Human sperm challenges one of the universal laws of physics, here’s how |

All of us are familiar with Newton’s Third Law: Action & Reaction. The third law states that for every action (force) in nature, there is an equal and opposite reaction. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A. In other words, forces result from interactions. Well, this may apply to almost everything in this world, but certainly not to human sperm. Yes, you read it right.
Human sperm propel themselves through viscous fluids, with their whip-like tails, and defy Newton’s third law of motion, according to a study published in PRX Life.

Kenta Ishimoto, a mathematical scientist at Kyoto University, and his colleagues are behind this novel discovery.
The team found that the sperms manage to propel forward through viscous fluids, rather than creating an equal and opposite reaction and being met with resistance. “This study explores a violation of Newton’s third law in motile active agents, by considering non-reciprocal mechanical interactions known as odd elasticity. By extending the description of odd elasticity to a nonlinear regime, we present a general framework for the swimming dynamics of active elastic materials in low-Reynolds-number fluids, such as wavelike patterns observed in eukaryotic cilia and flagella,” the study author notes.

They looked into the non-reciprocal interactions in sperm and other microscopic biological swimmers, to figure out how they slither through substances that should, in theory, resist their movement. To study this, the researchers analyzed experimental data on human sperm and also modeled the motion of green algae, Chlamydomonas, as both swim using thin, bendy flagella that protrude from the cell body and change shape, or deform, to drive the cells forward.

They found that the sperm tails have an ‘odd elasticity’ which enables them to propel forward without losing much energy to the fluid. However, they couldn’t fully explain the propulsion from the flagella’s wave-like motion. Hence the researchers also derived a new term, an odd elastic modulus, to describe the internal mechanics of flagella.

“From solvable simple models to biological flagellar waveforms for Chlamydomonas and sperm cells, we studied the odd-bending modulus to decipher the nonlocal, nonreciprocal inner interactions within the material,” the researchers said in a statement.
They added, “Odd elasticity is not a generic term for activity in solids, but rather a well-defined physical mechanism that generates active forces in solids or in other systems in which a generalized elasticity can be defined without using an elastic potential.”