Physics
Hawking Radiation
Quick fact
Hawking radiation is so weak that a black hole with the mass of our Sun would take about 10^67 years to evaporate—far longer than the current age of the universe.
Why this is interesting
You’ve heard that nothing can escape a black hole—not even light. So how could a black hole possibly vanish into a puff of radiation?
Read the full explanation
Understanding Hawking Radiation
Imagine empty space as a restless ocean, where pairs of particles and antiparticles constantly pop into existence and instantly annihilate each other. Near a black hole's event horizon, something strange can happen: one particle falls into the black hole while the other escapes. To an outside observer, the escaping particle looks like radiation coming from the black hole. The black hole loses a tiny amount of mass because the particle that fell in has negative energy (relative to the black hole). Over immense timescales, this slow drip of particles causes the black hole to shrink and eventually evaporate completely. This idea was shocking because it meant black holes are not truly black—they glow faintly.
A deeper explanation
Hawking radiation arises from applying quantum field theory to the curved spacetime around a black hole. The key is that the definition of a particle depends on the observer's motion. In the strong gravity near the horizon, what an inertial observer far away sees as a vacuum appears to a nearby observer as a sea of particle pairs. The black hole's gravitational field separates these pairs: one particle tunnels out (positive energy) while the other falls in (negative energy), reducing the black hole's mass. The emitted spectrum is thermal, characterized by the Hawking temperature T = ħc³/(8πGMkB). This temperature is inversely proportional to the black hole's mass, meaning smaller black holes emit more intensely. Hawking radiation implies black holes have entropy (Bekenstein-Hawking entropy) and obey thermodynamic laws. It also raises the infamous information paradox: if black holes evaporate, what happens to the information about matter that fell in? This tension drives ongoing research into quantum gravity.