For over 200 years, scientists have believed that heat naturally flows from hot objects to cooler ones.
But new research shows that under extreme conditions, heat doesn’t behave the way we expect—it can get stuck between materials, unable to pass from one to another.
Physicists Thomas White from the University of Nevada, Reno, and Cameron Allen, now at Los Alamos National Laboratory, have shown this surprising effect in a new study published in Nature Communications.
Their work involved studying how heat moves between two very different materials—metal and plastic—when both are exposed to incredibly high temperatures and pressures, similar to the intense environments inside stars, planets, or fusion reactors.
In their experiment, they used one of the most powerful lasers in the U.S., the Omega-60 laser in New York, to blast copper and generate intense X-rays.
These X-rays were then used to heat up a tungsten wire that was partially coated in plastic. The tungsten reached a scorching 180,000 degrees Fahrenheit, while the plastic coating stayed much cooler—only about 20,000 degrees.
The researchers expected the heat to quickly move from the hot metal into the cooler plastic. But to their surprise, it didn’t.
Instead, the heat was blocked at the boundary between the two materials. The reason? A phenomenon called interfacial thermal resistance, or ITR.
This happens when the electrons carrying heat from the hot material reach the boundary and bounce back instead of passing into the cooler material.
Though ITR is well known in electronics—like in cooling computer chips—this is the first time it has been observed in high-energy plasmas, the kind of matter found in fusion experiments and deep planetary interiors.
What’s even more astonishing is that this effect was first predicted over 200 years ago by mathematician Joseph Fourier, long before lasers or fusion research existed.
Now, it may help explain why current simulations of laser-driven fusion don’t always match real-life results. These experiments often use targets made of layers of different materials, and if heat isn’t crossing these layers as expected, it could throw off the entire process.
This discovery is a big step forward in understanding how energy moves in extreme environments.
As scientists work to develop fusion energy and other advanced technologies, knowing how heat behaves at these boundaries will be essential for building better, more accurate models—and ultimately, more efficient systems.
