ABSTRACT
Quantum gravity involves the unification of the principles of quantum theory and general relativity. Constructing such a theory constitutes one of the greatest challenges in theoretical physics. It is a particularly hard challenge for many reasons, both formal and conceptual, some of which I aim to elucidate in what follows. Even up until the 1970s, quantum gravity was, as Michael Duff remarks, “a subject ... pursued only by mad dogs and Englishmen” ((Duff, 1999), p. 185). That is something of an exaggeration, of course: for starters, when Duff speaks of
quantum gravity, he has in mind the particle physicist’s approach to the problem, according to which the gravitational interaction involves an exchange of gravitons (the quanta of the gravitational field). However, quantum gravity, understood as the general unificatory problem sketched above, has been pursed for around eighty years in some form or another, by many of the greatest physicists, many of whom were not English! The remark has more than a grain truth to it though; even now quantum gravity research is looked upon with some trepidation and, often, bemusement. This attitude stems primarily from the extreme detachment of quantum gravity research from experimental physics-a feature that leads Nambu (1985) to refer to quantum gravity research as “postmodern physics”! As a result, much of the research conducted in quantum gravity looks like an exercise in pure mathematics or, sometimes, metaphysics. This aspect makes quantum gravity especially interesting from a philosophical point of view, as I shall attempt to demonstrate throughout this chapter.
