New Study Challenges Newton’s Third Law of Motion: Sperm Cells and Algae Defy Symmetry in Movement
Researchers at Kyoto University have discovered that human sperm cells and single-celled algae can move through viscous fluids in a manner that seemingly defies Newton’s third law of motion. Kenta Ishimoto, a mathematical scientist, and his colleagues investigated the motion of these microscopic biological swimmers to understand how they navigate through substances that should, theoretically, resist their movement.
Newton’s third law states that for every action, there is an equal and opposite reaction. This principle highlights the symmetry in nature, where opposing forces counteract each other. However, certain systems, such as swimming sperm and flocking birds, display non-reciprocal interactions that violate this law.
Unlike inanimate objects, living organisms generate their own energy, which is continuously added to the system. For example, birds’ wings and sperm tails constantly generate energy to propel them forward. This constant influx of energy pushes the system far from equilibrium, allowing these organisms to defy Newton’s third law.
To understand the mechanics behind this phenomenon, Ishimoto and his team analyzed experimental data on human sperm cells and modeled the motion of green algae, known as Chlamydomonas. Both sperm cells and algae swim using thin, flexible flagella that change shape to propel them forward.
In highly viscous fluids, a flagellum’s energy would normally dissipate, preventing these cells from moving efficiently. However, the researchers discovered that sperm tails and algal flagella possess an “odd elasticity.” This unique property allows these flexible appendages to whip about without losing much energy to the surrounding fluid.
But the odd elasticity alone did not fully explain the propulsion achieved through the flagella’s wave-like motion. Through their modeling studies, the researchers introduced a new term called “odd elastic modulus” to describe the internal mechanics of the flagella, deciphering the nonlocal, nonreciprocal interactions within the material.
The implications of this study go beyond understanding the movement of sperm and algae. The findings could aid in the design of small, self-assembling robots that mimic living materials. Additionally, the modeling methods employed in this research could contribute to a better understanding of the underlying principles of collective behavior.
The study, titled “Deciphering odd bending elasticity in nature: From flagella waveform of Chlamydomonas and sperm cells to nonreciprocal inner interactions of biological materials,” has been published in PRX Life.