Sep 28 2012 3:00pm
Take a look at Gravity's Engines by Caleb Scharf, out now from FSG:
We’ve long understood black holes to be the points at which the universe as we know it comes to an end. Often billions of times more massive than the Sun, they lurk in the inner sanctum of almost every galaxy of stars in the universe. They’re mysterious chasms so destructive and unforgiving that not even light can escape their deadly wrath.
Recent research, however, has led to a cascade of new discoveries that have revealed an entirely different side to black holes. As the astrophysicist Caleb Scharf reveals in Gravity’s Engines, these chasms in space-time don’t just vacuum up everything that comes near them; they also spit out huge beams and clouds of matter. Black holes blow bubbles.
With clarity and keen intellect, Scharf masterfully explains how these bubbles profoundly rearrange the cosmos around them. Engaging with our deepest questions about the universe, he takes us on an intimate journey through the endlessly colorful place we call our galaxy and reminds us that the Milky Way sits in a special place in the cosmic zoo—a “sweet spot” of properties. Is it coincidental that we find ourselves here at this place and time? Could there be a deeper connection between the nature of black holes and their role in the universe and the phenomenon of life? We are, after all, made of the stuff of stars.
THE FEEDING HABITS OF NONILLION-POUND GORILLAS
Once upon a time there lived a great monster. It made its home deep inside a castle that was deep inside a huge forest. No one had ever seen the monster, but over the centuries and millennia there had been clear signs of it stirring. Legend told that it trapped all things that came near. In its lair even time itself became sticky and slow, and its hot blue breath would burn through the strongest shield. Few dared to venture into its realm. Those who did either returned emptyhanded with wide-eyed tales too strange to believe, or never came back at all. If you stood on the highest mountains in the land you could peer across the treetops and just see the haziest of outlines of the monster’s castle. Sometimes you might see a few strange clouds hovering over it, as if they were caught in a great swirl of atmosphere, and at night there might be an eerie glow reflected off the cool air. For years you’ve wondered about this enigmatic place and the monster within. Finally you decide that there is nothing else to be done but to go on your own quest, your own search for a glimpse of the beast. In this particular tale your starting point, and home, is our solar system, and the monster’s castle is deep in the galactic heart.
At first the going is easy on your journey. The stars are familiar and friendly. Out here in the Orion spur of the great spiral disk of the Milky Way, stellar systems are spaced with an average of about five to ten light-years between them. Finding a comfortable path through is not difficult. Even the rivers of dusty darkness between the galactic arms are easy to cross, and traveling the first twenty thousand or so light-years is a breeze. After a while, though, things begin to change. This is the beginning of the galactic axial hub. Like the distorted yolk of a huge fried egg, the central region of the galaxy inside about four thousand light-years is a gently bulbous but elongated structure. It contains a far higher density of old red and yellow stars than out in our suburbs. The woodlands begin to thicken up in here as we ease our way toward the inner sanctum. More and more stars begin to block the way, and we are constantly shifting our path in order to slide through.
Pressing on, we finally enter the true galactic core. Some six hundred light-years across, this interior forest is densely packed with stars buzzing around in their orbits. Compared to home, the skies are coated with star after star after star. At the edge of this core, where we first enter, stars are packed together a hundred times more densely than around our solar neighborhood. At the very middle, there are hundreds of thousands more than we are used to. The going is extremely tough and slow, and it gets worse and worse as we descend inwards. This is the oldest undergrowth, part of the ancient barrier to the center. Something else exists in here, too. A rather piecemeal and shabby disk of material encircles the entire core, made of hydrogen gas clouds. It blocks the view from some directions, and as we move farther down, another structure now begins to reveal itself. There is a flattened ring of gas rotating about the very center of the galaxy. It’s composed of atoms and molecules, and it is unlike anything else in the Milky Way. It is a rich and substantial formation, a hundred times denser than a typical nebula. Its outer edge is still some twenty light-years out from the galactic center, but its inner lip descends to within only about six light-years. Tilted at a rakish angle to the plane of the entire galaxy, it spins at about sixty miles a second. Most of it is hydrogen gas, but nestling in among this pure stuff are other compounds: oxygen and hydrogen in simple combination, molecules of carbon monoxide, and even cyanide. Every hundred thousand years or so, the inner part of this molecular ring makes one complete circuit around the center of the galaxy. This impressive structure at first looks serene, but closer inspection reveals the scars of terrible violence. Some great cataclysm has recently blasted the ring, pushing some of the gas into clumps and lumps and scorching other parts. It is a strange and ominous gateway.
