In 1969, British physicist Roger Penrose put forward a curious thought experiment. He proposed that by throwing an object into the edge of a rotating black hole, one could extract energy from the universe's most powerful gravity well.
Black holes, as predicted by Einstein's general theory of relativity, are surrounded by a special region called the "ergosphere." Here, the ferocious spin of an extremely massive object drags space itself around at speeds approaching the speed of light—a phenomenon known as frame-dragging. Suppose a particle entering this turbulent region of spacetime happens to split into two. If one fragment falls past the event horizon into the black hole's core while the other manages to escape, the physics yields a counterintuitive result.
Because the energy of the fallen fragment effectively behaves as negative relative to the black hole, the escaping fragment flies out into space carrying more energy than the original particle possessed. The black hole is stripped of a portion of its own rotational energy, which is surrendered to the outside world.
A research team at the Advanced Science Research Center (ASRC) at the City University of New York (CUNY) has brought this astrophysical theory—first proposed over half a century ago—onto an electronic circuit small enough to sit on a laboratory bench. Their study, published in Nature, does more than merely verify an extreme theory from astrophysics. It presents an entirely new paradigm for wave amplification, a technology that underpins the very foundation of modern communication infrastructure.
The Insurmountable Wall of Material Strength
Two years after Penrose's proposal, in 1971, Soviet physicist Yakov Zel'dovich extended this energy-extraction mechanism from particles to "waves." Imagine a wave—light, sound, or radio waves—striking a rapidly rotating object. If the object rotates faster than the wave's phase velocity, the wave will steal rotational energy and bounce back amplified, as a stronger wave.
This phenomenon, now known as "rotational superradiance," requires an extremely strict condition to occur. The frequency of the input wave, , must be smaller than the product of the wave's orbital angular momentum (azimuthal quantum number) $m$ and the object's angular velocity . Expressed mathematically, this is .
This condition presents a simple yet daunting physical hurdle. To amplify a wave, the target object must be spun at a ferocious speed exceeding the speed at which the wave itself propagates. Over the past decade, this phenomenon has been demonstrated in several laboratories. In 2017, the University of Nottingham achieved it using vortices in water surface waves in a tank, and in 2020, the University of Glasgow achieved 30% acoustic wave amplification using a rotating acoustic disk. Because water surface waves and sound waves travel relatively slowly, a physical motor's rotation could keep pace with them sufficiently.
However, this approach collapses the moment the target is switched to "electromagnetic waves" such as light or radio waves. Since the phase velocity of electromagnetic waves approaches the speed of light (about 300,000 kilometers per second), attempting to spin a physical metal cylinder or disk fast enough to reach that speed would cause the material to be completely shattered at the atomic level by centrifugal force. When the University of Southampton attempted electromagnetic field amplification in 2024 by rotating an aluminum cylinder, they could only reach a very limited low-frequency range. As long as one relies on physical motion, there exists an absolute, insurmountable limit imposed by material strength.
Superluminal "Synthetic Rotation" Housed in a Circuit That Doesn't Move an Inch
A research team at CUNY led by Andrea Alù and Hadiseh Nasari designed a device that fundamentally circumvents this constraint. Rather than mechanically rotating an object, they artificially created an environment that tricks electromagnetic waves into perceiving "extremely high-speed rotation."
The team constructed a ring circuit connecting three electronic resonators in a delta configuration. Each resonator incorporates a varactor diode, allowing its capacitance (its ability to store electric charge) to be rapidly varied by applying an external voltage. The researchers carefully modulated the electrical properties of these three resonators in sequence, with precisely staggered timing.
The device itself remains fixed on the workbench. However, because the "wave" of changing properties continuously circulates around the ring at high speed, the incoming 100 MHz radio wave perceives the entire circuit as if it were rotating at tremendous speed. This is the same mechanism by which the characters on a digital billboard in a city appear to move from right to left, even though in reality individual LEDs are merely flashing on and off in place.
