A massive aluminum cylinder weighing a ton hangs in a vacuum. A gravitational wave, a ripple in spacetime, passes through it, and the bar begins to tremble almost imperceptibly. That trembling can be heard as sound: gravitational waves turn into a phonon — a quantum of sound vibrations.
But when a graviton (the quantum of the wave itself) arrives not alone but in a special quantum state, coherent or squeezed, simple calculations yield a strange result. The probability of conversion exceeds one, as if a single graviton could give birth to more than one phonon. This violates a fundamental rule: the sum of all probabilities must always equal one. The authors solved the equations exactly, without approximations, and saw a different picture.
It turns out that for a coherent graviton, the conversion into sound does not proceed smoothly but in jerks: short bursts where the probability sharply rises are followed by long dips almost to zero. For a squeezed state, it's even more dramatic: after an initial rapid rise, the conversion almost completely stalls. This intermittent rhythm is not a glitch but a direct consequence of quantum mechanics. It could become a recognizable signature in attempts to register a single graviton in future experiments.
The most sensitive current gravitational-wave detectors, the laser interferometers LIGO and Virgo, do not catch individual gravitons but powerful bursts from black hole mergers. To spot a single graviton, you would need a detector the size of Jupiter or a clever quantum trick.
In Carl Sagan's novel Contact, an alien signal arrives as modulated gravitational waves — essentially a message encoded in the trembling of space itself.