Moving cautiously inside the ring, we take stock of what is happening around us. We are within an incredibly dense and constantly moving swarm of stars. It seems like chaos, yet through this noisy buzz we can see something distinctly peculiar happening up ahead. We pause in flight to watch as several of these innermost stars move along their orbits. Remarkably, these orbits are not only around something unseen ahead of us at the center, but they are extraordinarily fast as the stars swing by that invisible focal point. One star whizzes through its closest approach at velocities approaching 7,500 miles a second. That’s astonishing, considering that our homeworld, Earth, orbits the Sun at less than twenty miles a second, and even the planet M ercury moves at barely thirty miles a second. For the star to achieve an orbital velocity of that magnitude, it must be moving around a huge mass. We perform the calculation. Deep within a tiny volume at the galactic center is an unseen something that is 4 million times more massive than the Sun. There is nothing else this dark body can be except a colossal black hole.
How we have come to build this detailed picture of the environment at the center of our galaxy is a tale of technological prowess and skilled insight. One of the greatest achievements of astronomy in the late twentieth century and early twenty-first century has been the discovery that our own galaxy, the Milky Way, harbors a supermassive black hole at its center. It provides a vital context for the rest of our story, and a key reference point. But there are still limits to how much detail we can see when we peer this deep into the inner galactic sanctum. At present we have to rely on a number of indirect astronomical phenomena to tell us more. For example, tenuous hot gas is being measurably expelled from this tiny region. X-ray photons are also streaming out, and roughly once a day they flare up and brighten by a hundredfold. It’s tempting to imagine that somewhere inside this central core are moths flying too close to an open flame, and sometimes we see their unfortunate demise. Altogether these characteristics represent clear signs that matter is sporadically entering the maw of a brooding monster.
Figure 9. The innermost region of our own galaxy mapped at microwave frequencies. This image, spanning approximately twelve lightyears, reveals an extraordinary structure of irradiated gas centered on a bright object that astronomers associate with the central massive black hole. As the image suggests, this gaseous structure is in motion around and toward a central point.
We see another signature in the great loops of magnetized gas that surround this whole region, aglow in radio waves that flood out into the galaxy. They are part of the very same extraterrestrial radio signal that Karl Jansky first saw in the 1930s with his simple radio telescope in a field in New Jersey. Yet despite all this activity, the black hole at the center of the Milky Way is operating on a slow simmer compared to the brilliant distant quasars that can shine as brightly as a hundred galaxies. It’s a brooding, hulking beast, not a blazing pyre. But to really place it in context, we should size things up and compare this local environment to the rest of the cosmos.
To do that, let’s return briefly to our map of forever, still contained in the sack that was delivered to the doorstep two chapters ago. In our neighborhood of the universe, encompassing a mere 6 billion years or so of light travel time, the intensely bright quasars occur in only about one out of every hundred thousand galaxies. In other words, they are extremely rare creatures. For that reason, we should not be too surprised that the Milky Way isn’t one of the galaxies that contain a quasar. Those other galaxies with great radio lobes and ray-like jets extending outward are even more rare;the most prominent examples are over 10 million light-years from us. But at greater distances, further back in cosmic time, the situation is very different. In fact, between 2 billion and 4 billion years after the Big Bang, fiercely energetic quasars were a thousand times more common. We think that roughly one in a hundred galaxies held a quasar in its core at any moment. This was a golden age for these objects, powered by the voracious appetites of supermassive black holes.
No single quasar lasts for very long, however. W ith monumental effort, astronomers over the past several decades have surveyed and studied these enigmatic objects, and piece by piece they’ve reconstructed their history. Like paleontologists building the skeletons of long-gone creatures and covering them with reconstructed flesh, so too have astronomers rebuilt the lifestyle of the supermassive black holes that drive quasars. W e find that a typical quasar will only light up for periods that last between 10 million and 100 million years, a tiny fraction of cosmic history. Because of this, we know that more than 10 percent of all galaxies in the universe have actually hosted a brilliant quasar during their lifetimes. It just means that wherever or whenever we look, we never get to see them all switched on at once.
But why do quasars die out with cosmic time? It is a question that remains unresolved. Even this basic description of the cosmic distribution of quasars is the result of decades of intense research. (The history of that effort is a fascinating one, but a story for another day.) We can, however, make some reasonable speculations about the life cycles of quasars. First, they are powered by supermassive black holes that, as they devour matter, produce an output of energy far greater than in other environments. The electromagnetic shrieks of material falling into a black hole are what we see during this process. This suggests that the enormous energy of quasars is deeply connected to the availability of consumable matter and the rate at which it is being consumed. The more matter falls in, the bigger the hole can become, and the bigger the hole, the more energy it can extract from that matter. Eventually, though, this material seems to run out. Q uasars live fast and big and die after a blaze of glory that must depend acutely on the detailed nature of matter consumption by supermassive black holes.
The most distant quasars we know of (going back to within a billion years of the Big Bang) are typically also the most luminous. In other words, as the cosmic clock ticks, and new quasars come and go, they gradually become dimmer. The astronomical jargon used for this is “downsizing.” (Who says scientists don’t have a sense of humor?)All quasars, however, from the brightest to the faintest, are powered by the most massive of the supermassive black holes. They are the elite—the big guys. They also occur in the bigger galaxies in the universe. This is an important connection to make, because it begins to tie the evolution of supermassive black holes to the evolution of their host galaxies, their great domains.