This metamaterial technique of "synthetic rotation" allows the apparent angular velocity to be set arbitrarily high, completely ignoring the limits of physical materials. By venturing into an extreme rotational regime effectively equivalent to superluminal speeds, Zel'dovich's amplification condition for electromagnetic waves was satisfied for the first time in history.
A Circuit Design That Defies Convention—Where "Loss" Drives Amplification
When waves were sent into the synthetically rotating circuit, a decisive signal was observed. The moment the modulation speed was pushed past the threshold, the sign of the wave's "orbital angular momentum (OAM)" reversed. OAM is a property describing the degree of helical "twist" a wave exhibits as it propagates through space. This sign reversal is solid dynamical evidence that, in the effective rotating reference frame, the flow of time behaves as if it has been reversed.
Simultaneously with this sign reversal, the wave's intensity surged sharply. The maximum net amplification gain recorded in the experiment reached approximately 7.8 dB.
The research team applied Floquet theory to describe the behavior of this system—a mathematical framework for dealing with periodically varying systems. Just as pushing a child on a swing repeatedly at the right timing causes the oscillation to grow larger, a circuit modulated periodically in time and space injects energy into the incoming electromagnetic wave in a periodic manner. This forms not a frequency gap, but an "angular momentum band gap," through which only waves with a specific twist are selectively amplified.
Here, one counterintuitive fact came to light—the role of "loss."
In conventional electronic circuits and wave engineering, parasitic losses such as electrical resistance are typically treated as an unwanted factor that attenuates signals and should be eliminated. The prevailing assumption is that systems with less noise and lower loss are superior. The experimental data in this study revealed the exact opposite. In the regime of synthetic superluminal rotation, the greater the circuit's loss, the stronger the wave amplification effect became—and the wider the frequency band over which amplification occurred. This is because "loss," which dissipates energy from the system, becomes an essential channel through which energy flows efficiently from the external driving source into a specific wave mode.
| Comparison Item | Conventional Rotational Superradiance Experiments (~2024) | CUNY ASRC's Synthetic Rotation System (This Study) |
|---|---|---|
| Rotation Method | Physical mechanical rotation via motor, etc. | Non-moving synthetic rotation via changing properties of electronic resonators |
| Rotation Speed Limit | Dependent on material's breaking strength under centrifugal force | No limit (apparent superluminal states also possible) |
| Amplification Target | Water surface waves, sound waves (low phase velocity) | Electromagnetic waves (phase velocity near speed of light) |
| Treatment of Loss | Something to be eliminated as much as possible, as it attenuates waves | Becomes a channel for extracting energy from external drive |
A New-Principle Optical Router That Breaks Through the Heat Wall of Data Centers
Black hole physics, proven in a laboratory after half a century, holds the potential to shatter a serious bottleneck currently facing industry.
With the explosive proliferation of generative AI, traffic flowing through the world's data centers and communication networks is approaching its limits. Current infrastructure supporting optical fiber communication relies on devices such as erbium-doped fiber amplifiers (EDFAs) to compensate for signal attenuation. These use a "gain medium," such as a laser, to forcibly inject external energy into the wave. However, this approach consumes enormous amounts of power and is a major cause of the severe heat dissipation problems plaguing data centers.
In contrast, the amplification principle presented by CUNY's device is entirely different. By applying structural time modulation to the wave, the wave itself "extracts" energy from the system.
Even more importantly, this mechanism exhibits strong selectivity toward the wave's "degree of twist (OAM)." Since OAM can take integer values, it can in theory support infinite variations. "Space-division multiplexing communication," which carries separate data streams on each distinct shape of light's twist rather than on frequency (color), is regarded as the leading candidate technology for next-generation infrastructure.
If this synthetic rotation architecture were extended into higher frequency bands using all-optical modulation schemes, it would enable an unprecedented optical router—one capable of isolating signals with a specific OAM from noise within photonic integrated circuits handling massive data traffic, and amplifying them at low power.
The research team is already looking ahead to expanding this into larger ring networks with more resonators. The process of extracting energy in the extreme cosmos is poised to be incorporated into the next-generation communication infrastructure into which telecom operators and tech giants are pouring enormous investment—reborn as a new heart supporting the torrent of data.