

1 Introduction
Quantum mechanics and general relativity were discovered within a
decade of one another close to a century ago, and almost
immediately the search for a quantum theory of gravity had begun.
Ever since it has been a puzzle as to what theoretical framework
might ultimately reconcile these two theories with one another.
This reconciliation has proven to be difficult to achieve, and
although several promising proposals now exist none can yet claim
complete theoretical and experimental vindication.

