Does the superposition principle fail above some mass or complexity?
In plain words
Is there a size beyond which an object cannot be in two places at once even when perfectly isolated from its surroundings? No such limit has been found, but free-flight tests reach only about 170,000 atomic mass units.
Precise statement
For a free particle of mass m delocalized over $\Delta x$ for time t, with all known environmental decoherence (gas collisions, thermal photon emission and absorption, light scattering) accounted for, determine whether fringe visibility falls below the quantum prediction, i.e. whether an extra decoherence rate $\Gamma(m,\Delta x)>0$ exists. Progress is measured by the macroscopicity $\mu=\operatorname{log10}(\tau_e/1\,\mathrm{s})$ of Nimmrichter and Hornberger, with $\tau_e$ the largest classicalization time, referred to a single electron, that the experiment excludes for minimal macrorealistic modifications of quantum mechanics.
What would settle it
Interference at increasing $\mu$ with full decoherence budgets that either reveals unexplained loss of visibility or excludes modifications up to everyday-object scales.
Status in the literature
Unverified note
Sodium nanoclusters of about 170,000 u interfered at $\mu = 15.5$ in 2025, an order of magnitude beyond the previous record (arXiv:2507.21211).
Related problems
- More general than Spatial superposition of a levitated nanoparticle larger than its size
- More general than Matter-wave interference with particles above one million atomic mass units