Graviton WhisperQuantum Gravity / Force Carrier

Graviton Whisper has never been detected. That is the first and most important fact about them.

Gravity in Einstein's theory is the curvature of spacetime, and ripples in that curvature travel. The first formula on this page is general relativity in its weak-field form: a small wrinkle in spacetime obeys a wave equation, sourced by moving mass and energy. On 14 September 2015 the two LIGO detectors caught one, from two black holes merging over a billion light-years away. The peak strain was about 10⁻²¹, a change in each detector's four-kilometre arms of a small fraction of a proton's width. In the merger, about three solar masses' worth of energy left as gravitational waves. In 2017 a neutron-star merger was seen in both gravitational waves and gamma rays, and the two arrived within seconds of each other after travelling 130 million years: gravity's waves move at the speed of light to about one part in a thousand trillion.

Gravitational waves are classical, observed facts. The graviton is the hypothetical next step. Electromagnetic waves come in quanta: light comes as photons. If gravity is quantum too, its waves should come as gravitons, and the second line says what they would be like: massless, with energy h times frequency, and spin 2, because the field they ripple is a tensor rather than a vector.

Nobody knows how to catch one. Gravity is so weak that detecting a single graviton looks hopeless with any conceivable instrument, and some physicists argue it may be impossible in principle. Whether gravitons exist at all depends on how gravity and quantum mechanics are eventually reconciled, and that remains open.

Graviton Whisper does not bend space; in general relativity, mass and energy do that. If gravitons exist, they would carry the news, one quantum at a time.