Stephen Hawking: What Did He Predict, and What Is Still Unobserved?

Stephen Hawking is often introduced as the person who “discovered” that black holes evaporate. What he actually did was make a precise theoretical prediction. Whether real black holes radiate that way is still an open observational question. Our sticker Stephen Hawking is about the physics, not a victory lap over a detection that has not happened.

What did Hawking predict in 1974?

In “Black hole explosions?” (Nature 248, 30, 1974), Hawking argued that quantum fields near a black hole’s event horizon lead to a thermal flux of particles at temperature

T_H = ħκ/(2πk_Bc)

with κ the surface gravity (Hawking 1974^[1]). For a solar-mass black hole that temperature is tiny (~10⁻⁷ K), far below the cosmic microwave background, so stellar and supermassive holes are net absorbers today. Only hypothetical tiny holes would be hot enough to explode in the present universe.

A later paper sharpened the challenge to predictability when radiation carries away mass (Hawking 1976^[2]). Pedagogical and technical follow-ups include the tunneling derivation of Parikh and Wilczek (Parikh & Wilczek 2000^[3]; arXiv:hep-th/9907001^[4]). Britannica’s overview of Hawking radiation states the prediction in plain language (Britannica, Hawking radiation^[5]).

Has Hawking radiation been observed?

No — not from an astrophysical black hole. Label it clearly: Hawking radiation from real black holes remains unobserved. The expected power for stellar-mass and larger holes is far too small for current instruments.

Laboratory analogue systems (for example sound in fluids, or Bose–Einstein condensates) can mimic horizon kinematics and have reported Hawking-like spectra. Steinhauer’s 2016 Nature Physics experiment on an analogue black hole in a BEC is a landmark of that programme (Steinhauer 2016^[6]). Those results test analogue models. They are not detections of Hawking radiation from a gravitational black hole, and we will not call them that.

What about the information paradox?

Open. If a black hole formed from a pure state evaporates completely into thermal radiation, naive reasoning says information is lost — in tension with unitary quantum mechanics. Proposed resolutions (remnants, holography, replica wormholes, firewalls, and more) disagree. Our draft supporting post on the paradox tracks that debate; the short version for this page: the paradox is not experimentally settled, and theoretical consensus is not complete. See also the pillar Do Black Holes Evaporate?.

Did Hawking win a Nobel Prize?

No. The Nobel Prize in Physics 2020 went to Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity,” and to Reinhard Genzel and Andrea Ghez for the galactic-centre mass concentration (NobelPrize.org 2020 summary^[7]; 2020 press release^[8]). That citation is about classical GR and observations of a compact object, not about Hawking radiation. Hawking died in 2018; Nobel Prizes are not awarded posthumously under current statutes, and in any case radiation from real black holes was (and is) unobserved.

What else is he known for (briefly)?

• Singularity theorems with Penrose (classical GR) — separate from the 1974 quantum radiation paper.
• Black-hole thermodynamics: entropy proportional to horizon area, temperature proportional to surface gravity.
• Popular books that brought those ideas to a wide audience — not substitutes for the primary papers above.

Wear the horizon

Besides the Hawking sticker, the store’s horizon character is Eventide Prophet. Browse Relativity & Cosmos and Physicist Stickers.

Temperature and lifetime, in orders of magnitude

T_H scales as 1/M. A 10¹² kg hole would be hotter than the CMB and evaporating on interesting timescales; nothing of that mass has been observed. Stellar-mass holes (~few M_☉) have T_H in the nanokelvin range and evaporation times vastly longer than the age of the universe. So “black holes evaporate” is a correct theoretical sentence for isolated holes in empty space on absurd timescales — not a description of Sgr A* shrinking this year (Hawking 1974^[1]; Britannica^[5]).

Analogue ≠ astrophysical detection

Steinhauer’s BEC experiment reported thermal Hawking radiation in an analogue system (Steinhauer 2016^[6]). The fluid horizon is real physics and a clever test of the kinematics Hawking used. It does not instrument an event horizon in spacetime. We keep that distinction explicit because popular headlines often drop it.

Credit hygiene: Penrose 2020 vs Hawking 1974

Penrose’s Nobel citation is about singularity/trapped-surface theory showing black-hole formation is robust in GR (2020 summary^[7]; press release^[8]; Penrose facts^[9]). Genzel and Ghez observed stellar orbits at the galactic centre. None of those three citations is “for Hawking radiation.” Attribution posts on this blog exist to stop that smear: see Who Really Discovered It? and the evaporation pillar Do Black Holes Evaporate?.

What you can say with a straight face

• Hawking predicted black-hole radiance in 1974 (Nature paper^[1]).
• That radiation is unobserved for real black holes.
• Analogue experiments are analogues.
• The information paradox is open.
• Hawking did not win a Nobel; Penrose’s 2020 prize was for formation theory, not for Hawking radiation.

Tunneling derivations such as Parikh & Wilczek’s rewrite the same radiation as a classically forbidden trajectory across the horizon (Parikh & Wilczek 2000^[3]). They clarify thermodynamics; they do not constitute an observation.

FAQ

Is Hawking radiation the same as jets from black holes?

No. Astrophysical jets and accretion-disk emission are ordinary (if extreme) plasma physics outside the horizon. Hawking radiation is a quantum effect tied to the horizon itself.

Could we ever detect it?

Possibly from hypothetical microscopic black holes, or indirectly through future precision tests — both speculative. Stellar-mass holes are expected to be far too cold.

Why credit Penrose’s Nobel here?

Because popular blurbs sometimes blur “black holes” into one prize. Penrose 2020 is formation/singularity theory plus Genzel/Ghez observations — not a Hawking-radiation Nobel.

References

1. S. W. Hawking (1974), “Black hole explosions?” Nature 248, 30. https://www.nature.com/articles/248030a0
2. S. W. Hawking (1976), “Breakdown of predictability in gravitational collapse,” Phys. Rev. D 14, 2460. https://doi.org/10.1103/PhysRevD.14.2460
3. M. K. Parikh & F. Wilczek (2000), “Hawking Radiation As Tunneling,” Phys. Rev. Lett. 85, 5042. https://doi.org/10.1103/PhysRevLett.85.5042
4. M. K. Parikh & F. Wilczek (1999/2001), “Hawking Radiation as Tunneling,” arXiv:hep-th/9907001. https://arxiv.org/abs/hep-th/9907001
5. Encyclopaedia Britannica, “Hawking radiation”. https://www.britannica.com/science/Hawking-radiation
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. NobelPrize.org, The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/summary/
8. NobelPrize.org, Press release: The Nobel Prize in Physics 2020. https://www.nobelprize.org/prizes/physics/2020/press-release/
9. NobelPrize.org, Roger Penrose, Physics 2020, facts. https://www.nobelprize.org/prizes/physics/2020/penrose/facts/

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