Two massive particles hang motionless in the void, each at a crossroads: to be on the left or on the right. As long as nobody looks, they occupy both positions at once, and this state is called quantum coherence. Gravity pulls them toward each other, and the pull depends on where exactly each one ended up. Because of this, the shared fate of the particles begins to intertwine: now, knowing the position of one, you can say something about the other. This intertwining is called entanglement, and it does not come out of nowhere.
The authors of the new paper analyzed this process as an exchange of resources between two accounts. On one side, each particle holds a stock of coherence, and that stock can be measured. On the other side, the pair of particles holds a stock of entanglement. Gravity acts like a bank: it neither creates nor destroys the total capital, it merely moves it from one account to another. When coherence decreases, entanglement grows by exactly the same amount. For the richest initial states, an exact equality has been derived: the sum of the squares of these two quantities always equals one.
It was already known that gravity can entangle masses, but there was no understanding of where this entanglement comes from and how it relates to the initial properties of the particles. Now the mechanics are clear: gravity pumps coherence out of each particle into the shared state of the pair. This is not just a pretty picture but a rigorous mathematical result, testable in future experiments. To see gravitational entanglement, physicists will have to prepare particles in a perfect superposition and hold it while gravity does its work.
This perspective changes how experiments are planned. Scientists hoping to test the quantum nature of gravity now know that simply suspending two masses side by side is not enough: they also need to give them sufficient initial coherence. Otherwise the experiment is doomed to silence, like a bank with no deposits. Gravity remains the same old attraction, but now we can see how it handles quantum resources, moving them from one account to another.
Maximal entanglement between two masses is reached exactly when the local coherence of each mass vanishes completely, so gravity, as it were, pumps the quantum resource out of each particle and into their joint state.