200-Year-Old Physics Law Might Have Some Major Exceptions

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Heat transfer is among the oldest known laws of physics. First formalized by Netwon and then generalized by Jean-Baptiste Joseph Fourier, the eponymous Fourier’s Law has been unrivaled for centuries to explain how heat diffuses through a solid object. However, researchers have now discovered that the laws are definitely not complete.

It had been established that at the nanoscale Fourier’s law doesn’t explain all the transfer of heat. But still, the law could be seen as a generalization that works on the macroscale. Yet researchers were curious to see if those exceptions could also happen in something big – and it turns out they do under the right conditions.

“This research began with a simple question,” Steve Granick, Robert K. Barrett Professor of Polymer Science and Engineering at the University of Massachusetts Amherst and the paper’s senior author, said in a statement. “What if heat could be transmitted by another pathway, not just the one that people had assumed?”

Their simple idea was that heat can transfer through conduction in a solid but if that solid was transparent it could also transfer by radiation. They tested this in translucent polymers and inorganic glasses.

They placed these materials in a vacuum so air transfer didn’t play a role, and they heated up one side of these materials with a laser and measured with thermal cameras the diffusion of heat. Fourier’s Law alone could not explain what the team was able to observe.

“No one has tried this before,” added lead author Kaikai Zheng also from UMass Amherst. “There’s something unexpected happening within translucent polymers.”

“It’s not that Fourier’s Law is wrong,” Granick was quick to stress, “just that it doesn’t explain everything we see when it comes to heat transmission. Fundamental research like ours gives us an expanded understanding of how heat works, which will offer engineers new strategies for designing heat circuits.”

The team believes that the translucent materials allow for energy to radiate internally. And this radiation heats up imperfections in the material which become secondary heat sources that also radiate through the material. This is why opaque materials do not show such a deviation from Fourier’s law.

The paper is published in the Proceedings of the National Academy of Sciences.