Melvyn Bragg examines Quantum Gravity. Early in the 20th century physicists were startled by the realisation that the smallest things in the universe do not obey Newton’s laws of gravity. Ripe apples fall from trees, billiard balls roll mostly on the table and the moon orbits the Earth in thrall to its gravitational pull, but there is no such force of gravity at work in the world of very small things. It seems there is one set of rules for the realm of every day objects, and a very different set of laws for the quantum world - where tiny particles actually form the building blocks of all those larger things.But how can this be? It doesn’t appear to make sense. Physicists decided that there must be another theory - a much larger theory - that unites, incorporates and finally makes sense of these divided realms. And this has been the Holy Grail of physics ever since. With Dr John Gribbin, Visiting Fellow in Astronomy, University of Sussex; Lee Smolin, Professor of Physics, Centre for Gravitational Physics and Geometry, Pennsylvania State University and Visiting Professor of Physics at Imperial College, London; Dr Janna Levin, Advanced Fellow, Department of Applied Mathematics and Theoretical Physics, Cambridge University.