We study the thermal fluctuations of an optically confined probe particle, suspended in an aging colloidal suspension, as the suspension transforms from a viscous liquid into an elastic glass. The micron-sized bead forms a harmonic oscillator. By monitoring the equal-time fluctuations of the tracer, at two different laser powers, we determine the temperature of the oscillator, To. In the ergodic liquid the temperatures of the oscillator and its environment are equal while, in contrast, in a nonequilibrium glassy phase we find that To substantially exceeds the bath temperature.Understanding the slow dynamics of glasses is one of the most fascinating yet difficult challenges in statistical physics. One question which has attracted considerable interest is whether the dynamical fluctuations of a glass can be characterized by a non-equilibrium temperature. A rigorous thermodynamic temperature is strictly impossible to define for an aging glass, which remains far from thermal equilibrium even on long-time scales. Nevertheless various groups [1,2] have proposed "effective" temperatures with many of the properties expected for an equilibrium temperature. These ideas lead to the surprising prediction of two distinct temperatures in an aging glass [1]. The fast rattling modes of particles inside the cage constituted by neighbors thermalize rapidly to the temperature of the environment, T bath , while the much slower structural rearrangement of these cages are supposed to be characterized by a second temperature, T eff , which mean-field models predict should exceed T bath . To date most of the support for this striking two-temperature picture has emerged from simulation results [2] on idealized glasses. Experiments have so far produced conflicting results. Studies of colloidal glasses have reported that T eff increases [3], remains unchanged [4], or even decreases [5] with age in contrast to measurements on structural [6] and spin glasses [7] which have revealed effective temperature warmer than the bath temperature.In this Letter, we report the temperature of a micrometer-sized sphere immersed in an aging colloidal suspension, as the suspension transforms from a fluid to a glass. The particle, captured in an optical trap, constitutes a microscopic harmonic oscillator whose fluctuations probe the nonequilibrium dynamics of the aging glass. We measure the equal-time fluctuations of this local oscillator and show that, in an ergodic phase, the temperature of the oscillator T o equals the environment temperature T bath of the system, as required by equilibrium statistical mechanics. Significantly, when we repeat the measurements in an aging glass we find a higher temperature and T o > T bath .All experiments to date on colloidal glasses [3,4,5] have relied on active, driven measurements. We conduct our experiments instead in a quasi-static limit which has several advantages. First, the experiments are simpler because there is no need to characterize the complete time-dependent response of the system. Second, the lack of an ...
