# Vacuum expectation value

In quantum field theory the vacuum expectation value (also called condensate or simply VEV) of an operator is its average, expected value in the vacuum. The vacuum expectation value of an operator O is usually denoted by $\langle O\rangle$. One of the best known examples of an observable physical effect that results from the vacuum expectation value of an operator is the Casimir effect.
The observed Lorentz invariance of space-time allows only the formation of condensates which are Lorentz scalars and have vanishing charge[citation needed]. Thus fermion condensates must be of the form $\langle\overline\psi\psi\rangle$, where ψ is the fermion field. Similarly a tensor field, Gμν, can only have a scalar expectation value such as $\langle G_{\mu\nu}G^{\mu\nu}\rangle$.