I wish I could've made it to New York City a few weekends ago, when the streets and theaters there were briefly taken over by a glorious celebration of geekery: the first-ever World Science Festival. By all accounts it was a smashing success, with hundreds of thousands of attendees, and every single event sold out. As event organizer Brian Greene wrote recently, the World Science Festival was a chance to remind people that science is not just some stuffy list of facts to be memorized, it's a constantly unfolding human endeavor, indeed perhaps "the greatest of all adventure stories." Scientists aren't high and mighty sages on a hilltop, they're by and large just curious people trying to make some sense of the world around us. Science can be, should be, lots of fun, and at its best it can open up breathtaking vistas on the cosmos. It's a vast and subtle universe, and every so often something almost unbelievably surprising turns out to be true.

A couple months back, I spent a few afternoons volunteering in a fifth grade science class. In some ways, ten years old is a perfect age, when kids have started to develop more sophisticated reasoning abilities yet haven't yet lost the gleeful high-octane inquisitiveness of true youth. You surely know what I mean: that natural curiosity that just keeps asking "why? why? why?" far beyond the point at which any adult would stop, the fact-vacuuming insatiable thirst for knowledge that leads children to gloriously encyclopedic knowledge of dinosaur species, model rocket parts, and comic book continuity errors. There's nothing quite like a ten-year-old for asking question after question and vacuuming up the answers like mana from heaven. And in that fifth-grade classroom, that's exactly what they did, on a surprisingly wide range of topics befitting kids who've grown up surrounded by Star Wars and the Discovery Channel: "Dr. Perrin, why is Mars red?" "How far can you see with a telescope?" "How fast can a rocket go? Can we make warp drive?" "Have you ever been in outer space?" "Are there aliens?" "Has anybody ever seen a black hole?"

Ooooh, now there's a good question. Black holes! They bring together the allure of danger, the mystery of ultimate ends, the excitement of mindbogglingly catastrophic explosions—and the cutting edge of astrophysics. Black holes are the Tyrannosaurus Rex of astronomy: mysterious and dangerous, the end result of millions of years of evolution, perfect predators which hold our fascinated attention all out of proportion to their actual rarity.

The flip side of those kids' appetite for knowledge is that whatever I said, they would take for truth. From their perspective, that's just how the world is: adults hand you knowledge, especially when a real live scientist comes to class. (I must admit that receiving rock-star-like adulation from thirty ten-year-olds was kind of fun). But receiving knowledge like that, from Wise Grown Ups on high, tends to accidentally reinforce the stereotype of science as just a list of facts. Rarely do students get any real sense of the history of the knowledge they're receiving. Yet today's certain truth in textbooks was yesterday's debated tentative consensus was last week's wild new idea. Things that were miraculous to a prior generation become an unremarkable part of the world remarkably rapidly. I'm as much subject to this as are today's third millennial kids—and so are you. From my own perspective, for as long as I've known, an asteroid has always probably killed the dinosaurs, plate tectonics has been the blindingly obvious explanation for the coastlines of the continents, and there have always been computers. And black holes have always been real.

Yet each of those discoveries was made within just the last few decades. Plate tectonics only became widely accepted in the late 1960s, at about the same time as Gordon Moore penned his now-famous law about transistor densities. Heck, that dinosaur-killing asteroid idea dates to within my own lifetime! I was a few months old when the father-and-son team of Luis and Walter Alvarez published their landmark paper suggesting that an impacting asteroid was responsible for the demise of all those Tyrannosaurs. By the time my younger self was in obsessive dinosaur mode a few short years later, that idea had already gathered momentum as the leading hypothesis. Today's kids grow up knowing that without a doubt the impact occurred near what is now Chicxulub, Mexico precisely 65.95 million years ago. Thus goes science.

So, back to the question those kids asked me: "Has anyone ever seen a black hole?" It's a good question, after all. Isn't it by definition impossible to see such a thing, utterly black against the inky vastness of space? The answer that I gave in that classroom—and that I share with you now in somewhat greater detail—is that while no one has ever seen a black hole itself, we have seen many times the effects they have on their surroundings. We can track them through their gravity, and see stars and gas loop around them. We can even watch as glowingly hot gas pours down their insatiable maws. We study them hoping for insights into the difficult marriage of relativity and quantum mechanics, and today some can even contemplate creating black holes in the lab. The story of how black holes came to be known so well is a fascinating one, and serves, I think, as a great example of the grand scientific adventure which Brian Greene describes. Step by step and year by year, what was once supremely speculative (even science fictional!) has today become an accepted part of the scientific landscape, perhaps something we can turn into an experimental tool.

Black Holes in the Beginning: Where do stars go when they die?

