Importance of Experiments\nThat Can Test Theories Critically

General dynamic and thermodynamic properties of complex\nmaterials,\nincluding amorphous polymers and molecular glass-formers, have been\nestablished from the wealth of experimental data accumulated over\nthe years. Naturally, these general properties attract researchers\nto construct theories and models to address and explain them. Often\nmore than one theory with contrasting or even conflicting theoretical\nbases can equally explain a general property rather well. The correct\nexplanation becomes unclear, and progress is stopped. The resolution\nof the problem comes when an innovative experiment is performed with\ninsightful results that can critically test the premise and assumptions\nof each theory. This important role played by experimentalists is\nexemplified by the contributions of Mark Ediger in several general\nproperties considered in this paper: (1) dynamics of the components\nin binary polymer blends; (2) breakdown of the Stokes–Einstein\nand the Debye–Stokes–Einstein relations; (3) enhancement\nof surface mobility and in relation to formation of ultrastable glasses;\nand (4) the Johari–Goldstein β-relaxation in ultrastable\nglasses. Different theories proposed to explain these properties are\ndiscussed, including the Coupling Model of the author.