Gravity doesn't know how to be screened. Unlike electric fields, which can be hidden behind a metal mesh, gravity gets in everywhere. And everywhere it carries quantum noise along with it: the tiniest ripples of the field, gravitons, forever jostling any particle the way ocean waves rock a boat. For a long time physicists thought that for a fragile quantum superposition this rocking was dangerous in only one case: if the boat is enormous. More precisely, if its mass is comparable to the Planck mass, that is, 22 micrograms. That's the weight of a speck of dust, but for the quantum world such a mass is unimaginably large. It seemed that molecules and atoms had nothing to fear from gravitational noise.
But a boat has not only a keel, it also has a hold full of cargo. A particle, even the smallest one, isn't a point: it has internal structure. Electrons in a molecule can hop between orbits, the atoms inside it jitter. These internal motions, like unsecured cargo in the hold, respond to every oscillation of the gravitational field. And here's what turned up: on a long voyage, when the rocking lasts for hours, the swaying of the cargo starts to dominate the pitching of the whole hull. For a quantum superposition, that means death. The internal degrees of freedom turn gravitational noise into an inexorable process that destroys the quantum state even in a tiny molecule.
Previously the picture was different. For a point particle with no internal life, decoherence quickly saturates: the noise shakes the superposition up to a certain limit and then stops. A boat without cargo, no matter how much it's tossed, won't capsize. But cargo in the hold changes everything. It shifts with every jolt, and its motion is transmitted to the hull. In the end the rocking doesn't die down; it grows with time, as the cube of time. The quantum superposition is doomed.
The classical gravitational field created by a massive body, such as the Earth, slows this process down a bit. The Newtonian potential acts like an anchor: it slightly stabilizes the boat, slowing decoherence. But the anchor doesn't save it. In the long run, noise wins anyway. For systems with no internal dynamics, where the hold is empty, the anchor can even formally reverse the process, causing recoherence. But the time required for that is comparable to the age of the Universe. In practice this means that quantum fragility doesn't survive in the gravitational ocean.
For now this knowledge has no immediate applications. The effect is too weak for modern experiments: even the most precise interferometers won't notice decoherence within a day. But the calculation sharpens the boundaries of quantum mechanics in the presence of gravity. It shows that a system's internal structure fundamentally changes its fate. A boat with cargo in its hold will sooner or later capsize, no matter how small it is. Gravitational noise is inexorable, and internal structure only amplifies it until quantum fragility breaks.
For relic gravitons, which are assumed to be in a strongly squeezed state with a squeezing parameter on the order of 100, the decoherence time can decrease by a factor of $10^{29}$ compared to the vacuum state.
In Greg Egan's novel Quarantine, quantum superpositions and their collapse under the action of an observer play a central role. The idea that the gravitational environment can irreversibly destroy quantum coherence echoes this plot.