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

by Scharf, Caleb

  And here is a black hole, streaking along a remarkable trajectory at hundreds of miles a second. Some earlier event, perhaps during its formation, or perhaps in some frighteningly close encounter with another massive object, has ejected it from the galaxy it once called home. Its long journey is taking it out into intergalactic space, a lonely, dark voyager. Over there is a double pulsar. Twin neutron stars, each the mass of two Suns yet less than eight miles across, race about each other like the ends of a dumbbell spinning in space. Both are spinning furiously, taking mere fractions of a second to complete one rotation. Each is beaming out intense radio waves. And off in a majestic spiral galaxy is a small rocky planet orbiting a moderately bright star. Its three small moons glide around, tugging at the great equatorial ocean on its surface. In the gently lapping waters on its distant shores, a thick green carpet is slowly growing, home to countless microscopic forms that scuttle about their business.

  Our sack of universe stuff is huge, yet it represents a tiny, tiny fraction of the total observable universe. It also represents a very small slice of cosmic time strata. In the few hundred million years that it takes light to cross the sack, very little changes. Big hot stars have eaten through their hydrogen fuel and become bloated and old, or have exploded as gravity seeks to rearrange their equilibrium. New stars and planets have managed to form out of nebulous gas and dust. A few galaxies may have encountered each other, engaging in gravitational ballroom dancing that plays out over even longer timescales. Various small worlds have cycled through changes in environment, from ice ages to watery tropics. In the grand scheme of things though, on a truly cosmic scale, nothing much has happened.

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  So here we have it: a small piece of a map of forever. If we could see the rest, it would span the largest scales of the observable universe, all 13.8 billion years of it, yet also be filled with the infinitesimal detail of every jagged crumb of rock, every porous cloud of gas.

  In many respects, even our little map is still wildly incomplete. Big gaps exist. We’ve been able to probe some locations, but not others. We’ve been able to probe only so far back in time. Nonetheless, it’s a remarkable picture of the cosmos. It already takes in everything from the tenuous distribution of individual electrons, atoms, molecules, and microscopic dust all the way up through planets and stars. Then it proceeds onward to the great swarms that are galaxies, and farther to galaxy upon galaxy in clusters and superclusters, and finally to the walls, threads, and voids of the cosmic web. Some of it is delicately hued. The barest whispers of radiation emanate, whether in the ethereal realm of long-wave radio waves or the sparse but potent punch of the highest-energy cosmic particles—exotic subatomic crumbs whizzing through the cosmos in relativistic stasis, time apparently held almost still for them. Other parts are fiercely vivid, painted in the primary colors of intense infrared, ultraviolet, X-ray, and gamma-ray waves. Even the sea of photons from the early universe is extraordinarily rich: hundreds of them are thronging through every cubic centimeter of the cosmos.

  It’s a map of endless signposts and byways. What may appear to be dull, uninspiring little galaxies contain a myriad of treasures: new stars and planets, tangled magnetic fields, and intricate and always unique dynamics. Trillions of objects dance among themselves as gravity pulls the strings.

  A map like this serves many purposes. It helps satisfy our deepest longings for a sense of order and place. It allows us to identify, name, and study specific objects and phenomena. It allows us to quantify their relationships to the cosmographic surroundings, and to compare and contrast them to other cousins. Galaxies reveal more when we can compare them to other galaxies, nearby and distant, with common or unusual characteristics. Stars of different compositions all follow a pattern described by fundamental physics, yet exhibit enormous variety in their day-to-day behavior. Our map offers a vision of the universe complete enough for us to try to apply our theories and models of the cosmos, testing them against each other to deduce the universal nature of matter and energy and the underlying behavior of spacetime itself.

