Gravitational Waves Turn into Light express
A thrown stone sends ripples across water. A gravitational wave is the same kind of ripple, but in spacetime. It is produced, for example, by the merger of two black holes. In classical physics, this ripple is eternal: once created, it travels through the Universe without fading.
But gravity obeys quantum laws. The wave consists of tiny packets — gravitons (the particles that carry gravity). The researchers showed that a stream of gravitons can decay into pairs of photons — particles of light.
Classical physics forbids such a trick. This is a purely quantum phenomenon.
The decay leaves evidence. The born photons heat the cosmic microwave background (the cooled light of the Big Bang), add extra glow to intergalactic space, and can even split the nuclei of light elements like deuterium. The authors of the study estimated for the first time how strong the original gravitational waves must be for these traces to become noticeable. The constraints are still tight, but the principle itself opens a door: gravitational waves can be caught not only by detectors like LIGO, but also by their quantum echo in light.
Ripples on water disappear, turning into heat. Gravitational ripples are not eternal either: they can become light.
Original abstract
as written by the authors arXiv:2607.24930
We show that gravitational waves (GW), treated as coherent graviton states, decay into photon pairs in vacuum. The process, even if suppressed by $$G^2$$, is lifted by two effects combined: the expected factor of the graviton number squared, $$N^2$$, and the coherence of the wave. We perform the calculation describing both gravity and the photons as quantized fields, though we show that the effect admits a semiclassical description once the metric is solved to second order. We estimate the resulting rates for compact binaries and a stochastic background, including the effect from stimulated decay to the cosmic microwave background (CMB). In theories with light degrees of freedom, an analogous decay into them is also possible, and more relevant for ultralight dark matter, as it can entail huge occupation numbers. We derive first constraints on cosmological GW sources by the corresponding injection of photons from CMB spectral distortions, extragalactic backgrounds, and light-nuclei photofission. In summary, the decay of GWs into photons offers a new (challenging) handle on the detection of GW sources, and represents a new mechanism to generate other particles across cosmic history.
The graviton is so elusive that catching a single particle is unrealistic: it would require a detector the size of Jupiter. But collective effects, as in this work, make gravity accessible to study through its quantum habits.
In Carl Sagan's novel "Contact," an alien signal contains blueprints for a machine that uses gravitational waves. The decay of waves into light could be a key to deciphering such messages.