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.
Since a black hole has no surface (it's just a pit), defining where the exact edge is takes a little doing. Astronomers usually define the "edge" of a black hole as the point-of-no-return, the point where all paths through space are bent back into the black hole. This edge is called the event horizon, as any event that takes place within this limit is, in a real way, disconnected from the rest of our universe. We'll never know what goes on down there, as no information will ever find its way out. The word event horizon also has some connotations for the way time is warped around a black hole. Even time is stretched and twisted up around the event horizon, which gives you some idea of just how powerful these objects really are. And it's not just theory. Astronomers have observed an effect called frame dragging which, simply put, is a measurement of how much time is stretched near a black hole.
Now, to an observational astronomer, black holes pose an immediate problem. How do you observe something that gives off no radiation at all? In fact, how do we know black holes are out there in the first place?
Surprisingly, in the last few years black holes have become one of the easiest objects in space, especially at large distances, to observe. New black hole detections are routine, and astronomers are monitoring, in same cases on a daily basis, massive black holes as they swallow entire stars whole. How is it possible to observe something that gives off no light? The important step astronomers made was to get good at recognizing the telltale signs that a black hole is present, even if you can't see the actual beast. Since the 1960's, astronomers have noticed that some areas in space, especially the centers of distant, odd-looking galaxies, pour out unbelievable amounts of energy. Some of these galaxies, since named quasars, outshine our own substantial Milky Way galaxy by factors of thousands or millions. Ironically, astronomers now think that the cores of these objects, the brightest regions in the universe, are most likely gigantic black holes.
Nothing has changed in our basic understanding of black holes. There is still no way that anything can escape a black hole once it falls in. But it has to fall in first. That's the key.
What astronomers are observing is not light coming out of a black hole, but light emitted by material that is in the act of falling in to one. A good analogy is watching water spiral down a drain. You may not be able to see down inside the drain, but you can watch the water as it spins around before going down. And amazing thing happen to the matter as it goes down the drain. Remember how black holes spin space and time into a vortex close to the event horizon? The spun-up space and time act as a sort of natural particle accelerator. There is no way anything close to the event horizon can stand still, as space itself is spinning around like a tornado. As material gets swept up into the vortex, the acceleration imparts huge amounts of energy to the matter, which heats up to temperatures of millions of degrees. So, far from being black, the area right around the event horizon of a massive black hole pours out more energy than billions of suns.
The best place to observe super-heated matter around a black hole is in the X-ray region, since the majority of light coming from million-degree matter is emitted in X-rays. When the Chandra Space Telescope (named for Chandrasekhar) was launched a few years ago, one of its major goals was to probe the universe for this black-hole generated energy. And wow, did Chandra ever find it.
Not only are giant black holes lurking in the centers of almost every galaxy we know of, but Chandra has also found black holes at the edge of the known universe. Shining at us from billions of light-years away, we now know that there are literally billions of black holes out there. Some are only a few times the mass of our Sun, and can be seen ripping apart a neighbor star into bite-size chunks. Others are billions of times more massive, meaning that they have swallowed uncountable numbers of stars and solar systems, converting them to almost pure energy before spiraling them into oblivion.
In the end, I would have to say that black holes definitely live up to their reputation as being dramatic and mysterious. But I love the irony that these totally dark objects are now found routinely because they create the more brilliant beacons imaginable, and can be seen clear across the universe. It seems that black holes may not be quite so black after all.