Black holes die, but it takes a while
Q: Do black holes last forever? If not, how do they 'die'? And if not even light can escape a black hole, how do they
emit x-rays--Yiu Wai
A: Black holes die eventually because they radiate. In classical Newtonian physics, black holes are black. But on a quantum
mechanics level, they radiate an exceedingly small number of particles, mostly photons.
Right: [CXC/M.Weiss] Black hole draws gas into the abyss
Nobody has verified this weird phenomenon, called "Hawking Radiation", but it makes sense in a strange way.
According to Heisenberg's Uncertainty Principle, our space vacuum teems with invisible particles that flash into and out of
existence like virtual fireflies.
Suppose a pair of particle-antiparticles pops into being, conveniently enough, within effective range of the black hole's gravity.
Before the pair can annihilate each other, the black hole grabs the pair and, using the hole's rest-mass energy, pulls it into two particles. "...one particle has
negative energy and the other has positive energy," says Andrew J.S. Hamilton, astrophysical and planetary sciences professor at the University of Colorado.
"And the black hole swallows the negative energy, allowing the positive energy particle to go to infinity."
The black hole loses the energy contained in the escaped positive-energy particle, consequently loses an equal amount of rest mass, and eventually--after
enough positive-energy particles escape-loses all rest mass and dies.
Don't hold your breath. The death of a black hole can take ten raised to the 61st power times the age of the Universe for a 30 solar-mass black hole. Mini
black holes-the mass of a small mountain-can evaporate in less than the age of the Universe.
The black-hole glows extremely dimly. The Hawking luminosity of a 30 solar-mass black hole is a miserable ten raised to the minus 31 watts. Bigger black
holes are colder and dimmer.
So, on a quantum mechanics level, a black hole isn't completely black. It glows faintly.
Emitted x-rays are another matter completely. The black hole doesn't emit the x-rays. Rather it pulls hot gas away from its Sun-like companion star if it has
one. The gas forms a flattened disk swirling around the hole. The gas particles knock into each other zillions of times as they rotate and heat to extreme
temperatures: hot enough to emit x-rays. The gas particles reach the right temperature when they are close to the event horizon. (The event horizon is the
farthest distance from the black- hole body within which radiation is trapped.) Then they spit X-rays in all directions and less than a second later disappear. See
In the figure, the swirling material (white circles) is hot gas pulled from the black hole's companion star. The gas reddens and dims due to a gravitational red
shift as it approaches the event horizon (black center dot). Photons lose energy in the presence of a strong gravitational field and this causes a shift to lower
frequencies, i.e., a red shift. When the gas crosses the event horizon, it disappears.
(Answered by April Holladay, science correspondent, August 22, 2001)
U of Colorado, A. Hamilton: Hawking radiation
GFSC/NASA, Ask a space scientist: Black holes