Science › Black holes & quantum gravity
Hawking radiation: black holes evaporate
1974· Theory
Well supportedStrong evidence or math, but not fully tested.
Hawking's 1974 prediction that black holes slowly emit heat and, given enough time, evaporate completely.
See it on the Science map →How it works
Quantum theory says empty space constantly fluctuates. Near a horizon, these fluctuations become real particles: some escape outward as faint heat, while the black hole loses an equal amount of energy. Smaller black holes are hotter. One with the Sun's mass sits about 60 billionths of a degree above absolute zero, far colder than space, and would take roughly 10^67 years to evaporate.
Why it matters
It showed black holes have a temperature and entropy like ordinary objects, tying gravity to quantum physics and thermodynamics for the first time.
Think of it like
Like a building that slowly loses value through tiny, constant depreciation: invisible year to year, total over a long enough horizon.
How we know
Never observed from a real black hole; the signal is far too weak. Derived independently by many methods, and seen in lab analogs: sound waves in ultracold atoms (Steinhauer, 2016 and 2019).
Still a mystery
Whether the radiation secretly carries out the information that fell in is the heart of the information paradox.
Connections (4)
▸Quantum field theory applied near a horizon gives Hawking radiation: black holes evaporateWell supported
The relationship
Quantum field theory describes particles as ripples in fields that fill space. Hawking calculated how these fields behave on the curved spacetime of a forming black hole. The empty-space state long before collapse no longer looks empty to a distant observer afterward: it appears as a steady glow of particles at a temperature set by the black hole's mass.
How the link was found
Stephen Hawking in 1974–75, using quantum fields on a fixed curved background. William Unruh found a close cousin in 1976: accelerating observers also see a warm glow.
Is it proven?
The result has been rederived by many independent methods and is considered robust. Lab analogs in fluids and ultracold atoms show the same effect. Real black hole radiation is too faint to see.
Open questions and mysteries
The calculation treats spacetime as fixed. Near the end of evaporation that assumption fails, and the outcome is unknown.
▸The quantum vacuum isn't empty supplies the particle pairs in Hawking radiation: black holes evaporateWell supported
The relationship
Empty space briefly produces pairs of particles that normally vanish at once. Near a horizon, one partner can fall in while the other escapes. The escaping one becomes real radiation; the infalling one carries negative energy, shrinking the black hole. Hawking himself called this a helpful picture rather than the literal calculation, which uses fields, but it captures the idea.
How the link was found
Hawking offered the particle-pair picture alongside his 1974–75 calculation and popularized it in "A Brief History of Time" (1988).
Is it proven?
The vacuum's activity is measured directly through the Casimir force (measured precisely in 1997) and the Lamb shift (1947). The pair picture itself is an aid to intuition, not a separate test.
Open questions and mysteries
Exactly where the radiation originates, at the horizon or well outside it, is still discussed.
▸Stephen Hawking discovered Hawking radiation: black holes evaporateWell supported
The relationship
Hawking set out to disprove Bekenstein's claim that black holes have entropy, since that implied a temperature, and anything with a temperature must glow. Applying quantum field theory near a horizon, he found black holes do glow, with a temperature inversely proportional to their mass. He then realized they must slowly lose mass and eventually evaporate.
How the link was found
Stephen Hawking, "Black hole explosions?" in Nature, March 1974, with the full derivation in 1975.
Is it proven?
The calculation is widely rederived and accepted. Lab analogs, such as Jeff Steinhauer's sound-wave horizons in ultracold atoms (2016, 2019), show matching behavior. Direct observation is impossible with current tools.
Open questions and mysteries
Whether tiny primordial black holes from the early universe exist and are evaporating today, which would make the radiation visible.
▸Hawking radiation: black holes evaporate creates The black-hole information paradoxOpen question
The relationship
Hawking radiation's properties depend only on the black hole's mass, spin and charge, not on what fell in. As the black hole radiates, it shrinks; eventually it may vanish completely, leaving only this generic heat. If so, the details of everything that fell in are gone, which quantum theory forbids. The radiation is what turns a hidden puzzle into a real contradiction.
How the link was found
Stephen Hawking, 1976, as a direct consequence of his 1974 discovery.
Is it proven?
The contradiction is a solid theoretical result. Hawking's original calculation is approximate, and later work (Page 1993, islands 2019) suggests subtle correlations in the radiation do carry information out.
Open questions and mysteries
Where do those correlations come from physically, and could even an ideal observer ever decode them?