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US Scientists Break Kirchhoff’s 165-Year-Old Physics Law

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165-Year-Old Physics Law Falls – Pennsylvania State Scientists Rewrite the Science of Heat

In a groundbreaking scientific achievement, researchers at Pennsylvania State University have challenged Kirchhoff’s Law of Thermal Radiation – a fundamental principle of physics established in 1860. This discovery could open up transformative possibilities in energy harvesting, thermal control, and infrared technology.

Kirchhoff’s Law States:

A material’s ability to emit heat is equal to its ability to absorb it. However, the Penn State team disproved this with a 2-micron-thick metamaterial (a synthetic material engineered to have extraordinary properties) composed of five specialized semiconductor layers. Remarkably, this material emits more heat in one direction than it absorbs.

The team achieved a record emission contrast of 0.43 at the 10-micron wavelength band, which is twice as high as any previous attempt to disrupt this thermal equilibrium. In a reciprocal system, the emission contrast would be zero, which means no directional difference between heat emission and absorption.

Linxiao Zhu, assistant professor of mechanical engineering at Penn State, explained that physicists have suspected for years that Kirchhoff’s Law of Thermal Radiation might not always hold true. But until recently, no one had clearly proven it. The Penn State team directly measured a difference between how much heat a material gives off and how much it takes in, something the law says should be equal.

Zhu added that earlier attempts to break the law usually didn’t produce a big enough difference or work across a wide range of wavelengths. Both of these are crucial for turning the idea into real-world technology.

Key breakthroughs included:

Development of a specialized magnetic thermal emission spectrophotometer
Application of a powerful magnetic field
A thin-layer design suitable for integration into future technologies

Why it Matters:

This discovery could revolutionize solar power systems, enhance infrared sensors, enable advanced thermal diodes, and push the limits of thermodynamic efficiency — boundaries once thought impossible to surpass.

The potential applications of this technology include significantly improving energy harvesting efficiency. As co-first author Zhenong Zhang (doctoral candidate in mechanical engineering) explained, conventional solar cells (in accordance with Kirchhoff’s Law) must radiate some of the absorbed energy back toward the sun. This radiated energy is essentially lost.

Zhang elaborated:

“In standard, or reciprocal, solar cells, part of the captured sunlight is re-emitted back toward the sun, as required by Kirchhoff’s Law. This energy is wasted.”

He added:

“With nonreciprocal emitters, however, we could direct that emission elsewhere. For example, we could aim it at a second solar cell positioned to absorb this energy, thereby increasing the overall power conversion efficiency.”