The problem of identifying the system and the environment in the phenomenon of decoherence

MARIO CASTAGNINO; SEBASTIAN FORTIN; OLIMPIA LOMBARDI

Amsterdam

Congreso; Second Conference of the European Philosophy of Science Association EPSA09; 2009

European Philosophy of Science Association (EPSA)

In the literature on quantum mechanics, ‘decoherence’ refers to the quantum process that turns a coherent pure state into a decohered mixed state, which is diagonal in a well-defined basis.  The orthodox account of the phenomenon is given by the so-called ‘environment-induced decoherence’ (EID) approach, according to which decoherence is a process resulting from the interaction of a quantum system and its environment.  From the einselection view, the split of the Universe into the degrees of freedom which are of direct interest to the observer -the system- and the remaining degrees of freedom -the environment- is absolutely essential for decoherence.  However, since the environment may be external (a “bath” of particles interacting with the system of interest) or internal (such as collections of phonons or other internal excitations), the EID approach offers no general criterion for deciding where to place the “cut” between system and environment.  Zurek recognizes that this lack of a general criterion to discriminate between system and environment is a serious difficulty of his proposal. The main purpose of this paper is to argue that this “looming big” problem is actually a pseudo-problem, which is simply dissolved by the fact that the splitting of a closed quantum system into an open subsystem of interest and an environment is just a way of selecting a particular set of the relevant observables of the whole closed system. But since there are many different sets of relevant observables depending on the observational viewpoint adopted, the same closed system can be decomposed in many different ways: each decomposition represents a decision about which degrees of freedom are relevant and which can be disregarded in any case. Since there is no privileged or “essential” decomposition, there is no need of an unequivocal criterion for deciding where to place the cut between the system and the environment.