Science & Technology

Humans Can Run on Top of Water (on the Moon)

If you've ever gone canoeing, you may have noticed water striders, those small, long-legged bugs that easily stroll across the surface of the water without breaking the surface tension. In Central and South America, the slightly larger basilisk lizard can pull a similar trick, but it has to move pretty fast to do it. And we seem to remember a story about another guy way back when who was kind of famous for walking on water (not sure if that had anything to do with surface tension, though). But actually, a lot of people can perform that "miracle" — as long as they go to the moon first.

A Whole New Way to Moonwalk

What kind of people do this important research into the human capacity to traverse theoretical lunar swimming pools? The same kind of people who win Ig Nobel Prizes. The 2013 science gag prize in physics went to a team of Italian scientists who wanted to find an answer to that question. Their findings? It's possible — but difficult.

To get their results, the scientists devised a brilliant simulation of lunar gravity consisting of a springy harness hanging from the ceiling. They then equipped their study participants with swim fins to mimic the broad feet of basilisk lizards, because while low gravity, high speed, and broad feet can each on their own help with the endeavor, the best way to get results is to combine all three. The researchers rigged the harness to simulate various amounts of gravity and found that the most physically fit of their participants was able to stay afloat at about 22 percent of Earth's gravity. Reduce that amount down to 10 percent, and almost everybody was able to do it. Since the gravity on the moon is about 17 percent of Earth's gravity, that means that running across a lake is certainly possible — it's just closer to the "elite athletes only" side of the spectrum.

By the way, if you think that the sight of somebody strapped into a complicated harness, wearing floppy swim fins, and splashing frantically at the top of a pool of water would be a pretty funny sight, shame on you. This is serious science. And also, you're totally right.

River Running? Wild.

To pull off the same stunt on Earth, and without swim fins, a human being would have to run about 67 miles per hour (108 kilometers per hour). Usain Bolt's fastest speed ever was only about 28 miles per hour (45 kilometers per hour). If you could run fast enough, you'd need to spread your weight out so you didn't break the water's surface tension. Similarly, if you moved fast enough, you could be long gone by the time the surface was broken. That's how water striders and basilisks do it, but other animals have other methods of walking on water (depending on how strict you are about the word "walk").

If you've ever seen a sea-animal show at an aquarium, you know that dolphins prove you don't even need legs to "walk" on water. Humans train dolphins to do this all the time, but it's a behavior that's never been seen in the wild, except in dolphins who have interacted with other dolphins who learned the trick from humans. But lately, biologists have spotted wild dolphins doing it entirely on their own. Of course, this "walking" doesn't involve staying completely above the water line — instead, the dolphins propel themselves with their tail fins underwater, and it's that forward momentum that keeps them from falling through.

Another water-walking animal does so more naturally. The tiny bird known as the storm petrel is notable for appearing to walk calmly across the surface of the ocean. However, it's more of a half-hearted flight than a stroll — these birds are just carrying themselves right above the water so that they can hunt more effectively.

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Written by Reuben Westmaas March 30, 2018

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