Two observers, Alice and Bob, try to communicate faster than light. Alice creates a superposition of two massive bodies, like a coin that is heads and tails at the same time. Bob tries to pick up the signal with a rotating black hole, which starts to oscillate and emit gravitational waves.

But the black hole doesn't just relay the signal. Its gravitational waves are quantized, and quantum fluctuations blur the information the way static blurs a radio signal. Bob can't extract Alice's message faster than light. That means gravitational radiation has to be quantized, even when its source is a black hole with strong gravity.