Physicist Stephen Hawking has died at the age of 76, his family has said.
- Stephen Hawking was dubbed one of the most brilliant theoretical physicists since Albert Einstein
- He was given only two years to live when he was diagnosed with motor neurone disease in 1963
- Professor Hawking's worked ranged from the origins of the universe, through to the prospect of time travel
He died peacefully at his home in Cambridge in the early hours of this morning (local time), according to a statement from his family.
"His family have kindly requested that they be given the time and privacy to mourn his passing, but they would like to thank everyone who has been by Professor Hawking's side — and supported him — throughout his life," the statement said.
His children Lucy, Robert and Tim said they were deeply saddened by their father's passing.
"He was a great scientist and an extraordinary man whose work and legacy will live on for many years. His courage and persistence with his brilliance and humour inspired people across the world," they said.
"He once said, 'it would not be much of a universe if it wasn't home to the people you love'. We will miss him forever."
The University of Cambridge will be opening a book of condolence at Gonville and Caius College for those wishing to pay tribute to his life.
Professor Hawking sought to explain some of the most complicated questions of life while himself working under the shadow of a likely premature death.
In 1963 he was diagnosed with motor neurone disease and was given two years to live.
Professor Hawking went on to become a researcher at the University of Cambridge and Professorial Fellow at the Gonville and Caius College.
He was a Lucasian Professor at the university from 1979 to 2009, a position previously held by Isaac Newton in 1663.
Professor Hawking was dubbed one of the most brilliant theoretical physicists since Albert Einstein.
His work ranged from the origins of the universe itself, through the tantalising prospect of time travel to the mysteries of space's all-consuming black holes.
But the power of his intellect contrasted cruelly with the weakness of his body, ravaged by the disease he contracted at the age of 21.
Hawking was confined for most of his life to a wheelchair. As his condition worsened, he had to resort to speaking through a voice synthesiser and communicating by moving his eyebrows.
The disease spurred him to work harder but also contributed to the collapse of his two marriages, he wrote in a 2013 memoir My Brief History.
In the book, Professor Hawking related how he was first diagnosed: "I felt it was very unfair — why should this happen to me?
"At the time, I thought my life was over and that I would never realise the potential I felt I had. But now, 50 years later, I can be quietly satisfied with my life."
Hawking shot to international fame after the 1988 publication of A Brief History of Time, one of the most complex books ever to achieve mass appeal, which stayed on the Sunday Times best-sellers list for no fewer than 237 weeks.
He said he wrote the book to convey his own excitement over recent discoveries about the universe.
"My original aim was to write a book that would sell on airport bookstalls," he said.
"In order to make sure it was understandable I tried the book out on my nurses. I think they understood most of it."
Hawking's theories caused controversy
He caused some controversy among biologists when he said he saw computer viruses as a life form, and thus the human race's first act of creation.
"I think it says something about human nature that the only form of life we have created so far is purely destructive," he told a computer forum in Boston.
"We've created life in our own image."
He also predicted the development of a "race of self-designing human beings, who will use genetic engineering to improve their make-up".
Another major area of his research was into black holes, the regions of space-time where gravity is so strong that nothing, not even light, can escape.
When asked whether God had a place in his work, Professor Hawking once said: "In a way, if we understand the universe, we are in the position of God."
Hawking twice married and divorced
Professor Hawking married undergraduate Jane Wilde in July 1965 and the couple had three children.
But Professor Hawking said in his 2013 memoir that Ms Wilde became more and more depressed as her husband's condition worsened.
"She was worried I was going to die soon and wanted someone who would give her and the children support and marry her when I was gone," he wrote.
Ms Wilde took up with a local musician and gave him a room in the family apartment, Professor Hawking said.
"I would have objected but I too was expecting an early death," he said.
"I became more and more unhappy about the increasingly close relationship between [them].
"In the end I could stand the situation no longer and in 1990 I moved out to a flat with one of my nurses, Elaine Mason."
He divorced Ms Wilde in 1990 and in 1995 married Ms Mason, whose ex-husband David designed the electronic voice synthesiser that allowed Professor Hawking to communicate.
"My marriage to Elaine was passionate and tempestuous," he wrote in the memoir.
"We had our ups and downs, but Elaine's being a nurse saved my life on several occasions."
It also took its emotional toll on her, he noted, and the pair divorced in 2007.
Australian scientists pay tribute
Research fellow and outreach manager at Mt Stromlo Observatory at the Australian National University, Dr Brad Tucker, said Professor Hawking pushed researchers to challenge themselves and the unknown.
"He leaves having inspired many of us and having helped us to tackle the big questions that humans have asked for centuries," he said in a statement.
Associate Professor Alan Duffy, a research fellow in the Centre for Astrophysics and Supercomputing and lead scientist of the Royal Institution of Australia, said Professor Hawking enriched the lives of millions with his latest science and cosmic perspectives.
"Professor Hawking was an inspiration to me to become not just a scientist, but a communicator of that science," Professor Duffy said.
"His work as a cosmologist and discoveries in black hole physics were legendary.
"His best-known prediction, named by the community as Hawking Radiation, transformed black holes from inescapable gravitational prisons into objects that instead shrink and fade away over time."
Professor Duffy said Professor Hawking's sense of humour helped propel him into A-list celebrity stardom.
"Through it all, of course, his illness made his achievements near-superhuman," he said.
"How he manipulated Einstein's equations in his mind when he could no longer hold a pen I can't even begin to imagine."
Hawking's two concepts of time
Since 1974 Professor Hawking worked extensively on marrying the two cornerstones of modern physics — Einstein's General Theory of Relativity, which concerns gravity and large-scale phenomena, and quantum theory, which covers subatomic particles.
As a result of that research, Professor Hawking proposed a model of the universe based on two concepts of time: "real time", or time as human beings experience it, and quantum theory's "imaginary time", on which the world may really run.
"Imaginary time may sound like science fiction … but it is a genuine scientific concept," he wrote in a lecture paper.
Real time could be perceived as a horizontal line, he said.
"On the left, one has the past, and on the right, the future. But there's another kind of time in the vertical direction. This is called imaginary time, because it is not the kind of time we normally experience — but in a sense, it is just as real as what we call real time."
In July 2002, Professor Hawking said in a lecture that although his quest was to explain everything, a theory of determinism that would predict the universe in the past and forever in the future probably could not be achieved.