QED vacuum

The lowest-energy state of quantum electrodynamics, containing no real particles but exhibiting quantum-field fluctuations and virtual particle–antiparticle effects.

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Quantum vacuum: The lowest-energy state of a quantum field theory, not a classical void. The QED vacuum is the electromagnetic instance of this broader quantum state.
Quantum electrodynamics: The quantum field theory describing electromagnetic interactions between charged particles and photons. Its fields, interactions, and energy spectrum define the state called the QED vacuum.
Quantum electrodynamics precision tests: Experimental comparisons of QED predictions with high-precision measurements. Agreement tests calculations that include virtual processes associated with the vacuum.
Classical vacuum: An idealized empty region with no matter or fields in classical physics. Unlike the QED vacuum, it has no quantum-field fluctuations.
Vacuum polarization: The response of a quantum vacuum to electromagnetic fields through virtual charged-particle effects. Virtual electron–positron pairs alter how electric charge and fields behave.
Quantum field: A physical field whose excitations are described by quantum mechanics and special relativity. The vacuum is the lowest-energy state of QED’s electromagnetic and charged-matter fields.
Electron magnetic moment: A measure of the electron’s magnetic dipole strength and orientation. Its measured value tests QED corrections from interactions with the vacuum.
Thermal state: A quantum state characterized by thermal occupation of available energy levels. Thermal photons or charged particles make a state distinct from the zero-temperature QED vacuum.
Casimir effect: An attractive force between conducting boundaries caused by changes in quantum-field modes. It is a measurable consequence of how boundaries modify electromagnetic vacuum fluctuations.
Ground state: The lowest-energy state available to a quantum system. The QED vacuum is a ground state, rather than a region with literally nothing in it.