Could humans run on water?

Episode Summary

Executive Summary: The episode explores the physics of running on water through the lens of basilisk lizards, grebes, and human biomechanics. Fluid dynamicist Nicole Sharp explains how slapping motion, foot area, gravity, and fluid properties determine whether movement across liquid surfaces is possible, and why this could matter for robotics and future space-based sports more than for Earthly Olympics.

Main Topics: Nicole Sharp’s path into fluid dynamics (Priority: 4/5): Sharp describes how she became captivated by fluid dynamics during undergraduate studies, later building a career in aerospace engineering, research, and science communication through her FYFD blog. Fluid instabilities in nature (Priority: 5/5): Sharp highlights Kelvin-Helmholtz instability as a favorite example of fluid behavior appearing across many scales, from ocean waves and clouds to Jupiter’s atmosphere and galactic clusters. Why humans can’t normally run on water (Priority: 5/5): The discussion explains that Earth-bound humans would need to move at roughly highway speed to run on plain water, unless they increase effective foot area or alter conditions such as gravity. Basilisk lizards and the mechanics of water running (Priority: 5/5): Basilisk lizards avoid sinking by slapping oversized feet onto the water, generating vertical impulse rather than relying on buoyancy or surface tension. Engineering and robotics applications (Priority: 4/5): Understanding animal locomotion informs biorobotics and terrain-adaptive robots, especially for disaster response and movement across difficult surfaces like sand, gravel, or rubble. Space-based and reduced-gravity possibilities (Priority: 4/5): Water running becomes more plausible in reduced gravity, especially on the Moon, and could potentially be adapted to Titan’s hydrocarbon lakes; Richardson is cited as fast enough for Titan with the right conditions. Grebes, timing, and the limits of spectacle (Priority: 3/5): Grebes are presented as a larger water-running animal that uses a similar slapping mechanism, though their underwater foot motion remains hard to observe and human sports versions would require careful rules.

Key Arguments: Fluid dynamics is broadly relevant, extending beyond liquids to gases, plasma, and even granular materials like sand and snow. The Kelvin-Helmholtz instability demonstrates that the same physics can generate ocean waves, cloud patterns, atmospheric bands, and even structures in galactic clusters. A human on Earth would need roughly highway speed to run on plain water, making the feat impossible without modifications or environmental changes. The basilisk lizard succeeds because its oversized feet strike the water hard enough to create lift-like vertical impulse before the water closes around the foot. Larger foot area can substitute for speed, which is why floating lily-pad-like platforms allowed engineers to run across water in lab demonstrations. Animal locomotion studies are useful for designing robots that can move across mixed terrain and operate in disaster zones. Reduced gravity on the Moon makes water running more feasible, and a human with proportionally enlarged feet could sustain the motion for at least several seconds in experiments. Titan’s low gravity and liquid lakes make it a plausible off-world venue for analogous running events, though ethane is less supportive than water. Sha'Carri Richardson’s speed is sufficient for Titan-based water/ethane running according to the article’s calculations, but not for Earth water running without major changes.

Data Points: FYFD blog duration: 14 years - Nicole Sharp says she has run her fluid-dynamics blog for 14 years. FYFD article count: about 3,600 articles - Sharp estimates the blog has accumulated roughly 3,600 posts. Blog word count: over half a million words - The host notes the blog’s total output exceeds 500,000 words. Human water-running speed on Earth: about highway speed in a car - Sharp says that to run on plain water on Earth, a human would need approximately highway-speed running. Basilisk lizard mass: about 100 grams - Sharp explains the lizard is far too heavy for surface tension alone. Grebe mass: about 2 to 2.5 kilograms - Grebes are described as roughly 10 times heavier than basilisk lizards. Grebe step rate: about 20 steps per second - During their rushing display, grebes kick their feet extremely rapidly while running on water. Grebe water-running distance: about 20 meters - Grebes can run across water for roughly this distance during mating display. Human water-running speed on Titan: 8.7 metres per second - Sharp’s calculation for Sha'Carri Richardson running on Titan’s lakes. Richardson world championship speed: 9.3 metres per second - Her actual championship pace is cited as slightly faster than the Titan threshold. Richardson body mass: 45 kilograms - The article states her low weight helps make Titan water-running feasible. Richardson shoe size: UK size 5 - Her shoe size is mentioned as providing a favorable foot area for the Titan calculation. Olympic Games dates: 26 July to 11 August - The host references the Paris Olympics schedule.

Pivotal Quotes: "I am an aerospace engineer and a science communicator." — Nicole Sharp: Sharp introduces her professional background and dual role in engineering and outreach. "don't stop now. This is the first interesting thing any of you have said." — Nicole Sharp: She recalls becoming fascinated by fluid dynamics during an undergraduate lecture on airflow over an airplane wing. "you would only have to go about highway speed to be able to do it." — Nicole Sharp: Sharp summarizes the Earth-based speed requirement for running on water.

Implications: The episode shows that water-running is physically real in some forms, but Earth Olympics are unlikely without radical changes. The concepts are more relevant to robotics, reduced-gravity environments, and future off-world sports.