Heat Propagation: A Fourth Mechanism Allowing Heat Transfer from Cold to Hot

Abstract Fourier's law, governing heat conduction through temperature gradients, has been invalidated by heat waves observed in cryogenic heat-pulse experiments and reduced effective thermal conductivity (ETC) in nanoscale heat transfer. Despite persistent efforts over 150 years to find a replacement, this intriguing problem remains unresolved. Moreover, the root cause of reduced ETC and the generation mechanism and speed of second sound in hydrodynamic heat transfer have remained elusive. Our investigation reveals a groundbreaking discovery: a temperature-gradient-independent propagation of heat, encompassing both ballistic and hydrodynamic transfer, enables heat transfer from cold to hot without violating the Second Law of Thermodynamics. This identifies heat propagation as a fourth mechanism alongside conduction, convection, and radiation, revolutionizing traditional understanding of heat transfer beyond hot-to-cold scenarios. This discovery exposes the root cause for the breakdown of Fourier’s law and leads to a physics-based propagation-diffusion model to replace Fourier’s law. Incorporating the collective particle-wave duality of phonons, this model successfully replicates heat-pulse experiments and attributes reduced ETC to delayed heating effects. Furthermore, it challenges the established theory of hydrodynamic heat transfer driven by temperature gradients, offering an exceptionally efficient approach to thermal management, surpassing thermal conductivity limits, with profound implications across scientific and engineering disciplines.