Black Holes and Hawking Radiation

Spiral galaxy and black hole

Hawking radiation—sometimes also called Bekenstein-Hawking radiation—is a theoretical prediction from British physicist Stephen Hawking which explains thermal properties relating to black holes.

Normally, a black hole is considered to draw all matter and energy in the surrounding region into it, as a result of the intense gravitational fields; however, in 1972 the Israeli physicist Jacob Bekenstein suggested that black holes should have a well-defined entropy, and initiated the development of black hole thermodynamics, including the emission of energy, and in 1974, Hawking worked out the exact theoretical model for how a black hole could emit black body radiation.

Hawking radiation was one of the first theoretical predictions which provided insight into how gravity can relate to other forms of energy, which is a necessary part of any theory of quantum gravity.

The Hawking Radiation Theory Explained

In a simplified version of the explanation, Hawking predicted that energy fluctuations from the vacuum cause the generation of particle-antiparticle pairs of virtual particles near the event horizon of the black hole. One of the particles falls into the black hole while the other escapes before they have an opportunity to annihilate each other. The net result is that, to someone viewing the black hole, it would appear that a particle had been emitted.

Since the particle that is emitted has positive energy, the particle that gets absorbed by the black hole has a negative energy relative to the outside universe. This results in the black hole losing energy, and thus mass (because E = mc2).

Smaller primordial black holes can actually emit more energy than they absorb, which results in them losing net mass. Larger black holes, such as those that are one solar mass, absorb more cosmic radiation than they emit through Hawking radiation.

Controversy and Other Theories on Black Hole Radiation

Though Hawking radiation is generally accepted by the scientific community, there is still some controversy associated with it.

There are some concerns that it ultimately results in information being lost, which challenges the belief that information cannot be created or destroyed. Alternately, those who don't actually believe that black holes themselves exist are similarly reluctant to accept that they absorb particles.

Additionally, physicists challenged Hawking's original calculations in what became known as the trans-Planckian problem on the grounds that quantum particles near the gravitational horizon behave peculiarly and cannot be observed or calculated based off of space-time differentiations between the coordinates of observation and that which is being observed.

Like most elements of quantum physics, observable and testable experiments relating to the Hawking Radiation theory are almost impossible to conduct; additionally, this effect is too minute to be observed under experimentally achievable conditions of modern science—which includes the use of white hole event horizons created in laboratories—so the results of such experiments are still inconclusive to proving this theory.