More than half a century ago, Roger Penrose described a curious way to draw energy out of a spinning black hole. A particle entering the ergosphere — the region of space dragged around by the rotation — could split in two, sending one piece across the horizon while the other flew off carrying more energy than the whole had at the start. The physicist Yakov Zel'dovich later argued that a wave, rather than a particle, could be amplified in the same way if it met an object spinning fast enough.
Turning that idea into a laboratory result has long been difficult, because the rotation speeds involved lie far beyond what any real object can reach. Researchers at the Advanced Science Research Center at the CUNY Graduate Center report in Nature that they have sidestepped the problem entirely — by building a device that only pretends to spin.
Motion without movement
Rather than whirling matter around, the team assembled a ring-shaped network of electronic resonators and rapidly changed their properties in a carefully timed sequence. The adjustments swept around the loop as a travelling pattern, so that although nothing physically moved, incoming electromagnetic waves behaved as if they were meeting a medium turning at enormous speed — fast enough, in effect, to exceed the speed of light.
At those synthetic speeds, the researchers watched angular-momentum bandgaps open in the band structure of the system. Waves carrying the right rotational character drew energy out of the timed modulation and emerged amplified — selectively, within a band shaped by the device's own losses. The authors describe the result as a Floquet regime of rotational super-radiance.
"Our approach facilitates a new method of wave–matter interaction in which waves with selected rotational properties extract energy from synthetic time-engineered rotation," said Andrea Alù, the project's principal investigator. Lead author Hadiseh Nasari said the experiment "moves ideas about extreme rotational dynamics from theory to practice." Co-lead author Hady Moussa noted that the amplified waves reproduced "the essential physics of the Penrose–Zel'dovich process" using engineered metamaterials.
Because the platform recreates extreme rotation on a benchtop, the group sees it as a controlled setting for studying physics usually confined to astrophysics, alongside possible uses in wireless communications and in classical and quantum optics. The work was supported by the U.S. Department of Defense, the National Science Foundation and the Simons Foundation.