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A bit like Zeus flinging thunderbolts, physicists working on a mountaintop in Switzerland have used a high-powered laser to steer lightning. The advance could open the way to use lasers to protect airports, rocket launchpads, and other sensitive infrastructure, researchers say. Still, it remains unclear whether the million-dollar technology works any better than a relatively cheap lightning rod.
“It is inspiring,” says Matteo Clerici, a physicist at the University of Glasgow who was not involved in the work. “What will be the application of this? We can only speculate.”
Lightning occurs when static electricity builds up in storm clouds and begins to break down the surrounding air molecules. Paths of electrically weakened air spread like cracks in a car windshield. Once one such path reaches something on the ground or connects with other paths climbing from the surface, 30,000 amps of current gush through the jagged channel in a massive discharge that can blast a hole in a building and set it ablaze.
To help prevent such damage, people rely on a technology invented in 1752 by American polymath Benjamin Franklin: the lightning rod. Consisting of a pointed metal rod attached to a building’s roof and connected to the ground by a wire, the rod creates a strong electric field that draws lightning away from the building. When the rod is hit, the wire safely ushers current to the ground.
Since the first lasers emerged in the 1960s, scientists have thought of using them in a similar way to guide lightning, says Aurélien Houard, a physicist at the École Polytechnique. In theory, the laser beam would create a straight path of ionized air along which current could flow more easily. Yet early attempts with high-powered lasers failed because, within a short distance, the ionized air simply absorbed the laser light, leaving an air channel far too stubby to attract or affect lightning.
In the 1990s, physicists developed lasers that produced pulses just femtoseconds long. These shorter, lower energy pulses proved more effective at opening conductive channels, Houard says. The laser light ionizes some air, which then works like a lens to further focus the light into a long “filament” the width of a hair. The thin beam heats the air, driving away molecules and leaving a channel of lower density air, which better conducts electricity.
At least, it does in laboratory experiments. Efforts to control natural lightning in New Mexico in 2004 and Singapore in 2011 still failed to influence the paths of bolts, Houard notes.
Now, scientists led by Houard and Jean-Pierre Wolf, a physicist at the University of Geneva, have succeeded. They placed a femtosecond laser atop Säntis mountain in northeastern Switzerland next to a 124-meter-tall telecommunications tower. Like a giant lightning rod, the tower gets hit with lightning more than 100 times a year. The researchers shined their laser past the top of the tower from July to September 2021 during a total of more than 6 hours of thunderstorms.
The tower got hit at least 15 times during that period, including four times when the laser system was running. The researchers studied the strikes both with radio antennas flanking the mountain, which traced the lightning’s path, and with high-speed cameras. In all four lightning strikes taken with the laser on, the lightning followed the path of the laser beam before jumping to the tower, the 28-member team reports today in Nature Photonics. Thus, researchers steered about the last 50 meters of each bolt’s otherwise random trajectory.
The researchers succeeded where others hadn’t in part because their laser fired 1000 times per second, rather than 10 or fewer, Houard says. The rapid-fire pulses kept a stable conductive channel open even in the swirling atmosphere, he speculates. The other big difference? “We choose a specific location where the lightning is always hitting the same point,” he says.
The result is the culmination of a 5-year, €4 million European project, Houard says. To make it work, researchers had to disassemble their delicate laser, take it up the mountain piece by piece in a gondola, and enlist Switzerland’s biggest helicopter to assemble a building to house it.
Researchers still have a long way to go to prove the technique captures lightning efficiently. They also want to show that the laser can not only guide bolts, but trigger them to preemptively drain away the threat, Houard says. Still, Stelios Tzortzakis, a physicist at the University of Crete, says, “The work marks a great milestone.”
But can a $2 million laser compete with a dirt-cheap lightning rod? In some cases, maybe, Houard says. A lightning rod protects an area roughly twice as wide as the rod is tall, so the hope would be to make a very tall “virtual lightning rod” that would cover a bigger area than would be feasible using a metal lightning rod. Tzortzakis agrees, adding that the goal shouldn’t be to replace the conventional lightning rod, but to extend its range of coverage.
Houard says his team has discussed building a system to help protect Ariane rockets on the launchpad at Europe’s spaceport in French Guiana. The rockets are, of course, named after the Greek goddess Ariadne, princess of Crete and granddaughter of Zeus. What could be more fitting?