Hawking Radiation (1974): How Black Holes Slowly Evaporate

Hawking radiation was predicted by Stephen Hawking in a two-page Nature letter in March 1974. Combining quantum theory with general relativity, he showed that black holes are not completely black: they emit particles like a hot body, slowly lose mass and eventually evaporate, ending in a final burst. It has never been observed directly.

The question before the paper

In classical general relativity a black hole is the ultimate one-way door. Nothing that crosses its event horizon comes back out, so a black hole can only grow. In 1973 Jacob Bekenstein had proposed that black holes carry entropy, a measure of disorder, proportional to the area of their horizon. But in physics anything with entropy should also have a temperature, and anything with a temperature should glow. How could a black hole glow if nothing escapes it?

What the paper found

• Black holes emit particles. Applying quantum field theory, the physics of particles, to the curved spacetime around a black hole, Hawking found that it should emit particles as if it were a hot body.
• Bigger means colder. The temperature scales inversely with mass, about 10^-6 K for a black hole of one solar mass. That is far colder than the 2.7 K cosmic microwave background, so stellar black holes today absorb more than they emit.
• Black holes evaporate. By radiating, a black hole loses mass. Any primordial black hole lighter than about 10¹⁵ grams, formed in the early universe, would have evaporated by now.
• An explosive end. As a black hole shrinks it gets hotter and radiates faster. In its final 0.1 seconds it would release about 10³⁰ erg, hence the title’s question mark.

Hawking radiation explained

The key move was to treat matter quantum mechanically while keeping gravity classical: “quantum field theory in curved spacetime”. In quantum theory, empty space is never perfectly empty. Near a horizon, the warping of spacetime turns these quantum fluctuations into real particles that stream away from the black hole, while the black hole pays for them by losing mass. Hawking gave the full derivation in 1975, in Communications in Mathematical Physics.

Why it mattered

The result linked gravity, quantum theory and thermodynamics for the first time, and it gave physical meaning to Bekenstein’s black hole entropy. It showed that black holes, the most extreme prediction of general relativity, cannot be fully understood without quantum physics.

Evaporation also sets the final chapter of cosmic history. In the Black Hole Era, black holes are the last major concentrations of mass, and even supermassive ones vanish after roughly 10¹⁰⁰ years. Don Page turned Hawking’s prediction into precise emission and lifetime numbers in 1976, and Adams and Laughlin built them into their timeline of the far future.

What we know now

As of October 2026, Hawking radiation has not been observed from any astrophysical black hole; it is far too faint. Other parts of Hawking’s black hole physics have been tested. The 2025 gravitational-wave signal GW250114, recorded by LIGO, confirmed his 1971 area theorem: in a merger, total horizon area never decreases. A 2025 proposal by Falcke and colleagues suggests that similar radiation might even make dead stars evaporate. See the black holes guide and Deep Time.

Who wrote it

Stephen Hawking wrote the letter alone, and its title ends with a question mark: he presented the final explosion as a possibility to be tested. The letter has about 5,200 citations in Semantic Scholar as of October 2026.

Read the original

Hawking, S. W. (1974). “Black hole explosions?” Nature 248, 30. doi:10.1038/248030a0. The journal version requires a subscription. The 1975 full derivation is in Communications in Mathematical Physics 43, 199 (doi:10.1007/BF02345020).

Questions people ask

Has Hawking radiation been observed?

No. As of October 2026 Hawking radiation has never been detected from a real black hole. For black holes made by stars, the predicted temperature is about a millionth of a kelvin or less, far colder than the cosmic microwave background, so the signal is hopelessly faint.

How do black holes die?

Black holes die by evaporating through Hawking radiation. They lose mass extremely slowly, getting hotter and radiating faster as they shrink, and end in a final burst of energy. For supermassive black holes this takes around 10¹⁰⁰ years, far beyond the age of the universe.

Can black holes really evaporate?

According to quantum theory applied to curved spacetime, yes. Hawking showed in 1974 that every black hole emits radiation and loses mass. The prediction combines two well-tested theories, but it remains untested by observation because the radiation from known black holes is far too weak.