Do Black Holes Evaporate? Hawking Radiation, the Information Paradox and What We Haven’t Seen Yet

A black hole is defined by a boundary, the event horizon, from which nothing, not even light, can escape (NASA^[1]). So it is surprising that one of the most famous results in modern physics says black holes should shine. In 1974 Stephen Hawking calculated that, once quantum mechanics is taken into account, a black hole emits a faint thermal glow and slowly loses mass (Nature^[2]). This post explains the idea, the numbers, and the honest status: Hawking radiation has never been observed from a real black hole. Our Eventide Prophet tee stands at that horizon.

What is Hawking radiation?

Britannica summarises Hawking’s 1974 proposal this way: particle pairs arising near the event horizon may result in one particle escaping while its partner, carrying negative energy, falls in; the inflow of negative energy reduces the black hole’s mass “until it disappears completely in a final burst of radiation” (Britannica^[3]). Hawking’s own short paper in Nature was titled, characteristically, “Black hole explosions?” (Nature^[2]).

The “pair of particles” picture is a popular heuristic. The underlying calculation is about how quantum fields behave in the curved spacetime around a horizon, and it predicts a thermal spectrum, like the glow of a warm object.

How hot is a black hole?

The predicted temperature is

T_H = ħc³ / (8πGMk_B)

where M is the black hole’s mass, ħ is the reduced Planck constant, c the speed of light, G the gravitational constant and k_B Boltzmann’s constant. The key feature is the M in the denominator: bigger black holes are colder. Plugging in one solar mass gives about 6 × 10⁻⁸ K, sixty billionths of a degree above absolute zero. That is the formula on our Stephen Hawking sticker.

Why haven’t we detected Hawking radiation?

Because it is absurdly faint for any black hole we know about. The whole universe is bathed in the cosmic microwave background at 2.725 K (Fixsen 2009^[4]), tens of millions of times warmer than a solar-mass black hole. Such a black hole absorbs far more energy from the background than it emits, so today it is growing, not shrinking. As Natalie Wolchover writes in Quanta, Hawking’s calculation “cannot be tested directly; the radiation from actual black holes is far too faint to detect” (Quanta^[5]).

Only a black hole much smaller than any known one, with a mass more like an asteroid’s, would be hot enough to radiate noticeably. Some cosmologists have proposed that such small “primordial” black holes might have formed in the early universe. None has been confirmed, so this remains speculative.

Has anything like it been seen in the lab?

Yes, in analogues. In a flowing fluid, sound waves can be trapped behind a point where the flow outruns the speed of sound, a “sonic horizon.” In 2016 Jeff Steinhauer reported spontaneous Hawking-like radiation from such an analogue black hole in a Bose–Einstein condensate, with correlated, entangled partner particles (Nature Physics^[6]). These experiments test the mathematics of horizons. They do not show that astrophysical black holes evaporate, and physicists disagree about how much they tell us about real black holes (Quanta^[5]).

Do black holes have entropy?

Before Hawking’s result, Jacob Bekenstein argued in 1973 that a black hole must carry entropy proportional to the area of its horizon, so that the second law of thermodynamics would still hold when matter falls in (Phys. Rev. D^[7]). Hawking’s temperature made that idea precise. The Bekenstein–Hawking entropy is

S_BH = k_Bc³A / (4Għ)

where A is the horizon area. It is enormous: a black hole is, in this sense, the most entropy you can pack into a region. The link to the second law is the subject of our pillar Why Does Time Only Go Forward?

What is the black hole information paradox?

Quantum mechanics, as normally understood, never destroys information: in principle, the present state determines the past. But if a black hole radiates perfectly thermal (featureless) radiation and then disappears, what happened to the information about everything that fell in? That is the information paradox, and it is what our Eventide Prophet is brooding over.

