Stephen Hawking, the world-renowned physicist who battled a debilitating neurological disease for most of his life, has died at 76. Hawking was diagnosed with amyotrophic lateral sclerosis, or Lou Gehrig's disease, when he was 21. The progressive illness gradually robbed him of his ability to speak and move. But even as his body weakened, Hawking's mind remained as razor sharp as ever, producing groundbreaking work on the origins of the universe that made him one of the most celebrated scientists in history. In a career spanning more than 50 years, Hawking made several major discoveries that overturned traditional thinking about black holes and the Big Bang theory. He also became a popular symbol of human achievement in the face of adversity, and a leading advocate for using technology to fight disease. "My goal is simple," he once said. "It is a complete understanding of the universe, why it is as it is and why it exists at all." Hawking is survived by his wife, Elaine, and three children.