What comes to your mind when you are asked about measuring temperatures? For most of us, it might be a mercury thermometer that we use to check fevers, or maybe in our physics and chemistry laboratories. The mercury thermometer, in general, can measure temperatures in the range -37 to 356 degrees Celsius. Does this cover the range of temperatures we would see in our daily lives? How about measuring the temperature of something as hot as, say, the sun or the Earth’s core? The outer surface temperature of our sun is estimated to be as hot as 5500 degrees, and the innermost parts millions of degrees. The hottest part of Earth’s core is around 6000 Celsius.
Such extreme temperatures can also be observed in the laboratory, in the context of advanced material processing, space, and aerospace materials. Materials under such extreme conditions come outside the scope of conventional condensed matter systems, and measuring their exact temperatures is notoriously challenging.Â
Recent research by a team of scientists has overcome this challenge by developing a new technique to measure such high temperatures accurately. In the course of their work, they were in for a great surprise: the gold sample they were working on retained its solid structure up to temperatures approaching 19,000 degrees Celsius- nearly 14 times its melting point.
Moving atoms and temperature
To understand how this new method of temperature estimation works, first think about what temperature is. Atoms in matter are continuously in motion with certain speeds. There is a distribution of speeds for any collection of atoms. Temperature is a measure of the average kinetic energy atoms possess, or in simpler terms, a measure of how fast the atoms are moving on average.Â
When an object is kept in contact with another, such that it minimally perturbs the system, it equilibrates with the surrounding temperature, and this is carefully calibrated to give normal thermometers. Mercury expands/contracts upon heating/cooling, respectively, and this is calibrated to measure temperatures in mercury thermometers. Obviously, such a method is not suitable for measurements of the extreme temperatures in question.
Heating a gold sheet to almost 19000 °CÂ
A group of scientists from Stanford University and SLAC had been working on high-temperature measurements for decades. In a recent development, they used highly monochromatic X-rays to measure high temperatures produced by heating gold using intense ultrafast laser pulses. Such intense ultrafast laser beams can deliver large amounts of energy to atoms in an extremely short time, increasing their speeds and, in turn, raising the material’s temperature. Another high-energy monochromatic X-ray probe is shone onto the sample.Â
Such a technique is called a pump-probe measurement: First, a pump (ultrafast laser) is used to generate the state we would like to measure. A probe (X-ray) collects data at different time intervals to understand the changes. To better understand, we can think of an analogy to taking extremely fast photo snapshots, but now of a microscopic system. The X-rays scattered off the sample contain signatures of how fast the atoms are moving in the sample and hence, the temperature.
Faster atoms generate a broader shift in the scattered X-ray energy distribution. This width of energy distribution can be used to calculate the sample’s temperature. All this is done within several picoseconds: to get a feel for it, a picosecond is to one second what one second is to 30,000 years. The researchers were able to heat the gold samples to around 19000 degrees Celsius and measure the temperatures with minimal error.Â
The experiment is carried out in an extremely sophisticated setup: producing high-energy, high-specificity X-rays to measure minute broadenings requires accelerating electrons at high speeds over several kilometers, as at the Stanford Linear Accelerator Center.Â
Gold stayed solid at 19000K
But what is even more surprising is that the sample retained its solid structure up to temperatures approaching 19,000°C – nearly 14 times its melting point. Conventional understanding is that above the melting point, any solid changes into a liquid. However, fast heating can prevent this transition and allow the material to remain in the solid state.
It was long considered that a superheated solid phase cannot exist beyond the point at which the entropy of the solid exceeds that of the liquid, typically estimated to occur around 3 times the melting temperature for most materials. This experiment showed that if heated fast enough, there might not be a ceiling to the existence of the solid state.
The team showed that the heating rate in the experiment is a critical factor. Under equilibrium conditions, heating a solid induces lattice thermal expansion, which contributes significantly to the increase in its entropy. In contrast, given the ultrafast heating rates achieved in this experiment, the lattice expansion is insignificant in the probed timescales, rendering its contribution to entropy negligible.
As a result, the total entropy of the solid remains lower than that of the liquid, thereby resolving the apparent paradox. These findings indicate that, if the heating rate is sufficiently rapid, there may be no upper temperature limit for the existence of a solid.
This exciting experiment shows us how the basics of thermodynamics we study in high school can still bring out wondrous results. Using unprecedented advancements in accurate temperature measurement and precision spectroscopic techniques, more questions can be answered than ever before. This approach may be potentially applied to systems at high pressure and energy density, such as planetary interiors, where precise temperature determination remains a major challenge.
References:
White, T.G., Griffin, T.D., Haden, D. et al. Superheating gold beyond the predicted entropy catastrophe threshold. Nature 643, 950–954 (2025). https://doi.org/10.1038/s41586-025-09253-y
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Gopika Krishnan is a postdoctoral researcher in theoretical soft matter at the University of Illinois, Urbana-Champaign, USA. Her research focuses on exploring the motion of small molecules through large polymers. She’s passionate about weaving science and storytelling together to bring cutting-edge science and technology research closer to the public.
