Hawking radiation

Hawking radiation is a special kind of energy that is released just outside a black hole. This idea was developed by the famous scientist Stephen Hawking in 1974. Before this, scientists thought that once energy or light got too close to a black hole, it could never escape.

According to Hawking's model, this radiation happens because of tiny, invisible forces called quantum effects. The radiation is very, very faint and too weak for our best telescopes to see right now.

If Hawking radiation is real, it would slowly take away mass and energy from black holes. Over a very long time, this could make black holes shrink and eventually disappear. Very small black holes, called micro black holes, would disappear faster than big ones. If these tiny black holes exist somewhere in space, they might end in a big burst of energy, but we haven't seen this happen yet.

Background

Modern black holes were first predicted by Einstein’s 1915 theory of general relativity. We now have strong evidence for these amazing objects, called black holes, because of their very strong pull of gravity.

A black hole can form when enough matter or energy is squeezed into a tiny space, making the speed needed to escape faster than light. Since nothing can go that fast, nothing inside a certain distance from the black hole can get out. This distance marks the event horizon, the edge around the black hole where even light cannot escape.

Discovery

In 1971, scientists Yakov Zeldovich and Alexei Starobinsky suggested that rotating black holes might create and send out particles, comparing it to how spinning metal spheres give off energy. In 1972, Jacob Bekenstein proposed that black holes should have a kind of disorder called entropy, which relates to their size.

At first, Stephen Hawking did not agree with this idea. But after talking with Zeldovich in 1973, he combined these thoughts with his own work on how very small particles behave. In 1974, Hawking showed that black holes could send out tiny bits of energy, like warm glow, if we think about them using special science rules. This discovery was very important because it showed black holes are not completely black — they can slowly lose energy and shrink over time. This effect is now called Hawking radiation. It was supported by Bekenstein’s earlier work, and sometimes it is called Bekenstein–Hawking radiation.

Hawking radiation happens because of tiny changes in empty space. These changes can create pairs of particles, with one staying near the black hole and the other escaping far away.

Emission process

Hawking radiation happens because of special rules in physics called the Unruh effect and the equivalence principle. Near a black hole, someone trying to stay away from the black hole would need to move very fast. When moving very fast, a person would see tiny bits of energy, like heat, coming from around the black hole. Some of these bits of energy can escape far away, and this is what we call Hawking radiation.

A Schwarzschild black hole follows specific mathematical rules that help scientists understand how these tiny bits of energy behave. These rules show that the temperature of the radiation depends on the size of the black hole. Smaller black holes would give off more noticeable radiation, but the radiation from big black holes, like the ones we might find in space, is extremely weak and hard to detect.

Greybody factors

Greybody factors are special numbers that help us understand how the light or energy a black hole gives off is slightly different from the light given off by a perfect, glowing object. Because of tiny, invisible forces called quantum effects, a black hole can send out radiation that looks almost like the radiation from a perfect glowing object right at the edge of the black hole.

These greybody factors depend on the energy and spinning motion of the particles the black hole sends out. They also depend on whether the particles are one kind of tiny particle (called Bosons) or another kind (called Fermions). For black holes that have an electric charge, these factors can also change based on the charge of the particles being sent out.

Black hole evaporation

When particles escape, a black hole loses a tiny bit of its energy and mass. Because mass and energy are linked, this means the black hole has a limited lifespan. The time it takes for a black hole to disappear depends on its starting mass — bigger black holes last much longer.

For a black hole about the size of the Sun, it would take more than 10⁶⁷ years to vanish — far longer than the age of our universe. The tiny amount of energy a black hole gives off means its temperature rises as it shrinks, ending in a sudden burst of energy. Scientists are still figuring out what happens at this final moment.

Very small black holes, formed soon after the universe began, might have already disappeared. Only the tiniest of these could vanish this quickly, and even then, they need to be far lighter than Earth itself.

Problems and extensions

Trans-Planckian problem

The trans-Planckian problem is a challenge in understanding Hawking radiation. It happens because Hawking's original idea includes tiny particles with very short wavelengths near the black hole's edge. These wavelengths become shorter than the Planck length, a limit in physics. This happens because time seems to stop from far away, making particles look like they have very high energy when traced back.

When we look at these particles moving away from the black hole, their energy seems to grow huge as they get very close to the edge. This makes their wavelengths tiny, even smaller than the Planck length. Since we don't fully understand physics at such tiny sizes, some people find Hawking's idea hard to accept.

But many now think this problem is just a math trick in the way we describe black holes. Similar things happen with other ideas in physics. There are also other ways to explain Hawking radiation that try to fix this problem.

Large extra dimensions

Usually, we think black holes need to be at least as heavy as something called the Planck mass. But if there are extra, very small dimensions in space, the rules change. In these models, black holes could be much lighter and still work. The time it takes for such tiny black holes to disappear would be very short, about 10⁻²⁶ seconds.

In loop quantum gravity

Studies using a theory called loop quantum gravity look at the area around a black hole's edge. This theory shows that black holes might give off radiation in slightly different patterns than Hawking first thought. If we could see X-rays from very old, tiny black holes, we might notice these differences.

Tunneling picture

Some scientists describe Hawking radiation as tiny particles escaping, or "tunneling," through the black hole's edge. This idea matches what Hawking found but can be used for different kinds of black holes that change over time.

Experimental observation

NASA launched the Fermi space telescope in June 2008 to look for special flashes of light from tiny black holes called primordial black holes. As of January 2024, these flashes have not been found.

In 2023, the neutrino detector KM3NeT saw a very big burst of energy called KM3-230213A. One idea is that this could be from a tiny black hole disappearing, and some measurements match this idea if these tiny black holes make up a lot of dark matter.

Some theories suggest that the CERN Large Hadron Collider could make tiny black holes that disappear, but none have been seen there.

Because Hawking radiation is very weak, it is hard to see in real black holes. Scientists have tried to study something similar using sound in special materials called Bose–Einstein condensates, and they reported seeing something like Hawking radiation. In 2010, an experiment made something like a black hole in the lab and claimed to see radiation like Hawking radiation, but these results are still being checked and are not fully proven.