The role of the observer in physical theories in the sense of the observer’s viewpoint determining prominent features of the observed phenomena is discussed with reference to the problem of irreversibility and quantum measurement, the latter being closely related to the problem of classical behaviour of quantum systems. A completely subjective interpretation of irreversibility is proposed. It is claimed that irreversibility belongs only to phenomena as observed by a restricted observer who must project all phenomena on a restricted subset of the set of all possible states. The possibility of a completely unrestricted observer who does not see irreversibility is discussed.

The extraordinarily wide 22 GHz emission from W49A and two other galactic HII regions is discussed, and arguments presented why the entire width of these spectra cannot be attributed to the 6₁₆ → 5₂₃ transition of H₂O. It is suggested that most of the weak emission with large frequency shifts is due to stimulated Raman scattering of the strong central features appearing at the expected frequency of the water vapour line. It has not been possible to identify the scattering agent, but it is predicted that the spectra of millimetre-wave maser sources should also show such frequency-shifted features.

The problem of the subjective nature of entropy and its relation to information and irreversibility is examined in the light of the quantum measurement problem. The main thesis of the paper is that state collapse during a measurement and hence entropy increase in the observed universe is seen by observers who are only able to observe a restricted manifold of states determined by their concepts, language, etc., in short by their level of perception. The thesis leads to the assertion that any universe with a structure must evolve.

The problem of selection of preferred basis during passage from quantum to classical systems is treated with the help of a simple example of a 2-state system like the sugar molecule. A simple principle leading to this selection is stated and demonstrated in case of the chosen example. The principle, stated simply is that the preferred basis is the one in which the system environment interaction hamiltonian is diagonal.