Stephen Hawking

April 21, 2008: Dr. Stephen Hawking giving a lecture related to NASA's 50th anniversary at George Washington University in Washington, D.C. (AP/NASA, Paul E. Alers)

Stephen Hawking in July 1999. (AP)

Stephen Hawking attended to by a nurse during a visit to Hong Kong in June 2006. (AP)

April 2008: Stephen Hawking makes remarks at an event marking the 50th anniversary of NASA at George Washington University in Washington, D.C. (AP)

Stephen Hawking answers questions during an interview in Orlando, Fla., in April 2006, related to his zero-gravity flight. (AP)

Stephen Hawking in January 2008. (AP)

Stephen Hawking floats in zero gravity during a 'vomit comet' flight over Florida in April 2006. (AP)