally fundamental fields of physical theory: gravitation, cosmology, quantum theory, thermodynamics and information theory.

It starts with general relativity: the theory of gravitation that Albert Einstein devised in the 1910s to replace that of Isaac Newton.

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