  As the map of our known universe has been put together with increasing speed over the past hundred years or so, helped along by work like that of Harlow Shapley in 1918, and by Edwin Hubble’s later revelation that many nebule were actually distant galaxies, zooming away from us, we have been able to make some very bold statements. One that is critically important is the simple estimation of the population statistics of objects. Just as, on Earth, knowing how many humans there are, or how many trees or volcanoes, helps us assemble a description of our planet, understanding the nature of the cosmos hinges on statistics. With careful extrapolation, we can now estimate that the total number of galaxies in the observable universe probably exceeds 100 billion, and may be closer to 200 billion distinct systems. A single large galaxy such as our own Milky Way may contain upwards of 200 billion normal stars. Both for stars and for galaxies, the most numerous examples are also the smallest. About 75 percent of all stars in the Milky Way are less than half as massive as our Sun, and out in the universe, the majority of galaxies are classed as dwarfs, containing just a few hundred million individual stars.

  The numbers are somewhat flexible; estimation is as much art as science in this case. Nonetheless it’s a safe bet that there are 1,000,000,000,000,000,000,000 (1021 or a billion trillion) individual normal stars in the entire observable universe—and possibly ten to a hundred times more than that. It is a mind-bogglingly large number. It’s interesting to note that the total number of human beings ever born (or at least counting back to 50,000 or 100,000 years ago) is often estimated as about 110 billion. So, roughly speaking, there are about 10 billion stars in the universe for every human being who ever existed.

  We’re not going to run out of stars anytime soon. But the really intriguing question is, why is it this number at all? Why do we look up at a night sky with this particular balance between light and dark, between the scattered points of brilliance and the blackness of space? The propensity for our universe to make stars, and how efficiently it makes them at any point in cosmic history, is what determines the number of stars we can see—whether within our own galaxy or in the galaxies beyond, and even in the sparse terrain of intergalactic space. Since we are the product of generations of stars forging heavy elements, and are reliant on the energy of a star—our Sun—for maintaining the surface environment of Earth, it is critically important to know what the recipe is for making the universe this way.

  We’ve posited a deep connection between this question and the extraordinary phenomena that are black holes—a connection that implies that they play an active role in sculpting the universe. Our map of forever gives us a jumping-off point for following this line of reasoning. This is the universe as we see it today, yesterday, and through 14 billion years of history. The connections between this atlas and the gravity machines that have helped craft it are there for the taking. We just need to find them.

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  ONE HUNDRED BILLION WAYS TO THE BOTTOM

  On a sweltering September day in 1935, a twenty-car motorcade ploughed along a dusty road, heading toward a desert canyon. President Franklin Delano Roosevelt was on his way to dedicate the Hoover Dam. Waiting for him were the two Winged Figures of the Republic, sitting solemn-faced on their black plinths on the dam’s western side. Their muscular arms stretched up and fused with great blade-like wings of bronze, soaring thirty feet toward the crystalline blue heavens. To their south and east the world dropped away, a seven-hundred-foot-deep void opening up in the dry, rocky landscape. To the northeast, a glistening body of water lay between ancient cliffs. After arriving and being helped out of his car, Roosevelt took a good look around. For once he was genuinely speechless. The colossal, elegant structure surrounding him was staggering and overwhelming. He eventually began the official opening with a witty but heartfelt expression of appreciation for its monumental form: “This morning I came, I saw, and I was conquered…”

  The Hoover Dam
is a temple to gravity, its apex straddling the rocky border between Arizona and Nevada. The enormous forms of concrete and steel are poised in space and time, feeding off the energy of the matter in the Colorado River as it squeezes through this gorge. The dam is much more than a great engineering feat; it feels profoundly connected to the hidden glue and threads binding the universe together. It’s perhaps little wonder that when the United States Bureau of Reclamation sought the opinion of the famous architect Gordon B. Kaufmann on the dam’s design, he felt inspired to commission the organic curves of modernism and Art Deco that now characterize the structure, including the great winged sentinels. Set into the floor beneath these figures is an extraordinary map made of carved and polished stone. This great terrazzo mosaic of the stars and planets is constructed with exquisite precision, reflecting the celestial hemisphere as you would have seen it on the exact date in September 1935 when a humbled Roosevelt made his dedication. It’s designed to serve as a clock, a navigation aid across the ages. If in a thousand years archaeologists come across this map, by applying their astronomical knowledge they will be able to date precisely when it was positioned in the ground. Language is not a barrier. The stars provide their own translation. It’s impossible not to marvel at the wonderful sense of pride and optimism of the people who conceived and built the dam. Even postmodern cynicism is tempered in the presence of such a monument to human innovation and its cosmic connections.

