Invisibility Research Eyes Soldiers, Civilians and Armies

A research team at Nanjing University is developing a flat device intended to conceal civilians or soldiers from drone cameras, physicist Yun Lai said. The work grows from theories developed by Ulf Leonhardt, a physicist at Israel's Weizmann Institute of Science in Rehovot, where research into invisibility shares space with proposals for laboratory black holes and dark energy. The potential uses aren't limited to hiding from cameras: researchers have also discussed concealing submarines and manipulating waves to protect coastal areas.

Who Gets to Disappear?

Lai's team is working on a device intended to operate over distances of hundreds of meters. “From a distance, an object looks small, so it's easier to make it disappear,” Lai said. “The closer you want to cloak something, the more advanced the materials need to be.” He said there was no physical limit to the distance, while noting that making the technology work would require enormous funding.

Leonhardt's theoretical proposal would steer light around an object and return it to its original path, so an observer receives the light they would have seen if the object weren't there. He presented the idea in a paper published in Science in 2006. A few months later, scientists at Duke University reported creating an invisibility cloak that diverted microwaves around a concealed copper cylinder.

The cloak has sharp limits. Current versions work only across a narrow range of frequencies, not the entire visible spectrum. Even a cloak that covered visible light would stop its wearer from seeing outside, Leonhardt said, and it would be rigid. “Whoever wore it would be like a knight in armor,” he said. Making one for a person rather than a tiny object would be “extremely, extremely expensive.”

Leonhardt is skeptical that a Harry Potter-style cloak will become reality. “In a way, I'm glad about that, because I think it would mainly be used for bad things,” he said. “The Russians would be very happy to get their hands on something like that. You could enter anywhere and cause enormous damage without facing the consequences.” His warning sits beside the research's stated military applications, not outside them.

The Apparatus in the Lab

Metamaterials, whose engineered microscopic structures can give them unusual properties, can also bend sound waves around objects. Leonhardt said that could be useful for concealing submarines. “I'm sure there are armies working on this, but these are studies you don't hear about,” he said. Sébastien Guenneau of Imperial College London described marine and civil engineering as promising areas for applications. His team has experimentally demonstrated reducing ground vibrations and has proposed theoretical “water cloaks” to manipulate waves and protect areas from ocean swells.

The same research has a different proposed use: laboratory black holes. In an experiment proposed by Leonhardt and his first doctoral student, Dr. Paul Piwnicki, a laser beam enters a material where a vortex flows faster than light travels through it, sweeping the light inward. The researchers showed mathematically that the gravitational effects of this “optical black hole” are analogous to those of a real black hole, allowing tests of predictions about cosmic phenomena.

Leonhardt's work also depends on institutions and money. He left the University of St Andrews after becoming unhappy with researchers being judged by how much funding they brought in. At the National University of Singapore, he turned down an offer to head the physics department because he didn't want administrative responsibilities. He later received a grant from the Israel Science Foundation and substantial support from the Weizmann Institute for his speculative theory about dark energy.

Funding, Careers and Uncertainty

The career machinery around research appears in the account of Lai, who said he read Leonhardt's paper when he needed to publish in Physical Review Letters to obtain a position. He wrote two papers on cloaking that were published in PRL and continued his research career. Leonhardt also described stopping work in isolation because it was hurting his students, who needed results for publication and their careers.

In December 2024, Leonhardt and his student Ziv Landau published a theoretical model proposing that buoyancy also exists in the quantum vacuum. Student Dror Berechya tested the model against cosmological measurements in a doctoral dissertation that had been approved but had not yet been published in a scientific journal. Leonhardt said Berechya published nothing during his five years of doctoral studies. “And the results are spectacular,” he said.

Eugene Kolomeisky, a physicist at the University of Virginia, said Leonhardt's published paper was well grounded, but cautioned that fitting observations “doesn't necessarily mean very much,” because the observational picture keeps changing. He also said Leonhardt's choice of a minimum scale for the calculation had “an element of arbitrariness.” Leonhardt acknowledged the theory could be wrong: “It's a radical theory. We have good evidence, but it could certainly be wrong.”