It was probably an episode of Sci Fi Science, which covered many topics
but here is a list of all his shows
We Are the Guinea Pigs (1980)
Borders (1989)
Synthetic Pleasures (1995)
Einstein Revealed (1996)
Future Fantastic (1996)
Stephen Hawking's Universe (1997)
Bioperfection: Building a New Human Race (1998)
Exodus Earth (1999)
Me & Isaac Newton (1999)
Space: The Final Junkyard (1999)
Big Questions (2001)
Parallel Universes (2001)
Horizon: "Time travel" (2003)
Robo sapiens (2003)
Brilliant Minds: Secret Of The Cosmos (2003)
Nova: "The Elegant Universe" (2003)
Hawking (2004)
The Screen Savers (2004)
Unscrewed with Martin Sargent (2004)
Alien Planet (2005)
ABC News "UFOs: Seeing Is Believing" (2005)
HARDtalk Extra (2005)
Last Days on Earth (2005)
Obsessed & Scientific (2005)
Horizon: "Einstein's Unfinished Symphony" (2005)
Prophets of Science Fiction (2006)
Time (2006)
2057 (2007)
The Universe (2007)
Futurecar (2007)
Attack of the Show! (2007)
Visions of the Future (2008)
Horizon: "The President's Guide to Science" (2008)
Stephen Hawking: Master of the Universe (2008)
Horizon: "Who's Afraid of a Big Black Hole" (2009–2010)
Sci Fi Science: Physics of the Impossible (2009–2010)
Horizon: "What Happened Before the Big Bang?" (2010)
GameTrailers TV With Geoff Keighley: "The Science of Games" (2010)
How the Universe Works (2010)
Through the Wormhole (2011)