# How does a system interact with environment in quantum mechanics? When would this interaction occur? What is it?

As title says, how does a system interact with environment? I realize that this interaction can lead to interference terms and non-diagonal terms in density matrix being reduced (quantum decoherence). But what exactly is this interaction of system and environment? What would be the example of this interaction and when does this interaction occur?

• This does not make much sense, because quantum mechanics is about very small dimensions, and "system" and "environment" are blanket words for large ensembles of particles of these small dimensions. The density matrix formalism has been developed for this purpose. What do you mean by "system" and "environment"? – anna v Jul 27 '14 at 5:28
• For example, Lubos Motl, in his blog motls.blogspot.kr/2009/09/…, talks about interaction. "In general, the photon states are not exactly orthogonal. But when you calculate how quickly the process destroys the off-diagonal elements of the density matrix, it is extremely fast. Even the interactions with the cosmic microwave background are enough for a very tiny speck of dust to decohere within a tiny fraction of a second." – user55624 Jul 27 '14 at 5:33
• If wavefunctions of two "systems"(well, let us set aside QFT for now) have been not connected before interaction, how do they really interact? Is there some sorts of wavefunction of whole universe (universal wavefunction) that how things occur together non-locally or locally is probabilistically known? Or is there any theory that talks about how these interactions will take place between separate wavefunctions? – user55624 Jul 27 '14 at 5:36
• At this level with photon exchanges, there is nothing else long distance acting and as strong, no? – anna v Jul 27 '14 at 7:59
• It is just the contrary: Decoherence destroys interference and non-diagonal terms in the reduced density matrix... – Trimok Jul 27 '14 at 12:16

• yes a $\bf{Markovian}$ and time local master equation for a CPT map has to be Lindblad form, however to actually derive Lindblad from the underlying microscopic physics (tracing over is you how you do it) to study open systems with it you have to make several assumptions which are not generic for open systems. One is usually left with Redfield equation. – Bubble Sep 1 '14 at 10:45