A laser beam usually travels straight, like a jet of water from a hose. But what if you twist that jet like the thread of a corkscrew? You get a light spring — a beam that, as it moves, also spins around its own axis. At every point in space, its brightness traces a spiral, and if you place a flat screen in its path, you'll see a rotating spot of light.

Physicists measured how fast that spot races around the circle. It turns out you can set it almost arbitrarily — from a complete stop to values exceeding the speed of light in vacuum. Meanwhile, the beam itself moves forward at the ordinary speed of light; only the pattern it paints on the screen rotates faster, just as the shadow of a fast-moving finger on a wall can outrun the finger itself. They dubbed this speed the orbital group velocity.

Next, the scientists aimed such a spring at a thin layer of dense plasma — a gas where electrons are stripped from atoms. When a light spring with superluminal rotation passes through the plasma, it rocks the electrons so they oscillate in unison, as one coherent whole. This produces a burst of radiation in the terahertz range — between microwaves and infrared light. Such radiation safely penetrates clothing, paper, and plastic, but not metal or water, which is why it's used in airport scanners and medical diagnostics.

The new terahertz source is compact and tunable — by changing the twist of the beam, you can control the frequency and power of the radiation. It's like adjusting the water pressure in a hose without touching the tap, just by changing the shape of the jet.