Indeed, astronomers have found something else peculiar and critically important going on in galaxies. The mass of their huge black holes is generally fixed at one-thousandth of the mass of the central “bulge” of stars surrounding the galactic cores. These are typically the old stars that form a great buzzing cloud around galactic centers. Sometimes that central cloud can even dominate the whole galaxy. Careful astronomical measurements have revealed that a galaxy with a big bulge of central stars will also have a big central supermassive black hole, and a galaxy with a small bulge will have a smaller black hole—according to the 1,000:1 mass ratio. But while this relationship is strikingly clear in many galaxies, it is not entirely universal. For example, the Milky Way is pretty much “bulgeless.” Its central stars are in more of an elongated block or bar, not a swarm thousands of light-years across. And, as we’ve seen, our own supermassive black hole is a comparatively petite monster of 4 million times the mass of the Sun. By contrast, the nearby spiral galaxy of Andromeda has a great big bulge of central stars and contains a supermassive black hole that we think is 100 million times the mass of the Sun, neatly fitting the expected size. Why there should be this relationship between central stars and black holes is a mystery at the forefront of current investigations. We will find it to be of the utmost importance as we dig deeper into the relationship between black holes and the universe around them. But the next step in following this story is to get our hands dirty again with the business of feeding black holes.
We can make a number of broad arguments to describe how energy is produced from the distorted spacetime surrounding dense concentrations of mass in the cosmos. I made some of those in the previous chapter, and emphasized the power involved. The idea certainly sounds feasible:there’s plentiful energy to spare, but specific physical mechanisms are needed to convert the energy of moving matter into forms we can detect. Otherwise, it’s like stating that burning gasoline releases a lot of energy and therefore an engine could be driven by gasoline. That might be true, but it doesn’t demonstrate how an internal combustion engine works. In our case, the processes of energy generation and conversion are particularly complicated because of the exotic nature of black holes. Unlike an object such as a white dwarf or a neutron star, a black hole has no true surface. Matter that gets close to the event horizon will essentially vanish from sight for an external observer. There is no final impact onto a solid body, no final release of energy from that collision. So whatever is going on just outside the event horizon is absolutely critical to understand.
The early work on black hole energy generation by Z el’dovich and Salpeter in the 1960s, as well as that of Lynden-Bell, led to a number of theories about the mechanisms that could be at play. These involved a phenomenon known as accretion—the feeding of matter onto and into a body. But observation of the universe suggests that other things are going on as well. Something is responsible for producing the enormous energy-filled structures emitting radio waves from within galaxies, as well as the strange rayor jetlike features emanating from galactic cores. In this case, the bizarre spinning ring of material that we find surrounding our own galactic center actually offers a general clue to one piece of the puzzle. In order to see why, it’s time for us to properly consider the outrageous eating habits of black holes.
Although matter can fall straight down onto objects like planets, stars, white dwarfs, neutron stars, or black holes, in general it doesn’t. W hat it does tend to do is enter into orbits. One way to think about this is to imagine a swarm of nearsighted bees flying across a field in search of a good nectar-rich flower. One such happens to be in the middle of their path, its bright petals giving a beefriendly come-hither. A couple of lucky bees are lined up just right, and as the flower looms into their blurry vision, they simply land on it with a splat. The other bees, off to the sides, only barely notice something and have to swing their flight paths around to circle before coming in to land. In a sense, matter moving through curved space does the same kind of thing. If it’s not perfectly on track to the very absolute center of mass of a large object, the most bunchedup point of spacetime, it will tend to loop around and orbit. As we’ve seen, all matter tries to follow the shortest path through spacetime, but if that underlying fabric is warped then so too will be the path. If the components of that incoming matter can also bump and jostle each other, they can further rearrange themselves. Atoms and molecules, even dust and bigger chunks of material, will settle into orbiting a massive body in a flattened, disk-shaped structure. We see this occurring everywhere in the cosmos. The arrangement of planets in our own solar system is an excellent example of this phenomenon. The flatness of their orbits reflects the disk of gas and muck that they formed out of some 4.6 billion years ago. The rings we see around Saturn are another example. Time and again, matter captured by the influence of a dense and massive body ends up swirling into an orbiting disk. It certainly seems that the same thing must happen around a black hole.
But if a black hole just swallows matter up, light and all, then how does it produce energy? The trick is that when matter forms a disk around the hole, the material in the disk rubs against itself as it swirls around. It’s like spinning a stick against another piece of wood to start a fire. The pieces of wood are never perfectly smooth, and so friction between them results in the energy of the spinning motion being converted into thermal energy, and the wood gets hot. In an orbiting disk, the outer parts move much more slowly than the inner parts. This means that as the disk goes around and around and around, friction between the bands of moving material transfers the energy of motion into heating the matter. This has one very direct consequence:when you hold a hand on a spinning bicycle tire, the friction causes the tire to slow down and your hand to heat up. The same thing happens in the matter disk. The heated material loses orbital energy and spirals inward. Eventually, it gets to the event horizon and is accreted into the black hole, and it vanishes, sight unseen. But on the way toward that point, friction converts some of the tremendous energy of motion into photons and particles.
