“Black holes” conjure up curiosity and confusion in equal amounts. The concept sprang out of the mathematics of Einstein’s general relativity theory but has only recently attracted huge popular attention. Often black holes are portrayed as all-powerful destroyers that capture and crush everything around them. Thankfully for the Universe at large, this is not quite true.
Black holes could be said to be the weirdest things known to exist. The theoretical possibility of their existence has been around for almost a century, and we now have 30 years of strong observational evidence, yet astronomers still do not fully understand them.
A black hole containing four or five times the mass of the Sun would occupy a spherical volume just a few kilometers across. This would curve the fabric of space so sharply (see Was Einstein Right?) that the strength of gravity would change greatly even across the length of a human body. If you were in the vicinity, your feet would accelerate much faster than your head, stretching you on the gravitational equivalent of a medieval rack. The phenomenon is an extreme version of a tidal force (see Why Do the Planets Stay in Orbit?) and would eventually pull you apart in a process known, with black humor, as “spaghettification.” As your constituent atoms passed through the “event horizon,” the invisible boundary after which there is no escape, you would become part of the riddle that so far defies solution: what is inside a black hole?
The first inklings of the possibility of black holes—though they were not named as such—came as far back as the 18th century when geologist John Michell wrote to the Royal Society in London suggesting that a star 500 times larger than the Sun would generate so much gravity that not even light could escape from its clutches. Michell was inspired by the natural philosophers of his time, who were succeeding in measuring the finite, constant speed of light.
SPAGHETTIFICATION: ANYTHING FALLING INTO A BLACK HOLE WILL BE PULLED APART
The third Astronomer Royal, James Bradley, was the first to calculate the currently accepted value for the speed of light. Working at Greenwich, London, in 1728 he detected a strange movement in the position of the stars, amounting to just 1/200 of a degree. At first he thought that he had detected stellar parallax (see How Big is the Universe?) but soon realized that all the stars he measured had the same angular displacement, and so the effect could not be parallax, which would have varied with the star’s distance from Earth. He proposed that the movement was caused by the finite speed of light. In the same way that you have to slightly tilt an umbrella in front of you when walking through a shower because your motion makes the raindrops appear to be approaching at a slightly diagonal angle, so he was having to angle his telescope to compensate for the Earth’s motion through space. The angle of the tilt allowed Bradley to calculate the speed of light in relation to the speed of the Earth. He computed a figure of 186,000 miles per second (or around 300,000 kilometers per second).
Michell took Bradley’s figure and used Newtonian gravity to estimate the size of body needed to have an escape velocity (see Why Do the Planets Stay in Orbit?) equal to the speed of light, and came to his estimate of 500 times the mass of the Sun. The idea sparked a debate that rumbled for a number of years as astronomers mulled the possibility of such “dark stars.” Eventually the natural philosophers decided that Newton’s laws precluded light from being affected by a gravitational field and so light would always leave a celestial object, no matter how strong its gravity.
The matter rested for a couple of centuries until Einstein published his General Theory of Relativity in 1915 and showed that gravity did indeed affect light (see Was Einstein Right?). Less than two months after Einstein’s publication, German mathematician Karl Schwarzschild found that Einstein’s equations allowed celestial objects to become so dense that they create gravitational traps. The size of each trap, known as its “Schwarzschild radius,” is determined by the mass inside. For example, a black hole containing the mass of the Earth would have a Schwarzschild radius the size of a small coin, whereas a black hole containing billions of times more mass than the Sun would be as large as our Solar System. Once any object, or even light, had passed beyond this Schwarzschild radius—or “event horizon”—it could never escape.
Astronomers were forced to accept that black holes could exist, but the dilemma was how they could possibly observe something that emitted no light or any other type of radiation. The solution did not come until the early 1970s, when the first X-ray telescopes were lofted into space and revealed an extraordinarily bright X-ray source in the constellation of Cygnus, some 8000 light years away. After much analysis, it was decided that the source of X-rays was a superheated cloud of gas spiraling into a black hole, dubbed Cygnus X-1. As the gas accelerated in the immensely strong gravitational field outside the event horizon, it was heated to millions of degrees and began to emit X-rays.
Since then numerous observations have been made of a black hole “feeding,” for example by ripping apart a companion star. The blue supergiant star HDE 226868 is about 30 times more massive than the Sun and 400,000 times brighter. The black hole next to it contains only between 5 to 10 times the mass of the Sun but has a gravitational field so strong that it has pulled the blue supergiant into an egg shape and is now greedily stripping it of gas. The gas falls from the giant star and enters a brief spiral orbit around the black hole, forming what astronomers call an accretion disk. In the maelstrom, magnetic fields compress some of the gas into jets that seem to shoot away from oblivion. Most of the gas, however, ends up heading into the black hole like water spiraling down a plug hole and then disappears forever.
