Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

by Scharf, Caleb

  Lots of cosmic phenomena are connected to the existence of life, but some are a little more important than others. Black holes are on that list, and it’s because of their unique nature. No other object in the universe is as efficient at converting matter into energy. No other object can act as a great electrical battery capable of expelling ultra-relativistic matter across tens of thousands of light-years. No other object can grow so massive yet still be so comparatively uncomplicated. A black hole is a dent in spacetime described by just three fundamental quantities: mass, spin, and electrical charge.

  Take a look at your hand. It contains atoms of carbon, oxygen, and nitrogen that were forged a million miles below the surface of another star billions of years ago. Your hand also contains hydrogen that was present at the very beginning of the universe. All these elements have felt the forces of black holes. And right now, a tiny fraction of the vast electromagnetic sea of photons racing through the universe is reaching down through our atmosphere, hitting those ancient atoms of your flesh. Some of these photons originated in the fearsome spinning of matter around black holes, or from the accelerated jets of particles rushing at near light speed out into the cosmos. We are awash in their radiation, but it is nothing new to the atoms in your hand. As the cosmic dark ages lifted 13 billion years ago, some of the primordial hydrogen in your body was likely buffeted and tickled by the radiation of feeding black holes. Billions of years later, the stars that built your heavy elements existed because of the history of gravity and energy in one zone of what was becoming the Milky Way galaxy.

  This fertile corner of the cosmos has been governed by all that has gone on around it, including the behavior of the black hole at our galactic center. The very places that have sealed themselves away from the rest of the universe have served as one of the most influential forces shaping it. We owe so much to them.

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  THERE IS GRANDEUR

  It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us … There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

  The English naturalist Charles Darwin wrote these much-repeated words about life on Earth, but they resonate deeply with our twenty-first-century vision of the universe around us. From atoms and molecules forged in successive stellar generations to galaxies of varied shapes, sizes, and hues, layers of cosmic structure have been molded through time by enormously efficient and hungry black holes, gravity’s marvelous engines. Human beings arrive in less than an eye blink of all that rich history. Here we are, self-aware organisms contemplating our place in the cosmos, at the improbable crossroads of a multitude of pathways and possibilities. It’s easy to see the grandeur of the entangled bank writ large across this universe.

  Our modern understanding of nature is by no means complete, but it has become startlingly rich as we probe further into the intricacies and interconnections of the universe. Of all cosmic phenomena, however, it is the most extreme that hold a particular fascination, and black holes are the ultimate extreme. Among all the conceptions of the human mind, we have really outdone ourselves with these objects—they are fantastical, dream-like, and mythological in stature. But they are much more than just a tall tale—they are a vital and active part of all that we see around us.

  There is good reason to think that there are hundreds of billions, perhaps trillions, of black holes scattered throughout the universe. They are endpoints for matter, providing critically influential anchors for the environments surrounding them, even though they are minute on a cosmic scale. Imagine that we could sense directly the curvature and distortion of spacetime as if it were a simple three-dimensional landscape. We would find a universe of gently undulating hills, valleys, and small indentations—peppered with the sharpest little pinholes, so fathomlessly deep that the walls plunge out of view as we peer in. It is a most curious topography: elegant curves punctured by fearsome holes that pin down the very fabric of spacetime and flood it with geysers of radiation and particles.

  Why does our universe make these dreadful holes in itself? The fundamental laws of physics tell us that spacetime is capable of being constricted and expanded, curved and dragged into motion. These laws also describe the behavior of electromagnetic radiation and the fuzzy and paradoxical quantum world of the subatomic. Together these rules define matter’s critical thresholds, the densities and pressures that break through to extreme states. The same principles show us, first, how gravitation builds dense structures out of normal matter. Such gatherings of matter, poised between forces fighting to collapse and expand them, are incubators for chemical and atomic interactions among their contents, and in some cases they get hot enough to ignite nuclear fusion: a star is born. Eventually a few of these objects, stockpiling mass and succumbing to gravity, reach an incredible density that distorts spacetime irreparably. They drop, sink, burrow, and implode all the way out of what we consider to be normal existence. They leave behind fearsome trails in spacetime, like unplugged drains into the underworld, from which even light cannot escape.

  The energy that these tormented corners of space spew back into the cosmos affects almost everything that we see. It influences youthful and turbulent galaxies as well as our own quite special Milky Way. Breaking just one of the crisscrossing strands of cosmic history and energy that connect us to black holes could subvert the entire pathway to life here on our small rocky planet. And it is from this little world that we have doggedly pursued an understanding of the universe around us. The story of this pursuit is both inspiring and sobering. Through our struggles with the most rudimentary challenges of survival and our own penchant for conflict, we have nonetheless reached out for greater knowledge. The sobering part is just how little we understand. This is especially true when it comes to the specific and unpredictable real-world consequences of general cosmic rules. The fundamental physical laws give off such an aura of completeness that we may feel that if we know the laws, we know the universe. A Theory of Everything is a popular conceit and certainly a noble aspiration. But any scientist will tell you that the sense of triumph at solving a beautiful equation lasts only as long as you’re willing to ignore the colorful wealth of complexity and the endless surprises of combination, permutation, and chance that fill nature.

