Renowned physicist Stephen Hawking is not well enough to attend a conference held to celebrate his 70th birthday, a University of Cambridge official said on Sunday.

Prof. Hawking’s remarkable career is being honoured as part of a daylong conference on cosmology being hosted at the university.

But the celebrity scientist, who suffers from Lou Gehrig’s disease, will not be in attendance, according to Vice-Chancellor Leszek Borysiewicz.

Mr. Borysiewicz told conference attendees that Prof. Hawking was released from hospital on Friday and that “unfortunately his recovery has not been fast enough for him to be able to be here.”

He didn’t specify the nature of Prof. Hawking’s condition, although he said he believed Hawking would be well enough to meet some of the attendees over the next week.

Prof. Hawking uses a wheelchair and is dependent on a computerized voice system for communication.

Mr. Borysiewicz said he hoped that Hawking would follow the proceedings via videolink.

“If you’re listening Stephen, happy birthday from all of us here today,” Mr. Borysiewicz said to a round of applause.

Prof. Hawking was diagnosed with Lou Gehrig’s disease known as motor neurone disease in the U.K. when he was 21. Most people die within a few years of the diagnosis, but Hawking has defied the odds and gone on to revolutionize the field of theoretical astrophysics and become one of the best-known scientists since Albert Einstein.

An expert on black holes, Prof. Hawking also gained wide acclaim for popularising astronomy in best-selling books such as A Brief History of Time and The Universe in a Nutshell.

Other speakers scheduled for Sunday include Astronomer Royal Martin Rees, Nobel Prize-winning physicist Saul Perlmutter, and Hawking’s longtime collaborator Kip Thorne. Among the topics due to be discussed are supernovae, black holes, and dark matter.

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