Theories of Quantum Gravity

Ju ly 20 0 8 je a n -f ra n co is P o d ev in on the very smallest scales—the voids in between the smallest known elementary particles—by quantum laws and possibly explain it in terms of some fundamental constituents. Superstring theory is often described as the leading candidate to fill this role, but it has not yet provided an answer to any of these pressing questions. Instead, following its own inner logic, it has uncovered ever more complex layers of new, exotic ingredients and relations among them, leading to a bewildering variety of possible outcomes. Over the past few years our collaboration has developed a promising alternative to this much traveled superhighway of theoretical physics. It follows a recipe that is almost embarrassingly simple: take a few very basic ingredients, assemH ow did space and time come about? How did they form the smooth fourdimensional emptiness that serves as a backdrop for our physical world? What do they look like at the very tiniest distances? Questions such as these lie at the outer boundary of modern science and are driving the search for a theory of quantum gravity—the long-sought unification of Einstein’s general theory of relativity with quantum theory. Relativity theory describes how space time on large scales can take on countless different shapes, producing what we perceive as the force of gravity. In contrast, quantum theory describes the laws of physics at atomic and subatomic scales, ignoring gravitational effects altogether. A theory of quantum gravity aims to describe the nature of space time KEY CONCEPTS