Caleb Scharf's Gravity's Engines is a vivid and accessible exploration of black holes, those terrible things we fear but don't really understand. Here, Scharf gives us a primer on what exactly a black hole is.

Black holes have quite a reputation. They’re where economies go to die, where confounding bureaucracies take your paperwork, and where science fiction can conveniently stuff villains or place grand finales.

The conceptual birthplace of these most extreme of astrophysical objects sounds almost as unlikely as their properties. A clergyman at a small parish in northern England two hundred and twenty-nine years ago made the astonishing prediction that objects in the universe might be so massive that light itself would become trapped by gravity. These ‘dark stars’ were the brainchild of one John Michell, now a curiously obscure figure in the history of science, despite the fact that before running a church he was a highly respected scientist at Cambridge who, among other things, helped give birth to modern seismology and counted luminaries such as Benjamin Franklin as personal acquaintances.

Time passed and Michell’s idea was largely forgotten, until scientists began grappling with the implications of Einstein’s relativity at the start of the 20th century. The first modern inklings that the universe might be able to build objects blacker than the blackest black also came from the most unlikely of places – the trenches of the Russian front in 1915. Writing to Einstein from his dreadful military post, the young scientist Karl Schwarzschild outlined a special mathematical solution to the equations of general relativity. It described the way spherical bodies distorted space and time around themselves, and suggested something extraordinary: If matter could be made dense enough, squeezed to incredible compactness, it would hide itself away from the universe. Buried inside a horrific dent in space it would not only appear to stop the passage of time at its outskirts (the event horizon), but would stretch escaping light waves out to nothingness. The degree of compactness is genuinely astonishing; the entire Earth would have to be squashed into a sphere barely half an inch across to disappear inside its own event horizon. Einstein hated the notion that anything might do this, but fifty years later astronomers began to see possible routes by which nature could build such objects, and the modern black hole was born.

Since then we’ve discovered near-irrefutable proof for the existence of black holes birthed from the used up remains of massive stars. We’ve also discovered that giant black holes, ranging from millions to tens of billions of times more massive than our Sun, exist near the centers of most galaxies. Although the horizons of these supermassive objects would still fit neatly inside our solar system, they exhibit a most extraordinary reach – not with gravity, but with energy. In fact, the way that black holes get noticed in the universe is by producing enormous amounts of energy whenever anything is unfortunate enough to fall towards them. The reason is simple in essence, but highly complex in practice. The physical rules of the cosmos dictate that matter follows the shortest path it can through space and time. But space and time are distorted by mass, stretched towards it and bunched up around it. Gravity is simply what this distortion does to the motion of objects. On Earth the distortion causes matter to accelerate towards the planet’s center, but as we all know, the ground gets in the way. For a black hole there is no ground, no material surface. All the mass, sometimes billions of star’s worth, is a very, very long way down inside the distorted space. So falling matter accelerates on, and on, and on, until it approaches the speed of light itself. In this way the smallest speck of stuff, a mere grain of dust or molecule of matter, can gain the moving energy of a nuclear weapon.

Matter falling and swirling towards a black hole collides and crashes like water noisily slurping down the drain, releasing vast amounts of energy back out to the surrounding universe. Black holes can also spin and, rather unexpectedly, possess electrical charge - properties resulting in energy being produced with even greater efficiency, sometimes being projected away in fearsome jets of subatomic particles that reach for hundreds of thousands of light years. A fast spinning giant black hole can convert matter to energy fifty times more efficiently than nuclear fusion.

For the last few decades, astronomers have mapped and charted these beacons of destruction. So-called quasars, blazars, and active galaxies are all different views of the same basic phenomenon. They have also seemed rather aloof, spectacular but disconnected from the rest of things. However this perception has changed in the past twenty years. It now appears that the energy produced by matter falling into black holes has played a pivotal role in regulating and tempering the production of stars and the formation of galaxies across cosmic time. Remarkable new astronomical discoveries have found some places where energy from giant black holes inflates vast bubbles of hot intergalactic matter, and generates booming waves through tenuous cosmic gas with a power of a trillion, trillion, trillion watts. Such noise slows the condensation of matter and inhibits the formation of new stars, and in the very young universe of more than ten billion years ago it may even prevent some teenage galaxies from ever growing to their full potential.

We have also discovered that our own Milky Way contains a black hole four million times the mass of the Sun at its center. We can see it swinging the orbits of stars around itself, and we have begun to realize that it too has episodes where it gorges on matter and expels energy. This behavior may have influenced the specific mix of stars and elements around us, an unsettling possibility since it could mean that our own existence is connected via a complex thread to the physics of black holes. We will learn much more in the years to come.

How do black holes work? It turns out that they work very hard indeed.