# Here’s a simple explanation of Stephen Hawking’s greatest contribution to science

Mathematical physicist and cosmologist Stephen Hawking was best known for his work exploring the relationship between black holes and quantum physics.

A black hole is the remnant of a dying supermassive star that’s fallen into itself; these remnants contract to such a small size that gravity is so strong even light cannot escape from them.

Black holes loom large in the popular imagination – schoolchildren ponder why the whole universe doesn’t collapse into one. But Hawking’s careful theoretical work filled in some of the holes in physicists’ knowledge about black holes.

Why do black holes exist?

The short answer is: Because gravity exists, and the speed of light is not infinite.

Imagine you stand on Earth’s surface, and fire a bullet into the air at an angle. Your standard bullet will come back down, someplace farther away.

Suppose you have a very powerful rifle. Then you may be able to shoot the bullet at such a speed that, rather than coming down far away, it will instead “miss” the Earth. Continually falling, and continually missing the surface, the bullet will actually be in an orbit around Earth.

If your rifle is even stronger, the bullet may be so fast that it leaves Earth’s gravity altogether. This is essentially what happens when we send rockets to Mars, for example.

Now imagine that gravity is much, much stronger. No rifle could accelerate bullets enough to leave that planet, so instead you decide to shoot light.

While photons (the particles of light) do not have mass, they are still influenced by gravity, bending their path just as a bullet’s trajectory is bent by gravity. Even the heaviest of planets won’t have gravity strong enough to bend the photon’s path enough to prevent it from escaping.

But black holes are not like planets or stars, they are the remnants of stars, packed into the smallest of spheres, say, just a few kilometers in radius.

Imagine you could stand on the surface of a black hole, armed with your ray gun. You shoot upwards at an angle and notice that the light ray instead curves, comes down and misses the surface! Now the ray is in an “orbit” around the black hole, at a distance roughly what cosmologists call the Schwarzschild radius, the “point of no return.”

Thus, as not even light can escape from where you stand, the object you inhabit (if you could) would look completely black to someone looking at it from far away: a black hole.

But Hawking discovered that black holes aren’t completely black?

My previous description of black holes used the language of classical physics – basically, Newton’s theory applied to light. But the laws of physics are actually more complicated because the universe is more complicated.

In classical physics, the word “vacuum” means the total and complete absence of any form of matter or radiation.

But in quantum physics, the vacuum is much more interesting, in particular when it is near a black hole. Rather than being empty, the vacuum is teeming with particle-antiparticle pairs that are created fleetingly by the vacuum’s energy, but must annihilate each other shortly thereafter and return their energy to the vacuum.

You will find all kinds of particle-antiparticle pairs produced, but the heavier ones occur much more rarely. It’s easiest to produce photon pairs because they have no mass. The photons must always be produced in pairs so they’re moving away from each other and don’t violate the law of momentum conservation.

Now imagine that a pair is created just at that distance from the center of the black hole where the “last light ray” is circulating: the Schwarzschild radius. This distance could be far from the surface or close, depending on how much mass the black hole has.

And imagine that the photon pair is created so that one of the two is pointing inward – toward you, at the center of the black hole, holding your ray gun. The other photon is pointing outward.

(By the way, you’d likely be crushed by gravity if you tried this maneuver, but let’s assume you’re superhuman.)