PROFESSOR Stephen Hawking was too ill to attend a public symposium to celebrate his 70th birthday today.

But was able to keep up with the event entitled 'The State of the Universe' as it was broadcast live on the internet.

Cambridge University's Professor Hawking was due to give a speech looking back on his life but a tweet from the university explained: "Prof Hawking not well enough to attend today but is watching the live stream. Happy birthday Prof and get well soon!"

Earlier this week it emerged his speech, using an infrared sensor mounted on his cheek and voice technology, has slowed down dramatically recently while advisors try to persuade him to switch to newer technology.

The symposium held in Cambridge today had Twitter filled with debate using the hashtag #Hawking70 as world-renowned physicists gave speeches.

Ahead of the event, members of the scientific community paid tribute to Professor Hawking.

Given only two years to live when he was diagnosed with a form of Motor Neurone Disease in 1963, Professor Hawking has defied medical expectations to become one of the most brilliant theoretical physicists since Einstein.

Currently director of research at the Department of Applied Mathematics and Theoretical Physics at the university, where he also founded the Centre for Theoretical Cosmology, he previously (1979 - 2009) held the Lucasian Professorship of Mathematics at Cambridge, a post once held by Newton.

The author of A Brief History of Time, which was an international bestseller, his other books for the general reader include A Briefer History of Time, the essay collection Black Holes and Baby Universes, and The Universe in a Nutshell.

Professor Hawking has more than a dozen honorary degrees and was awarded the CBE in 1982. He is a fellow of the Royal Society and a Member of the US National Academy of Science.

Justin Rattner, chief technology officer, Intel Corporation, which has organised the symposium alongside the university's centre for theoretical cosmology, will introduce Stephen Hawking's speech on Sunday. He said: "With more than half a century of remarkable research Professor Hawking has continually pushed the boundaries of humankind's understanding of the cosmos.

"More than any other scientist in recent history, his ability to engage people in the process of scientific discovery through his books, lectures, and television programs has opened countless inquisitive minds to a Universe full of possibilities. Thank you, Stephen, and the best of wishes on your 70th Birthday!"

Lord Rees, the Astronomer Royal, said: "It is wonderful that we are celebrating Stephen's 70th birthday. It's a chance to thank him for the many insights he's given us about the universe, and for all he's done to present scientific ideas to a wide public - and above all for the inspiration he's offered to millions by achieving so much, against all the odds."

Professor Kip Thorne, the acclaimed American theoretical physicist and long-standing collaborator with Stephen Hawking, said: "When Stephen lost the use of his hands and could no longer manipulate equations on paper, he compensated by training himself to manipulate complex shapes and topologies in his mind at great speed. That ability has enabled him to see the solutions to deep physics problems that nobody else could solve, and that he probably would not have been able to solve, himself, without his newfound skill."

Professor Sir Leszek Borysiewicz, Vice-Chancellor of the University of Cambridge, said: "I am proud that the world's best-known scientist is a Cambridge colleague."

Other speakers at the event included the Astronomer Royal Lord (Martin) Rees, Professor Saul Perlmutter (University of California, Berkeley, 2011 Nobel Prize in Physics), and one of the world's leading theoretical physicists, Professor Kip Thorne (California Institute of Technology).