The theory used to describe the physics of elementary particles. According to this theory, quantum fields are the ultimate reality and particles are merely the localized quanta of these fields. A consequence of the Heisenberg uncertainty principle. For example, the number of photons in a box with perfectly reflecting walls is constantly varying because of quantum fluctuations. Quantum fluctuations can cause particles to appear and disappear.
Some theories hold that the entire universe was created out of nothing, in a quantum fluctuation.
To date, there is no complete and self-consistent theory of quantum gravity, although successful quantum theories have been found for all the forces of nature except gravity. A general term used to describe attempts to quantize gravity. The elementary particle of the gravitational field is the graviton. String theory is an example of a theory of quantum gravity. The counter-intuitive weirdness of the concept results in part from the limitations of the particle metaphor in describing a phenomenon that is also in many respects a wave.