[Phys-L] Violation of Newton's third law in motile active agents?

[Antti Savinainen <antti.j.savinainen@gmail.com>]

Hi,

I came across an article which claims that Newton's third law doesn't apply
to "active motile agents." It also claims that they would exhibit
"nonconservativity of energy." The article is freely available:

https://journals.aps.org/prxlife/abstract/10.1103/PRXLife.1.023002

Here is an excerpt from the introduction:

"Motility is one of the main features of living matter, from a single cell
to a swarm of birds or a human crowd. In the
last few decades, the dynamics of motile active agents, both individual and
collective behavior, have been intensively studied, giving rise to a
rapidly expanding research field in physics bridging nonequilibrium
statistical physics, biophysics, and continuum mechanics, now known as
active matter and living
matter physics. A crucial feature of these systems is that inner activity
units convert energy into mechanical forces. In turn,
Newton’s third law may be violated when we regard it as an open system,
with its mechanical energy being injected from
microscopic active units. Therefore, the mechanical interactions between
the units can be non-reciprocal."

As far as I can tell, Newton's third law, expressed in terms of momentum
flow, is pretty fundamental in physics.

Any thoughts?

Regards,

Anti Savinainen, PhD
Finland