Physicists will tell you that four forces control the universe. Of these, gravity may the most obvious, but it is also the most mysterious. Newton managed to predict the force of gravity but couldn't explain how it worked at a distance. Then Einstein picked up on the simple premise that gravity and acceleration are interchangeable to devise his mind-bending General Relativity, showing how matter warps space and time. Not only did this explain how gravity worked - and how apparently simple gravitation has four separate components - but it predicted everything from black holes to gravity's effect on time. Whether it's the reality of anti-gravity or the unexpected discovery that a ball and a laser beam drop at the same rate, gravity is the force that fascinates. AUTHOR: Brian Clegg is the author of How to Build Your Own Time Machine. He holds a physics degree from the University of Cambridge and has written regular columns, features and reviews for numerous magazines. He lives in Wiltshire, England, with his wife and two children.