Hawking radiation describes the black body radiation that black holes emit through hypothetical particles formed near and at the event horizon. It’s a type of radiation that, on average, reduces the mass of black holes, making it an extraordinary property of black holes.
Hawking radiation indicates that the temperature of black holes is inversely proportional to their mass. In other words, the smaller a black hole is, the more radiation it will emit, the hotter it will glow, and the faster it will evaporate.
Despite never having been directly observed and being very difficult to detect, Hawking radiation is a prediction supported by unified models of General Relativity and quantum mechanics. This radiation is called “Hawking Radiation” because in 1974, the great physicist Stephen Hawking was the first to question whether such radiation could exist in his paper titled “Black hole explosions?“.
Stephen Hawking was asking a fundamental question: Nothing, not even light, can escape from black holes; but could black holes emit heat? The answer to this question is enormously important because if Hawking radiation is proven to be real, our knowledge about the nature and lifespan of black holes will fundamentally change: If black holes do radiate, contrary to what was previously thought, it means they shrink over time. As these already extremely dense objects become even smaller through this radiation, the smallest black holes would violently explode with the slightest heat contact, while larger ones would slowly evaporate and disappear over trillions of years.
Why Should Black Holes Radiate?
When matter enters a black hole, it becomes completely isolated from the rest of the Universe. Before Hawking, scientists thought that objects falling into a black hole could never escape, making black holes a one-way street. According to them, black holes didn’t emit any matter, energy, or information. However, this meant the disappearance of entropy, the measure of disorder that physicists refer to.
The ability of matter to vanish inside black holes would make the Universe less disordered (or more ordered), suggesting that this property of black holes violated the second law of thermodynamics.
Hawking disagreed with this view. According to Hawking, black holes obeyed the second law of thermodynamics, and their entropy had to increase over time. This was a critical acceptance because anything with entropy must have a temperature! In other words, entropy is just another way of describing heat energy that always emits radiation. If the event horizon had entropy, it would have to glow somehow. This meant black holes couldn’t be as black as they appeared.
However, Hawking wasn’t the only physicist trying to solve the black hole problem.
Jacob Bekenstein, then a physics student at Princeton University, showed that when matter falls into a black hole, the surface area of the event horizon—the region most affected by the black hole’s incredible gravity—should increase slightly. He demonstrated that this change in surface area was equivalent to the entropy that would otherwise be lost, a proposal that could solve the paradox.
Nevertheless, Hawking wasn’t entirely convinced by this explanation either. Therefore, he tried to determine the temperature of black holes through his calculations. To do this, he combined the predictions of Einstein’s General Theory of Relativity, which explains how gravity works on large scales, with quantum mechanics, which describes how the Universe works at the smallest scale. These two theories have still not been fully unified today, and one of physicists’ greatest dreams is to reach a Theory of Everything that can explain the Universe from the smallest to the largest scale in one go. Hawking had to use both theories because both help us explain how things work at the event horizon of black holes.
In his efforts to disprove Bekenstein’s seemingly absurd proposal, Hawking discussed this topic with other physicists and tried to use mathematical models to show it was impossible. However, far from disproving Bekenstein, Hawking found that black holes indeed glow with a kind of “cold light.” He communicated this to the masses with these immortal words:
The message of this lecture is that black holes ain’t as black as they are painted. They are not the eternal prisons they were once thought. Things can get out of a black hole both on the outside and possibly to another universe. So if you feel you are in a black hole, don’t give up – there’s a way out
How Do Black Holes Produce Hawking Radiation?
The physical process behind particles emanating from the event horizon around a black hole is quite complex and requires a solid understanding of the mathematical foundation of quantum field theory.
But if we try to explain it simply, you first need to understand this: The vacuum of space is, in many ways, not empty at all! Of course, you can imagine removing all matter, all radiation, all energy quanta from a region of space, until what remains is close to “nothing” that we can remove from this Universe.
However, even in this case, the “zero-point energy” of that empty space—the lowest energy level a quantum mechanics-based system can have—is not zero. Even if you remove everything you can, space itself has an inherent amount of energy that is non-zero.
How Do Black Holes Produce Hawking Radiation?
One way to conceptually understand and better visualize this is through particle-antiparticle pairs. Note: These virtual particles are not real particles; they’re just a conceptual way to imagine the non-zero vacuum energy. However, according to quantum mechanics, particles and antiparticles are said to constantly come into and out of existence. We’ll emphasize this point again shortly.
