Scientists say sperm appear to break one of Isaac Newton’s laws of motion.
Most people picture sperm as tiny, tadpole-like swimmers searching for an egg.
But researchers now report that sperm seem to violate a physics rule scientists have used for centuries.
The finding comes from Kenta Ishimoto, a mathematical scientist, and colleagues at Kyoto University.
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They investigated sperm behavior in light of Newton’s third law of motion, which states, “for every action, there is an equal and opposite reaction.”
That law implies forces come in pairs that oppose each other.
You can see it when two same-sized marbles collide and transfer force, bouncing apart.
Sperm act differently.
The team discovered sperm propel themselves through viscous fluids without producing an equal and opposite reaction that cancels their motion.
Ishimoto and his colleagues examined these non-reciprocal interactions.
They analysed experimental data on human sperm and modelled the motion of the green alga Chlamydomonas.
Both sperm and the algae use a flagellum — the tail-like appendage — to generate forward motion.
Normally, thick fluids dissipate energy and slow such motion.
The researchers found sperm flagella possess an unusual property they call “odd elasticity.”
That property lets the tails whip and continue generating thrust without wasting much energy to the surrounding liquid.
But odd elasticity alone didn’t fully explain the propulsion from the flagellar wave.
To clarify matters, the team introduced a new term, “odd elastic modulus”, to characterise flagella’s internal mechanics.
As the flagellum bends in response to the fluid, it avoids the equal-and-opposite reaction and conserves energy.
“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.”
“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.”
The researchers say their results could inform the design of tiny, self-assembling robots that mimic living materials.
The work might also help explain principles behind collective behaviour.
The discovery is fascinating — and it leads into another sperm-related fact that might unsettle some readers.
Elon Musk has reportedly been an active sperm donor, according to a New York Times report.
Musk has tweeted about falling birth rates and framed the trend as a major threat.
He wrote: “A collapsing birth rate is the biggest danger civilization faces by far.”
He added: “Many countries are already well below replacement rate, and the trend is that almost all will be.
“This is simply a fact, not a ‘debunked theory.’ 2.1 kids is replacement rate, and obviously the world as a whole will soon drop below that point.”
The New York Times reported the Tesla CEO has offered his seed to friends and acquaintances over the years.