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Arthur T Knackerbracket has processed the following story:
Wafer-thin sheets of gold shot briefly with lasers can be heated up to 14 times their melting point while remaining solid, far beyond the theoretical limit, raising the possibility that some solids may have no upper melting point at all.
Superheating is a common phenomenon where a solid can heat up beyond its melting point, or a liquid can heat up past its boiling point, without changing state. For example, a cup of water heated in a microwave can reach temperatures above 100°C (212°F), as long as the cup is sufficiently smooth and still. However, as soon as the cup is jostled, the water will violently boil.
For solids, many physicists have proposed an upper limit for superheating, at a temperature around three times the standard melting point in kelvin. This point is called the entropy catastrophe, which is where the entropy, often defined as the amount of disorder in a system, for the solid state would become larger than if the substance were liquid. If the substance remained solid above this temperature, then it would violate the second law of thermodynamics, which says that entropy cannot decrease over time for most systems.
[...] It would also be interesting to see whether this applies to other solids apart from gold, says Vinko, and whether there is any upper limit to heating before melting. “The thing that’s intriguing here is to ask the question of whether or not it’s possible to beat virtually all of thermodynamics, just by being quick enough so that thermodynamics doesn’t really apply in the sense that you might think about it.”
Journal Reference: 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
DOI: 10.1038/s41586-025-09253-y
(Score: 3, Insightful) by VLM on Tuesday July 29 2025, @09:49PM (1 child)
Gold melting point is about "a thousand degrees C" and its coefficient of thermal expansion is 14 ppm/degree C and they claim to have reached 14 times MP so 196000 ppm thermal expansion or 0.196 or about 1/5. Thats a pretty monsterous pressure wave.
Note that off the top of my head (aka I'm probably wrong) isn't the couple femtosecond laser pulse best described by millionths of an inch "thick" so its a short sharp pulse.
If a bulk material thermalizes and the material can flow a couple mean free paths then "temperature" means something but the temperature of something whacked by a laser like that is like trying to define the "temperature" of the plasma inside a helium neon laser tube; its problematic by most definitions. Whats the "temperature" of the innards of a gas thyratron switching tube at the picosecond it turns on; well it depends on what you mean by "temperature" because unmixed quantities of energy over a very short time period are not well defined...
(Score: 3, Insightful) by c0lo on Tuesday July 29 2025, @11:59PM
The technique they used for estimating the temperature is illuminating the sample with a monochromatic X-ray and examining the broadening (by Doppler) of the reflected scattered spectrum - the difference between cold/hot state allows inferring a "temperature" change. The theoretical model they used is the phonon-phonon interaction, of which they say "the phonon–phonon equilibration rate is an order of magnitude higher than the electron–phonon equilibration rate in gold, allowing us to ascribe a single temperature (Ti) to the phonon subsystem" (ie they say the electron gas ΔE is negligible to the ΔE in the lattice shaking wildly due to the extreme laser pulse).
Gotta give it to them, the reflected hot spectrum resembles enough a Maxwell distribution [nature.com] for a decent range for the "resemblance" values.
Note: TFSciA is open access, if you want to read it.
https://www.youtube.com/@ProfSteveKeen https://soylentnews.org/~MichaelDavidCrawford