- Hawking radiation is thermal radiation spontaneously emitted by black holes, due to quantum effects near their event horizon.
- It gradually decreases the rotational energy and mass of black holes.
- However, it’s just a theory; no such radiation has been observed yet.
In 1974, Stephen Hawking shocked the physics world by showing that black holes can emit sub-atomic particles until they exhaust their energy and evaporate completely.
Before that, black holes were generally considered as perfect black objects from which no particle and radiation can escape. However, Hawking’s paper titled ‘Particle creation by black holes’ (published in 1975) gave a whole new perspective to study black holes.
What Is Hawking Radiation? (Explained In Simple Terms)
By taking quantum field theory into account, Stephan Hawking showed that black holes emit radiation — known as Hawking radiation — near their event horizon.
In laymen’s terms, Hawking radiation is thermal radiation that is spontaneously emitted by black holes. It gradually decreases the rotational energy and mass of black holes. Thus, inactive black holes (that do not consume matter) are expected to shrink and eventually vanish. This process is also called black hole evaporation.
Hawking radiation was a controversial discovery, but by the end of the 1970s, it was broadly accepted as a major breakthrough in theoretical physics.
A Little Background
In the early 1970s, research done by James Bardeen, Stephen Hawking, and other physicists led to the formulation of black hole thermodynamics where the behavior of a black hole can be exampled by relating area to entropy, mass to energy, and surface gravity to temperature.
Hawking, using a lot of math, combined insights from both Einstein’s theory of relativity and quantum mechanics. While the theory of relativity describes gravity in regions of large scale and high mass (galaxies, stars), quantum mechanics focuses on non-gravitational forces in regions of small scale and low mass (molecules, atoms).
Scientists have been trying to combine these two major theories for decades: they have been trying to develop a theory of everything that could fully explain and link together all physical aspects of the universe.
It turns out both theories come into play at the black hole’s event horizon, a boundary beyond which nothing can escape (not even light).
Stephen Hawking used quantum field theory to show that black holes should radiate like a black body. And like many other objects in our universe, black holes shrink and die. His calculations showed that both rotating and non-rotating black holes emit radiation. He even turned his findings into a bit of advice:
“If you feel you are in a black hole, don’t give up. There’s a way out.” — Stephen Hawking at the public lecture in Stockholm, Sweden.
How Does Hawking Radiation Work?
The explanation is quite trippy. As per the quantum mechanical theory, particles and their counterparts (antiparticles) constantly pop in and out of existence throughout the universe.
These particle-antiparticle pairs are also created near the black hole’s event horizon, and they annihilate each other quickly. However, it is possible for one particle to fall into the black hole before the annihilation can happen, in which case the other particle (its counterpart) escapes as Hawking radiation.
Hawking radiation as particle pairs are generated near a black hole
This means Hawking radiation doesn’t come directly from the black hole itself; instead, it’s a result of virtual particles being ‘uplifted’ by the intense gravitation of the black hole into becoming real particles.
The particle that fell into the black hole should have had negative energy, while its counterpart should have had positive energy (with respect to an outside observer). In this way, it would appear that the black hole has just released a particle, losing its mass.
The energy from outside the event horizon produces the radiation, which means the black hole must lose mass to compensate | Credit: E. SIEGEL
This phenomenon can also be seen as a quantum tunneling effect, in which the pair (of particle and antiparticle) forms from the vacuum and one of the pair tunnels outside the event horizon.
Black Hole Information Paradox
The key difference between the thermal radiation emitted by a black body and black hole radiation (as evaluated by Hawking) is that the former is statistical in nature.
Thermal radiation carries information about its source body, whereas Hawking radiation doesn’t seem to carry such information: it solely relies on the mass, charge, and angular momentum of the black hole.
So what happens to the matter engulfed by black holes? As per our understanding of general relativity, the information is destroyed. But if this is the case, it would violate the laws of quantum mechanics.
This puzzle is called the black hole information paradox.
In 2015, Stephen Hawking presented an idea about how this paradox can be solved. He suggested that information is not actually stored inside the black hole, but in its boundary – the event horizon.
The information is stored in the form of super translations, a hologram of the ingoing particles. It is released in the quantum fluctuations that black holes create, albeit in useless, chaotic form.
Hawking Radiation Is A Slow Process
Illustration of a black hole as shown in movie Interstellar
Hawking radiation temperature is inversely proportional to the mass of the black hole. Therefore smaller black holes emit more radiation and dissipate faster than the larger ones.
Calculations show that a black hole with one solar mass would take 1067 years to evaporate; the supermassive black hole at the center of the Milky Way would require 1087 years, and even more massive ones in the universe could take as long as 10100 years.
In 2008, NASA launched a space observatory named Fermi Gamma-ray Space Telescope, which is currently searching for evaporating primordial micro black holes from their estimated Hawking radiation.
Scientists believe that micro black holes might be experimentally generated in an artificial environment at CERN’s Large Hadron Collider. If successful, they may observe black hole evaporation as well as confirm some of the theoretical predictions of superstring theory regarding gravity.
Many scientists have presented intriguing methods to test Hawking radiation within the framework of analog gravity, but it hasn’t been observed yet.
In 2010, a team of researchers claimed that they had observed optical light pulses closely related to Hawking radiation in a lab experiment. Their claim, however, remains unconfirmed and questionable.