Passive Propulsion via Quantum Diffraction and Geometric Asymmetry: A Mechanism for Generating Net Force from Isotropic Atmospheric Pressure

We propose a passive propulsion mechanism in which a rigidly connected asymmetric nanostructure generates a net propulsive force under atmospheric pressure. As a concrete and illustrative realization, consider a double-slit wall — containing two nanoslits each of width a — and a single-slit wall containing one nanoslit of width 2a, placed facing each other in parallel, separated by a distance on the order of the mean free path of the gas, rigidly connected, and placed under atmospheric pressure. Since the total slit width is equal on both sides, the number of gas molecules incident on the slits of each wall per unit time is equal. However, owing to the difference in quantum diffraction patterns between the two aperture configurations, the number of gas molecules per unit time that enter from the double-slit side and exit through the single slit without collision differs from the number traversing in the opposite direction without collision. As a result, the balance of atmospheric pressure acting on the two walls is broken, and the structure acquires a net propulsive force. This propulsion mechanism operates without combustion, electrical input, or moving parts. At standard atmospheric temperature and pressure, the de Broglie wavelength of air molecules is approximately 0.002nm — small compared to realistically achievable nanoslit widths of a few nanometers, so that the quantum diffraction effect per molecule is not pronounced. Nevertheless, under atmospheric pressure the number of air molecules impinging on the nanostructure per unit time is enormous, and the cumulative effect over a large number of molecules can give rise to a practically effective propulsive force. By arranging large numbers of units in a collective array, macroscopic thrust becomes achievable. Furthermore, we propose an alternative realization based on edge diffraction, in which two edges, A and B, are placed witha vertical separation a (a → 0 or a < 0 is in principle possible), a horizontal offset s (s ≫ a), and a wall height L (L ≫ a). By Babinet’s principle, the edge-diffraction component is maximised in the limit a → 0 where the aperture closes, providing the theoretical basis for enhanced diffraction efficiency without nanoscale fabrication. This configuration does not require nanotechnology and is in principle realisable using micro- or mesoscale manufacturing processes, making large-scale integration straightforward. The proposed mechanism cannot be dismissed by appeal to the principle of detailed balance. Regardless of which process pairs detailed balance applies to in the present geometry, no physical mechanism exists that would cancel the momentum asymmetry N_A・W_A≠N_B・W_B — where N_A = N_B by the isotropy of atmospheric pressure acting as an inexhaustible molecular reservoir, and W_A≠W_B by the laws of quantum mechanics. The system is therefore not in thermal equilibrium — a state that would require the net momentum flux to vanish. The proposed mechanism is applicable to any assembly of nanoscale elements — including nanopores, arrays of nanorods or nanowires, clusters of nanoparticles, or any other arrangement of scatterers — that satisfies two conditions: the elements are effectively connected as a single mechanical unit (whether by rigid physical connection, chemical bonding, Coulomb interaction, or any other force that causes them to move as one), and the distinct sub-assemblies present structurally distinct collective quantum diffraction patterns. The opposing faces need not be limited to two; three or more sub-assemblies arranged in arbitrary spatial configurations — for instance, in a triangular or tetrahedral arrangement — are equally valid realizations. Although the present work focuses on atmospheric pressure as the ambient medium, the mechanism is not limited to gases. It applies in principle to any isotropic medium — whether gas, liquid, electromagnetic radiation, or other wave field — that provides an inexhaustible reservoir of waves or particles exhibiting wave-particle duality. We note that simply scaling funnel or ratchet pores down to nanoscale dimensions does not give rise to the present mechanism. Geometrically, these configurations do not produce quantum-diffraction-induced deflection events that result in collisions with an opposing wall. Atmospheric pressure, sustained on a planetary scale by solar energy, provides an inexhaustible passive molecular reservoir. Since the mechanism originates solely from the geometric asymmetry of the nanostructure, it imposes no constraints on material composition and operates at any altitude within the atmosphere where pressure is sustained. The present framework further suggests that naturally occurring asymmetric structures — from biological ion channels to crystal lattices — may be subject to, or constrained by, nonreciprocal momentum fluxes under ambient pressure, offering new perspectives on passive transport in biological systems and structural stability in condensed matter. Quantitative verification via molecular dynamics simulation and experimental measurement, and the theoretical formulation from first principles, remain essential next steps. Note: A patent application has been filed for the edge-diffraction-based passive propulsion device and method described in this paper (Japanese Patent Application No. 2026-079977).