  Black holes are a really good example. Yes, the workings of black holes and the reasons why they occur in this universe are intimately tied to the most fundamental physical laws of relativity, quantum mechanics, and even thermodynamics. But the actual influence of their presence on the basic character of galaxies, stars, and the matter in them is not an obvious consequence. We may imagine the ways in which a black hole can reveal itself, through its gravitational impact on spacetime, its generation of energy, or its influence on surrounding matter, but only discovery can tell us what nature actually does.

  There is little doubt in my mind that the influence of energy feedback from black holes has played a key role in shaping the universe into the way we see it today. Even more critically, this phenomenon was a potent force in the early stages of cosmic evolution when the first galaxies were assembling. The present state of our own Milky Way has been influenced by the balances of radiation and particle energy that participated in clearing the pervasive fog of cold hydrogen and helium gas from the universe during its first 100 million years. That process seems intimately linked to the growth of the first supermassive black holes, as does the surprising but undeniable relationship between the size of those objects and the bulging ste
llar swarms that surround them at the centers of galaxies. Such relationships may also have a deeper connection to the nature of the elusive dark matter that dominates the mass of our universe. But there is something else too, and that is how black holes have contributed to the special, life-friendly circumstances we find ourselves in.

  When our own solar system was forming some 4.5 billion years ago out on a peripheral spiral arm of the Milky Way, the environment and elemental richness of our cosmic birthplace would have been very different and perhaps far less fertile were it not for the universal impact of black holes on their surroundings. It’s also now apparent that our own galactic center harbors a moderately massive black hole that belongs to a class of systems still gently growing, capturing, and eating matter episodically. Although we are relatively sheltered, a modest wash of radiation out here on the galactic rim every few hundred thousand years could temporarily modify the atmospheric chemistry of a small rocky planet. Even small changes can have big consequences. A little more or less ozone, a little more or less watery precipitation, and the fortunes of a particular organism could take a turn for the better or worse. A few million years down the line, the results could have amplified to dramatically alter the course of evolutionary history on a planet. The stage that the Milky Way is in right now, a seemingly transitional period in the galactic “green valley” before our collision with the Andromeda galaxy in a few billion years, is connected to our central gravity engine. Exactly what this implies is for us to discover.

  Astrophysicists have observed and struggled to understand many other fascinating aspects of black holes’ impact on their surroundings, such as the nitty-gritty of how matter descends to a black hole, fighting against the outpouring of radiation from material already deep down in the warped spacetime. This game can play out in many ways. A superhot zone called a corona can form above and below a disk of accreting material. Tenuous but scorching matter boils off the disk, filling this region through magnetically controlled channels—an environment much like the surface of our own Sun, but much more extreme. For massive black holes this is a major boost to generating X-ray light, since the great disks on their own are only hot enough to glow in the ultraviolet. We’ve seen strange pulsations of energy when we’ve looked deep inside these systems. These rhythmic patterns of outflowing photons betray the ongoing fight between matter and radiation. Like the bubbling skin on boiling milk, radiation pushes matter away from the inner zones around a black hole until it’s heated to a disintegrating burst and flops back down, only to begin the cycle again.

  Astrophysicists have also tried to understand whether there is a maximum size for black holes. As they grow, they may simply become so good at generating energy that they push away new incoming material, limiting their own size. In effect, a great photon wind blows whenever matter is accreted by the hole, and that wind stops more food from reaching the throat. It’s like trying to feed a roaring bonfire. The more fuel you manage to throw on, the farther you need to back away. Such an impasse might occur when a black hole reaches 10 billion times the mass of our Sun, roughly the size of the largest holes yet scrutinized. Pushing yet another limit, some of the most massive black holes appear to spin at close to the maximum rate allowed by physics.

  Some data and theories even hint at opportunities for stars to be born within the great gathering disk of material accreting into a hole. Kinks and disturbances in the circulating matter could allow for its localized agglomeration into new objects. Instead of just destroying the arrangement of matter, the black hole environment could conceivably encourage a new start. What a strange and alien environment this might be for the birth of a stellar system. Could there be planets around these stars? We don’t know yet, but we can only imagine what the night skies might be like on such worlds. There is also new and intriguing evidence that some black holes, billions of times the mass of the Sun, have been flung out of their parent galaxies. Ejected during the final stages of the merger of black hole pairs, these mysterious objects race outward, escaping their galactic confines for the desolate emptiness of intergalactic space.

