  A theory is presented for calculating the effect of the electromagnetic field
on the centre of mass of a macroscopic dielectric body that is valid in both
quantum and classical regimes. We apply the theory to find the classical
equation of motion for the centre of mass of a macroscopic object in a
classical field, and the spreading of an initially localized wave-packet
representing the centre of mass of a small object, in a quantum field. The
classical force is found to be consistent with the identification of the
Abraham momentum with the mechanical momentum of light, and the motion of the
wave-packet is found to be subject to an acceleration due to the Casimir force,
and a time dependent fluctuating motion due the creation of pairs of
excitations within the object. The theory is valid for any dielectric that has
susceptibilities satisfying the Kramers-Kronig relations, and is not subject
to arguments regarding the form of the electromagnetic energy-momentum tensor
within a medium.