Figure 10. An artistic impression of a disk of material orbiting a black hole and glowing with light. In the background is a vista of stars and galaxies. To simplify things, the disk of matter is shown in a very pure state:no dust or other debris, just thin gas. It becomes denser and hotter as it swirls inward, heated by friction. At the very center is the dark event horizon, and the light in its near vicinity is bent by passing through this extremely distorted spacetime to form what looks like an eye. In fact, we’re seeing the light of the disk that would otherwise be hidden from us on the far side of the hole, curved around as if by a giant lens.
Exactly what causes this friction is still a significant mystery. The force of atoms bumping randomly into one another simply doesn’t suffice to explain what we observe happening out in the universe. Ripples and whirls of turbulence in gas may help roughen the frictional forces within the inner speedy parts of a disk, but they too are not quite enough. It may be that magnetic fields produced from the electrical charges and currents of material in the disk act like a great source of stickiness to produce the necessary friction.
Whatever the precise cause, there is absolutely no doubt about what happens when matter is ensnared this way. As it spirals inward through the disk, the friction generates huge amounts of thermal energy. Toward the inner regions, an accretion disk around a supermassive black hole can reach fearsome temperatures of hundreds of thousands of degrees. Powered by the huge reservoir of gravitational energy from the curved spacetime around a supermassive black hole, the matter in a single disk can pump out enough radiation to outshine a hundred normal galaxies. It’s the ultimate case of friction burn. As Lynden-Bell originally saw in 1969, this is an excellent match to the energy output astrophysicists have seen in the brilliant quasars and inferred from the great structures of radio emission from many galaxies. This mechanism is also tremendously efficient. You might think that such a prodigious output would require a whole galaxy’s worth of matter, but it doesn’t. An accretion disk around a big black hole needs to process the equivalent of only a few times the mass of the Sun a year to keep up this kind of output. Of course, this adds up over cosmic time spans, but it’s still a remarkably leanburning machine. And there’s even more going on, because spacetime around a black hole is not of the common garden variety.
We’ve touched on the effect a spinning mass has on its surroundings, the tendency to drag spacetime around like a twister. This phenomenon was one piece of the mathematical solution that Roy Kerr found to Einstein’s field equation for a spinning spherical object. It’s actually a more general description of mass affecting spacetime
Figure 11. A Hubble Space Telescope image of the very center of an elliptical galaxy known as N G C 4261 that is 100 million light-years from us, still within our general cosmic “neighborhood.” At the pixelated limits of even the Hubble instruments, this image shows a darker disk of thick gas and dust lying within the light of stars at this galaxy’s core. The disk is tilted by about 30 degrees toward us and is some three hundred light-years across. It surrounds a supermassive black hole 400 million times the mass of our Sun (100 times the mass of the black hole at the center of the Milky Way). This material is slowly feeding into the bright disk of accretion-heated, rapidly orbiting matter seen as a point in the very center. That innermost disk—leading directly to the event horizon—may be only a few light-months across. Radio telescopes also detect huge jets emerging from the top and bottom of this system and reaching out for more than thirty thousand light-years on each side.
that also encompasses Karl Schwarzschild’s original solution for a motionless object. Any spinning mass will tug at spacetime. Even the Earth does this, but to an extent that is extremely difficult to detect. However, things get pretty interesting when it comes to a black hole and the enormous stress it places on spacetime around its compact mass. In particular, because of light’s finite speed, there is a distance away from a rapidly spinning black hole at which photons traveling counter to the twister-like spacetime could actually appear to stand still. This critical point is farther out than the distance we call the event horizon, from which no particles of light or matter can escape.
With all this in mind, a spinning black hole actually has two locations, or mathematical boundaries, around it that are important to know about. The outermost is this “static” surface where light can be held in apparent suspension, motionless. It’s the last hope for anything to resist being swept around and around by the spacetime twister. Then the surface inward from that is our more familiar event horizon. Between these two surfaces is a maelstrom of rotating spacetime. It is still possible to escape from this zone, but you cannot avoid being moved around the black hole, since spacetime itself is being pulled around like a thick carpet beneath your feet. This rather spooky region is known as the ergosphere from the Latin ergon, which means “work” or “energy.” Furthermore, neither the outer surface of this ergosphere nor the inner event horizon is spherical. Just like those of a balloon full of liquid, the horizons and surfaces around a spinning black hole bulge out toward their equators, forming what is known as an oblate spheroid.
