Chapter One

The classical electromagnetic vacuum is simply the state in which all moments of the electric and induction fields identically vanish, and thus the fields themselves identically vanish. Hence, in classical electrodynamics the interaction of matter with the electromagnetic field - including field-assisted interaction between matter systems - always requires excited (source-attributed) fields, which, as is known, can be described in terms of positive semi-definite probability distribution functions in phase space. In quantum electrodynamics the situation is quite different, because the noncommutativity of canonical conjugate field quantities necessarily implies nonvanishing moments. As we know, the quantum electromagnetic vacuum can be regarded as the state in which all normally ordered fieldmoments identically vanish. Clearly, the anti-normally ordered field moments cannot do so due to virtual photon creation and destruction - an effect in which the noise of the quantum vacuum becomes manifest.

Since the electromagnetic vacuum cannot be switched off, its interaction with atomic systems cannot be switched off either, thereby giving rise to a number of observable effects such as spontaneous emission, the Lamb shift, intermolecular energy transfer and the van der Waals force. Both virtual and real photons can be involved in the atom-field interaction. Whereas the interaction of ground-state atoms with the electromagnetic vacuum processes via virtual photon creation and destruction, the creation of real photons always requires excited atoms. A typical example of the first case is