Deep inside stars, giant planets, and even Earth’s core, matter exists in a strange in-between state—neither solid nor fully plasma. Known as warm dense matter, this extreme form of matter shapes how planets generate magnetic fields and how nuclear fusion unfolds.
However, despite being common across the universe, warm dense matter has remained a mystery because it’s even hotter than the surface of the Sun, and impossible to touch with conventional instruments.
Now, after nearly a decade of work, a team of researchers has finally found a way to directly measure one of its most important properties, electrical conductivity, without ever touching it. The breakthrough reveals a novel way to explore environments that were previously beyond experimental reach.
How light solved a problem that heat made impossible
Until now, scientists could only estimate how well warm dense matter conducts electricity. Traditional methods require wires or probes to be physically connected to a material, but that approach fails instantly when temperatures soar to thousands of degrees.
As a result, much of what physicists believed about warm dense matter relied on indirect clues and theoretical models that were difficult to test. The study authors tackled this challenge by removing contact altogether. Instead of touching the material, they used light.
First, the researchers took a thin aluminum sample and blasted it with a powerful laser, rapidly heating it to about 10,000 kelvin, nearly twice the temperature of the Sun’s surface. At this point, the aluminum entered the warm dense matter state.
Next came the key innovation: the team fired terahertz radiation, a form of light with very short wavelengths, at the glowing-hot sample. This light induced an electric field inside the aluminum without making physical contact.
By carefully measuring how the material responded to that field, the researchers could calculate its electrical conductivity directly. According to Siegfried Glenzer, a senior study author, “this is the most accurate technique yet for measuring conductivity in warm dense matter to date.”
What the team observed surprised them. As the aluminum heated up, its ability to conduct electricity dropped sharply—not once, but twice. The first drop happened as expected when the material shifted from solid metal to warm dense matter. The second drop, however, had never been clearly seen before.
To understand why, the researchers turned to another powerful instrument at the Department of Energy’s SLAC National Accelerator Laboratory and performed ultrafast electron diffraction. By firing high-energy electrons through the sample, they captured atomic-scale snapshots of the aluminum as it changed on timescales of a millionth of a millionth of a second.
These measurements revealed that the second conductivity drop occurred when the atomic structure of the warm dense aluminum suddenly lost its orderly arrangement and became disordered.
Now scientists can explore the extreme
This new contactless method does more than solve a technical problem. It gives physicists a reliable way to test and refine models of matter under extreme conditions. Such models are essential for understanding stars, planetary interiors, and fusion energy.
It could help scientists better explain how Earth’s magnetic field is generated and improve designs for nuclear fusion experiments, where materials face similar extremes.
There are still limitations, though. For instance, so far, the method has been demonstrated only on aluminum, which is a relatively simple metal. However, the team is already planning to expand its work.
“I’m looking forward to making these measurements on more complicated materials, as well as materials relevant to Earth’s core, like iron,” Benjamin Ofori-Okai, first author of the study and a postdoctoral researcher at Stanford University, said.
For decades, warm dense matter was something physicists knew existed but could barely study. With light now serving as a precise, non-contact probe, that barrier is finally breaking—bringing some of the universe’s most extreme environments within experimental reach for the first time.
The study is published in the journal Nature Communications.