Atom interferometry, spacetime and reality

Einstein's general theory of relativity dominates the science of the very large, including gravity and cosmology. Quantum theory reigns supreme on the scale of atoms and smaller. What is more natural than to try to unify these two theories into a single quantum theory of gravity? Modern theories of gravity, including superstring theory, give quantum properties to space and time. These theories have been inspired by Dirac's principles of beauty and elegance. But Dirac had the advantage of experimental evidence, and modern theories of quantum gravity do not. Moreover, beauty and elegance are subjective – they are a guide to the correct theory, not a guarantee. For any scientific theory, experiment is the final test.