Black holes such as Cygnus X-1 are known as “stellar” black holes. They contain several times the mass of the Sun and are formed when very massive stars explode as supernovae (see What Are Stars Made From?). The supernova is triggered when the inert iron core of the star collapses to become a neutron star. As the outer layers of the star come crashing down, igniting the supernova explosion, the neutron star at the core is pummeled and some of the outer material is absorbed, increasing the core’s mass, which can increase the gravitational field so much that the core becomes a black hole.
While such stellar black holes are the most prevalent kind, there are now known to be other, larger, black holes with different formation mechanisms. The next size up is termed an “intermediate mass” black hole. In common with the stellar black holes these orbit the center of their galaxy and contain a few hundred or a few thousand solar masses. Astronomers are not sure how these form; possibly they result from several stellar-sized black holes merging together.
ANATOMY OF A BLACK HOLE: A BLACK HOLE IS NOT A SIMPLE OBJECT BUT IS MADE UP OF SEVERAL COMPONENTS
Thirdly, there are the “supermassive” black holes, containing anything from millions to billions of times the mass of the Sun. A supermassive black hole is thought to sit at the center of every galaxy, but taking up no more volume than an average solar system. In 90 percent of galaxies the central supermassive black hole is inactive but, in the other ten percent, it is constantly feeding from surrounding celestial objects and this drives an extraordinary engine of activity that can be seen across billions of light years of space.
Vast quantities of radiation are released by an active galaxy, all derived from matter heating up before it plunges into the supermassive black hole at the galaxy’s center. The most powerful active galaxies generate more energy per second than a trillion Suns, with the result that the active nucleus outshines the rest of the galaxy by a hundred times or more. This brilliance masked the nature of an active galaxy for some time; when astronomers caught their first glimpses of them during the 1950s, they saw the star-like active cores and assumed that they were peculiar stars in our own Galaxy. They called them “quasi-stellar” objects, from which the present name quasar is derived.
“[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.”
JOHN WHEELER 20TH CENTURY PHYSICIST
The true identity of quasars was revealed in 1962, when astronomers discovered that they are in fact incredibly distant, so could not be stars but had to be intensely powerful galaxies. As the distribution of these highly active galaxies was charted, it was revealed that all quasars are in the far reaches of the Universe and none exist nearby. Because light takes so long to travel across space this means that quasars are ancient objects—they seem to have populated the Universe in their greatest abundance around 10 billion years ago. This leads astronomers to conclude that quasars are a phase that every galaxy passes through, brought about when the supermassive black hole at its center has a lot of matter to consume.
Less powerful active galaxies can be found throughout the Universe at all distances. Some may be aging quasars whose food source is almost used up. When the supermassive black hole finally devours everything within its reach, the active galaxy quiets to become a normal galaxy, such as our own. But there is nothing to stop the black hole coming back to life if more matter falls into its clutches. According to calculations, one medium-sized star like the Sun wandering too close to the galactic center is all that would be needed to reignite the activity and keep the black hole spewing energy for a year. So, the current population of active galaxies must be transient. If we came back in a million years’ time, some presently active galaxies would have become inactive, while other currently quiet ones would be blazing with energy.
Seeing a black hole that is not feeding was once thought to be impossible but that view is now changing, thanks to technological advances in radio telescopes. Within a decade astronomers anticipate being able to look for a black hole’s “silhouette” against the background of bright stars. This is far from easy. Our own Galaxy’s central supermassive black hole, known as “Sagittarius A*” (pronounced A-star), is estimated to be about 4.5 million solar masses, all squeezed into an event horizon some 27 million kilometers (17 million miles) in diameter; this is only about half the distance of Mercury from the Sun. From our vantage point on Earth, the silhouette of Sagittarius A* would appear no larger than a soccer ball on the surface of the Moon.
By combining the simultaneous observations of radio telescopes across the world, however, astronomers hope to be able to distinguish the silhouette of the black hole, and even to catch sight of a few clouds of gas straying into its gravitational maw. By watching the paths that these clouds take to their destruction, it will be possible to measure how fast Sagittarius A* is spinning. If, as is theorized, a black hole is spinning, general relativity tells us that it will create a vortex in the fabric of space—imagine the twisted mass that can be created by spinning a spoon in a jar of honey. This twisted region around a black hole is called the “ergosphere.” Observations of gas clouds caught in the ergosphere of Sagittarius A* would open up a whole new way to investigate not only the black hole, but also the validity of general relativity in such an extreme environment.