From our modern perspective, it's hard to remember how truly outlandish the idea of black holes seemed at first. Yet for most of the 20th century, black holes were just a preposterous mathematical curiosity, equations carried to an absurd extreme which certainly had nothing to do with to the actual universe around us. Only recently, after decades of drama, has it finally come to be accepted that black holes really, truly do exist—and the full implications of that are still being debated.

The genesis of this story begins about 90 years ago, just after Einstein presented his theory of general relativity to a stunned world. Einstein himself famously struggled with the advanced mathematics of relativity, and never found any exact mathematical solutions to the equations of general relativity that he derived (though his approximate solutions were sufficient to solve the mystery of Mercury's precessing orbit, for instance). The first exact solution of Einstein's field equations was found by a German physicist and astronomer named Karl Schwarzschild. Almost immediately after reading Einstein's general relativity paper in November 1915, Schwarzschild tried to apply this new theory of gravity to better understand the gravity of stars. He learned the methods and mathematics of relativity, systematically made simplifying assumptions (there is only one star in the universe, and it's a sphere!), and within days had found a perfect mathematical description of that star's gravity. (He did all this, amazingly enough, while serving on the Russian front in World War I, and suffering from a rare disease called pemphigus too. Sadly, this combination proved deadly only a few months later.) You've surely seen those cartoon schematics of space-time stretched out like rubber sheets; Schwarzschild's solution is the real deal, mathematics that describes precisely how curved spacetime gets for a given amount of mass. Einstein was thrilled to see this mathematical tour-de-force, and Schwarzschild's solution remains today a staple of any relativity class.

But there was a tiny problem. If the radius of the hypothetical star became too small, the resulting equations failed, in effect trying to divide by zero. Neither Einstein nor Schwarzschild were concerned with this—the problematic radius was unrealistically tiny. You'd have to cram the entire 700,000-km-across sun down into a ball only 3 km across to run into this problem. So clearly this was just a mathematical curiosity that would go away with better understanding of physics down the line; it couldn't possibly have anything to do with reality. Lest this seem like a curious attitude, remember the context of early 20th century physics: After many years in which the best theories of the day accidentally predicted that all atoms everywhere should instantly vanish in a puff of ultraviolet light, the dawn of quantum mechanics was redrawing all the maps. Atoms were stable again, yet their weights and properties were mysterious and radioactivity still utterly defied understanding. Even the mere idea that the sun was powered by fusion was still four years in the future. In that context, one little mathematical hiccup from a theory, which required you to plug in a completely ridiculous radius, was the least of anyone's worries.

And so black holes were promptly ignored, for decades. Through the 1920s into the 1930s, the real focus of physics was the quantum revolution. In those heady years, Heisenberg, Bohr, Schroedinger and other giants of physics puzzled out the nature of atoms and the origin of the periodic table. Meanwhile, led by Eddington, the first generation of true astrophysicists applied these ideas to discern the life and death of stars. (One of the key figures in working out the internal structure of stars was Martin Schwarschild, son of the aforementioned Karl. Science was a much smaller community in those days!) Far from being a purely academic exercise, all this knowledge soon thereafter contributed to the century's most famous applied physics project. Yet in 1939, few had heard of J. Robert Oppenheimer, author of a landmark paper on continued gravitational contraction of dying stars. He argued that "When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse . . . this contraction will continue indefinitely." Thus, for a massive enough star, eventually collapse down to that remarkably small, problematic "Schwarzschild radius" was not only possible, it was inevitable. Such collapsing, dying stars must fall toward the Schwarzschild radius, gradually contracting more and more slowly due to general relativistic time dilation yet continuing inwards and inwards until . . . until what, precisely? The mathematics of relativity failed. No one knew.

The Golden Age: Do Hippie Black Holes Have Long Hair?

The dawn of the atomic era once again diverted all those brilliant minds for some time, and further progress in understanding these collapsing "frozen stars" had to wait until the 1960s. That tumultuous decade was a golden age of general relativity research, with one after another mathematical wunderkind dramatically advancing the state of the art. The seed for this revolution was planted by David Finkelstein, who in 1958 realized that the mathematical breakdown of Schwarzschild's equations could be avoided by recasting those equations in a different coordinate system. With this new approach he discovered that the critical Schwarzschild radius was not a physical barrier to collapse, but rather a new kind of entity which he called an "event horizon," meaning a surface past which light cannot travel and all events remain hidden. Yet we can infer what the inside of that region must be like, by using the Schwarzschild metric or an improved solution, developed in 1963 by a New Zealander named Roy Kerr, which allows for more realistic rotating stars. And what those equations predict is the most surprising part of this story yet.