Spinning black holes open up a bag of mathematical wonders. M ost of these don’t concern us for the purposes of our quest to understand the far-reaching effects of matter consumption, but they’re fascinating and lead to some of the most outrageous concepts in physics. For example, the true inner singularity in a spinning black hole—that central point of infinite density—is not point-like at all, but rather smears into the shape of a ring. Not all routes inward arrive directly at this singularity, and objects may miss this bizarre structure altogether. Wormholes through to other universes and time travel are tantalizing possibilities in some cases, although the very presence of foreign matter or energy seems to thwart these hypothetical phenomena. It is intoxicating and magical stuff, but the most important piece that’s relevant to our present story is that there is in fact a maximum rate at which a black hole can spin.
In that sense, black holes are remarkably similar to everything else in the universe. At a high enough rate of spin, the event horizon would be torn apart, and the true singularity would be exposed and naked. That’s not a good thing for our theories of physics. Singularities are best kept hidden behind event horizons. If they weren’t, then, in technical terms, all hell would break loose. Luckily, nature seems to prevent black holes from ever getting past this point, although, as we’ll see, they get awfully close. In the 1980s the physicist Werner Israel demonstrated that the universe must conspire to stop a black hole from ever gaining maximum spin. Once a black hole has reached close to the highest rate of rotation, it becomes effectively impossible for incoming material to speed it up any more. Matter quite literally cannot get close enough through the centrifugal effect of the spinning ergosphere. This means that any further interaction with the external universe will typically act to slow down, not speed up, a maximally spinning black hole. In this way it is kept from tearing apart. Perhaps not surprisingly, this limit to spin occurs when the rotational velocity close to the event horizon approaches the velocity of light.
This brings us back to the English physicist and mathematician Roger Penrose’s marvelous insight in 1969 that the rotational energy of a black hole can be tapped into via the surrounding spacetime twister. This mechanism is important because the accretion disk of material surrounding an eating black hole continues all the way into the ergosphere. It’s perfectly fine for it to do so—it’s still outside the event horizon. Within this zone, the relentlessly dragging spacetime will force the disk to align itself with the equatorial plane of the spinning hole. The same kind of frictional forces that allow the matter to shed energy will still be at play, and that energy can still escape the ergosphere. So matter in the disk continues to accrete through the ergosphere and inward to the event horizon. As the spinning black hole grows from eating this matter, it will also gain the spin, or angular momentum, of that material. Keeping all this in mind, we’d expect the most massive black holes in the universe to also be rotating the fastest, all the way up to the limit of maximal spin. This could be a terribly important factor in the next phenomenon we need to think about, which is all about siphoning off that spin.
Jets of matter are a phenomenon we find in many situations here on Earth as well as out in the cosmos. We can start off by thinking about the jet of water that comes out of a hose. Water under pressure is confined in a tube, and when it emerges it has a tendency to just keep going in the same direction. The same principle holds elsewhere. For example, on a relatively small cosmic scale, as young stars gather up matter and become more and more compact, they too can propel flows or jets of material. These are impressive-looking structures when seen through a telescope. Particles of matter are accelerated out in northern and southern beams at velocities of about 60 miles a second. Eventually, they crash into tenuous interstellar gas and dust many light-years away, producing bright splashes of radiation. Supermassive black holes can produce jets of matter as well, but their nature is quite literally of a different order. Particles in this case travel outward at close to the speed of light— what is called an ultra-relativistic state. These are the extraordinarily fine and narrow lines or rays emanating from some galactic cores. They are also often associated with the rare, but impressive, radio-emitting dumbbell structures around galaxies that we encountered previously. Visually, we’re tempted to think that the jets are somehow creating the dumbbells, but to be sure we have to better understand their origin and nature.
Just how jets of incredibly accelerated matter are formed is one of the most enduring problems of modern astrophysics—not, however, for want of ideas. Scientists have put forth a wide variety of possible mechanisms as contenders, many of which are at least superficially plausible matches to what we see in the universe. But the devil is in the details. Two basic things have to happen for nature to make a jet of matter. The first is that a physical process has to generate rapidly moving material. In the case of jets from black holes, these particles are streaking away at very close to the speed of light and seem to emanate from the poles of a spinning and spheroidal horizon. The second requirement is for this stream of ultra-highvelocity matter to be funneled into an incredibly narrow beam that can squirt out for tens of thousands of light-years. It’s like a magical hose that forces all the water molecules to shoot out in nearperfect alignment so that you can accurately drench your neighbor at the far end of the street, if so inclined.
