gamma-ray radiation
/ GAM-uh ray /
At the very far end of the electromagnetic spectrum, beyond even X-rays, lies the most energetic light there is: gamma rays. Their wavelengths are smaller than an atom, and a single gamma-ray photon can carry millions or even trillions of times the energy of a photon of visible light. Where ordinary light gently warms, a gamma-ray photon hits like a bullet, capable of shattering atomic nuclei.
Because they are so energetic, gamma rays come only from the universe's most extreme events: matter falling into supermassive black holes, the collapse and explosion of massive stars, the collision of neutron stars, and the decay of radioactive nuclei freshly forged in supernovae. The most dramatic are gamma-ray bursts, flashes that for a few seconds can outshine everything else in the gamma-ray sky combined, marking the birth of a black hole halfway across the universe. Gamma rays also reveal where cosmic-ray particles are being accelerated to near the speed of light.
Gamma rays are absorbed high in Earth's atmosphere — again sparing life below — so they are studied from space, or indirectly from the ground by catching the faint flash of particles a gamma ray triggers when it crashes into the upper atmosphere. They cannot be focused by mirrors or lenses at all; gamma-ray telescopes instead track the direction of each individual photon as it interacts inside the detector, so a gamma-ray 'image' is built one rare, precious photon at a time.
On 17 August 2017 a gravitational-wave detector felt two neutron stars merge, and 1.7 seconds later a satellite caught a short gamma-ray burst from the same spot — the first time the same cosmic collision was 'heard' in gravity and 'seen' in gamma rays.
Gamma rays flag the universe's most violent events — and now help anchor multi-messenger astronomy.
There is no sharp wavelength where X-rays end and gamma rays begin; the distinction is partly by energy and partly by origin (nuclear or high-energy processes). The names overlap, and the boundary is a convention, not a law of nature.