The Penrose Process: How to Steal Energy From a Black Hole

In 1969, the physicist Roger Penrose worked out how to steal energy from a black hole. Not metaphorically — an actual method, on paper, for coming away with more energy than you started with. For fifty years it stayed on paper, because testing it seemed to require a spinning black hole in your laboratory. This month, a team finally pulled it off — with no black hole, and, strangest of all, nothing physically spinning at all.

First, a Spinning Black Hole

A black hole is a region where gravity is so strong that nothing, not even light, can escape once it crosses the boundary called the event horizon. That much most people know.

The key detail here is that black holes spin — often ferociously fast, inherited from the rotating star that collapsed to form them. And a spinning mass does something a stationary one doesn't: it drags space and time around with it.

This is a genuine prediction of Einstein's general relativity called frame dragging. Picture a heavy ball spinning in a vat of honey. The honey right next to the ball gets caught up and swirls along with it. Spacetime does the same thing near a rotating black hole — it gets dragged into rotation, whether anything is there to feel it or not.

What is the ergosphere?

Close enough to a spinning black hole, this dragging becomes so severe that you cannot stay still no matter what you do. There is no engine powerful enough to hold your position, because space itself is being swept around faster than you could ever fight against. You are forced to rotate along with the black hole.

That region — outside the event horizon, but where standing still is impossible — is the ergosphere. The name comes from the Greek ergon, "work," because it's the region from which you can extract energy. And crucially, it sits outside the point of no return: you can venture into the ergosphere and still get back out. That's what makes the whole trick possible.

The Penrose Process, Step by Step

Here is Penrose's idea, stripped of the mathematics.

1. You send an object into the ergosphere.
2. Once inside, it splits into two pieces — imagine firing a projectile, or splitting a spacecraft in half.
3. You arrange the split so that one piece is thrown into the black hole, against the direction of spin.
4. The other piece flies back out of the ergosphere — and it escapes carrying more energy than the original object had when it went in.

That extra energy isn't created from nothing, which would break the deepest law in physics. It's taken from the black hole's rotation. The piece that fell in did so in a way that slightly slowed the black hole's spin, and that lost spin energy came back out with the escaping piece. You leave a fraction richer; the black hole spins a fraction slower.

The intuition that helps most is a slingshot. Space agencies routinely fly probes close to a moving planet to pick up speed — the probe steals a tiny amount of the planet's orbital motion and flies off faster, while the planet is slowed by an utterly imperceptible amount. The Penrose process is that same bargain, run against the rotation of spacetime itself instead of the motion of a planet.

Then Zel'dovich made it about waves

A few years later, the physicist Yakov Zel'dovich realised the same bargain should work for waves, not just objects. Send a wave at a body spinning fast enough and the wave should bounce back amplified, having drained a little energy from the rotation. He even predicted it would work with an ordinary spinning cylinder of the right material — no black hole required, in principle.

In principle. The catch was brutal: to amplify a wave this way, the cylinder would have to spin so fast that it would tear itself apart long before reaching the necessary speed. The prediction sat there, correct-looking and untestable, for half a century.

The Clever Part: Faking the Spin

This is where the new experiment gets genuinely ingenious.

The team, led by Andrea Alù at the CUNY Advanced Science Research Center, reasoned that nothing actually needs to move. What matters to an incoming wave isn't that atoms are physically rotating — it's that the wave experiences a rotating environment. And you can manufacture that experience without spinning anything.

They built a stationary ring of electronic resonators — small circuits whose electrical properties can be switched rapidly. Then they changed those properties in a carefully timed sequence around the ring, so that a pattern of change travelled around it like a wave going round a stadium as people stand and sit in turn.

Nobody in a stadium wave runs around the stadium; only the pattern moves. In the same way, the metal and circuits in the ring stayed completely still, while the pattern of their changing properties swept around at enormous effective speed. To an electromagnetic wave entering the ring, the system looked exactly like something rotating — "synthetic rotation."

When they sent electromagnetic waves into this fake-rotating ring, some came out amplified — carrying more energy than they went in with, extracted from the "rotation" exactly as Penrose and Zel'dovich had predicted. Fifty-plus years after the theory, the effect had finally been seen directly, published in Nature in July 2026 under the name Floquet rotational super-radiance.

Real Black Hole vs Lab Version, Side by Side

It's worth being precise about what was and wasn't done. Nobody made a black hole, and nobody extracted energy from a real rotating spacetime. What the experiment did was reproduce the mechanism — the amplification of waves by a rotating environment — in a system governed by the same underlying wave physics. That's how a great deal of physics gets tested when the real object is out of reach: you build a stand-in that obeys the same equations and study that.

Common Misunderstandings to Avoid

• "They made energy from nothing." No — the surplus energy is drained from the rotation, whether that's a black hole's spin or, in the lab, the power driving the modulation. Energy is conserved throughout.
• "They created a black hole." No black hole, no gravity, no danger. They built an electronic system that mimics one specific behaviour of a rotating black hole.
• "Something moved faster than light." Only a pattern did, and patterns carry no mass and no information faster than light. Nothing physical broke the speed limit.
• "This means we can power cities from black holes." Not remotely. The value is scientific — a new kind of wave amplifier, and the first hands-on confirmation of a famous prediction — not an energy source.

Why It Connects Back to Curved Spacetime

Underneath the electronics, the reason any of this works comes back to the central idea of Einstein's general relativity: mass and motion shape spacetime, and rotating mass drags spacetime around with it. The ergosphere, and the energy locked in a black hole's spin, are direct consequences of that curving and dragging.

What the experiment shows is that this behaviour isn't a quirk of gravity alone. It's a deeper property of how waves interact with any rotating environment — so deep that you can reproduce it with circuits on a bench, no gravity in sight. A prediction born from the geometry of black holes turns out to describe a stadium wave of electronics. That kind of unexpected reach, from the most extreme objects in the universe down to a lab ring, is exactly what makes relativity worth understanding.

The best way to build a feel for the gravity side of the story is to watch masses move and spacetime respond. To see how gravity choreographs orbits, slingshots and the dance of multiple bodies, try the N-body gravity simulator, and to see how a mass warps the space and time around it — the curvature that gives a black hole its ergosphere in the first place — explore the spacetime curvature simulator.

The study — "Observation of Floquet rotational super-radiance" by Nasari, Moussa, Kasahara, Thielens and Alù — was published in Nature in July 2026 by the Advanced Science Research Center at the CUNY Graduate Center.