From Classical Theory to Quantum Gravity

While it has been generally accepted for decades that general relativity and quantum theory are inconsistent, and that therefore general relativity must yield to a new theory of gravity compatible with quantum principles, the way to the new theory is still very unclear. A major obstacle is the lack of any experimental indicators as to where general relativity might break down. I speculate on the different ways in which a quantum theory including gravity might emerge from present-day theory and use these speculations to examine a number of promising lines for new experiments.