Theoretical physicist Stephen Hawking was diagnosed with amyotrophic lateral sclerosis (ALS) at aged 21 and was given less than five years to live. (Mike Hutchings/Reuters)

Renowned physicist Stephen Hawking, who suffers from Lou Gehrig’s disease, was too sick to attend a conference celebrating his 70th birthday at the University of Cambridge in Britain.

Diagnosed at age 21 with the motor neuron disease and confined to a wheelchair, the celebrated scientist has defied medical predictions that he would die at an early age. He speaks through a computerized voice system.

Cambridge University Vice Chancellor Leszek Borysiewicz told conference attendees on Sunday that the scientist could not attend after being released from hospital on Friday: "Unfortunately his recovery has not been fast enough for him to be able to be here."

Borysiewicz said Hawking might follow the proceedings via videolink.

"If you're listening Stephen, happy birthday from all of us here today," Borysiewicz said as the audience applauded.

While Borysiewicz did not divulge more details, he indicated Hawking might be well enough later in the week to meet with some of his colleagues and admirers.

An expert on black holes, Hawking has carved out an acclaimed career in astrophysics, writing several popular books including A Brief History of Time and The Universe in a Nutshell.

Much of his work involves marrying relativity and quantum theory — space and time with how the tiniest particles in space behave to explain how the universe is governed.

Hawking’s list of accomplishments and awards is long. Born in 1942 in Oxford, England, he became one of the youngest fellows of the prestigious Royal Society at age 32 and in 1979, he was appointed Lucasian Professor of Mathematics at Cambridge University.

He was anointed a commander of the Order of the British Empire in 1982 and bestowed the Presidential Medal of Freedom in 2009, the highest civilian honour in the U.S.

Diagnosed with amyotrophic lateral sclerosis (ALS), Hawking has lived with the disease longer than anyone. Most sufferers live less than five years after being diagnosed.

Hawking has said that his mainstream popularity can be traced to his physical state: "People are fascinated by the contrast between my very limited physical powers,and the vast nature of the universe I deal with."

A man of high science, Hawking is not beyond participating in pop culture, with cameos on Futurama, The Simpsons and the TV series Star Trek: The Next Generation.

U.S. theoretical physicist Kip Thorne, Hawking’s longtime collaborator, said when his friend lost use of his hands and could no longer do equations by using pen on paper, he "compensated by training himself to manipulate complex shapes and topologies in his mind at great speed."

The twice-divorced Hawking has three children with his first wife.

With files from The Associated Press