In
quantum field theory, the
vacuum state (also called the
vacuum) is the
quantum state with the lowest possible
energy. Generally, it contains no physical particles. The term "
zero-point field" is sometimes used as a synonym for the vacuum state of an individual quantized field.
According to present-day understanding of what is called the vacuum state or the quantum vacuum, it is "by no means a simple empty space"[1], and again "it is a mistake to think of any physical vacuum as some absolutely empty void."[2] According to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence.[3][4][5]
The QCD vacuum of quantum chromodynamics is the object of study in the Relativistic Heavy Ion Collider and the Large Hadron Collider, and is related to the so-called vacuum structure of strong interactions.[6]
If the quantum field theory can be accurately described through perturbation theory, then the properties of the vacuum are analogous to the properties of the ground state of a quantum mechanical harmonic oscillator (or more accurately, the ground state of a QM problem). In this case the vacuum expectation value (VEV) of any field operator vanishes. For quantum field theories in which perturbation theory breaks down at low energies (for example, Quantum chromodynamics or the BCS theory of superconductivity) field operators may have non-vanishing vacuum expectation values called condensates. In the Standard Model, the non-zero vacuum expectation value of the Higgs field, arising from spontaneous symmetry breaking, is the mechanism by which the other fields in the theory acquire mass.