Normally, when these virtual particle pairs begin to exist, they cannot maintain their existence for long, and the pair quickly annihilates each other. In fact, shortly after the Big Bang, more matter than antimatter was produced for an unknown reason, allowing things to exist in the Universe. Without this imbalance, the Universe would have quickly annihilated itself.
At the boundary of black holes, things don’t work as they normally do.
Around a black hole, there are three regions where these particle-antiparticle pairs can emerge:
- A region where both particles in the pair appear outside the black hole’s event horizon, exist, and re-annihilate each other.
- A region where both particles in the pair appear inside the black hole’s event horizon, exist, and re-annihilate each other.
- A region where both particles in the pair appear outside the black hole’s event horizon, but one falls into the black hole while the other escapes.
Yes, this is an extremely simplified explanation. However, although this description doesn’t fully define where Hawking radiation comes from or what its energy spectrum is, it’s one of the simplest visualizations that correctly explains the qualitative properties.
In his quite popular book, Hawking explains his theory as virtual particles affected by extreme gravity, where one particle of the pair escapes from the black hole, causing mass loss due to the strong gravitational effects that give negative energy to the other particle.
The one falling into the black hole is said to effectively have negative energy. The one escaping from the black hole has positive energy. The particle that manages to escape from the black hole is what causes what we call Hawking Radiation. Since the energy of the particle pair falling into the black hole is effectively negative, it means energy is escaping from the black hole somewhere; in other words, the energy (and therefore mass) of the black hole is gradually decreasing.
Again: The important thing to remember at this point is that these “pairs” we’re talking about aren’t physically real. In reality, what comes out of the black hole is a black body radiation spectrum. This spectrum is related to the size of the black hole’s event horizon. Also, the scattered spectrum is mostly in the form of extremely low-energy photons, and smaller black holes radiate faster. Therefore, as a black hole evaporates and shrinks, it evaporates increasingly faster.
The significance of this is: A particle pair that emerges inside the black hole cannot add mass to the black hole because the total energy there is always the same. Ultimately, the energy of particle-antiparticle pairs comes from the space around them. However, if you have energy resulting in real radiation moving away from the black hole originating from outside space, this energy must come from the black hole itself and reduce its mass. This is how Hawking radiation works, and this is why black holes gradually evaporate.
In summary, Hawking Radiation occurs when one of a particle-antiparticle pair—which normally come into existence from nothing and return to nothing within the fabric of spacetime—falls into the black hole while the other escapes from the black hole. Because these particles are separated by an enormous gravitational difference, they cannot annihilate each other, thus affecting the mass of the black hole.
How Does Black Hole Evaporation Happen?
As we’ve mentioned, if black holes radiate energy back to the rest of the Universe, their eventual “evaporation” and disappearance becomes inevitable. It’s possible to detect this spectrum emitted from the black hole by examining the black hole’s thermal equilibrium state and the extreme redshift events that occur very close to the event horizon (where quantum entanglement phenomena must also be taken into account).
Virtual particles that form under quantum effects very close to a black hole almost always exist as a pair of photons. One of these photons cannot pass the event horizon and becomes trapped inside the black hole; the other escapes from the black hole and travels toward the rest of the Universe.
Looking at this process more closely, here’s what we see: An extreme redshift event occurs in the photon formed at the black hole’s event horizon. Meanwhile, the photon escaping from the black hole almost “shatters.” Interestingly, the intensity of this escaping photon increases slightly. This intensity increase causes the formation of a particle called a partner wave that carries negative energy, which gets caught in the black hole’s strong gravity and falls into the black hole.
The photon escaping from the black hole adds positive energy to the rest of the Universe; however, note that no matter has actually left from inside the black hole. On the other hand, the partner wave falling into the black hole is subject to conservation laws and therefore undergoes the same black body radiation as the escaping photon. Therefore, the radiation of this wave carries no information about the internal conditions of the black hole.
The mass and rotational energy of black holes that emit Hawking Radiation decrease over time, and they eventually disappear through a process called black hole evaporation. Therefore, black holes that don’t receive mass from external sources must eventually cease to exist. For all black holes except very small ones, this evaporation process occurs incredibly slowly.
