Nothing beats black holes for audience entertainment. I use them shamelessly myself. Whenever a lecture feels like it's going stale, I bring up black holes. Everyone has heard of black holes, and even grade-school kids have their own theories and fantasies about how these mysterious objects work, or what might happen if you fell into one.

In all honesty, I can't think of anything more dramatic than bottomless pits of gravity that rip holes in the fabric of time and space, swallowing up entire stars, even light, never to be seen again. Black holes are especially intriguing to us because we don't yet understand the physics of what goes on inside them (just how can a black hole swallow a million stars and pack them all into no space at all?).

But for a long time now we've at least had a good grasp on how and why black holes form. In the 1930's an Indian astrophysicist named Subramanian Chandrasekhar did some startlingly simple calculations, and found out that there is a limit to how much gravity 'matter,' at least as we know it, can support. And while nothing on the surface of the earth comes close to exceeding this matter-gravity limit, some of the more dramatic objects in the universe indeed do.

In the middle of a star, violent, uncontrolled fusion reactions that are millions of times more powerful than the combined power of our entire nuclear arsenal take place every single second. The only reason a star doesn't fly apart from the force of the explosions is that it is too big -- the massive gravitational field generated by the star holds everything together. This balance, fusion explosions forcing out and gravity forcing in, sustains stars throughout their lifetimes.

But as huge as stars are, they have only a finite amount of nuclear fuel to burn through. Eventually, a star runs out of fuel and the nuclear reactions in its core cease. But all the mass (and gravity of the star) is right where it always was, still pressing down toward the center. With nothing to support the crush of gravity, things can quickly get out of control. In the case of a relatively small star like the Sun, the entire body of our star will get crushed into a hot, super-dense little cinder about the size of the Earth. The force of gravity will squeeze the Sun until every teaspoon of material weighs about as much as an 18-wheel truck, but at that point, the crush of gravity will stop.

For more massive stars, it's not that easy. When a star about three times the mass of the sun dies, gravity takes complete control. Not even the fundamental structure of matter can stand up to the colossal forces present, and, quite literally, a bottomless pit is formed. A black hole has been born.

What is so appealing to children and scientists alike is the fact that a black hole really is a bottomless pit that can suck in anything. The defining characteristic of black holes is that they are, well, black. The reason for this blackness is that even light can't escape a black hole. And we're not just talking about visible light, but any kind of radiation. Whether it's an X-ray, a radio wave, or light visible to human eyes, nothing eludes the clutches of a black hole.

When I first learned about black holes, this made no sense to me. After all, gravity is defined as the force of attraction between two objects with mass. Gravity holds us down to the surface of the earth, or pulls planets into orbit around a star. You have to have matter: real, solid, stuff to be attracted by gravity. Light has no mass; it just flies around freely through the universe, unaffected by gravity. So how can a black hole suck down light?

That's where Einstein comes in. Einstein showed that while it's usually okay to ignore the effect of gravity on massless particles (like light), things can get tricky around objects like black holes. And the reason is quite shocking.

The gravity of a black hole is so intense; it twists the very fabric of space and time into a violent, rotating, tornado-like vortex. So, while light may very well be massless, it still has to travel through space. And close up to a black hole, space itself is bent and stretched into a bottomless pit. There is no path that light can travel through space and emerge from the inside of a black hole.

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