Sperm cells caught breaking a law of Newton
The third law of Newton, also known as the “Law of Action and Reaction,” states that for every action, there is an equal and opposite reaction. However, this universal law does not apply to some biological swimmers like sperm cells. In a groundbreaking study, scientists analyzed Chlamydomonas algae and human sperm cells, identifying non-reciprocal mechanical interactions they termed “strange elasticity” that do not adhere to Newton’s third law.
Both Chlamydomonas and sperm cells use hair-like appendages called flagella to move. These appendages extend from the cell, acting almost like a tail, to propel them forward. They change shape as they interact with the surrounding fluid to achieve movement.
The scientists explained that flagella exhibit non-reciprocal motion, meaning they do not cause an equal and opposite reaction in the environment, thus violating the fundamental law of physics. Non-reciprocal interactions are observed in rebellious systems such as bird flocks, particles in fluids, and swimming sperm cells.
Since birds and cells generate their own energy with each wing flap or tail movement, the system moves away from equilibrium, and the usual rules do not apply. Highly viscous fluids would dissipate the energy of a flagellum, hindering sperm cell or unicellular algae mobility. However, these elastic flagella somehow manage to move.
Sperm cells and algae are not the only ones with flagella; many microorganisms possess them. This suggests that there may be other law-breaking cells yet undiscovered by science. Researchers believe this finding could be beneficial in designing small, elastic robots capable of defying the laws of physics.
This story was written in Spanish by Perla Vallejo in Ecoosfera.
