Two floats on the waves. Throw a stone into water, and the ripples will spread, rocking both floats one after another — not simultaneously. In the quantum world, such rocking is called entanglement: particles, like floats, begin moving in sync. The role of water is played by the gravitational field, whose trembling, quantum fluctuations, carry influence from one particle to another. Gravitational waves are like ripples here.

Physicists, using the method of Feynman, calculated this effect for two massive bodies held by forces similar to springs. It turned out that entanglement occurs with a delay: time is proportional to distance. That's exactly how long it takes for gravity to 'run' at light speed. So, even in the micro-world, curved spacetime doesn't violate causality.

The effect is so weak that it can't be measured yet. But if particles are specially prepared by squeezing their state (like a tightly stretched spring), the connection gets slightly stronger. More importantly, the calculation shows that gravity behaves like a quantum field, and that's a step toward the long-awaited theory.