In quantum fiel theory, the zero-point field is the lowest energy state of a field, i.e. its ground state, which is non zero. This phenomenon gives the quantum vacuum a complex structure, which can be probed experimentally; see, for example, the Casimir effect. The term "zero-point field" is sometimes used as a synonym for the vacuum state of an individual quantized field. The electromagnetic zero-point field is loosely considered as a sea of background electromagnetic energy that fills the vacuum of space, and is often regarded merely as a curious outcome of the quantum mechanical requirement, namely the Heisenberg uncertainty principle, that the lowest allowable energy level in a harmonic oscillator mode is not zero but ħw /2, where w is the characteristic frequency of the oscillator.