A wide variety of amorphous materials exhibit similar behavior in their low temperature thermal properties regardless of their chemical composition. Examples include a linear specific heat and a T2 thermal conductivity. These features have been attributed to tunneling two level systems (TLS). However, the standard TLS model has not been able to explain the universally small value of phonon scattering reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound velocity. For example, the mean free path of phonons in amorphous SiO2 is about 60 microns at 0.5 K. Why do phonons go so far? We present an answer based on aspects of the standard model that have either been ignored or not fully appreciated. We find good agreement between experiment and theory for a variety of individual glasses.
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