In 1993 Don Page analysed how information would have to come out if evaporation obeys ordinary quantum rules. His result implies that the radiation’s entanglement entropy should rise and then fall again over the black hole’s lifetime, a shape now called the Page curve (Phys. Rev. Lett.^[8]). Quanta’s 2016 article records the opposing view too: Bill Unruh, a pioneer of analogue black holes, interprets the robustness of the Hawking calculation as evidence that information is lost (Quanta^[5]).

Many theorists now expect information to be preserved, and theoretical work on the problem continues. But the paradox is not resolved experimentally, and exactly how information escapes is still argued about. We mark it open.

What do we actually know about black holes?

A lot, and almost none of it depends on Hawking radiation:

• They form. Roger Penrose shared the 2020 Nobel Prize “for the discovery that black hole formation is a robust prediction of the general theory of relativity,” and Reinhard Genzel and Andrea Ghez shared it for discovering a supermassive compact object at the centre of our galaxy (NobelPrize.org^[9]).
• They merge. The 2017 Nobel Prize recognised “decisive contributions to the LIGO detector and the observation of gravitational waves,” and the first detected signal came from two merging black holes (NobelPrize.org^[10]; Phys. Rev. Lett.^[11]). Our Graviton Whisper tee is about those ripples.
• They cast shadows. In 2019 the Event Horizon Telescope released the first image of a black hole’s shadow, in the galaxy M87, about 6.5 billion times the Sun’s mass and 55 million light-years away (EHT^[12]).

For the classical side, tidal stretching near a black hole (“spaghettification”) is the joke behind Spaghetta, and light orbiting at 1.5 times the horizon radius is the Photon Sphere hoodie. Our earlier post Physics Myths Busted tackles wormholes and time travel.

Will a black hole ever evaporate completely?

If Hawking’s calculation is right, yes, eventually. For stellar or larger black holes, “eventually” is far longer than the present age of the universe, and evaporation cannot even begin in earnest until the cosmic background has cooled below the black hole’s temperature. What happens in the final moments, when the black hole becomes tiny, depends on a quantum theory of gravity we do not yet have.

Key terms in plain English

• Event horizon: the boundary of a black hole, inside which nothing, not even light, can escape (NASA^[1]).
• Hawking radiation: the faint thermal emission predicted by Hawking in 1974 (Nature^[2]). Predicted, not observed.
• Hawking temperature: the temperature of that emission, inversely proportional to the black hole’s mass.
• Bekenstein–Hawking entropy: the entropy of a black hole, proportional to the area of its horizon (Phys. Rev. D^[7]).
• Information paradox: the conflict between quantum mechanics, which preserves information, and a black hole that appears to evaporate into featureless radiation.
• Analogue black hole: a lab system, such as a flowing condensate, with a horizon for sound rather than light (Nature Physics^[6]).

Why did Hawking’s result surprise physicists?

Classically, a black hole is the ultimate one-way street: things go in and nothing comes out, so it should have no temperature at all. Bekenstein’s entropy argument hinted that black holes behave thermodynamically, but a body with entropy and energy should also have a temperature, and anything with a temperature should radiate. Hawking’s calculation supplied that missing piece by combining general relativity with quantum field theory, two theories that are normally applied to completely different scales. That is why the result is treated as a crucial clue to quantum gravity, even though nobody has detected the radiation. It is one of the few places where both theories must be used at once, and where their combination gives a definite, if currently untestable, prediction.

Could we ever detect it?

Not from any black hole we know of. Stellar black holes are far colder than the cosmic microwave background (Fixsen 2009^[4]), and supermassive ones, like the one imaged by the Event Horizon Telescope at about 6.5 billion solar masses (EHT^[12]), are colder still. Detection would need a black hole far smaller and hotter than any confirmed, which is why searches for hypothetical small primordial black holes attract interest. Until then, the best tests are indirect: theoretical consistency and laboratory analogues (Quanta^[5]). This is one of the clearest examples in physics of a prediction that is widely trusted and still unconfirmed.

