Since the revolutionary development of both theories in the early twentieth century, it is fair to say that general relativity and quantum mechanics have had a rather hostile relationship to one another. One reason for this is simple a matter of scale: gravitational effects, described by general relativity, are essentially negligible in particle interactions. It is rather straightforward to calculate that the gravitational force between a pair of electrons is 39 orders of magnitude smaller than the electrical force. That is,
This is a very small ratio! In interactions between quantum particles, then, gravity plays no role whatsoever. Furthermore, on cosmological scales at which gravity is the dominant force, quantum mechanics has no noticeable effect.
The problem is more than just the relative size of forces, however. Quantum mechanics and general relativity just don’t seem to fit together very well, like pieces of completely different jigsaw puzzles. There are many challenges in formulating a perfectly consistent theory of quantum gravity, and we only mention one of these here. The force of gravity is determined by the distance between two massive objects, but the Heisenberg uncertainty relation of quantum mechanics states that this distance is fundamentally an uncertain quantity. An attempt to reconcile gravity and quantum mechanics is one of the fundamental driving forces behind the continually-controversial string theory.
But there have been some crude experimental glimpses at the relationship between gravity and quantum mechanics. In 1975, a collaboration between Purdue University and Ford Motor Company, of all places, researchers measured the effect of gravity on the wave properties of matter*. Though they verified quantum-mechanical predictions, their results also left some perplexing theoretical and experimental questions that are still being investigated today.