When Derek Muller took an experimental land yacht for a spin this spring, he wasn't aiming to stir up scientific controversy. He certainly wasn't trying to win US$10,000 in a bet.

Muller, the creator of the Veritasium YouTube channel, likes to break down funky science concepts for his 9.5 million subscribers. So in May, he published a video about a vehicle called Blackbird that runs on wind power.

Created by Rick Cavallaro, a former aerospace engineer, Blackbird is unique because it can move directly downwind faster than the wind itself for a sustained period.

Any sailor worth their salt can tell you that a boat can do that by cutting zigzag patterns; that's called tacking. But the idea that a vehicle can beat the breeze traveling straight downwind, no tacking involved, is controversial.

"I knew this was a counterintuitive problem. To be perfectly honest with you, when I went out to pilot the craft, I didn't understand how it worked," Muller told Insider.

Blackbird is so counterintuitive, in fact, that less than a week after Muller released his video (below), Alexander Kusenko, a professor of physics at UCLA, emailed to inform him that it had to be wrong. A vehicle like that would break the laws of physics, Kusenko said.

"I said, 'Look, if you don't believe this, let's put some money on this,'" Muller said. He suggested a wager of $US10,000, never imagining Kusenko would take it.

But Kusenko agreed, and in the weeks that followed, they exchanged data and argued about Blackbird. They even brought in several of science's biggest names, including Bill Nye and Neil deGrasse Tyson, to help decide who was right.

In the end, Muller emerged victorious.

'I never saw a way I could lose'

Days after Muller suggested the wager, Kusenko sent him a document with the bet's terms, Muller said.

"Everything was always super airtight, I never saw a way I could lose," Muller said.

But Kusenko was equally confident. "Thanks to the laws of physics, I am not risking anything," Kusenko told Vice last month. He did not respond to Insider's request for comment.

Kusenko gave Muller an hour-long presentation explaining why he was certain the YouTuber had been taken in by bad science.

The professor said Blackbird was most likely taking advantage of intermittent wind gusts that helped the vehicle speed up. He outlined his objections on a page of his UCLA website, though it has since been taken down.

For his part, Muller sent Kusenko data from the driving test in his video, which was filmed in the El Mirage lake bed in California. During that drive, Blackbird accelerated over two minutes – a feat that would have been impossible if it had relied on sporadic gusts.

The vehicle reached a speed of 27.7 mph (45km/h) in a 10-mph (16km/h) tailwind.

Muller even contracted Xyla Foxlin, a fellow YouTuber, to build a model cart similar to Blackbird that could be tested on a treadmill. Indeed, Foxlin showed that her wind-powered model could go faster than the wind.

Muller documented this back-and-forth in a follow-up video (below) that he released in June.

"Kusenko was so sure he was right. He wanted to make it public," Muller said.

How Blackbird works

In 2010, Google and Joby Energy sponsored Cavallaro and a team of collaborators from San Jose State University to build Blackbird. The team demonstrated that the vehicle could travel downwind 2.8 times as fast as the wind, a record confirmed by the North American Land Sailing Association.

The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades – they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward.

"I never even imagined a decade later that a physics professor would still be arguing how it's impossible," Cavallaro, a chief scientist at Sportvision, told Insider.

After three weeks of debate, Kusenko acknowledged that Blackbird could go slightly faster than the wind, but he maintained that it was for only short periods. If a gust of wind sped up the land yacht and then quickly died down, he said, it would appear that Blackbird was traveling faster than wind.

"The resolution of our bet was not as clean as I'd hoped," Muller said. "Kusenko coughed up the 10 grand, let's leave it at that."

Cavallaro, too, wanted more acknowledgment of his vehicle's capabilities.

Kusenko "conceded on a technicality – that the vehicle moves marginally faster than the wind temporarily," Cavallaro said. "I offered him another US$10,000 bet, because his technicality is entirely wrong, but I know I won't be hearing from him."

Muller's two videos have each garnered at least 6.8 million views and 41,000 comments, with many agreeing with Kusenko that it's impossible for Blackbird to go faster than the wind. Some viewers have even asked the YouTuber if he'd make follow-up wagers.

"It breaks a lot of people's brains," Muller said. "Clearly it got Kusenko too."

This article was originally published by Business Insider.

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