Quantum fluctuation is the temporary appearance of energetic particles out of nothing, as allowed by the Uncertainty Principle. It is synonymous with vacuum fluctuation.
The Uncertainty Principle states that for a pair of conjugate variables such as position/momentum and energy/time, it is impossible to have a precisely determined value of each member of the pair at the same time. For example, a particle pair can pop out of the vacuum during a very short time interval.
|The uncertainty principle is illustrated with a schematic representation on the left. An extension is applicable to the "uncertainty in time" and "uncertainty in energy" (including the rest mass energy mc2). When the mass is very large (such as a macroscopic object), the uncertainties and thus the quantum effect become very samll, classical physics is applicable once more.|
In classical physics (appliable to macroscopic phenomena), empty space-time is called the vacuum. The classical vacuum is utterly featureless. However, in quantum mechanics (appliable to microscopic phenomena), the vacuum is a much more complex entity. It is far from featureless and far from empty. The quantum vacuum is just one particular state of a quantum field (corresponding to some particles). It is the quantum mechanical state in which no field quanta are excited, that is, no particles are present. Hence, it is the "ground state" of the quantum field, the state of minimum energy. The picture on the left illustrates the kind of activities going on in a quantum vacuum. It shows particle pairs appear, lead a brief existence, and then annihilate one another in accordance with the Uncertainty Principle.