Funnily enough, there appear to be a variety of ways for nature to perform an extraordinary trick like this, and a big part of the challenge has been to figure out which mechanism is at play. For the extreme environments around a black hole, the answer seems to involve magnetism. When James Clerk Maxwell formulated his laws of electromagnetism back in the mid-1800s, he crystallized a description of how moving electrical charges, or currents, produce magnetic fields. These same rules apply to an accretion disk, the whirling hot plate of sauce around a black hole. A structure like this will be full of electrically charged matter. It’s easy to imagine why it has to be. The temperature of its inner regions is so high that atoms are stripped of their electrons. Positively and negatively charged particles are racing around in orbit about the hole, and as a result, great currents of electricity are flowing. It seems inevitable that powerful magnetic fields will be produced, and as is their nature, they will extend away from or into the structures surrounding the black hole. As the material in the disk spins around and around it will pull these magnetic fields with it, but it will pull them most efficiently close to the disk itself, and less so above or below. It’s not unlike taking a fork to a plate of spaghetti. The strands of pasta are the lines of magnetic field or force. The tip of your fork is like the sticky swirling disk of matter. Spin the fork into the spaghetti. The strands begin to wrap around, because the fork is pulling against the ones still lying on your plate. Above and below the disk around a black hole the strands of magnetic spaghetti are twisted into a funnel-like tube, leading away from both poles. It becomes a narrow neck of escape. Particles that boil off from the disk get swept up into these pipes of densely packed magnetic spaghetti and are accelerated even further as they spiral outward through and within this corkscrew. This should work incredibly well at producing a jet of matter. But to accelerate particles to close to the speed of light may need something still more. It may need a turbocharger.
When Roger Penrose demonstrated the principle of how rotational energy could be extracted from a black hole through the ergosphere, it may have seemed like an esoteric and immensely impractical idea to most of us. But there is another property of black holes that makes such energy extraction a very real possibility, and further supports Penrose’s original idea. Scientists now think that a black hole can behave like an electrical conductor, which is an utterly counterintuitive idea in that the event horizon is supposed to hide all information from us. Indeed, only the mass and the spin of a hole are manifest through their effect on the curvature of the surrounding spacetime. At first glance there doesn’t seem to be a way to paint any more colors onto these objects, to give them any more properties. Yet there is one more piece of trickery that can occur because of the incredible distortion of spacetime just outside the event horizon.
Figure 12. A sketch of one way that a narrow jet of matter may be created by a spinning black hole. Magnetic field lines (“spaghetti strands”) that are anchored in the disk of accreting matter around the hole tend to twist and wind up, creating a tube-like system that “pinches” gas and particles into a jet as they race outward.
Imagine you have in your possession an electrically charged object, such as a single electron. You can tell that it’s electrically charged because if you move another electrically charged object around it, you can feel a force between the two. Like charges repel, and opposite charges attract. That force is transmitted through spacetime by photons, and it is all part and parcel of electromagnetic radiation. Now, let’s say I’m going to whisk that electron away, place it just outside the event horizon of a black hole, and ask you to come along and look for it by sensing the electric field. Most likely, you’re going to get somewhat confused, because the extremely curved spacetime at the horizon can bend the paths of photons, and hence of electrical forces, completely around itself. Even if the electron is placed on the opposite side of the hole from where you are, its electrical field will be bent around to your side. It doesn’t matter what direction you approach the black hole—you’ll still feel the electric force of the electron. It is as if the electrical charge has been smeared across the entire event horizon. The hugely distorted spacetime is creating an electrical mirage, except it is better than a mirage. It is equivalent to the black hole having acquired an electrical charge.
This is exactly the way that an electrical conductor behaves—say, a piece of copper wire, or a chunk of gold ingot. An electrical charge on these materials exists only on their surfaces. The truly remarkable consequence is that a spinning black hole, surrounded by magnetic fields, produces a difference in electrical potential, or voltage, between its poles and the regions toward its equator. The physicists Roger Blandford and Roman Znajek first demonstrated the idea that a black hole can do this in 1977. A spinning hole will quite literally become a giant battery. But unlike the little battery cells you put in a flashlight or a camera, where there is a oneor two-volt difference between the “+” and the “−”, a spinning supermassive black hole can produce a pole-to-equator difference of a thousand trillion volts. Surrounded by hot and electrically charged gas from the accretion disk, this voltage difference can propel enormous currents. Particles are accelerated to relativistic energies and funneled up and away through the twisted magnetic tubes above and below the black hole. This is driven by the enormous store of rotational energy in the black hole. Theoretical calculations show that this alone can produce an output equivalent to the radiation of more than a hundred billion Suns. It may still be that more than one mechanism is at play across the universe for producing accelerated jets of matter, but this one is a leading contender for black holes. It also means that when we see a jet, we are seeing a signpost to a charged and fast-spinning black hole.
These jets of particles are relentless. They drill outward as they climb away from the black hole, and there is little in a galaxy that can stop them. They simply bore their way out through the gas and dust within the system and carry on into the universe. Intergalactic space is not entirely empty, however. Although incredibly sparse, atoms and molecules still exist out in the void, and over thousands of light-years the particles in the jet collide with these rare bits of matter. As a result, the very leading end of a jet sweeps up this material before it like someone hosing dirt off the sidewalk. But this intergalactic gas and dust cannot move as fast as the ultra-relativistic particles squirted out by the black hole, and eventually there is a cosmic pile-up of speeding matter. This train wreck of material builds into an intense spot where the jet particles are bounced, reflected, and diverted from their straight paths. It’s not unlike shooting a hose at a hanging bedsheet: it gives a little, but mostly the water sprays out to the sides and back at you.
