It’s not easy to surprise physicists with a phase change. Yet when Tom White first reviewed the data from a new experiment that used lasers to heat gold, he had to pause and double-check his results.
He had good reason to be skeptical. White and his colleagues had just observed solid gold reaching an astonishing 19,000 kelvins (33,740 degrees Fahrenheit or 18,726 degrees Celsius) — more than 14 times its melting point — without melting. For a brief instant lasting just trillionths of a second, the atoms held their crystalline form, stubbornly refusing to liquefy under conditions once thought thermodynamically impossible.
“We were surprised to find a much higher temperature in these superheated solids than we initially expected,” said White, a physicist at the University of Nevada, Reno. “This wasn’t our original goal, but that’s what science is about — discovering new things you didn’t know existed.”
How is this possible without breaking the laws of physics?
The laws of thermodynamics rule that there’s a hard upper limit to how much heat a solid can absorb without transforming into a liquid. Go beyond that limit, and theory warned of an “entropy catastrophe” — a tipping point where the disordered state of a solid would exceed that of a liquid, a direct violation of the Second Law of Thermodynamics.
That threshold was pegged at about three times a material’s melting temperature. Anything higher was considered impossible — until now.
In a study published in Nature, White and his team used a finely tuned laser to heat a film of gold just 50 nanometers thick. Within 45 femtoseconds — less time than it takes light to cross the width of a human hair — the sample’s atoms began vibrating furiously.
Using the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in California, the team blasted the superheated gold with pulses of ultrabright X-rays. The scattered X-ray photons revealed just how fast the atoms were vibrating, allowing researchers to directly measure the temperature of the ions.
Their results shocked even the most seasoned experimentalists.
Skirting a Thermodynamic Collapse
The idea that a solid could persist in such an ultra-hot state upends more than 40 years of theoretical work. In 1988, physicists Hans Fecht and William Johnson introduced the concept of an “entropy catastrophe,” arguing that no solid should exist with more disorder than its liquid counterpart.
But these limits only apply in certain conditions when materials are heated slowly enough to maintain thermal equilibrium. White’s experiment avoided that trap. By heating the gold faster than the atoms could rearrange themselves — at rates exceeding a quadrillion degrees per second — the team essentially froze the atoms in place.
“It’s important to clarify that we did not violate the Second Law of Thermodynamics,” White said. “What we demonstrated is that these catastrophes can be avoided if materials are heated extremely quickly — in our case, within trillionths of a second.”
At the higher heating rate of 6 × 10¹⁵ kelvin per second, the gold reached 19,000 K (about 14 times its melting point). Even at a lower heating rate, the material hit 13,800 K, still shattering the supposed 3× limit.
A New Thermometer for the Extreme
Key to the breakthrough was the use of inelastic X-ray scattering — a method akin to taking an atomic-speed radar reading. When X-rays hit vibrating atoms, their frequency shifts slightly due to the Doppler effect. The broader the spread of frequencies in the scattered photons, the hotter the atoms.
Bob Nagler, a staff scientist at SLAC and co-lead of the study, called it “a decades-long problem” finally solved.
“We have good techniques for measuring density and pressure of these systems, but not temperature,” Nagler said. “In these studies, the temperatures are always estimates with huge error bars, which really holds up our theoretical models.”
This experiment changed that. Using just a few hundred scattered X-ray photons collected over dozens of shots, the researchers extracted reliable temperature readings directly and without depending on simulations.
Nagler believes this is just the beginning. “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.”
Moving Beyond the Gold
Understanding how materials behave at extreme temperatures and pressures is essential for everything from designing fusion reactors to modeling planetary interiors. Until now, temperature was the least certain variable in those models.
Researchers who study “warm dense matter,” an exotic state found inside giant planets and during the first instants of fusion reactions, have long operated with uncertainty around thermal measurements. This study provides a new, direct method to calibrate those models.
Inertial fusion energy research needs to know how hot fusion fuel targets get when they implode. Now, we finally have a way to make those measurements.
Even more intriguingly, the researchers believe silver may also surpass its entropy limit, based on preliminary data.
To understand why the gold didn’t melt, it helps to consider how phase changes typically unfold. In conventional melting, heat causes atoms to jiggle out of alignment and disrupt the orderly crystal lattice. But in this experiment, there was simply no time.
The gold lattice never had a chance to expand. Bragg peaks — patterns of X-rays reflecting from the atomic planes — stayed firmly in place during the first few picoseconds. That lack of expansion, the researchers argue, prevented the onset of the entropy catastrophe. Without expansion, the entropy of the solid never crossed that of the liquid.
“The crossing of the two entropy curves is effectively eliminated by ultrafast intense heating,” the study concludes. “Superheating may not have an upper bound.”
Rethinking the Rules
So, where does this leave the entropy catastrophe?
Like many good scientific ideas, it’s not discarded — but revised. The limit remains valid under conditions of thermal equilibrium. But when matter is pushed out of equilibrium, as in this experiment, new regimes emerge.
The team’s data shows that gold can exist as a solid far beyond what was thought to be its thermodynamic breaking point — if the heating is fast enough.
Now, the challenge is to understand how widespread this phenomenon might be. Could other metals behave the same way? Could this be used to design materials with new properties? And how does this affect our models of matter in stars and fusion plasmas?
For now, one thing is clear: the limits of heat just got a lot hotter.