The new concept of quantum reference systems is introduced, and a corresponding new foundation of nonrelativistic quantum mechanics is given in terms of a set of postulates. The resulting theory gives an explanation for the measurement without assuming an a priori classical background. Schrödinger's cat paradox and the EinsteinPodolsky-Rosen paradox gain resolution, too. It is also shown that, despite of the violation of Bell's inequality, quantum mechanics is a local theory.

We present the general dielectric formalism for Bose-Einstein condensed systems in an external potential at finite temperatures. On the basis of a model arising within this framework as a first approximation in an intermediate-temperature region for the large condensate we calculate the damping of low-energy excitations in the collisionless regime. ͓S1050-2947͑98͒50511-2͔PACS number͑s͒: 03.75.Fi, 67.40.Db, 05.30.Jp G 1,1 ͑ r ជ,,rЈ ជ ,Ј͒ϭG 2,2 ͑ rЈ ជ ,Ј,r ជ, ͒ ϭϪ͗T ͓ ͑r ជ,͒ † ͑rЈ ជ ,Ј͔͒͘, G 1,2 ͑ r ជ,,rЈ ជ ,Ј͒ϭϪ͗T ͓ ͑r ជ,͒ ͑rЈ ជ ,Ј͔͒͘;and G 2,1 (r ជ,,rЈ ជ ,Ј) is obtained by replacing by † in G 1,2 (r ជ,,rЈ ជ ,Ј). In the Fourier transforms G ␣,␤ (r ជ, rЈ ជ ,) and (r 1 ជ ,r 2 ជ ,) the frequency takes on only the discrete values

The presence of undamped harmonic center of mass oscillations of a weakly interacting Bose gas in a harmonic trap is demonstrated within the dielectric formalism for a previously introduced finite temperature approximation including exchange. The consistency of the approximation with the Kohn theorem is thereby demonstrated. The Kohn modes are found explicitly, generalizing an earlier zero-temperature result found in the literature. It is shown how the Kohn mode disappears from the single-particle spectrum, while remaining in the density oscillation spectrum, when the temperature increases from below to above the condensation temperature. 03.75.Fi,05.30.Jp,67.40.Db

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