Inside the event horizon lies a bizarrely warped region of spacetime, where time and space have literally swapped places. For anything unfortunate enough to fall within the event horizon, it is no more possible to avoid falling into the ultimate center than it is for objects in regular spacetime to avoid moving forward in time. All matter within the event horizon—yes, including all that mass of the collapsing star—apparently becomes compressed into an infinitesimally small point of infinite density.

Stop for a moment and really think about that: a point of infinite density yet finite mass, whose awesome gravity can stop light and twist the very structure of space and time beyond all recognition. The mind boggles. It's hard to even imagine such an object. The name "frozen star" doesn't come close to describing such a bizarre entity. John Wheeler, another titan of 20th-century physics, coined a new name in a public science lecture he gave in December of 1967: a black hole.

What's just as remarkable as a black hole's strangeness is its simplicity. The birth of a black hole apparently erases all the details of the infalling matter, leaving behind merely three properties: mass, spin, and charge. Famously, "black holes have no hair", being simultaneously as massive as stars yet as simple as subatomic particles. Conditions within black holes are so weird that it seems possible that they could give rise to time machines (as explored by Kip Thorne and colleagues) or even baby universes (as described by Lee Smolin in The Life of the Cosmos_). Meanwhile, the fate of information and objects falling into black holes was worked out by Stephen Hawking and his students, leading eventually to his famous discovery of the faint quantum radiation from black holes that now bears his name (and this is an aside, but if you've not read Gregory Benford's charming account of a dinner conversation he had with Hawking on black holes, the fate of universes and Marilyn Monroe, then by all means stop reading this column and go learn from the true masters instead!)

All this weirdness led to plenty of healthy and continued skepticism that black holes really would exist. Perhaps there was some overlooked factor, a relativistic get-out-of-jail-free card that would prevent black holes from forming in the real world. Yet by 1970, Hawking and his colleague Roger Penrose had proven that singularities are an unavoidable part of all solutions to Einstein's field equations. No matter which mathematical approach to relativity you take, sooner or later there's no getting around black holes.

The Observational Era: How can you see darkness itself?

So by the end of the 1960s, our modern theoretical understanding of black holes was firmly in place. From an ignored mathematical hiccup, black holes had blossomed into a key part of our understanding of the fates of stars, and a seemingly inevitable part of the universe. But the final arbiter in science is experimental evidence, so it was high time to hunt for black holes. But forget about needles in the haystack: how can you see a tiny utterly black region in the endless night of deep space?

The trick to seeing something that's entirely black is to observe how it affects its surroundings. And here I should clarify something: black holes are not cosmic vacuum-cleaners as commonly portrayed by Hollywood. They no more reach out and suck in everything around them than the Earth's gravity "sucks in" all the satellites from the sky. It's just as possible for objects to orbit around black holes as it is for our own moon to sail overhead on a calm summer's night. In order for orbiting material to actually fall into the black hole, there must be enough of that material that it starts to interact and collide with itself, with the resulting collisions pushing some matter inward toward the hole. The high-speed collisions within such an accretion disk can heat gas to millions of degrees, causing it to glow ferociously in X-rays. Thus we have two ways to detect black holes: watch for stars orbiting around something massive yet invisible, or seek out tiny points of X-ray glare.

The X-ray approach paid off first. The earliest X-ray telescopes were lofted above the atmosphere on suborbital rockets (like today's Spaceship One) for brief glimpses of the high energy sky. In 1964, one of those rocket missions found eight X-ray sources in the constellation Cygnus. The brightest of these, called Cygnus X-1, was soon realized to be coming from the direction of an apparently normal giant star, one which should not be capable of producing such brilliant X-ray glare. This alone suggested that the star must have a hidden companion, a hypothesis which was confirmed in 1971 by two independent groups of astronomers from Toronto and Cambridge, England. Their radial velocity measurements allowed the mass to be estimated (just as we estimate the much smaller masses of unseen planets today). The hidden source weighed in at 20 or 30 times the mass of the sun. The case for its being a black hole seemed increasingly solid. In 1975, a famous scientific bet was made, between Hawking and Thorne on whether or not Cygnus X-1 was indeed a black hole (Hawking took the "no" side of the bet even though he thought the answer was yes, reasoning that if his black hole hopes were dashed then at least he'd won the bet for consolation) The observational evidence kept adding up as the years rolled by, with both the timing of X-ray flickers and improved radial velocities strengthening the original conclusions for Cyg X-1. Meanwhile, quite a few other similar systems turned up too. Faced with all this evidence, in 1990 Hawking broke into Thorne's office to leave a signed concession of the bet (and you thought scientists never had any fun).