The deflected jet particles are still extraordinarily “hot,” moving at close to the speed of light. Now they start to fill up space, still pushing other matter aside and outward into a shell- or cocoon-like structure that encompasses the jets, the galaxy, and the black hole. This is precisely what creates the enormous radio-emitting dumbbells extending for thousands of light-years around certain galaxies. The radio emission is coming directly from the jet particles themselves, as they cool off over tens of millions of years. H ow this cooling works is part of a fundamental physical mechanism in nature that was actually first discovered here on Earth, and almost by accident.
Since the late 1920s physicists have been studying the most basic subatomic building blocks of matter in particle accelerators. The idea behind these devices is simple in essence, and harks back to the earliest experiments with electricity and magnetism. A particle like an electron has an electrical charge, and so we can use electric and magnetic fields to move it around. We can then propel or accelerate it to extremely high speeds. As the particle gets closer and closer to the speed of light, all the wonderful effects of relativity come into play. Physicists have learned to exploit this and use the terrific energy carried by an accelerated particle to smash and crash into other particles, converting energy into new forms of matter and making the apparatus a microscope of the subatomic.
The exotic new particles generated in these experiments can be extremely unstable. For example, one of the simplest and most readily produced is the particle called a muon, sometimes described as a heavy electron. The muon is also electrically charged, but it is not stable and has a half-life of existence of about two microseconds before it turns into an electron, a neutrino, and an antineutrino. If you want to study the muon, you’d better be pretty quick on your feet. But if you accelerate a muon to close to the speed of light, you can give yourself all the time you need. The muon’s clock will appear to slow down, and its brief lifetime can be extended to seconds, to minutes, and even longer. All you have to do is keep it moving fast. One of the ways to do this is to propel particles around and around a circular loop of magnets and electrical fields. The Large Hadron Collider and many of the other major particle accelerators in the world follow this design. It’s a great solution for keeping your subatomic pieces under control. The problem is that a constant force must be applied to the particles to keep them flying around in a circle. W hen this force is applied using magnetic fields, for example, then in order to change direction the particles will try to dispose of some of their energy. This streams out as photons, and that happens even when the particles are not moving particularly fast. But when they’re barreling around at close to the speed of light, a whole new regime opens up.
In the late 1940s, a group of researchers at General Electric in Schenectady, New York, were experimenting with a small device called a synchrotron, a cleverly designed circular particle accelerator. (In order to push particles to higher and higher velocities, the synchrotron tunes its electric and magnetic fields to “chase” them around and around. It’s like a wave machine for subatomic surfers. It sends a perfect ripple of electromagnetic force around the track to constantly propel the particles and keep them zipping around a circular path. It synchronizes with them, just as its name implies.) The GE physicists were pushing their synchrotron to the limit to test its abilities. The experiment used an eight-ton electromagnet surrounding a circular glass tube about three feet in diameter. By cranking up the power, the scientists were pushing electrons in the tube to velocities close to 98 percent that of light, hoping to probe deeper and deeper into the atomic nuclei of matter.
One afternoon, a technician reported an intense blue-white spot of light pouring out of one side of the glass vacuum tube just as they reached peak power. Surprised by this, the scientists fired up the accelerator once more, and again, at the highest power, it lit up a brilliant spot of light. They had inadvertently discovered a very special type of radiation predicted just a year earlier by two Russian physicists. The excited scientists at GE quickly realized what they were seeing, and since the phenomenon had previously been only a theory with no agreed-upon name, they christened it with the practical but rather unimaginative label of “synchrotron radiation.”
They had discovered that when charged particles moving close to the speed of light spiral around magnetic fields and are accelerated in a sideways direction, they pump out radiation with very special properties. This is a distinct “relativistic” version of the energy loss experienced by any charged particle getting buffeted by magnetic forces. Remarkably, from this experiment in the 1940s comes the key to appreciating how the beams of matter from black holes cool off over cosmic time. In these splashing jets, the energy of motion in particles like electrons and the single protons of hydrogen nuclei is being converted into natural synchrotron radiation. It runs the gamut from radio frequencies to optical light and higher and higher energies like X-rays. It also comes with some quite unique characteristics. The ultra-high velocity of a synchrotron radiation–emitting particle results in the radiation pouring out as a tightly constrained beam in the direction it’s moving in, just like the spot of light from the GE experiment. If you were standing off to the side you would not see anything. Stand in the path of the beam, though, and you’d be scorched by the intense radiation. Out in the universe this property is very clearly manifest. Jets from supermassive black holes are quite difficult to see from the side— they are thin and faint. But once the jet particles splash into the growing cocoon around a galaxy, their synchrotron radiation lights up in all directions:the glow of the dragon’s breath.
