Professor Stephen Hawking has celebrated his 75th birthday at a specially-organised series of public lectures about gravity and black holes.
The physicist and cosmologist gave a talk reflecting on his life and scientific work at Sunday’s symposium at Cambridge University.
It followed an afternoon of lectures from other distinguished scientists including Professor Brian Cox.
Prof Hawking, who turned 75 in January, joked: “Actually, for those keeping count, I’m closer to 75-and-a-half. But that shouldn’t get in the way of a good celebration.”
In his talk, titled My Life In Physics, he recalled his diagnosis of amyotrophic lateral sclerosis (ALS) at the age of 21.
“At first I became depressed,” he said. “I seemed to be getting worse really rapidly.
“There didn’t seem any point working on my PhD because I didn’t know I would live long enough to finish it.
“But then the condition developed more slowly and I began to make progress in my work.
“After my expectations had been reduced to zero, every new day became a bonus and I began to appreciate everything I did have. While there’s life there’s hope.”
The event concluded with the full auditorium singing Happy Birthday to Prof Hawking.
Prof Hawking said he had not anticipated the success of his book, A Brief History Of Time.
"I thought I might make a modest amount to help support my children at school and the rising costs of my care but the main reason was because I enjoyed it," he said.
He continued: "I never expected A Brief History Of Time to do as well as it did.
"Not everyone may have finished it or understood everything they read.
"But they at least got the idea that we live in a universe governed by rational laws that we can discover and understand."
He said mankind must continue to go into space "for the future of humanity".
"I don't think we will survive another 1,000 years without escaping beyond our natural planet," he said. "I therefore want to encourage public interest in space."