Stephen Hawking, the brilliant British theoretical physicist who overcame a debilitating disease to publish wildly popular books probing the mysteries of the universe, has died, according to a family spokesman. He was 76. 

Considered by many to be the world's greatest living scientist, Hawking was also a cosmologist, astronomer, mathematician and author of numerous books including the landmark "A Brief History of Time," which has sold more than 10 million copies.

With fellow physicist Roger Penrose, Hawking merged Einstein's theory of relativity with quantum theory to suggest that space and time would begin with the Big Bang and end in black holes. Hawking also discovered that black holes are not completely black but emit radiation and will likely eventually evaporate and disappear. 

Hawking suffered from ALS (amyotrophic lateral sclerosis), a neurodegenerative disease commonly known as Lou Gehrig's Disease, which is usually fatal within a few years. He was diagnosed in 1963, when he was 21, and doctors initially only gave him a few years to live.

The disease left Hawking wheelchair-bound and paralyzed. He was able to move only a few fingers on one hand and was completely dependent on others or on technology for virtually everything -- bathing, dressing, eating, even speech.

Hawking used a speech synthesizer that allowed him to speak in a computerized voice with an American accent.

"I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many," he wrote on his website.

"I have been lucky that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope."