Quantum gravity is a physical theory incorporating quantum theory and the principles of general relativity to describe Planck scale microstructures of spacetime. Quantum gravity includes all the constants of general relativity and quantum theory, such as c (the velocity of light in vacuo), ℏ (the reduced Planck’s constant), and G (Newton’s constant), to describe units of mass, length and time at the Planck scale.
Uniting quantum theory and general relativity to mathematically describe quantum gravity is technically challenging due to limitations of experimental capabilities and the prima facie incompatibility of general relativity with quantum theory. Quantum gravity has fluctuating geometry, referred to as 'spacetime foam', and is an example of the facie incompatibility of general relativity with quantum theory. This incompatibility arises from quantum theories not having definite values and expressing Heisenberg’s Uncertainty Principle, while general relativity uses definite values to describe its theories. For example, if a physicist focuses on measuring the spatial geometry associated with quantum gravity it will not have a definite trajectory. This occurs because in quantum theory when researchers increase their observations on one specific property it gives rise to a decreased specificity of its canonically conjugate property; making quantum gravity a highly technical and difficult phenomenon to describe mathematically in a theory that satisfies the requirements of both quantum theory and general relativity (as of May 2019).