Researchers at Kyoto University have made a groundbreaking discovery about the behavior of human sperm cells in viscous fluids. Contrary to long-held assumptions based on Newton’s Third Law of Motion, sperm cells seem to defy physics and glide effortlessly through dense liquids like honey.
Newton’s third law states that for every action, there is an equal and opposite reaction. However, when it comes to swimming organisms like sperm, this law doesn’t apply in the same way. The flexible flagella of the sperm bend and move in a non-reciprocal manner, generating propulsion without the typical equal and opposite force from the fluid.
This phenomenon can be attributed to a property called odd elasticity, which describes how the tail of the sperm bends in a specific way that bypasses the usual reaction from the surrounding fluid. In essence, this allows the swimmer to create asymmetrical waves that don’t provoke an equal-and-opposite response from the fluid, enabling them to move forward with minimal energy loss.
The implications of this discovery are far-reaching, extending beyond sperm cells to inspire new designs for small robots and machines that mimic biological systems. By studying how sperm cells operate in viscous environments, scientists may develop more efficient self-propelled robots that can operate in similar conditions – potentially changing industries like medicine, environmental monitoring, and space exploration.
Furthermore, this research opens the door for a broader understanding of collective behavior in nature. The ability of individual organisms to defy traditional laws of physics has implications for how forces operate in living systems, and may provide new insights into interactions between crowds or flocks of animals.
Source: https://dailygalaxy.com/2025/07/scientists-find-sperm-cells-breaking-one-of-physics-most-famous-laws-newtons-law