This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Heat, it turns out, has a memory. A cooling cup of coffee may not seem particularly thoughtful.
At the scale of a kitchen, heat appears to follow a straightforward rule: it moves from warmer places to cooler ones. Leave the cup unattended long enough, and the disappointing result offers convincing evidence that this rule works. But shrink the system to the dimensions of a modern computer chip—or observe it over just trillionths of a second—and this simple description can become incomplete.
Heat flowing at one place and time may still carry the influence of a temperature disturbance that occurred earlier or somewhere else in the material. In other words, heat can retain a kind of physical memory. A new theoretical framework developed by Jianjun "JJ" Dong, the Thomas and Jean Walter Professor in the Department of Physics at Auburn University, and Yi Zeng, an Auburn alumnus who earned his Ph.D. in mechanical engineering in 2019 and is now a research scientist at the Department of Energy's National Laboratory of the Rockies, provides a unified way to describe that memory.
Their study, "Unified Statistical Theory of Heat Conduction in Nonuniform Media," was published in Physical Review B. "Heat does not remember in the way that we remember a person or an event," Dong said. "Its memory is stored in the microscopic motion of the material.
The heat flowing at this moment can still carry information about a temperature disturbance that occurred earlier." For roughly two centuries, scientists and engineers have relied on Fourier's law to describe heat conduction. It assumes that heat flow at a particular location responds immediately to the temperature gradient at that same location. This local and instantaneous description works remarkably well for familiar objects and at ordinary scales.
It helps engineers predict how buildings retain heat, how engines cool and how warmth spreads through cookware. In simple terms, Fourier's law describes heat flow as responding to conditions "here and now." At very small length scales and short times, however, "here and now" may no longer tell the complete story. In crystalline solids, heat is often carried by collective atomic vibrations called phonons.
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