A quantum particle can be a wave, and pass through two paths at once. But if something 'peeks' at which path it took, the wave disappears. This effect is called loss of interference. Usually, photons or air molecules do the peeking. The authors of the new study asked: what about gravitons?

Gravitons are hypothetical particles that carry gravity. No one has ever caught one, but if they exist, any movement of mass should generate them, like ripples on water. The authors calculated how much such 'gravitational ripples' could reveal a particle's path, and destroy its wave properties. It turns out, almost not at all. In ordinary vacuum, the effect is so weak it's imperceptible: the interference loss factor is a tiny number from 10⁻⁸⁹ to 10⁻⁶¹, depending on mass, size, and duration of the experiment.

But what if the vacuum isn't empty but filled with 'squeezed' gravitons — a special state that might have remained after inflation in the early universe? Then the effect strengthens, but it still remains negligible: at most 10⁻²⁷. Even the most sensitive interferometers with atoms or molecules won't feel such interference. Gravity, it turns out, is remarkably delicate: it doesn't spoil the quantum picture of the world.