Meanwhile, radio astronomers were on the trail of an even more astounding back hole in the far distant universe. Here's another surprise of astrophysics: when conditions are just right, sometimes clouds of hot interstellar gas can become natural microwave lasers, firing out brilliant beams of microwave emission across the universe (sounds crazy, I know—but true!). These astrophysical masers are so intensely bright that radio telescopes can track their positions with extraordinary accuracy, even in other galaxies. In 1994, astronomers Makoto Miyoshi, Jim Moran, and colleagues observed such masers deep in the core of a galaxy called NGC 4258, and discovered that they are orbiting around something with the mass of 35 million suns packed into an region no larger (and probably much less than) a quarter of a light year across. Surely a whopper of a black hole! There had already been suggestions that such "supermassive" black holes existed in many galaxy cores, based on the X-ray glow of accretion disks and vast outflows shot out from the nuclei of certain "active" galaxies, but NGC 4258 provided the first truly unambiguous evidence for supermassive black holes. By the end of the 1990s, it was clear that such supermassive black holes probably lurk at the hearts of nearly all galaxies. And here's yet another example of how rapidly the cutting edge becomes commonplace: The discovery of the black hole in NGC 4258 was front page news in 1994, yet five years later I weighed it myself during an undergraduate astronomy lab class. The secrets of the distant universe, turned into homework assignments.

So what about our own galaxy? It's actually harder to observe the center of the Milky Way than to observe other galaxies, because to do so we must look through 26,000 light years of stars, gas, dust, and miscellaneous interstellar junk. But starting about ten years ago, infrared observations have been able to pierce through all the obscuration and reveal the center of the galaxy. No X-ray glare from accretion disks or spontaneous microwave lasers the size of solar systems here, the center of our galaxy appears to be a pretty quiet place, though stars are packed together ten thousand times denser than out in the boonies where we live. In 1994 first came evidence for the faint infrared flicker of a thin stream of matter passing through an accretion disk into the black hole, found by Arizona astronomers Laird Close and Don McCarthy. Shortly thereafter, the opening of the behemoth 10-meter Keck telescope provided a clearer view, allowing the motions of individual orbiting stars to be tracked. A team of Californian astronomers led by Andrea Ghez used these motions to pin down the black hole's mass at 3 million suns. In the decade since, her team and a competing one led by Reinhardt Genzel in Germany have continued to ratchet up the precision of their studies, leading to ever better knowledge of our galaxy's central black hole, and how it affects its surroundings.

What's it all about?

I tell you all these names not to overwhelm you with history, but to make the point that science is a living process, an ongoing human endeavor. Each generation was faced with one question or another: how do stars work? What happens when they die? Will the collapse keep on going or not? Is there really a singularity in there, and what happens to things that fall in towards it? Can we actually observe this happening somewhere? The answers to these questions often seemed remarkably implausible. Any given scientist was only able to fit into place one or two of the puzzle pieces, but year by year, decade by decade the overall picture has become clearer.

Today, we are faced with questions every bit as hard, if not harder. Do black holes in the early universe serve as nuclei around which primordial galaxies accrete, or do baby galaxies produce central supermassive black holes as byproducts? What is the nature of the "dark energy" that is causing the expansion of the universe to accelerate, perhaps forever? Is there life on other worlds? Can we even take good care of life on this one? None of those answers are apparent yet, and some have wondered whether we're reaching a point where the answers are too hard to find, perhaps even an end to science. But I very strongly doubt that's the case. Einstein wrote that he never expected anyone to find an exact mathematical solution to the equations of general relativity, yet Schwarzchild, Kerr, and others have done just that time and again. And I bet if you'd asked Oppenheimer in 1939 whether it would be possible to observe one of his collapsing stars, or Hawking in 1970 whether black holes might possibly be manufactured on Earth within his lifetime, both of them would have been equally skeptical. The scientific road ahead is not always clear, and there are wrong turns aplenty along the way, yet the road winds onward nevertheless. For every critic who argues about the end of science, there's another who argues instead that we're in a rapid runaway of knowledge toward the other kind of Singularity. I suspect (and hope) the truth lies somewhere in between. Out there somewhere, there's probably a bright little kid growing up amidst all the wonders of the modern age, a kid who takes black holes for granted but is curious about dark energy, or fusion power, or antigravity hover cars or something, and who will lead the way to the next unbelievable idea that's too strange to be true—yet ends up being true anyway.

For further reading:

The Wikipedia page on black holes is very thorough, and includes a much more detailed description than I've given here of the strange physical processes occurring in and around black holes.

Kip Thorne's book Black Holes and Time Warps gives a first-hand account of the golden age of black hole research as seen by one of the key players. Wheeler, Thorne, and their colleague Misner together wrote Gravitation, a sprawling graduate text massive enough to have an appreciable gravitational field of its own. Not for the uninitiated!

For more on the observations of black holes at the center of this and other galaxies, the UCLA galactic center group has a public web page.