Human sperm cells do not care about Newton’s third law of motion, and scientists don’t know why

Newton's Third Law of Motion posited that for every action in the natural world, there is an equal and opposite reaction. Beyond being a memorable line for heroes seeking vengeance, Newton's 17th-century genius formulated this law to describe how forces function, emphasising that interacting pairs of objects always exert equal and opposite forces on each other.

Newton's "laws" revolutionised science, and they remain valid in most cases, with some exceptions in the microscopic realm or when objects approach the speed of light. However, there is another domain where this Third Law appears to break down, leaving scientists perplexed. Surprisingly, this occurs right within our bodies.

Human sperm cells have unique characteristics, with a peculiar blob-like head and an extraordinarily long tail. Despite their appearance, these sperm cells defy the laws of physics simply by existing.

Human sperm cells and the Chlamydomonas algae both move by wiggling their tails, or flagella. While this motion may seem innocuous, physicists are baffled as to how it works. In theory, the fluid surrounding the sperm cells should dissipate all the energy they exert, essentially rendering them stationary.

However, the inherent elasticity of the sperm cells' shape-shifting flagella allows them to move without losing a significant amount of energy to the surrounding environment. This extraordinary property, which defies Newton's Third Law, is termed the 'odd elastic modulus.' This term is relatively new, introduced to the scientific community in around 2019 to help comprehend microscopic interactions.

The precise mechanisms at play in these agile swimmers are known only to the intricate processes of evolution. If we can calculate the odd elastic modulus for various microorganisms, we may gain insight into what enables them to defy Newton's laws. Furthermore, this knowledge could aid scientists in developing soft, elastic robots that can fully exploit this "non-reciprocity."

The research findings have been published in PRX Life and can be accessed here.