Stephen Hawking, the most famous physicist of his time, has died at the age of 76. A family spokesman said Hawking, who was British, died peacefully early Wednesday at his home in Cambridge, England. Hawking was a theoretical physicist. He was known worldwide for working to explain subjects like the beginnings of the universe and the complexities of black holes. He was diagnosed with a disease called amyotrophic lateral sclerosis, or ALS, at age 21. At the time, doctors predicted he would only live a few years. But he proved them wrong by surviving for more than 50 years more and continuing his scientific work. The disease eventually put him in a wheelchair and took away his ability to speak. For many years, Hawking communicated by using a voice synthesizer. In his 2013 book, “My Brief History,” Hawking wrote about first learning of the illness: “I felt it was very unfair - why should this happen to me?” He added: “At the time, I thought my life was over and that I would never realize the potential I felt I had. But now, 50 years later, I can be quietly satisfied with my life,” he wrote.