QED vacuum

The quantum electrodynamic vacuum or QED vacuum is the field-theoretic vacuum of quantum electrodynamics. It is the lowest energy state (the ground state) of the electromagnetic field when the fields are quantized. When Planck's constant is hypothetically allowed to approach zero, QED vacuum is converted to classical vacuum, which is to say, the vacuum of classical electromagnetism.

Another field-theoretic vacuum is the QCD vacuum of the Standard Model.

The QED vacuum is subject to fluctuations about a dormant zero average-field condition: Here is a description of the quantum vacuum:

The quantum theory asserts that a vacuum, even the most perfect vacuum devoid of any matter, is not really empty. Rather the quantum vacuum can be depicted as a sea of continuously appearing and disappearing [pairs of] particles that manifest themselves in the apparent jostling of particles that is quite distinct from their thermal motions. These particles are ‘virtual’, as opposed to real, particles. ...At any given instant, the vacuum is full of such virtual pairs, which leave their signature behind, by affecting the energy levels of atoms.

It is sometimes attempted to provide an intuitive picture of virtual particles based upon the Heisenberg energy-time uncertainty principle:

(where $Δ E$ and $Δ t$ are energy and time variations, and $ħ$ the Planck constant divided by 2 $π$ ) arguing along the lines that the short lifetime of virtual particles allows the "borrowing" of large energies from the vacuum and thus permits particle generation for short times.

This interpretation of the energy-time uncertainty relation is not universally accepted, however. One issue is the use of an uncertainty relation limiting measurement accuracy as though a time uncertainty $Δ t$ determines a "budget" for borrowing energy $Δ E$ . Another issue is the meaning of "time" in this relation, because energy and time (unlike position $q$ and momentum $p$ , for example) do not satisfy a canonical commutation relation (such as $[q, p] = iħ$ ). Various schemes have been advanced to construct an observable that has some kind of time interpretation, and yet does satisfy a canonical commutation relation with energy. The many approaches to the energy-time uncertainty principle are a continuing subject of study.

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