  Arguments about the conservation laws of energy and momentum in the
micro-world being statistical or strict began in 1924, and conflicting
viewpoints remain today. The former is mainly supported theoretically, but the
latter has been proved by many experiments. Here we explain that in principle,
the strict conservation law of momentum always holds in the entangled state
form of the momentum eigenstates of a closed composite system with interactions
among subsystems, by expanding the total wave function to the sum of the
products of momentum eigenstates of subsystems. Common scenario is that one of
the two subsystems of a composite system is large or strong, its state remains
approximately unchanged in a short time and the entangled state can be
approximately written as a product state, which can be easily deduced from the
paper of Haroche's group in 2001. The considered micro-subsystem can be
approximately represented as the superposition of its different momentum
eigenstates; therefore, the approximation can be used to explain why the law
holds statistically for the subsystem and for any single particle due to
neglecting the interactions with the large subsystem or the environment. So the
two momentum conservation laws reasonably hold without conflicts.
