Working almost entirely in his head as motor neurone disease stripped him of movement and then speech, Stephen Hawking showed with Roger Penrose that classical general relativity drives the early universe toward a singularity, and then derived that black holes are not perfectly black but slowly radiate and evaporate. That second result — Hawking radiation — is theoretically powerful but too faint to detect, which is why it never earned him a Nobel Prize. A Brief History of Time made cosmology a global popular passion and his synthesized voice one of the most recognized sounds on earth.
A black hole was supposed to be a perfect trap. Anything that fell in could never get out, not even light, so it should be utterly black and last forever. In 1974 a young physicist already living with severe motor neurone disease did the mathematics in his head and reached an answer that startled everyone, including himself. Stephen Hawking found that a black hole must very faintly glow. Over almost unimaginable stretches of time, it slowly leaks its mass away and finally disappears. Black holes, it turned out, are not entirely black.