The alpha-relaxation behaviour of polymers and glass forming viscous liquids is well described by the Vogel-Fulcher-Tammann equation, introducing a Vogel temperature T0 at which the relaxation time tau(alpha) diverges (T0 almost-equal-to T(g) -50 K). With the recent development of the mode coupling theory a new critical temperature T(c) is introduced located about 50 to 80 K above T(g). The relevance of the various temperatures is discussed on the basis of dynamic light scattering studies and dielectric relaxation data. The dynamic and static light scattering experiments revealed some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (1) In static light scattering the intensity I(q --> 0) is no longer proportional to the isothermal compressibility. (2) This excess scattering I(exc) shows a strong q-dependence (q = (4pin/lambda)sin(theta/2)) corresponding to a correlation length xi in the range of 20-200 nm. (3) The Landau-Placzek ratio I(Rayleigh)/2I(Brillouin) is much too high, compared with the results of light scattering theories. (4) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (GAMMA approximately q2) is detected with relaxation rates GAMMA about 10(-4) to 10(-7) lower than those of the alpha-process at a given temperature. These effects are caused by long density fluctuations indicating a nonhomogeneous distribution of free volume. The redistribution of free volume in space causes the new ultraslow mode. A tentative model is proposed which describes the long range density fluctuations as a result of the coexistence of molecules with two different dynamic states, which show up in the Fabry-Perot and Raman spectroscopy.
