Scientists at Kyoto University have uncovered that human sperm appear to defy Newton’s third law—“every action has an equal and opposite reaction”—by swimming through thick fluids using a mechanism dubbed “odd elasticity.” When sperm whip their flexible tails (flagella), they interact with the fluid in a non-reciprocal way, avoiding the expected equal-and-opposite counterforce.

In experiments and models involving both human sperm and green algae (Chlamydomonas), researchers identified internal bending forces within flagella that bend just enough to propel sperm forward without losing much energy to fluid resistance. This behavior stems from an odd elastic modulus—a newly defined physical property describing how the tail’s mechanical interactions break classical reciprocity.

Some commentators note the intriguing nuance:

“Such movements…should result in reactions from the fluid … but that was not the case.” 
“Odd elasticity basically allows the cells to move their flagella without expending a significant amount of energy in their environment.”

This discovery helps resolve how microscopic swimmers eclipse the “scallop theorem” and sustain efficient motion in viscous environments. Beyond biological insight, the breakthrough could inform engineering of tiny elastic microswimmers—robots that mimic sperm movement for applications like drug delivery or minimally invasive surgery.

While the headlines suggest a dramatic “violation,” researchers emphasize that the cells still obey fundamental physics; their behavior merits a broader framework of odd elastohydrodynamics, which generalizes Newtonian laws to active, energy-consuming microscopic systems.