Unraveling the Secrets of Sperm and Algae: Defying Newton’s Laws

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Scientists have discovered that sperm and single-celled algae, which have whip-like tails and strange flexibility, can move through viscous fluids with minimal energy loss. This contradicts Newton’s third law of motion, which states that for every action, there is an equal and opposite reaction. However, the natural world is unpredictable, and non-reciprocal interactions can occur in complex and disorderly systems. Researchers Kenta Ishimoto and his colleagues discovered that these microscopic swimmers have an “odd elasticity,” allowing them to whip through the fluid with minimal energy loss. This property, known as the “odd elastic modulus,” is critical to understanding how these creatures move.

The findings have implications for robotics and collective behavior, as they show promise in creating miniature robots made from biomimetic components. By delving into the principles underlying these out-of-the-ordinary motions, scientists can explore new areas of inquiry, such as bio-inspired robotics and improved understanding of collective behavior in complex systems. The potential applications of these findings extend beyond sperm and algae, demonstrating the potential for new areas of inquiry in the field of motion.

The Enigma of Non-Reciprocal Interactions

Sir Isaac Newton established the groundwork for the study of the interaction between matter and the forces that operate upon it in 1686 with the formulation of his laws of motion. The third law of Newton, also known as “the law of universal gravitation,” declares that “for every action, there is an equal and opposite reaction.” It suggests that the natural universe is perfectly symmetrical, with equal and opposite forces. A simple example of this law in work is the collision and rebound of two marbles.

However, physical systems do not always follow these symmetrical norms, and the natural world is notoriously unpredictable. In complex and disorderly systems, such as flocks of birds, suspensions of particles in fluid, and, intriguingly, swimming sperm, non-reciprocal interactions arise.

The Odd Elasticity of Sperm and Algae

Researchers like Kenta Ishimoto and his colleagues had to figure out the hidden logic behind the mobility of sperm and single-celled algae. These microscopic swimmers get by with whip-like tails on sperm and bendy flagella on algae. The rules of physics indicate that these appendages should quickly lose energy in very viscous fluids, limiting their mobility. However, reality contradicts this.

The researchers found that the tails and flagella have a ‘odd elasticity,’ a property that allows them to whip through the fluid with minimal energy loss. Since this peculiar quality cannot account for the thrust on its own, we must introduce a new concept: the “odd elastic modulus.” This word refers to the internal workings of these appendages, which are critical to grasping how they move.

Implications for Robotics and Collective Behavior

The implications of this work go far beyond sperm and algae. They show promise in a number of areas, including the creation of miniature robots that can assemble themselves out of biomimetic components. Scientists can open up new areas of inquiry by delving into the principles underlying these out-of-the-ordinary motions.


Newton’s principles of motion are put to the test by the world of microorganisms that swim in water. Non-reciprocal interactions are seen in sperm and algae, which have whip-like tails and strange flexibility. The potential applications of these findings are many, ranging from bio-inspired robotics to improved comprehension of collective behaviour in complex systems.


How does odd elasticity contribute to the motion of sperm and algae?

The whip-like tails and flagella of sperm and algae are able to propel themselves through viscous fluids thanks to their unusual suppleness.

Are there practical applications for the findings of this study?          

Yes, the study can help with the development of self-assembling, miniature robots and the analysis of group dynamics in complicated systems.

What led scientists to introduce the term ‘odd elastic modulus’?         

The word was coined to describe the underlying mechanisms at work in tails and flagella, illuminating previously obscure aspects of their movement.

How does this research challenge Newton’s third law of motion?

The research demonstrates that the biological swimmers’ non-reciprocal interactions allow them to travel in ways that break the symmetry rules of Newton’s third law.

What are the potential future developments in this field?

The findings have interesting implications, including the potential for improved bio-inspired robotics and a greater grasp of collective behaviour in complex systems.

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