What began as a joy ride in a wind-powered vehicle has spiraled into a $10,000 wager, witnessed by the likes of Bill Nye and Neil deGrasse Tyson, over a physics problem that has generated heated debate for more than a decade.
On one side of the bet is Derek Muller, creator of the popular science channel Veritasium on YouTube. On the other is Alexander Kusenko, a professor of physics at the University of California, Los Angeles (UCLA) and a visiting senior scientist at the Kavli Institute for the Physics and Mathematics of the Universe in Japan.
Both men are utterly convinced that they will emerge as the winner.
“My confidence never wavered,” said Muller in a call. “But we're not at the end of this thing yet. It's over when I'm able to convince him. That's what I'd like to see.”
Kusenko is so sure of his position that he did not even attempt to negotiate the $10,000 stakes proposed by Muller. “Thanks to the laws of physics, I am not risking anything,” he said in an email. “So, I could accept any bet, however large or small the amount might be.”
This scientific gamble was sparked by Muller’s recent video, entitled “Risking My Life To Settle A Physics Debate,” in which he took the experimental land yacht Blackbird for a spin.
Conceived and developed by engineer Rick Cavallaro, the vehicle has earned enormous interest and controversy over the claim that it can travel directly downwind faster than the wind itself, using no other acceleration source other than wind. This should be impossible, according to many scientists, including Kusenko. Wind-powered vessels can exceed wind speeds when they are oriented at an angle to the wind direction, they argue, but not when a vessel is exactly parallel to the downwind force.
For this reason, Blackbird has attracted admirers and naysayers since it first claimed to achieve this seemingly inconceivable feat in 2010. At the suggestion of some of his viewers, Muller decided to tackle the longstanding ambiguity over Blackbird’s abilities for Veritasium.
“There was so much debate in forums and people just couldn't seem to get a definitive answer,” Muller said. “There definitely seemed to be reputable people on both sides of the debate, and so [viewers] asked me to look into it and see if I can make a definitive video on the topic, which was the goal here.”
The video, posted on May 29, documents Muller’s test-drives in Blackbird, and his own conviction, by the end of the experience, that the vehicle does in fact exceed downwind speeds. Kusenko got in touch with Muller to express his misgivings about those claims. The pair had discussed science and physics in the past, and Kusenko said that he normally enjoys Veritasium’s content, but that he had to take issue with Muller’s Blackbird demonstration. Kusenko has since laid out his objections in a series of slides at this website.
“The physics explanation was clearly wrong, and the experiment was not properly designed to answer the question they wanted to answer,” Kusenko said. “So, I sent Derek a friendly email pointing out the problems with this video.”
The men exchanged a few messages disagreeing about the principles and conclusions of the video, which ultimately culminated in the proposal of a wager. At Muller’s invitation, Bill Nye, Neil deGrasse Tyson, and Sean Carroll, a research professor of physics at Caltech, served as witnesses of the agreement and participated in its ratification.
The victor depends on Muller’s ability to successfully demonstrate (or not) “a model vehicle with the same principle of operation as the Blackbird” which “will constitute sufficient evidence that Blackbird can sail faster than the wind downwind as described,” according to the agreement. Muller noted that there are logistical challenges to setting up further test demonstrations with Blackbird, as it requires specific weather and track conditions, so he plans to produce a smaller model that can be tested in more controlled conditions on a treadmill.
“Any model is good, as long as the model is proven to be equivalent to the real thing,” Kusenko said. “The burden of proof is on the model builder. To be honest, I don't see any model succeeding because, as I show in my slides, the laws of physics make sustained sailing downwind faster than the wind impossible.”
Kusenko believes that Blackbird only appears to travel faster than wind due to the variable wind speeds of an outdoor test, like the one depicted in Muller’s video. In these conditions, he said, a wind gust could result in an acceleration of the vehicle, which would continue at that speed through inertia even as the wind decelerates, causing it to look as if it is traveling faster than the current wind speed.
“The car really does travel faster than wind for a short time, after the wind speed drops,” Kusenko explained. “Of course, the car is decelerating, and it can only move faster than the wind for a limited time, until the car speed drops below the current wind speed. How long this time window is depends on the design of the car, as well as on the difference in wind speeds before and after.”
Cavallaro, Blackbird’s creator, has said that the vehicle can cruise faster than the wind not due to any breach in the laws of physics, but because it uses a unique propeller design. Muller pointed out that Blackbird showed that it could travel at almost triple the downwind speed in 2010, setting a record that was confirmed by the North American Land Sailing Association.
“They were going 2.8 times the wind speed, and in fact, accelerating over the period of measurement,” Muller said, “I look at that data and I just say, you cannot make an argument that it was due to wind shear, wind gradients, or a gust.”
“I feel that the data that is out there does not support [Kusenko’s] conclusions,” he continued. “That is not to say that what he's saying is wrong—it's just that it doesn't explain the observations. If you really look into the observations and the data that exists, his analysis falls short.”
The researchers dispute many other points about the concept, including the threat that Blackbird’s claims actually pose to established physics. For Kusenko, the notion of a vehicle traveling faster-than-wind downwind in a sustained state—as opposed to due to a temporary speed burst—is a clear violation of the law of energy conservation.
“If this kind of design were possible, there would be a market for it,” Kusenko pointed out. “For one thing, sailboats crossing the ocean and running with the trade winds could use help in the form of some mechanical propeller adding to the thrust of the wind. Unfortunately, this is impossible, which is why there are no sailboats out there utilizing propellers in this fashion.”
Muller mentions in his video that the concept has earned a reputation as an unruly breaker of physical laws. However, in his view, the vehicle’s unique propeller design obeys the law of energy conservation in its own head-spinning way, by picking up energy from the wind behind it. In fact, Muller thinks that Kusenko’s claim—that traveling faster-than-wind downwind in a sustained fashion is impossible—is the real challenge to the basic laws of physics.
“If he's right, I think it's Earth-shattering in that many, many physicists would have been wrong, and there would be huge implications for basic things that are taught in textbooks,” Muller said. “If I’m right, I would say that the physics is vindicated in a very counterintuitive way.”
Kusenko, who expects to win, plans to donate the $10,000 to Kudu, an online learning platform that he co-founded. Muller, who also expects to win, hasn’t decided what he would do with the windfall just yet, but is considering charities to support.
It’s not clear how long it will take to complete the wager and determine a winner. Both men expressed hopes that the other would concede if evidence clearly pointed one way or the other. Despite their high-stakes dispute, they did manage to find one point of agreement: regardless of the outcome of the wager, it will make for an invaluable learning experience.
“This is what I love about science,” Muller said. “We only make progress when we're able to not fool ourselves and not do things the way that they initially appear, but the way they really are. That's what I'm really excited for: to show my audience, and the public more generally, how the scientific method can move us forward.”
“In these partisan and tribal times, I think it's great to hold up examples of when intelligent people can disagree with each other and instead of just name-calling, we can agree on some terms to move forward,” he added.
Kusenko also emphasized the value of the wager as a science communication and education tool.
“This story is instructive on many levels, and it will make a good example to discuss in my UCLA physics classes and in Kudu (http://kudu.com) physics courses,” Kusenko agreed. “It touches on the use of energy conservation, on the design of an experiment which may or may not be answering the question it was intended to address, etc.”
“I like this story for its own sake, regardless of whether Kudu will ever see Derek Muller's money or not,” he concluded.