Watch NOVA Season 31 Episode 4 Online

Part 2, "String's the Thing," opens with a whimsical scene in a movie theater in which the history of the universe runs backwards to the Big Bang, the moment at which general relativity and quantum mechanics both came into play, and therefore the point at which our conventional model of reality breaks down. Then it's string theory to the rescue as Greene describes the steps that led from a forgotten 200-year-old mathematical formula to the first glimmerings of strings—quivering strands of energy whose different vibrations give rise to quarks, electrons, photons, and all other elementary particles. Strings are truly tiny, being smaller than an atom by the same factor that a tree is smaller than the solar system. But, as Greene explains, they are able to combine the laws of the large and the laws of the small into a proposal for a single, harmonious theory of everything.