Quantum mechanics and macrosystems

Anthony J. Leggett

Eriksson, Karl-Erik

A small quantum scattering system (the microsystem) is studied in interaction with a large quantum system (the macrosystem) described by unknown stochastic variables. The interaction between the two systems is diagonal for the microsystem in a certain basis, and it leads to an imprint on the macrosystem. Moreover, the interaction is assumed to involve only small transfers of energy and momentum between the two systems (as compared to typical energies/momenta within the microsystem). This makes it suitable to carry out the analysis in scattering theory, where the transition amplitude for the whole system factorizes. The interaction taking place within the macrosystem is assumed to depend on the stochastic variables in such a way that, on the average, no particular channel is favoured. The result is then, in the thermodynamic limit of the macrosystem, that the whole system bifurcates and the microsystem ends up in a state described by one of the basis vectors (in the mentioned basis). The macrosystem ends up in an entangled state tied to this basis vector. For the ensemble of macrosystems, the interaction with the microsystem leads, on the average, to the usual decoherence and diagonal density matrix for the microsystem. The macrosystem can be interpreted as representing a measurement device for performing a measurement on the microsystem. The whole discussion is carried out within quantum mechanics itself without any modification or generalization.

Günther Ludwig

A. J. Leggett

Vladimir A. Skrebnev, R. N. Zaripov

An experimental study of the applicability of mechanics equations to describing the process of equilibrium establishing in an isolated spin system was performed. The time-reversion effects were used at the experiments. It was demonstrated, that the equations of mechanics do not describe the spin macrosystem transition to the equilibrium. The experimental results correspond to the theory which is based on the non-equilibrium thermodynamics methods and takes into account the quick decay of cross-correlations in the systems.

N. G. van Kampen