Black Holes Slowly Evaporate Over Time — Stephen Hawking Proved It in 1974

Black holes are not truly black. In 1974, Stephen Hawking published a four-page paper in Nature demonstrating that black holes must emit a faint thermal glow, now called Hawking radiation, and that over astronomical timescales this emission causes them to lose mass and eventually vanish entirely. The smaller the black hole, the hotter it glows and the faster it disappears. For a black hole with the mass of our Sun, that process would take roughly 10^67 years, far longer than the current age of the universe. But for a primordial black hole with the mass of a mountain, compressed into a space smaller than a proton, the evaporation timeline ends right around now, with a final explosive burst of high-energy gamma rays.

Before Hawking's 1974 result, the reigning view was captured by the phrase physicists still use: black holes have no hair. Once matter falls in, the outside universe can know only three things about a black hole: its mass, electric charge, and angular momentum. Nothing was supposed to come out. In 1972, Jacob Bekenstein had argued that black holes possess entropy proportional to their surface area, which implied they had a thermodynamic temperature. Most physicists, including Hawking initially, resisted that idea because a hot object must radiate. Hawking set out to disprove Bekenstein and instead confirmed him, and went much further. Applying quantum field theory in curved spacetime, he derived an exact temperature for a black hole and showed the radiation was truly thermal, carrying real energy away from the object.

How It Works

The mechanism is rooted in quantum vacuum fluctuations. Empty space is not truly empty; it seethes with pairs of virtual particles and antiparticles that pop into existence and almost immediately annihilate each other. At the event horizon of a black hole, the geometry of spacetime is so severely warped that these pairs can be torn apart before they recombine. One particle falls inward across the horizon. To conserve energy, that infalling particle must carry negative energy relative to a distant observer, effectively reducing the black hole's mass. The other particle escapes to infinity as real radiation. To an outside observer, the black hole appears to emit a steady thermal spectrum of particles, exactly as if it were a hot body.

• Solar-mass black hole: Hawking temperature of approximately 6×10^-8 K, colder than the cosmic microwave background at 2.7 K. It actually absorbs more radiation from the universe than it emits. Total evaporation time: ~2×10^67 years.
• Primordial black hole (mountain mass, ~10^11 kg): Formed in the first fraction of a second after the Big Bang. Such an object would have a Hawking temperature near 10^11 K and a lifetime of about 13.8 billion years, the current age of the universe. It would be finishing its evaporation today, releasing its final energy in a sharp burst of GeV gamma rays.
• Micro black hole (1 TeV mass): Purely theoretical, but if produced in a particle collider it would evaporate in roughly 10^-26 seconds, essentially instantaneous and harmless.

Why It Matters

Hawking radiation sits at the junction of general relativity, quantum mechanics, and thermodynamics, and it exposes a deep conflict between them. General relativity describes the event horizon as a smooth, featureless surface from which nothing escapes. Quantum mechanics says the radiation carries a thermal spectrum, which by definition contains no information about what fell in. This leads directly to the black hole information paradox: if a star collapses into a black hole that then evaporates entirely into structureless thermal radiation, the quantum-mechanical information encoded in the star appears to be destroyed. Resolving this paradox has occupied theorists for fifty years and remains one of the central open problems in theoretical physics, driving work on string theory, loop quantum gravity, and the holographic principle.

From an observational standpoint, astronomers scan the sky for the gamma-ray signatures a dying primordial black hole would produce. Instruments like the Fermi Gamma-ray Space Telescope have searched for point sources with the spectral shape Hawking's equations predict. No confirmed detection has been made, but upper limits from these searches constrain how many primordial black holes formed in the early universe and what fraction of dark matter they could constitute.