Entropy from an event horizon
The story begins with an odd thermodynamic proposal rather than radiation itself. In 1972, Jacob Bekenstein argued that black holes must carry entropy, and specifically that this entropy scales with the area of a black hole’s event horizon rather than its volume, an idea that seemed to require black holes to have a temperature as well, since entropy and temperature are linked concepts in thermodynamics. At the time this was a striking claim, because a black hole with a genuine temperature ought, by ordinary physical reasoning, to radiate energy, which seemed to directly contradict the classical picture of a black hole as an object from which nothing, not even light, can escape.
Hawking’s 1974 calculation
Stephen Hawking took this problem seriously and, applying quantum field theory to the region near a black hole’s event horizon, showed in papers published in 1974 and 1975 that black holes should indeed emit radiation, behaving mathematically like a thermal blackbody with a temperature inversely proportional to the black hole’s mass. This meant smaller black holes should be hotter and radiate more intensely, while larger black holes should be correspondingly colder, a genuinely surprising theoretical result that reconciled Bekenstein’s entropy proposal with quantum theory and gave black hole thermodynamics a complete, self-consistent physical basis for the first time.
Why big black holes are not visibly shrinking
One immediate consequence is that the black holes astronomers actually observe are not visibly evaporating. For a black hole with the mass of the sun, the predicted Hawking temperature works out to a tiny fraction of a degree above absolute zero, far colder than the roughly 2.7 kelvin cosmic microwave background radiation that fills the universe and bathes every black hole in space. Because that background radiation delivers more energy to a stellar-mass black hole than the black hole loses through Hawking radiation, such black holes are currently gaining mass on balance rather than shrinking, and this situation is expected to persist until the background radiation itself cools further as the universe continues to expand.
Only tiny, ancient black holes could have evaporated
Only much smaller black holes could plausibly have evaporated within the age of the universe so far, and the only candidates for such small black holes are hypothetical primordial black holes, thought to have possibly formed from extremely dense regions in the very early universe rather than from the collapse of a star. Calculations suggest only primordial black holes below a specific, comparatively tiny mass threshold could have evaporated completely by the present day, and researchers including those working with instruments such as the Fermi gamma-ray space telescope have searched for the burst of radiation a final evaporation event would produce, without confirmed detection to date.
No direct detection yet, but laboratory analogues
No astronomical observation of Hawking radiation itself has been confirmed, since the effect is expected to be extraordinarily faint for any black hole large enough to have actually formed and persisted. Physicists have instead built laboratory analogue systems that reproduce the same underlying mathematics in a different physical setting, including sonic black hole analogues using flowing fluids and experiments using light in specially structured optical media, and some of these experiments have reported detecting a Hawking-radiation-like effect, though the interpretation and significance of specific results, particularly an early optical experiment, remain debated among specialists in the field.
A paradox about where information goes
The theory also generates a genuine and still unresolved puzzle known as the black hole information paradox. If a black hole eventually evaporates entirely through Hawking radiation, and that radiation is calculated to carry no detailed information about whatever matter originally fell into the black hole, then the information describing that original matter appears to be permanently destroyed, a direct conflict with a basic principle of quantum mechanics that information should never be truly lost. Proposed resolutions include the idea that the radiation secretly does encode the missing information in a way not yet fully understood, or that some remnant persists after evaporation, but no single account has been established as correct, leaving this as a live, actively worked problem at the boundary of quantum mechanics and gravity.
