Does quantum mechanics apply to macroscopic objects? How to define macro?

TL;DR

The paper investigates whether quantum mechanics applies to macroscopic objects by analyzing the center-of-mass (c.m.) motion as a free-particle quantum problem. It derives the standard Gaussian c.m. wave-packet evolution, showing the width evolves as $\langle x^2\rangle_t = d^2 + (\frac{\hbar t}{2Md})^2$, revealing that the mass–width product $Md$ governs spreading and that macroscopic masses should spread negligibly, yet macroscopic objects are never observed in broad c.m. states. It argues that macroscopic definiteness is not simply explained by decoherence and discusses gravitational self-trapping proposals (e.g., the Diosi–Penrose framework) and alternative viewpoints that QM has a limited domain of applicability requiring higher-level theories. The paper emphasizes the potential of micro-miniaturization experiments to illuminate the micro–macro threshold and the proper definition of the quantum-to-classical boundary, offering critical reflections on the interplay between quantum theory, gravity, and classical observations.

Abstract

According to Quantum Mechanics a narrow wave-packet of the center of mass of any macroscopic object should smear out after some time. The problem is usually waved out by assuming that due to their heavy masses this occurs over astronomical times. Without a clear definition of macroscopic objects this remains ambiguous. On the other hand, Quantum Mechanics allows and energetically even prefers largely smeared out c.m. states that never been seen. Why is this the real state of the world we know? Does it suggest a micro-macro threshold with different theoretical descriptions? Does offer micro-miniaturization a hope for answering these questions?