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 one-or 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. How this cooling works is part of a fundamental physical mechanism in nature that was actually first discovered here on Earth, and almost by accident.
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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. When 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 directio
ns: the glow of the dragon’s breath.
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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. Gas, 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, 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.
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.
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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 determine 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.
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BUBBLES
Carl Sagan once said that to make an apple pie from scratch you must first invent the universe. He was right. And in inventing the universe you will need to build all the objects and structures that we find in it. These are the planets, stars, white dwarfs, neutron stars, black holes, gas, dust, galaxies, galaxy clusters, and superclusters. Eventually, when this cosmic mix has cooked for long enough, the molecular arrangements will emerge to produce that apple pie. But how does the universe actually build all this stuff? It’s a question for the ages. We have always wondered how our surroundings came to be. Perhaps we’ve sat around our fires and shelters asking one another about the looming silhouette of a great mountain against the twinkling stars, or the brightly lit disk of the Moon. Where did that all come from? For that matter, where and how did we spring out of these monumental forms?
The origin and evolution of objects and structures in the universe is a central and critical question for modern astrophysics, and it is arguably one of the biggest unfinished puzzles in science. In truth, one reason it is not yet complete is because it is a hugely complex problem that stretches both our physics and our imaginations to the limit. We may come up with clean and elegant fundamental rules for how the physics of the universe works, but nature’s application of them is often extraordinarily messy. That is also, of course, part of the fun. For us it’s also a key issue for dealing with the effects that black holes have on the universe, and we really need to get an understanding of the cosmic laboratory in which they are at play. To do this, we can split a big problem into simpler parts to understand it better. In this case, the growth of cosmic objects naturally divides into two large pieces. One is about construction; the other is about preventing that.
Let’s deal with constr
uction first. The glue that the universe builds cosmic structures with is gravity. Gravity springs from Einstein’s mathematical framework for how mass distorts our stiff but flexible spacetime to create its own future paths. The number and variety of objects we see in the universe is in part determined by the effect of gravity on the tiny bumps and kinks of matter that we started out with almost 14 billion years ago. Had the baby universe been perfectly smooth and uniform, it would have remained dull and boring. With no seeds of structure there can be no growth. Exactly how many bumpy and kinky seedlings there were in the very young universe, and where they came from, is part of another fascinating story. For now it’s enough to say that we think we have a pretty good idea of what they looked like. We also feel pretty confident that most of the matter in the universe is dark. This soup of ghostly but massive particles is distinct from the kind of stuff that stars, planets, and human beings are made of. It is the combination of dark and normal matter coalescing and moving around owing to gravity’s glue that provides a very big piece of the building plan for our universe.
We also know that eventually the continuing expansion of spacetime following the Big Bang will put an end to any construction. The immense stretching out of space will isolate material into islands. More and more, matter will end up in small dense objects or dispersed farther and farther apart until nothing new is formed. That, however, is very far in the future.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 12