Heat has always been something we thought we understood. From baking bread to running engines, the idea seemed simple: heat spreads out smoothly, like water soaking through a sponge. That simple picture, written down by Joseph Fourier 200 years ago, became the foundation of modern science and engineering.
But zoom into the nanoscale—inside the chips that power your smartphone, AI hardware, or next-generation solar panels—and the story changes. Here, heat doesn't just "diffuse." It can ripple like sound waves, remember its past, or flow in elegant streams like a fluid in a pipe. For decades, scientists had pieces of this puzzle but no unifying explanation.
Now, researchers at Auburn University and the U.S. Department of Energy's National Renewable Energy Laboratory have delivered what they call a "unified statistical theory of heat conduction."
"Fourier's law was written 200 years ago; this breakthrough rewrites the rules for how heat conducts in the nanoscale and ultrafast world of today," said Prof. Jianjun (JJ) Dong, Thomas and Jean Walter Professor of Physics at Auburn University.
The new theory, recently published in Physical Review B, links the chaotic jiggling of atoms—the vibrations that carry heat—to the strikingly unusual ways heat moves in tiny, complex materials. Instead of relying on fragmented models for different scenarios, Dong and co-author Dr. Yi Zeng (National Renewable Energy Laboratory) developed a single comprehensive framework that explains it all: diffusion, waves, ballistic transport, and the quirky behavior at interfaces between materials.
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