A short timeline of black-hole thermodynamics

• 1973: Jacob Bekenstein argues that black holes carry entropy proportional to horizon area (Phys. Rev. D^[7]).
• 1974: Stephen Hawking predicts that black holes radiate and slowly lose mass (Nature^[2]).
• 1993: Don Page analyses how information could emerge in the radiation (Phys. Rev. Lett.^[8]).
• 2015–16: LIGO detects gravitational waves from merging black holes (NobelPrize.org^[10]).
• 2016: Jeff Steinhauer reports Hawking-like radiation from an analogue black hole in a condensate (Nature Physics^[6]).
• 2019: the Event Horizon Telescope releases the first image of a black hole’s shadow (EHT^[12]).
• 2020: the Nobel Prize recognises black-hole formation theory and the discovery of the Milky Way’s central compact object (NobelPrize.org^[9]).
• Still open: a direct detection of Hawking radiation from a real black hole, and the final word on the information paradox.

Notice the pattern. The observational milestones, gravitational waves and the shadow image, confirm general relativity’s picture of black holes. The quantum milestones are theoretical or analogue. That gap is exactly why Hawking radiation remains one of the most important untested predictions in physics.

The bottom line

Black holes are real, they merge, and they cast shadows. Whether they also glow and evaporate is a theoretical prediction that most physicists accept and nobody has observed. And if they do evaporate, what happens to the information that fell in is still one of the great open questions in physics.

FAQ

Has Hawking radiation been proven?

No. It is a widely accepted theoretical prediction (Nature^[2]) that has never been observed from an astrophysical black hole (Quanta^[5]). Laboratory analogues show the same horizon mathematics (Nature Physics^[6]).

Why are smaller black holes hotter?

Because the Hawking temperature is inversely proportional to mass: T_H ∝ 1/M.

Is the information paradox solved?

Not definitively. Theoretical work continues, building on Page’s analysis of how information could escape (Phys. Rev. Lett.^[8]), but it remains open.

Can a black hole destroy information?

That is exactly the open question. Most theorists today expect not; some, like Unruh, have argued it can (Quanta^[5]).

See every black-hole design in the Relativity & Cosmos collection, and read next: Why Is the Speed of Light the Limit?

References

1. NASA Science, “Black Holes”. https://science.nasa.gov/universe/black-holes/
2. S. W. Hawking (1974), “Black hole explosions?” Nature 248, 30. https://www.nature.com/articles/248030a0
3. Encyclopaedia Britannica, “Hawking radiation”. https://www.britannica.com/science/Hawking-radiation
4. D. J. Fixsen (2009), “The Temperature of the Cosmic Microwave Background,” ApJ 707, 916. https://arxiv.org/abs/0911.1955
5. Natalie Wolchover, “What Sonic Black Holes Say About Real Ones,” Quanta Magazine (2016). https://www.quantamagazine.org/what-sonic-black-holes-say-about-real-ones-20161108/
6. J. Steinhauer (2016), “Observation of quantum Hawking radiation and its entanglement in an analogue black hole,” Nat. Phys. 12, 959. https://www.nature.com/articles/nphys3863
7. J. D. Bekenstein (1973), “Black Holes and Entropy,” Phys. Rev. D 7, 2333. https://doi.org/10.1103/PhysRevD.7.2333
8. D. N. Page (1993), “Information in black hole radiation,” Phys. Rev. Lett. 71, 3743. https://doi.org/10.1103/PhysRevLett.71.3743
9. NobelPrize.org, The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/summary/
10. NobelPrize.org, The Nobel Prize in Physics 2017 (gravitational waves). https://www.nobelprize.org/prizes/physics/2017/summary/
11. LIGO Scientific & Virgo Collaborations (2016), “Observation of Gravitational Waves from a Binary Black Hole Merger,” Phys. Rev. Lett. 116, 061102. https://doi.org/10.1103/PhysRevLett.116.061102
12. Event Horizon Telescope, press release, 10 April 2019. https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.