01/08/2012 01:18 pm ET Updated Jan 08, 2012

Stephen Hawking Too Ill To Attend 70th Birthday Symposium

Stephen Hawking is missing his own birthday party. The famed British physicist, who turned 70 on January 8, was unable to attend a scientific conference held in his honor at the University of Cambridge. Why? According to Cambridge Vice-Chancellor Leszek Borysiewicz, "Stephen has been unwell and was only discharged from hospital on Friday."

Although Hawking has been remarkably healthy for decades following his diagnosis with Lou Gehrig's disease, a.k.a. amyotrophic lateral sclerosis, he has shown a number of worsening symptoms and unrelated medical problems in the last several years.

The debilitating disease, which causes atrophy in nerve cells that control muscle activity, left him largely paralyzed since his early 20s, and his storied career as a physicist and science communicator has come in spite of a bleak prognosis. Hawking lost his already meager ability to speak in 1985, when he underwent a tracheotomy after contracting pneumonia. Now he communicates via a speech synthesizer that he controls with one finger on his left hand and a muscle in his cheek.

But even Hawking's few usable muscles are now deteriorating, leaving him sometimes able to produce only one word per minute. This has prompted an effort to develop a speech system that reads signals directly from Hawking's brain.

It's unclear whether Hawking's most recent health problems are related to ALS. The professor was rushed to a hospital in 2009 for a chest infection, but no details of his current treatment have been released.