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Who is your favorite physicist?

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Author’s Note

Black holes are often described as the ultimate traps of the universe. Once something falls in, it can never come back out. Not light. Not matter. Nothing. In the 1970s, when Stephen Hawking claimed that black holes slowly glow and evaporate, physicists were stunned. It sounded impossible. If nothing can escape a black hole, how could it emit radiation? In a high school physics class, we usually learn that black holes are regions of spacetime with gravity so strong that the escape velocity exceeds the speed of light. That definition makes them sound permanent and unchanging. What Hawking discovered was something much deeper and much stranger. The radiation does not come from inside the black hole at all. It comes from empty space just outside it. So the real question is not “how do black holes emit radiation?”
It is: why/how does empty space near a black hole create particles out of nothing?

Introduction

A black hole is defined by a boundary called the event horizon. This is the point beyond which nothing can escape. Cross it, and you are gone forever. That is why black holes are supposed to be perfectly black. They absorb everything and emit nothing. But quantum physics quietly changes this picture. According to quantum theory, empty space is not truly empty. It is filled with tiny energy fluctuations that never completely disappear. When this restless quantum vacuum exists next to the extreme gravity of a black hole, something extraordinary happens. The black hole begins to lose mass and energy. This slow leakage is called Hawking radiation.

Empty Space Is Not Empty

In classical physics, a vacuum is just nothing. In quantum mechanics, a vacuum is alive with activity. Because of the uncertainty principle, energy can briefly fluctuate into existence. This allows pairs of particles to appear momentarily. One particle and one antiparticle pop into existence, exist for a tiny fraction of a second, and then annihilate each other and disappear. These are called virtual particles. Normally, this process leaves no trace. It is like borrowing energy from the universe and paying it back immediately.

What Changes Near a Black Hole

Now place this quantum vacuum next to a black hole’s event horizon. Virtual particle pairs still form exactly as they do everywhere else in space. But now gravity enters the story. Sometimes, one particle in the pair falls into the black hole while the other escapes into space. When that happens, the escaping particle becomes real. It no longer annihilates with its partner. To conserve energy, the black hole must lose a tiny amount of mass. That escaping particle is Hawking radiation. So nothing actually escapes from inside the black hole. The radiation is created just outside the event horizon.

Why the Black Hole Loses Mass

This is the part that feels like a cheat. The particle that falls into the black hole effectively carries negative energy, as measured from the outside universe’s point of view. That reduces the black hole’s total energy. And since energy and mass are equivalent, according to Einstein’s famous equation:

Losing energy means losing mass. So every escaping particle shrinks the black hole just a little bit.

Black Holes Have Temperature

One of Hawking’s most shocking results was that black holes have a temperature. The temperature of a black hole depends on its mass. The equation for Hawking temperature is

where M is the mass of the black hole, G is the gravitational constant, c is the speed of light, ℏ is the reduced Planck constant, and k_B​ is Boltzmann’s constant.

It looks frightening, but the message is simple. As the mass M increases, the temperature decreases. Put simply, Big black holes are extremely cold. Small black holes are extremely hot.

Why Big Black Holes Evaporate Slowly

For a black hole with the mass of the Sun, the temperature is about 0.00000006 kelvin. That is far colder than the cosmic microwave background. This means stellar black holes actually absorb more energy from the universe than they emit. So they grow, not shrink. But tiny black holes behave very differently. The smaller the black hole becomes, the hotter it gets. As it shrinks, it radiates faster. Eventually, in theory, it ends in a violent final burst of energy.

Why This Was So Revolutionary

Hawking radiation connects three pillars of physics: Quantum mechanics, General relativity, and Thermodynamics. As it tells us, black holes have temperature, entropy, and black holes can die. This shattered the old idea that black holes are eternal objects. It also created one of the deepest problems in modern physics, called the black hole information paradox. If black holes evaporate completely, what happens to the information about what fell in?

Takeaway

Black holes are not perfectly black. They slowly leak energy because quantum fluctuations create particle pairs near the event horizon, gravity separates those pairs, one escapes, and the black hole loses mass to pay the energy bill. In other words, black holes glow because empty space itself is unstable near extreme gravity.