Lasers Shot At Sky Can Guide Lightning

A powerful trillion-watt laser shot at the sky can generate lightning rods in the air that can guide lightning strikes to keep them from causing havoc, a new study finds.

To date, the most common and effective form of protection against lightning is the lightning rod invented by Benjamin Franklin in 1752. These pointed electrically conductive metal rods intercept lightning strikes and guide their electric current safely to the ground.

However, a key drawback of a conventional lightning rod is that the radius of its area of protection is roughly equal to its height. Since there are practical limits to how tall one can build a lightning rod, this means they may not prove useful at protecting large areas, including sensitive infrastructure such as airports, rocket launchpads and nuclear power plants, says study senior author Jean-Pierre Wolf, a physicist at the University of Geneva.

“This is the first demonstration that lightning can be controlled by a laser.”
—Jean-Pierre Wolf, University of Geneva

Scientists first suggested using lasers to generate lightning rods in the air nearly 50 years ago. “The idea is to create a very long lightning rod with the laser,” Wolf says.

In the new study, researchers conducted experiments during the summer of 2021 at the top of Mount Säntis, which at 2,502 meters above sea level, is the highest mountain in the Alpstein massif of northeastern Switzerland. The laser was activated every time storms were forecast between June and September, with air traffic closed over the area during these tests.

Wolf and his colleagues sought to protect a 124-meter transmitter tower equipped with a traditional lightning rod at the summit belonging to telecommunications provider Swisscom. This tower is struck by lightning about 100 times a year, and scientists had previously equipped it with multiple sensors to analyze these strikes.

Near the tower, the researchers installed a near-infrared laser the size of a large car. It fired pulses each packing about a half-joule of energy and a picosecond (trillionth of a second) long roughly a thousand times a second, with a peak power of a terawatt (trillion watts). (It also shot a visible green beam to help show the laser’s path.)

“Imagine transporting a 10-ton laser to 2,500-meter altitude on a mountain with helicopters, making it run in very harsh conditions, tracking lightning in extreme weather like winds up to 200 kilometers per hour, heavy rain, hail, temperatures varying from -10 degrees to 20 degrees Celsius in the same day, and then, when it works, you get a massive lightning bolt some tens of meters next to you—and you’re so happy,” Wolf says.

The laser pulses can alter the refractive index of the air—the quality of a material that controls how quickly light travels within it. This can make the air behave like a series of lenses.

After crossing this lensing air, the intense, short laser pulses can rapidly ionize and heat air molecules, expelling them from the path of the beam at supersonic speeds. This leaves behind a channel of low-density air for roughly a millisecond. These “filaments” possess high electric conductivity and can thus serve as lightning rods, and can range up to 100 meters long. The researchers could adjust the laser to create filaments that appear up to a kilometer from the machine.

In experiments, the scientists created filaments above, but near, the tip of the tower’s lightning rod. This essentially boosted the rod’s height by at least 30 meters, extending its area of protection so that lightning would not strike parts of the tower otherwise outside the rod’s shelter, says study lead author Aurélien Houard, a research scientist at the Superior National School of Advanced Techniques in Paris.

The laser operated for more than six hours during thunderstorms happening within three kilometers of the tower. The tower was hit by at least 16 lightning flashes, all of which streaked upward.

Four of these flashes occurred while the laser was operating. High-speed camera footage and radio and X-ray detectors showed the laser helped guide the course of these discharges. One of these guided strikes was recorded on camera and revealed it followed the laser path for nearly 60 meters.

During tests carried out on the summit of Mt. Säntis by Jean-Pierre Wolf and Aurélien Houard’s team, the scientists noted that lightning discharges followed laser beams for several dozen meters before reaching the Swisscom telecommunications tower (in red and white).Xavier Ravinet/UNIGE

“This is the first demonstration that lightning can be controlled by a laser,” Wolf says.

Although lab experiments had suggested that lasers could help guide lightning strikes, previous experiments failed to do so in the field over the past 20 or so years. Wolf, Houard and their colleagues suggest their new work may have succeeded because of the pulse rate of their laser was hundreds of times greater than prior attempts. The more pulses are used, the greater the chance one might successfully intercept all of the activity leading up to a lightning flash. In addition, higher pulse rates are likely better at keeping filaments electrically conductive, they added.

Wolf noted their work is not geoengineering research. “We are not modifying the climate,” he says. “We deflect lightning to protect areas.”

In the long term, the scientists would like to use lasers to extend lightning rods by 500 meters. In addition, they would likely to run experiments at sites such as airports and rocket launchpads, Wolf notes.

The researchers detailed their findings 16 January in the journal Nature Photonics.