Viscosity: The Unique Criterion Distinguishing Quantum and Classical States

Since the early development of modern physics, physical phenomena have traditionally been classified according to spatial scale, with macroscopic systems described by classical theories and microscopic systems by quantum mechanics [2-5]. However, the discovery of macroscopic quantum phenomena, such as superfluidity and superconductivity, demonstrates that quantum behavior is not restricted to microscopic length scales [9-11].

This observation points to a more fundamental distinction. The terms “macroscopic” and “microscopic”merely describe differences in spatial scale rather than differences in the physical state of matter. The more essential classification is between “classical states”and “quantum states”, which should be regarded as two distinct physical states of matter, analogous to the solid, liquid, and gaseous states. They do not represent different sets of physical laws; instead, they are different manifestations of the same underlying laws arising from changes in the physical environment and the quantitative conditions of the system.

In this work, we propose that the fundamental distinction between classical and quantum states is determined by the presence or absence of “viscosity”. A classical state possesses finite viscosity, through which thermal motion continuously dissipates ordered collective motion. By contrast, a quantum state is characterized by vanishing or extremely low viscosity, allowing long-range coherent collective motion to remain stable against thermal dissipation. This difference gives rise to the markedly different physical behaviors observed in classical and quantum systems.

Based on this viewpoint, we investigate the physical origin of viscosity and establish a unified theoretical framework based on the competition between intrinsic motion and thermal motion. Within this framework, the effects of particle mass, interparticle interactions, and particle symmetry on viscosity are analyzed systematically, with the aim of providing a unified physical description of both classical and quantum states.