  Despite the incredible rate of progress in astronomy and our increasing ability to reach out and explore the cosmos, there is one thing we have not yet managed to do: look at a black hole up close and personal. Is it even possible? Clever astronomical techniques have been devised as surrogate probes of the inner recesses of spacetime surrounding the event horizon, but they still offer an incomplete view. One of these methods, owing much to the English astronomer Andrew Fabian, uses the emission of X-ray photons from iron atoms as they swirl about within the accreting material around a black hole. The curved and spinning spacetime imprints a very particular mark on the energy of these photons, which is itself very specific to the iron atoms. Some photons are blueshifted—boosted in energy; others are reduced in energy and redshifted, but this process is not evenhanded. The extreme speed of the spinning material takes it into a relativistic regime where light from material moving toward us is not only blueshifted to higher energies, but is also enhanced in density. It’s called “beaming,” or more colloquially, the “headlight effect.” The result is that we see far more light coming from the side of a disk spinning toward us than from the side spinning away. Altogether, the photons from around the black hole come out a little tilted and drunken, with energy askew. Add in the other effects of the extremely curved spacetime, and you’re left with a dirty fingerprint that can tell us about black hole mass, spin, and the nature of the disk surrounding it. This is a tricky measurement to make, though, and what we see is not an image but a collection of the energies of these battered photons.

  Other efforts seek to exploit the innate structure of the cosmos as a medium for receiving messages from the deepest gravitational cusps. If a pair of neutron stars or black holes come to orbit each other closely enough, they can merge in a spectacular crescendo, setting ripples in motion through spacetime itself that transfer energy out into the universe. These are known as gravity waves, and they propagate outward into the cosmos at the speed of light. Gravity waves stretch and squeeze space a minute amount in a distant place like our solar system. As they pass through us they quite literally alter the underlying yardstick of physical dimensions. Physicists and astronomers have been devising systems to try to detect them.

  It’s an extraordinarily difficult task, but the potential payoff is enormous. Kip Thorne describes it as listening to a gravitational symphony, one that will tell us the masses of the merging black holes and the nature of their merger, and will provide an ultimate test of our mathematical description of the whirling spacetime at their edges. Experiments like the tongue-twisting Laser Interferometer Gravitational-Wave Observatory (thankfully LIGO for short) employ a similar technique to the one used by Michelson and Morley in 1887 to try to detect changes in the time light takes to travel a set path. In this case, however, the path itself is vulnerable to change as gravity waves race through our neighborhood. Unlike that modest-size early experimental device, a LIGO observatory consists of pairs of 2.5-mile-long tubes through which lasers bounce back and forth. Oriented at 90 degrees to each other, they are designed to sense changes in the actual physical distance that the photons have to travel. A passing gravity wave will quite literally alter that length.

  Two such observatories exist, working in tandem: one in Hanford, Washington, and the other 1,865 miles away in Livingston, Louisiana. Their level of precision is mind-boggling. Physicists can detect changes in the dimensions of a laser’s path that are smaller than one-thousandth of the size of a proton. As they sift through the myriad sources of confusion and noise—even waves crashing on shorelines hundreds of miles away can produce a signal—scientists are getting tantalizingly close to detecting astrophysical events. For example, they are hoping to confirm the prediction that objects such as a pair of small black holes or neutron stars emit a shrieking “chirp” of gravitational energy just as they come together, orbiting furiously around each other. Plans to constr
uct a space-based gravity wave detector employing a similar technique are currently on hold because of budgetary constraints. If it’s built, the Laser Interferometer Space Antenna, or LISA, would consist of three spacecraft marking off a triangle in interplanetary space, an astonishing 3 million miles on a side. In astronomers’ view, the effort and expense of setting such a remarkable net would be worth it: LISA would be able to hear deep rumbles as supermassive black holes merged in distant galaxies, as well as the hum and buzz of millions of whirling pairs of dense stellar remnants within the Milky Way.

  What of the event horizon, though—the very interface between our familiar universe and the one lost within? It is just outside that final gateway that matter gives up the energy that plays such a vital role throughout the cosmos. Observing this directly would be an ultimate victory. We could see exactly how things really work: the accreting disk of material, the twisted coils of a spin-driven jet of particles. Such a close-up would quite literally reveal the inner workings of these gravitational machines. But even the identifiable small black holes in our galaxy are thousands of light-years away—impossibly distant, tiny, and impenetrable. Perhaps. Perhaps not. You need to go talk to the right people.

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  In the early 1990s, Keith Gendreau spent his time as a graduate student at MIT working on a new type of camera. It wasn’t for holiday snapshots, but for capturing X-ray photons from the universe and turning them into an image. It was cutting-edge technology. You took the type of silicon-based device used in digital cameras, a tiny chip divided up into even tinier pixels, and you redesigned and repurposed it. An X-ray photon whacks into the silicon atoms, dumping energy that pushes electrons out of semiconductor stasis. Locate and count those electrons, and you could begin to build an image of the source of the photons. Previous generations of X-ray imaging devices relied on arrays of gas-filled cells and electrostatic grids. Moving to silicon ushered in a whole new era of high fidelity. The only hitch was that you had to do this in space.