DECOHERENCE OF QUANTUM-FIELDS - POINTER STATES AND PREDICTABILITY

We study environmentally induced decoherence of an electromagnetic fie ld in a homogeneous, linear, dielectric medium. We derive an independe nt oscillator model for such an environment, which is sufficiently rea listic to encompass essentially all linear physical optics. Applying t he ''predictability sieve'' to the quantum field, and introducing the concept of a ''quantum halo,'' we recover the familiar dichotomy betwe en background field configurations and photon excitations around them. We are then able to explain why a typical linear environment for the electromagnetic field will effectively render the former classically d istinct, but leave the latter fully quantum mechanical. Finally, we su ggest how and why quantum matter fields should suffer a very different form of decoherence.