The Strange Nature of Sperm

Human sperm cells caught breaking Newton’s third law of motion

The motion of sperm. Image provided by IFLScience

Newton’s third law of motion states that every action or force has an equal and opposite reaction. That is, if object A exerts a certain amount of force on object B, B exerts the same amount of force on A. Despite being thoroughly proven, the law is violated in specific nonequilibrium situations. The most recently found example is the case of sperm.

Kenta Ishimoto, a mathematical scientist at Kyoto University in Japan, and his colleagues directed a study involving Chlamydomonas algae and human sperm cells. Both algae and sperm cells use flagella, hairlike appendages protruding from the cell surface, to move, creating waves through the liquid environment. The researchers expected the equal and opposite force exerted by the fluid to slow down the cells’ movement.

Contrary to expectations, the team uncovered a phenomenon demonstrating unreciprocated physical interactions, which they termed “odd elasticity.” More specifically, they found that flagella did not lose energy by avoiding the equal and opposite reaction from the fluid. As a cell’s odd elasticity score, or modulus, increases, its flagellum can produce waves better without energy loss and, therefore, allows greater cell movement—defying Newton’s law of physics.

This discovery extends beyond biological curiosity. Flagella is a common trait among many microorganisms, which indicates that this rule-breaking situation can be found in many more natural scenes. The property of odd elasticity has created a lot to reconsider for physicists, but its discovery can help scientists design robots with similar properties and understand various real-life motions and their principles.