Physicists Just Found a 67-Year-Old “Demon” And It Could Change How We Power the World

Somewhere inside a small, silvery crystal in a university laboratory, something was hiding. It had no mass. It carried no electric charge. It passed through matter without leaving a trace that any conventional instrument could read. For nearly seven decades, physicists knew it should exist theoretically, at least, yet every attempt to catch it came up empty. Some scientists stopped trying altogether.

So when a research team at the University of Illinois Urbana-Champaign sat down to study a rather unremarkable metal, finding what they found was just about the last thing any of them expected.

A Physicist’s Bold Prediction

Back in 1956, theoretical physicist David Pines put forward an idea that most of his peers found, at best, speculative. Pines argued that electrons inside certain metals could do something strange. Under the right conditions, electrons from two separate energy bands inside a solid could combine and oscillate in opposition to each other’s peaks, lining up with valleys, effectively canceling out their own charge. What would emerge from this odd behavior, Pines said, would be a new kind of particle altogether.

He called it a “demon,” short for “distinct electron motion.” Not because it was dangerous or mysterious in any supernatural sense, but because it defied the normal rules. A demon, as Pines described it, would be massless and carry no electric charge. It would interact with nothing that physicists typically used to probe matter. And because it carried no mass, it could, in theory, exist at any temperature, including room temperature. It was a radical claim. And for 67 years, no one could prove it.

Why It Stayed Hidden So Long

To understand why demons proved so difficult to detect, it helps to know how physicists usually look for things inside materials. Most experiments rely on light, specifically on how different particles interact with photons. Charge is what makes that possible. When a particle carries an electric charge, light can push and pull it, scatter off it, and leave a measurable signature.

Pines’ demon, by design, carries no charge at all. Light passes straight through it, producing nothing. Standard optical experiments, which account for the vast majority of condensed matter research, return a blank result. A demon could sit inside a metal for centuries without ever showing up on a conventional readout. What physicists needed was a completely different type of experiment, one that didn’t rely on light at all.

An Unlikely Candidate

Enter strontium ruthenate, a layered metal with a puzzling reputation. For years, physicists had noticed that it behaves remarkably like a high-temperature superconductor without actually being one. It sits in a strange scientific gray zone, sharing properties with some of the most exotic materials in physics while resisting the label that would make it truly interesting.

That made it worth studying. Peter Abbamonte, a physics professor at the University of Illinois Urbana-Champaign, wanted to understand why strontium ruthenate had these superconductor-like qualities. So his team set out to map the metal’s electronic properties using a specialized technique called momentum-resolved electron energy-loss spectroscopy, or M-EELS. Rather than probing the material with light, M-EELS fires electrons directly at a crystal and measures the energy of those electrons as they bounce back. It was, at the time, a fairly unusual method, and it was applied to a material that hadn’t been studied this way before. What happened next had nothing to do with what anyone was actually looking for.

“We Basically Laughed It Off”

As the team sorted through their data, something odd kept appearing. Deep in the low-energy readings, there was a modeof an electronic signal that didn’t fit. It moved too slowly to be a surface plasmon, a well-known phenomenon in which electrons at a material’s surface ripple like waves. But it moved far too quickly to be an acoustic phonon, a sound-like vibration carried through the atomic lattice.

Nobody had a clear explanation. “At first, we had no idea what it was. Demons are not in the mainstream. The possibility came up early on, and we basically laughed it off,” said co-author Ali Husain, now a research scientist at the quantum technology company Quantinuum. “But, as we started ruling things out, we started to suspect that we had really found the demon.”

To confirm the suspicion, the team brought in condensed matter theorist Edwin Huang, a postdoctoral scholar at the university, to calculate the electronic structure of strontium ruthenate from scratch. Huang’s calculations had no adjustable parameters, no fine-tuning, no fitting to match the results already in hand. When he ran the numbers, he found exactly what Pines had described in 1956: two electron bands, labeled beta and gamma, oscillating in opposite directions with nearly equal strength, producing a mode with no mass and no charge. Pines’ demon had been there the whole time. It just needed the right instrument pointed in the right direction.

