The early work on black hole energy generation by Zel’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 ray-or 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. What 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 bee-friendly 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 bunched-up 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 lean-burning 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 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.
Figure 11. A Hubble Space Telescope image of the very center of an elliptical galaxy known as NGC 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.
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. Most 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.
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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-high-velocity 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 near-perfect 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.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 11
