Astrophysicist Stephen Hawking died on Wednesday aged 76. Professor Hawking was diagnosed with a form of motor neurone disease (also known as amyotrophic lateral sclerosis and Lou Gehrig's disease) when he was 21, but that did not stop him becoming the foremost scientist of his generation.
Tributes poured in after the news of his death. Fellow scientists Richard Dawkins posted on Twitter, repeating the words of Wordsworth when Sir Isaac Newton died: "Silent face, the marble index of a mind forever voyaging through strange seas of thought, alone."
Scientist Neil deGrasse Tyson also took to Twitter to say: "His passing has left an intellectual vacuum in his wake. But it's not empty. Think of it as a kind of vacuum energy permeating the fabric of spacetime that defies measure."
Professor David Wiltshire of the University of Canterbury was one of Hawking's students at the University of Cambridge in the mid 1980s and knew him well. He talks to Guyon Espiner.