Hawking Radiation, Explained Simply

Hawking radiation is the faint glow that black holes give off because of quantum effects near their edge. It means that a black hole is not perfectly black: it has a real temperature, it slowly loses mass over time, and, given long enough, it can evaporate completely. Stephen Hawking predicted it in 1974, and it is the discovery that made his name among physicists, because it was the first time anyone had shown how gravity, quantum mechanics and thermodynamics fit together in a single object.

Here is what that actually means, and why it matters so much.

The puzzle Hawking started with

By the early 1970s, physicists agreed that a black hole was a one-way door. Anything crossing the event horizon, the boundary of no return, was lost forever, and nothing could come back out. A black hole could only ever grow.

Hawking had himself proved a result that fit this picture: his area theorem showed that the surface area of a black hole's horizon can never shrink. But that theorem had an awkward echo. In thermodynamics there is another quantity that can only ever increase: entropy, a measure of disorder. The young physicist Jacob Bekenstein suggested the resemblance was no accident, that a black hole's area really is a kind of entropy.

Hawking initially thought this was wrong. If a black hole has entropy, then it must have a temperature; and anything with a temperature must give off radiation. But black holes, by definition, could not give off anything. Trying to prove Bekenstein wrong, Hawking did the calculation properly, and discovered that Bekenstein was right.

What Hawking found

When you take the empty space just outside the event horizon and apply the rules of quantum theory to it, the black hole turns out to emit a steady stream of particles, with a precise temperature that depends only on its mass. This is Hawking radiation.

The temperature is inversely related to mass, which leads to a strange and important consequence:

• A big black hole is extremely cold and radiates incredibly slowly.
• A small black hole is hotter and radiates faster.

As a black hole radiates, it loses mass; as it loses mass, it gets hotter and radiates faster still. The process accelerates, so a black hole's life ends not with a whimper but with a final flare of radiation. For an ordinary black hole formed from a collapsed star, though, this takes far, far longer than the current age of the universe; its temperature is a tiny fraction of a degree above absolute zero.

The popular picture, and its health warning

Hawking offered a vivid way to imagine it, and it has been repeated ever since. Empty space is never truly empty: pairs of "virtual" particles are constantly flickering into existence and annihilating each other. Right at the event horizon, the story goes, a pair can be split: one particle falls in while its partner escapes. The escaping particles are the radiation; the one that falls in carries negative energy, so the black hole loses a little mass.

It is a beautiful image and it gets the bookkeeping right. But Hawking and others were always careful to flag that it is a simplification. The real derivation is subtler, involving the way quantum fields are stretched and reshaped by the black hole's gravity over time. The particle-pair story is a useful picture, not the actual mechanism, worth knowing if you want to understand the idea honestly rather than just repeat it.

Why it was such a big deal

Before 1974, gravity and quantum mechanics were two separate empires that physicists could not get to agree. Hawking radiation was the first concrete result where they not only coexisted but produced something new and specific: a black hole with a definite temperature and a definite entropy, now called the Bekenstein–Hawking entropy.

That single result tied together three of the great pillars of physics: general relativity (gravity), quantum mechanics and thermodynamics. It has been a guiding star ever since. A huge amount of modern theoretical physics, including major parts of string theory and the study of quantum information, is in some sense an attempt to fully understand the implications of what Hawking found.

Why it never won a Nobel Prize

The Nobel Prize in Physics is awarded for results confirmed by experiment or observation. The problem with Hawking radiation is brutally simple: for any real black hole we know of, it is far too faint to detect. A stellar-mass black hole's Hawking temperature is billions of times colder than the faint afterglow of the Big Bang that bathes all of space, which completely drowns it out.

Physicists have tried to sneak up on the effect using analogue black holes, which are laboratory systems, such as carefully engineered flows of ultracold atoms or fluids, that trap sound or light the way a black hole traps light. In 2016 the physicist Jeff Steinhauer reported evidence of the analogue of Hawking radiation in such a system. These results are tantalising and support the theory, but they are not the same as catching it from a genuine black hole. Until someone does, Hawking's most famous prediction remains unconfirmed, which is why the prize eluded him.

The problem it left behind

Hawking radiation also created the deepest puzzle of his career. If a black hole slowly radiates away and disappears, what happens to the information about everything that fell into it? Quantum mechanics insists information cannot be destroyed, yet Hawking's radiation seemed to carry none of it out. This is the black hole information paradox, and physicists are still arguing about the answer today. It is covered in full on its own page.

For all that it remains unconfirmed, Hawking radiation changed how physicists think about reality. It took the blackest, simplest object in the universe and revealed it to be warm, complicated, and quietly leaking itself back into the cosmos.

The mechanism is often pictured using virtual particles flickering at the edge of the horizon.

Because this radiation has never been directly detected, it is also the reason Hawking never won a Nobel Prize.

Want to see the numbers for yourself? Try the black hole calculator.

For the original research, see Hawking's key scientific papers.

For how this prediction is faring in current research, see his legacy today.

For a paragraph-by-paragraph reading of the original, see the annotated 'Black hole explosions?' (1974).

References

1. Hawking, S. W. "Black hole explosions?" Nature 248, 30 to 31 (1 March 1974). DOI: 10.1038/248030a0. The first announcement of the result.
2. Hawking, S. W. "Particle Creation by Black Holes." Communications in Mathematical Physics 43, 199 to 220 (1975). DOI: 10.1007/BF02345020. The full, rigorous derivation.
3. Bekenstein, J. D. "Black Holes and Entropy." Physical Review D 7, 2333 (1973). DOI: 10.1103/PhysRevD.7.2333. The proposal that black holes carry entropy proportional to horizon area, which Hawking's radiation result placed on a firm physical footing.

This page explains the physics for general readers and is not a substitute for the original papers above.