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Gamma rays are like cheetahs: they are the charismatic megafauna of the particle world. They are light of the maximum possible potency, usually defined as a wavelength shorter than 10–11 meter—a realm where light’s wave nature is hard to observe and its particulate nature stands out. Each gamma photon has an energy of more than 100 kilo-electron-volts (keV), 100,000 times more than a photon of visible light. The mightiest gamma ever recorded packed a punch of 100 tera-electron-volts (TeV), far outgunning anything particle physicists can blast out with their most powerful instrument, the Large Hadron Collider.
Creating such extreme particles takes commensurately extreme processes: the collision of particles moving at nearly the speed of light; the annihilation of matter and antimatter, which converts their mass entirely into energy, per Einstein’s famous equation E = mc2 the leakage of energy out of black holes; and the release of nuclear energy in radioactive decay or fusion reactions. (Technically, all photons emitted by atomic nuclei are classified as gamma rays, even the rare ones of less than 100 keV.) As extreme as these processes may be, we bask in their glow every day: sunlight started off as gamma radiation in the sun’s core and degraded into visible light during its tortuous passage through the overlying layers of gas.
The gammas of greatest interest to astronomers, though, come from things that are dead, dying or deadly. When a massive star blows up in a supernova explosion, its debris sparkles with gamma rays, and the stellar corpse left behind—a neutron star or black hole—has such intense gravity that it drives the ongoing generation of gammas. At the centers of galaxies, black holes with the mass of a billion stars suck in material, some of which does not fall all the way in but squirts back out as jets that set off shock waves and generate gammas. Some gammas have yet to be traced to a source and might come from the decay or annihilation of particles not yet known to science. If the starry night sky seems tranquil and gentle, gamma rays betray the true violence of the universe.