In a landmark experiment, scientists have directly measured the temperature of atoms in “warm dense matter” for the first time, overturning four decades of theory and redefining the limits of how hot solids can get without falling apart.

Led by researchers at SLAC National Accelerator Laboratory and University of Nevada, Reno, the study reveals that materials like gold can withstand extreme temperatures when heated fast enough.

The study offers a method to measure temperatures in matter as hot as planetary cores and fusion reactors, systems where temperature has always been the hardest variable to pin down.

“We have good techniques for measuring density and pressure of these systems, but not temperature,” said Bob Nagler, staff scientist at SLAC.

“In these studies, the temperatures are always estimates with huge error bars, which really holds up our theoretical models. It’s been a decades-long problem.”

Gold breaks the rules

To get around the problem, the researchers developed a new method that avoids indirect models. At SLAC’s Matter in Extreme Conditions (MEC) instrument, they used a laser to superheat a gold sample just nanometers thick. A flash of ultrabright X-rays followed.

As the atoms vibrated from the heat, the X-rays scattered, shifting in frequency in a way that directly revealed their temperature.

“Finally, we’ve directly and unambiguously taken a direct measurement, demonstrating a method that can be applied throughout the field,” said Tom White, associate professor of physics at the University of Nevada, Reno.

The numbers shocked the team. Gold, typically expected to melt around 1,337 kelvins, reached a staggering 19,000 kelvins while retaining its solid crystalline structure.

That’s 14 times its melting point and far beyond the theoretical threshold where solids are supposed to fall apart.

“We were surprised to find a much higher temperature in these superheated solids than we initially expected, which disproves a long-standing theory from the 1980s,” White said. “This wasn’t our original goal, but that’s what science is about—discovering new things you didn’t know existed.”

Implications for fusion science

The finding also challenges a long-held assumption known as the entropy catastrophe, the idea that there’s a hard upper limit to how far you can push solid matter before it melts or vaporizes suddenly.

The gold didn’t break down because it was heated so quickly that it had no time to expand or lose its structure.

“It’s important to clarify that we did not violate the Second Law of Thermodynamics,” White said with a chuckle.

“What we demonstrated is that these catastrophes can be avoided if materials are heated extremely quickly—in our case, within trillionths of a second.”

Nagler believes researchers may have already crossed this limit in past experiments, without realizing it, because they couldn’t measure temperatures directly. With this method, that changes.

“If our first experiment using this technique led to a major challenge to established science, I can’t wait to see what other discoveries lie ahead,” he said.

The study is published in the journal Nature.