  The microscopic mechanism of thermal transport in liquids and amorphous
solids has been an outstanding problem for a long time. There have been several
different approaches to explain the thermal conductivities for these systems,
for example, the Bridgman's formula for simple liquids, the concept of the
minimum thermal conductivity for amorphous solids, and the thermal resistance
network model for amorphous polymers. Here, we present a ubiquitous formula to
explain the thermal conductivities of liquids and amorphous solids in a unified
way. The calculated thermal conductivities using this formula without fitting
parameters are in excellent agreement with the experimental data for these
systems. Our formula is not only providing detailed implications on microscopic
mechanisms of heat transfer in these systems, but also solves the discrepancies
between existing formulae and experimental data.