There may be a fourth type of black hole, at the opposite end of the scale. These are tiny “primordial” black holes, thought to have been created during the Big Bang when the space–time continuum was so crushed that minuscule regions could seal themselves off from the rest of the cosmos. Such a primordial black hole has never been detected although, in the 1970s, the celebrated physicist Stephen Hawking suggested a way in which we might see them, and at the same time prove that black holes are not completely black. He did so using another cornerstone of modern physics: quantum theory.
First propounded in the early decades of the 20th century, quantum theory describes the Universe on its smallest scales. Central to its explanation is that energy comes in discrete packets, so what may look like a beam of light is actually made up of a multitude of tiny particles called photons. Each one of these photons carries a well-defined amount of energy; for example, a photon of blue light carries twice the energy of a photon of red light. Quantum theory also tells us how particles behave, and one of its tenets is that a particle is difficult to pin down to a specific place. Physicists can calculate where they expect a particle to be, but it could be just as easily somewhere else in a small region around this calculated position. This means that particles traveling close to a boundary can sometimes appear to have spontaneously jumped across it, a phenomenon known as “tunneling.” It is manifest in actual observations: it makes fusion in stars possible at temperatures lower than would normally be needed, because the closely packed atomic nuclei occasionally find themselves close enough to their neighbor to fuse together and release energy.
According to Hawking, a particle can tunnel from the interior of a rotating black hole and escape, thus lowering the black hole’s mass. As the black hole loses mass, so the process continues and speeds up, until the black hole disappears in a sudden release of gamma rays. The most likely black holes that “evaporate” in this way are the primordial ones, because they will be evaporating faster than they are capable of consuming. Yet, although gamma ray satellites have been on the lookout for this behavior, nothing has yet been seen. It is possible that the Large Hadron Collider in Switzerland will create minute black holes in particle collision processes, which may evaporate in a fraction of a second and give scientists their first glimpse of this process.
Although black holes are now an accepted part of the pantheon of celestial objects, there is still unease about them in astronomical circles. One of the uncanny things is that mathematically they share a striking similarity to the Big Bang. At first, there may not seem to be anything in common between a black hole, which sucks things out of existence, and the Big Bang, which created the Universe and set it expanding. However, to a mathematician they share an identical feature: a point of infinite density and zero volume, known as a “singularity.” Inside a black hole, the singularity is presumed to be the last resting place of matter because gravity crushes that matter into smaller and smaller volumes. This presents a problem for physicists because as the volume approaches zero no theory can be used to study the resulting singularity.
There is also something known as the black hole “information loss” problem. To the outside Universe, only three properties of the black hole are visible: its mass, its electric charge and its angular momentum (rotation). Any other information, such as what fell in, appears to be lost. This goes against the grain of one of the deepest principles of physics: that of reversibility. If you drop something into a bowl of water and let it dissolve, in principle someone could analyze the water, and identify what has been dropped in there. They could even separate the substances again by boiling off the water and thus reconstruct the original material. This is reversibility. In the case of a black hole, once something crosses the event horizon we cannot then discover what it was, let alone recover it. All the stars and planets that have been devoured have been erased from the Universe: their composition, their temperature, their density—all are gone. Not even particles tunneling out, according to Hawking, can provide us with the missing information.
Physicists hope that their efforts to unify gravity with the other forces of nature using string theory (see Was Einstein Right?) will allow them to investigate this conundrum. Indeed, string theory may even have given them their first clue.
String theory suggests that black holes do not have singularities but that their volume from the center out to the event horizon is a highly compressed ball of subatomic “strings,” the fundamental building blocks of nature that according to the theory give us particles of matter. These compressed strings would store the fundamental information about the objects that have fallen into the black hole, so no information has actually been lost.
In this view, matter does not pass through the event horizon on its way to the singularity; instead it compresses itself onto the surface of the “fuzz ball” and merges with the other strings. Think of it as layers of paint, but instead of each successive layer overwriting the last one, they run together, and the black hole’s Schwarzschild radius grows a little larger to make room for the latest arrivals. In relativity this is explained as the curvature of space becoming a little steeper, because the black hole has swallowed more mass. In string theory, the black hole simply expands a little to accommodate the new information.
Black holes are the most extreme celestial objects that we know and, as such, continue to challenge our understanding of the most fundamental laws of physics. Even if a black hole is not truly black, and not a hole but a fuzzy ball of quantum strings, everyone agrees about one thing: you definitely would not want to fall into one.