A translator gets distracted by every rustle in the room, and an hour-long meeting stretches into two. Quantum mechanics works the same way: any measurement disturbs the system, wiping out part of the information. Physicists have long suspected that the gravitational radiation from black holes should obey the same rules, but proving it for strong fields kept failing.

Now a thought experiment has been put forward with two observers, Alice and Bob. Alice creates a superposition of two massive bodies, like a coin tossed in the air that is heads and tails at the same time. Such a superposition produces a tidal field that Bob tries to measure with a rotating black hole. The black hole, like a giant detector, starts to oscillate and emit gravitational waves.

It turned out that Bob cannot extract information about Alice's state faster than light. Quantum fluctuations of the gravitational waves emitted by the black hole limit the overlap of coherent states, and the decoherence function tends to zero under spacelike separation. This means that quantizing gravitational radiation is necessary for quantum mechanics and general relativity to stay consistent, even in strong fields.

A distracted translator ultimately conveys the meaning accurately, only with a delay. So it is with a black hole: its quantum fluctuations do not let you bypass the speed-of-light limit, but they leave a trace in the form of gravitational waves. This is the first step toward proving that quantum effects must also show up in astrophysical sources, such as merging black holes.