Physicists Superheated Gold to Hotter Than the Sun’s Surface and Disproved a 40-Year-Old Idea
A thin piece of gold reached 33,740 degrees Fahrenheit, which is more than 14 times higher than its melting point, by being rapidly heated—and it didn’t melt
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Researchers recently heated a solid sample of gold to 19,000 Kelvin (33,740 degrees Fahrenheit)—which is hotter than the surface of the sun—without melting it.
The process is called superheating, and as if that achievement wasn’t impressive enough on its own, the work claims two other major breakthroughs: Researchers directly measured such scalding temperatures for the first time ever, and they disproved a long-established prediction about superheating of solids.
It’s difficult to measure the temperatures of extremely hot things, including “warm dense matter,” such as the sun’s plasma, the cores of planets and the insides of human-made fusion reactors. Researchers have had to estimate these temperatures with huge margins of error. That is, until now.
Key concept: What is superheating?
Superheating occurs when a solid is heated past its melting point and stays solid—or when a liquid is heated past its boiling point but doesn’t vaporize.
As detailed in a study published last week in the journal Nature, an international team of scientists superheated a thin slice of solid gold with a laser. They then shot the sample—which was only 50 nanometers thick—with a pulse of X-rays that bounced off the gold’s superheated, vibrating atoms. By measuring the change in the reflected X-rays’ frequency, the researchers were able to measure the gold atoms’ velocity and temperature.
The temperature of the gold “was extremely surprising,” lead author Thomas White, a physicist at the University of Nevada, Reno, tells Scientific American’s Clara Moskowitz. “We were totally shocked when we saw how hot it actually got.”
That’s because a long-standing theory from the 1980s suggests the superheating of solids cannot surpass around three times the solid’s melting temperature. This limit, known as the “entropy catastrophe,” is the point at which solids were thought to spontaneously melt, according to a University of Nevada, Reno, statement.
Even though some previous research had demonstrated that rapidly heating materials could avoid these limits, “the entropy catastrophe was still viewed as the ultimate boundary,” White explains in a SLAC National Accelerator Laboratory statement.
Needless to say, at 19,000 Kelvin, the solid gold sample blew past that boundary, heating up to more than 14 times its melting point, which is about 1,300 Kelvin. The team suggests the speed of the heating likely kept the gold from expanding. They blasted the gold to its record-setting temperature in just 45 femtoseconds, or 45 millionths of a billionth of a second.
“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,” Sam Vinko, a physicist at the University of Oxford in England who did not participate in the study, tells New Scientist’s Alex Wilkins.
The team notes that the second law of thermodynamics, which states that disorder increases with time, still stands—their work did not disprove it. That’s because the gold atoms reached their extreme temperature before they had time to become disordered, White tells Nature’s Dan Garisto.
Even still, researchers are now faced with a question they had considered all but completely solved nearly four decades ago, per New Scientist: How hot can something really get before it melts? If a material is heated quickly enough, there might be no limit, per the SLAC statement.
This would be a much more difficult question to investigate without the team’s new approach. In fact, the study’s “biggest lasting contribution is going to be that we now have a method to really accurately measure these temperatures,” Bob Nagler, a staff scientist at SLAC, tells Scientific American.
Nagler and his colleagues’ work paves the way for studying more “warm dense matter,” such as modeling the inside of planets and fusion reactors.
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