Introduced by John Madden

A self-consistent unified picture of the natural world is the long-term goal for physics. Yet when physicists took at natural phenomena, nature stubbornly presents four different, seemingly disparate forces - the gravitational, the electro magnetic and the two types of nuclear forces. Professor Steven Weinberg, of Harvard University, has been responsible for developments that have brought together two of these, and he discusses the possibilities of complete unification of the laws of physics.

followed by an interlude