Conventional decoherence (usually called 'Environmental
Decoherence') is supposed to be a result of correlations
established between some quantum system and the environment.
'Intrinsic decoherence' is hypothesized as being an essential
feature of Nature - its existence would entail a breakdown of
quantum mechanics. A specific mechanism of some interest is
'gravitational decoherence', whereby gravity causes intrinsic
decoherence.
I will begin by discussing what is now known about the mechanisms of
environmental decoherence, noting in particular that they can and do
involve decoherence without dissipation (ie., pure phase decoherence).
I will then briefly review the fundamental conflict between Quantum
Mechanics and General Relativity, and several arguments that suggest
how this might be resolved by the existence of some sort of 'gravitational
decoherence'. I then outline a theory of gravitational decoherence
(the 'GR-Psi' theory) which attempts to give a quantitative discussion of
gravitational decoherence, and which makes predictions for
experiments.
The weak field regime of this theory (relevant to experimental
predictions) is discussed in detail, along with a more speculative
discussion of the strong field regime.