One of the most significant debates of our time is whether our macroscopic world (i) naturally emerges from quantum mechanics or (ii) requires new physics. We argue for the latter and propose a simple gravitational self-decoherence mechanism. For this purpose, we postulate the existence of a Heisenberg cut such that particles with masses $m$ much smaller and larger than a critical mass $M_{\mathrm{C}}$ (of the order of the Planck mass $M_{\mathrm{P}}$) would be necessarily treated according to quantum and classical rules, respectively. Our effective model is designed to capture the new physics that free quantum particles would experience as their masses approach $M_{\mathrm{C}}$. The purity loss for free quantum particles is evaluated and shown to be highly inefficient for quantum particles with $m\ll M_{\mathrm{C}}$ but very effective for those with $m\sim M_{\mathrm{C}}$. The physical picture behind it is that coherence would (easily) leak from heavy enough particles to (nonobservable) spacetime quantum degrees of freedom. Finally, we contextualize our proposal with state-of-the-art experiments and show how it can be tested in a future Stern-Gerlach-like experiment.

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• Open quantum systems & decoherence
• Quantum formalism
• Quantum-to-classical transition