Diverse material classes exhibit qualitatively similar behavior when made viscous upon cooling toward the glass transition, suggesting a common theoretical basis. We used seven different measurement methods to determine the mechanical relaxation kinetics of a prototype molecular glass former over a temporal range of 13 decades and over a temperature range spanning liquid to glassy states. The data conform to time-temperature superposition for the main (alpha) process and to a scaling relation of schematic mode-coupling theory. The broadband mechanical measurements demonstrated have fundamental and practical applications in polymer science, geophysics, multifunctional materials, and other areas.viscous liquids | broadband mechanical spectroscopy | mode-coupling theory T he extraordinary slowing down of viscous liquid dynamics upon cooling toward the glassy state plays a key role in myriad contexts, including polymer processing, survival of living organisms in extreme cold, amorphous metal synthesis, and many others. Glass-forming liquids display a number of common features, despite quite different chemistry ranging from hightemperature covalently bonded glass formers to van der Waals liquids that typically form glasses below room temperature (1-6). This commonality makes the research area attractive from a theoretical point of view and motivates the detailed study of selected liquid systems in the hope of revealing generic features of glass-forming liquids.Viscoelastic relaxation behavior derives from two distinct and sequential processes common to all glass-forming liquids. The primary or "alpha" structural relaxation dynamics, which dictate the time scales for molecular diffusion and flow, are nonexponential in time, typically extend over several decades of time scales at a single temperature, and shift dramatically from picoseconds at high temperatures and low viscosities to many seconds as the sample is cooled and the glassy state is approached. This behavior gives rise to broad loss peaks in elastic compliance spectra, covering an extended frequency range at any temperature and shifting from gigahertz to millihertz frequencies as the temperature is lowered (Fig. 1). In addition to the temperature-dependent alpha relaxation dynamics, local rearrangements of molecules within existing cage geometries of the molecules, the so-called (fast) "beta" relaxation processes, result in a higher-frequency feature in the loss spectrum.This scenario applies in the simplest cases when structural relaxation is slowed down through obstruction among neighboring molecules as in van der Waals liquids but not through extensive networks as in hydrogen-bonded liquids or entanglements as in polymers. In general, additional so-called Johari-Goldstein beta processes are observed (7,8). For the present study, we chose a liquid with no such additional processes, to make the analysis as simple as possible.A variety of semiempirical models (9-14), as well as the firstprinciples mode-coupling theory (MCT) (15, 16), have been developed in a...