  Irreversibility remains one of the least understood concepts in physics. One
of the main reasons is the fact that the fundamental laws of classical and
quantum physics are time symmetric, whereas macroscopic processes evolve in a
preferred time direction. This long-standing conflict of ideas goes to the
heart of the so-called Loschmidt's paradox. Here, we address this dichotomy
from a Newtonian perspective of the dynamics of the center of mass of a
many-body system, which was found to impose constraints on the time evolution.
We demonstrate that despite the time symmetric behavior of the microscopic
constituents of the system, in many typical cases the external conditions lead
naturally to the unidirectional macroscopic time evolution, independent of the
internal interactions and how far the system is from equilibrium. We also
illustrate our findings by direct calculation of the center of mass dynamics
for a nonequilibrium steady state system partially free from external influence
and for a system following the route to equilibrium.
