Equilibration and the Eigenstate Thermalization Hypothesis as Limits to Observing Macroscopic Quantum Superpositions

Macroscopic quantum superpositions are widely believed to be unobservable because large systems cannot be perfectly isolated from their environments. Here, we show that even under perfect isolation, intrinsic unitary dynamics with the eigenstate thermalization hypothesis suppress the observable signatures of macroscopic coherence. Using the GHZ state as a representative example, we demonstrate that while fully correlated measurements can initially distinguish a macroscopic superposition from its corresponding classical mixture, generic many-body evolution renders them operationally indistinguishable for most times during the evolution. By analyzing both distinguishability measures and established quantifiers of macroscopic quantumness, we find that equilibration not only hides coherence from accessible observables but also suppresses macroscopic superpositions themselves. These results identify unitary thermalization, independent of environmental decoherence, as a fundamental mechanism that limits the emergence of macroscopic quantum effects.