So now we’ve arrived at a pretty good description of the ways in which our black hole monsters consume matter and belch their energy into the cosmos. G as, dust, and even stars and planets that are swept into the accretion disk of a black hole can be torn apart by gravitational tides and friction-heated to very high temperatures. This heat causes the disk alone to glow with the power of many galaxies. The quasars are the most powerful examples of this, and they represent a bird’s-eye view into the center of a disk surrounding a black hole. They are also extraordinarily efficient, eating just a few times the mass of our Sun per year in raw cosmic material. The spacetime twister of spinning black holes cranks up this phenomenon to a new setting on the amplifier, and it also gives rise to another energy outlet: ultra-relativistic jets of matter that streak across thousands, sometimes millions of light-years. We think that spinning,
Figure 13. A Hubble Space Telescope image of a jet coming from the center of the galaxy called M87. This is a giant elliptical galaxy 54 million light-years from us. Amid the dandelion-like haze of hundreds of billions of stars, the jet extends outward more than five thousand light-years, glowing in blue-tinged visible light that is the synchrotron radiation of electrons moving at close to the speed of light. The black hole producing this jet is 7 billion times more massive than our Sun and is eating about a Sun’s worth of matter every year.
electrically charged holes may be required to launch these sprays across the cosmos, and when they splatter into the intergalactic grasslands, their careening particles push aside great cocoons, glowing hot with synchrotron radiation. In this way a black hole that would actually fit inside the orbit of Neptune can produce these potent structures that extend over a hundred thousand light-years. That is as if a microscopic bacterium suddenly squirted out enough energy to inflate a balloon more than a mile wide. The monster is tiny, but its breath is enormous. The next challenge is to begin to investigate what this particularly virulent exhalation does to the universe. But before that it is worth pausing for a brief recap—and to consider again the nature of what we’re dealing with.
Black holes really are like something out of a fairy tale. The great American physicist Kip Thorne, who has played a central role in the development of black hole theory and the quest to find these objects, puts it nicely: “Of all the conceptions of the human mind, from unicorns to gargoyles to the hydrogen bomb, the most fantastic, perhaps, is the black hole . . .” In my brief version the story of these massive monsters began with the nature of light—something so commonplace, seemingly mundane, and part of our everyday existence. Yet the reality of light is actually quite fantastical. Here is a phenomenon that can be described in terms of electric and magnetic forms that behave both like waves and then as particles, moving through the vacuum of the universe like a snaking rope made of sand. Not only that, but it is light’s constant pace that actually defines what we mean by space and time. Furthermore, the properties of matter that we call mass and energy do something extraordinary:they influence the very essence of this spacetime. They distort it, curve it, warp it. Reality is twisted and bent to make paths that we cannot comprehend with our biological senses but that we are literally compelled to follow as we move through space. Out in the universe it is these paths that underlie the vast neuronal forms of the cosmic web of matter as it coalesces and condenses into structures. Those structures fragment and flow into smaller structures. Eventually, because of the particular balance of forces and phenomena in this universe, matter can accumulate and concentrate to such an extent that it seals itself away from the outside.
Primal creatures are born in this process. Young and ancient black holes are the magical boxes that gobble up unwary passersby. Their event horizons are like punctures in spacetime, places that drain all the colorful and complex beauty of the cosmos out of sight. In a different universe, with different rules, this might happen quietly and discreetly. In this universe, our universe, it’s usually a painful and ferocious process. We now know that matter does not go gently into the night. And like beasts grown out of other beasts, the black holes we find at the centers of galaxies have become monsters that sit inside their great castles. Their sheer size allows them to consume enough matter with enough violence that they light up the cosmos like flares tossed to the roadside. These monsters are a long way away and they’ve been around almost forever, a fascinating fact of life but one that we might at first assume to be unimportant to us. Yet in ancient fairy tales and myths, giants helped carve the world into its present form and provided the landscape we enjoy. Now they lie dormant, except for the rare occasions when something stirs them back to life. Perhaps we need to consider if this isn’t also true of those real-life giants out in the cosmos.
Our investigation into this question through the history and life cycle of black holes is vibrant, and it continues as scientists race to new theories and observations. Many of us find it particularly intriguing because of the interplay between so many strands of scientific inquiry. In many respects that has always been the hallmark of black hole science. Both relativity and quantum mechanics were necessary to explain how black holes could actually come into existence, and astronomy operating at multiple parts of the electromagnetic spectrum is necessary to find the signposts to real black holes out in the universe. Although currently neither the physics of accretion disks nor that of astrophysical jets is complete, there may be deep connections between the microscopic scales that help deter mine things like friction in accretion disks and the vast scales of cosmic structure. It may be that there will be a “Eureka!” moment when we finally understand precisely what happens in these environments. It may also be that the physics is just too complex and variable between different instances, and a single crystal-clear description will elude us.
These challenges already tell us that black holes can be very messy eaters. But oh, what eaters they are! Whether or not we can pin down their precise table manners, we can most definitely see the consequences of what they do to the universe around them. It is the story of those consequences that will reveal some of the deepest and most puzzling characteristics of the universe that we have yet encountered.
Gravity's Engines © Caleb Scharf 2012