The Concept of Hawking Radiation From Black Holes

Rishabh Nakra

Admin and Founder of ‘The Secrets Of The Universe’ and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing a Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity & Quantum Mechanics.

This article on Hawking radiation from black holes is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.

Background

In 1973, world-renowned physicist Stephen Hawking visited Moscow. There, he was convinced by fellow scientists to look into the behavior of rotating black holes. After almost a year of studying the emission phenomena linked to black holes, Hawking’s calculations lead to a surprising result. While he couldn’t win a Nobel Prize until his predictions were experimentally proven, the physicist has always been praised and valued in the scientific community. 

The Radiation

Before Hawking, scientists were comfortable with two types of radiation concerning the black holes. The first one was the well-known, general phenomenon of black-body radiation, that happens anywhere in the universe. The second one was the idea that rotating black holes were radiating energy, decelerating their motion. However, his theory now suggests that every black hole loses energy at a specific rate (Hawking radiation), related strongly to the body’s mass and temperature.

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The calculations behind this phenomenon are based on quantum physics, yet one can find more approachable explanations. Both are based on the conditions in the vicinity of a black hole’s event horizon that determine the creation of virtual particle-antiparticle pairs (by the black hole’s gravitational energy or vacuum fluctuations). 

Firstly, if the gravitational pull acts on the virtual particles in such a way that they become real particles, they can also escape the vicinity of the black hole. The flow of such particles represents a flow of mass from the black hole to its exterior, therefore a rate of energy loss by evaporation. 

Secondly, we can consider that, due to energy conservation, one of the pair’s particle has to have negative energy. It is possible that due to a quantum physics phenomenon (quantum tunneling) the positive energy particle escapes the black hole, while the negative energy particle flows into the event horizon, decreasing the overall energy of the black hole (and as a result, its mass too). 

The Equation of Hawking Radiation

Considering the simplified case of a Schwarzschild black hole (one that has the greatest radius possible for its gives mass), one can empirically find an equation relating Hawking radiation to the body’s parameters and universal constants by means of dimensional analysis. A measure of this loss of energy is often the Hawking temperature, a quantity analogous to the classical temperature used when computing the blackbody radiation (which depends directly on the fourth power of temperature).

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The final equation would be the one depicted below, highlighting that the Hawking temperature and the black hole’s mass are inversely proportional. This means that (perhaps paradoxically) the smaller the black hole, the more radiation it emits. Therefore, only very small black holes can emit substantial Hawking radiation and eventually evaporate. 

Detection of Hawking Radiation

It is important to recall that for all discovered and localized black holes, this effect is too small to be measured under experimentally achievable conditions. Because of that, Hawking radiation has not been detected yet. In the meantime, physicists are building and analyzing analogous systems.

In September 2010, it was thought that a laboratory-created body, simulating a white hole’s event horizon radiated an optical analog to Hawking radiation, although to this date no official confirmation of the accuracy of this experiment exists. One of the latest predictions also links sonic black holes (for which sound perturbations are analogous to light in a gravitational black hole) to a form of perfect fluid flow that could simulate Hawking radiation. 

While it is not yet known how and when will Hawking radiation be detected, scientists are diligently working towards this purpose. What happens after such radiation is discovered is also unknown, but one can only hope that it will open a new era when it comes to understanding black holes.