What Makes a Demon Different

At its core, a demon is a quasiparticle, not a fundamental particle in the way an electron or proton is, but a collective behavior of many electrons that acts as a single unit. Plasmons, which are ripples through a sea of electrons, are quasiparticles too. What sets a demon apart is how it forms.

In most metals, plasmons require a specific minimum energy to get started. That energy threshold creates a practical temperature floor below a certain point; plasmons simply can’t form. Pines argued that a demon, because it draws electrons from two bands that cancel each other’s charge, faces no such threshold. With no mass to overcome and no energy minimum to clear, a demon can exist at any temperature. That includes the temperatures at which humans live and work. For physics, that distinction carries enormous weight.

The Superconductor Problem

Superconductivity, the ability of a material to conduct electricity with zero resistance, ranks among the most sought-after properties in modern science. Materials that achieve it could transmit electricity without losing any energy to heat, a limitation that costs power grids billions of dollars every year.

Scientists have known about superconductivity since 1911, and for decades, they worked with a well-established explanation called BCS theory. Under BCS theory, superconductivity happens when quantum vibrations in a crystal lattice, known as phonons, cause electrons to pair up. These paired electrons, called Cooper pairs, move through a material in a coordinated way that eliminates resistance.

BCS theory works well at very low temperatures. But it runs into trouble with a class of materials called high-temperature superconductors, which achieve zero resistance at temperatures far above what BCS can account for. Something else must be pairing those electrons, and for years, physicists have debated what.

Pines himself believed demons might be part of the answer. A massless, chargeless particle that can exist at room temperature and interact with electrons across multiple energy bands has exactly the right profile to push electrons together in ways that phonons cannot. If demons do play a role in high-temperature superconductivity, understanding how they work could point researchers toward materials that superconduct at or near room temperature.

Room-temperature superconductors remain one of the most pursued goals in physics. Achieving them would not just improve power grids. It would reshape quantum computing, medical imaging technology, and virtually every system that depends on moving electricity efficiently across long distances.

Serendipity, Not Strategy

When Abbamonte reflected on how his team arrived at this discovery, he was candid about what it took.

“Demons have been theoretically conjectured for a long time, but experimentalists never studied them,” he said. “In fact, we weren’t even looking for it. But it turned out we were doing exactly the right thing, and we found it.”

That honesty cuts against a popular image of scientific discovery as a precisely aimed process. Abbamonte and his team weren’t executing a decade-long plan to detect Pines’ demon. They were doing something relatively simple: looking at a material that hadn’t been closely examined with a technique that wasn’t widely used. They found something unexpected because they were willing to look somewhere new.

Abbamonte credited that attitude as much as any technical skill. His team used M-EELS, a non-standard method, on strontium ruthenate, a non-standard subject. Had they stuck to more familiar tools and more familiar materials, the data that revealed the demon might never have been collected.

One Metal Down, Many More to Go

Strontium ruthenate may not be the only metal hiding a demon. According to the research team, many multiband metals whose electrons occupy more than one energy band could produce the same conditions that generate demons. Whether those demons play a direct role in producing superconductivity in any of those materials remains an open question. Mapping the full behavior of demons, how they move, how they interact with other particles, and how their properties change under different conditions, will require years of follow-up work.

But the fact that a demon now has a confirmed, measured existence changes what that research can look like. For decades, theorists could only speculate about a particle they couldn’t directly study. Experimentalists now have a real signal to work with.

“It speaks to the importance of just measuring stuff,” Abbamonte said. “Most big discoveries are not planned. You go look somewhere new and see what’s there.”

Sixty-seven years after David Pines named his theoretical particle and dared colleagues to find it, someone finally did. And what they may have found, hiding inside a modest silver crystal, could be a key to one of the biggest unsolved puzzles in modern physics, one with consequences that reach far beyond any laboratory.