The essence of two fundamental physical theories, quantum mechanics and general relativity is concentrated in their basic equations. The mutual consistency of both theories is one of the biggest open problems in physics and its philosophy.
The philosophical essence of that problem might be represented as follows. General relativity refers to smoothmotion in a curvedspace, which can be thought as constituted by the propagation of a deformed spherical wave in the usual Euclidean 3D space. Quantum mechanics needs to represent any quantum leap as a discretemotion and uses the separable complex Hilbert space, which is never curved in principle.