Quantum superpositions decay — a particle in a superposition of two locations gradually loses coherence as its environment records “which-path” information. Photons, phonons, and air molecules all serve as decohering agents. In principle, you can shield against all of them: put the system in vacuum, cool it to near absolute zero, isolate it from electromagnetic radiation. But you cannot shield against gravity. Gravitons — if they exist as quantum excitations of the gravitational field — permeate everything and couple to all mass-energy.
Fragkos, Bose, and Marshman (arXiv 2602.22517, February 2026) analyze gravitational decoherence of a composite particle — one with internal degrees of freedom like vibrational or rotational modes — held in a spatial superposition. The interplay between gravitons and the classical Newtonian potential produces a richer picture than either alone.
On short timescales, graviton emission and absorption dominate the decoherence. The process is analogous to bremsstrahlung: a massive particle in a superposition of positions radiates gravitons at different rates from each branch, and the emitted graviton field records which branch the particle occupies. This was already understood for structureless point particles.
The new result concerns long timescales and internal structure. When the composite particle has dynamical internal degrees of freedom, the graviton-mediated decoherence becomes inevitable in the long-time limit — even for microscopic masses that would normally be considered too light for gravitational effects to matter. The internal modes provide additional channels for the graviton bath to extract which-path information. The decoherence is not just stronger; it is qualitatively unavoidable.
But the classical Newtonian potential pushes back. For particles without internal structure, the static gravitational field can produce recoherence — a partial restoration of quantum coherence that the gravitons destroyed. The classical field carries no which-path information; it couples identically to both branches of the superposition and can interfere constructively with the decohered state. Gravity decoherence; gravity recoheres. The quantum and classical channels of the same force compete.
For composite particles, the internal degrees of freedom break this recovery. Recoherence requires that no subsystem has recorded the which-path information, and internal modes always record it eventually. The Newtonian potential slows the decoherence but cannot reverse it. Gravity wins on all timescales when the system has structure.