Focus section on quantum gravity - 25 years of quantum gravity

Abstract

The problem of quantum gravity has been with us for over 80years. After quantum theory was established in the 1920s, it was successfully applied to the electromagnetic field. Over the years there have been many attempts to bringgravity into the fold. There has been work on the Hamiltonian formulation of general relativity, perturbative approaches to quantum gravity and more. Much intellectual effort went into understanding conceptual and technical problems stemming from the general covariance of the theory. However, in earlier decades, the subject of quantum gravity was relatively on the fringes of theoretical physics research, pursued by a small and diverse community of people.In the mid 1980s the situation changed dramatically. The subject of quantum gravity came to the forefront of fundamental physics research, no longer a backwater but the mainstream. Quantum gravity was widely acknowledged as the last frontier of fundamental physics and attracted the brightest young people. Unlike in previous decades, workers in this area were no longer isolated groups or individuals ploughing lonely furrows, but organised into coherent `programmes' for a concerted attack on the problem.The main programmes coincidentally were all formulated in the mid1980s. The two `programmes' covered in this section are string theory and loop quantum gravity. String theory was born an offshoot of Hadronic models in particle physics and reflects theparticle physicists view that gravity is just one more interactionto be encompassed by a unified theory. Loop quantum gravityreflects the general relativist'sconviction that gravity is different and should not be treated asa perturbation about Minkowski spacetime. Each of these approaches has its proponents, adherents and critics. It is now about a quarterof a century since these programmes started. It is perhaps a goodtime to take stock and assess where we are now and where each of these programmes is headed.The idea in this focus sectionis to get a comparative perspective on these programmes, by asking ourreviewers to critically evaluate progress in their programmes over the last 25 years (1986–2011).This section features invited review articles from physicists who have been associated with these programmes from theirinception. They were invited to write a retrospective review: what were the initial hopes? To what extent have these hopes been realised? What were the major successes, surprises, and disappointments? The emphasis is on what has come out of the programme rather than technical developments internal to the programme.We hope that the reader, whatever his/her persuasion,will be able to form a panoramic view of quantum gravity researchtoday within these two programmes. We hope to complement thisview with a topical review of causal sets in the future.