In science fiction, the term quantum singularity is used to refer to many different phenomena, which often approximately resemble a gravitational singularity in the scientific sense in that they are massive, localized distortions of space and time. The name invokes one of the most fundamental problems remaining in modern physics: the difficulty in merging Einstein's Theory of Relativity (which includes singularities within its models of black holes) and quantum mechanics. In fact, since singularities are infinitely small according to relativity, they are expected to be quantum mechanical by their nature; a theory of quantum gravity would be required to describe this behavior, and no such theory has yet been completed. On the other hand, only one kind of singularity has ever been observed (