Adrian Bejan | Gravity does matter to the fish, from Design in Nature

In this video, Adrian Bejan challenges a widespread assumption in scientific literature that gravity does not matter to the fish. He revisits his earlier work on locomotion, which united flyers, runners, and swimmers under one principle: effort is divided into overcoming resistance horizontally and lifting weight vertically. But the fish remained an anomaly. Drawing from both personal experience and physics, Bejan delivers a breakthrough: when a fish moves, it lifts water, and that lifted water makes gravity matter.

• Bejan recounts the origin of his theory, from his 2000 book Shape and Structure, to a pivotal moment in 2004 at Monte Verità, when biologist Jim Martin encouraged him to extend his theory of flying to running and swimming.

• He explains how the fish, once thought to escape the vertical effort of gravity, in fact must displace water when they move. This displacement lifts a volume of water equivalent to the fish’s own body, creating a bulge on the water surface, a visible manifestation of vertical effort.

• The pushed boat becomes Bejan’s thought experiment and proof: a body moving in water, like a boat or a fish, expends energy not only on drag but on lifting water. Without accounting for this, the motion appears endless, but in reality, it stops.

• Bejan uses dimensional reasoning to show that kinetic energy from motion becomes gravitational potential energy stored in the lifted prism of water, explaining why a pushed boat (or a gliding swimmer) comes to rest.

• He extends this insight to the pendulum in water, showing that oscillations stop because of both drag and the lifting of fluid, reinforcing that gravity is active even in aquatic environments.

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Umit Gunes, Ph.D.
Assoc. Prof.​ |​ Yildiz Technical University
Editor​ | International Communications in Heat and Mass Transfer
Guest Editor​ |​ Philosophical Transactions of the Royal Society A
Guest Editor​ |​ BioSystems
​Web | umitgunes.com