  The Hoover Dam is built around the very same basic principles that govern the behavior of matter anywhere in the universe. Standing on its great walkway, you are far closer to gravitational physics than you might suspect. The weight of water trapped against its wall by gravity represents an enormous store of energy. Trillions of gallons fill the vast reservoir of Lake Mead to its north. This mass of water not only provides the impetus that is used to drive the huge electrical generators at the dam’s base, but also pushes against the convex face of the dam, transferring forces into the natural rock walls to either side and ensuring that the concrete seals tightly and securely. Following universal rules, the waters of the Colorado seek the shortest path to fall along, deep inside the Earth’s distorted space and time. Here, in the cusp of land between Arizona and Nevada, is a beautiful and visceral example of a phenomenon that is critically important for understanding the cosmic nature of black holes.

  This great dam can extract as much as 2,000 megawatts of electrical power from the water rushing through its base and spinning its turbines. This adds up to an astonishing 4 terawatt-hours roughly every year—enough to power a small city. By sealing off the natural gravitationally driven flow of water, it has created a literal mountain of liquid, poised to release energy as it flows downward. We call this power source hydroelectricity, and across our planet it has become a critical resource in helping us maintain our lives. In some places it is truly indispensable. Thousands of miles away from Nevada, the country of Norway is blessed with an extraordinary and beautiful topography of mountains, containing more than thirty thousand elevated lakes that tap directly into natural planetary cycles. Water vapor is lofted into the atmosphere through solar-driven evaporation, returning as rain to these high lakes and rivers, where gravity then accelerates it downward. Hydroelectricity in Norway generates a total of over 130 terawatt-hours per year, providing effectively all the country’s electrical power needs.

  We’ve learned how to extract gravity’s impressive energy in these special circumstances, but to understand how this same principle works in the most extreme environments of the universe, we need to stretch our minds a little further. Gravity as we experience it here on Earth is a weak version of the phenomenon. Cosmic equivalents of the Hoover Dam, or Norway’s mountain lakes, involve a whole different order of physics that builds on these terrestrial examples. I’ve already introduced the idea that gravity is really a side effect of the curvature of space and time, and now is the moment to tackle that concept head-on.

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  When Einstein produced his work on special relativity in 1905, he profoundly altered the way in which we conceived of the universe. Space and time, up until this point, had been considered separate entities by physicists, but Einstein revealed that they were intimately connected. He was able to reconcile our model of nature with what we saw around us by accepting the finite and unchanging speed of light and the invariance of the laws of physics in each and every frame of reference. But the tricky conceptual price was that space and time had to be both variable and inseparable. Together, they were something new that became known as spacetime. This implies a host of physical effects that hinge on the innate limitations to measurement and interaction implied by the finite speed of light. An object moving past us at a high constant speed appears shorter along its direction of motion. It also appears to have greater inertia. If it carries a clock, we see time on that object passing more slowly. Events that appear simultaneous to one observer might not to another who is moving past. In the wake of such a profoundly new worldview, it was little wonder that Einstein worried endlessly about tidying up the loose ends, especially those that would allow relativity to apply to any natural situation.

  He was particularly concerned with gravity. According to Newton, the force of gravity accelerated objects. But that force depended on the distance between objects, and in Einstein’s new relativistic spacetime, distance was a flexible quantity. An imaginary astronaut speeding toward a planet would measure a shorter distance between the space capsule and the looming world than would an observer standing off to the side. Each would then deduce a different gravitational pull on the astronaut—but this could not be true, since it would violate relativity’s assertion that physical laws remain the same everywhere. Einstein was deeply puzzled. How could nature choose the right distance or frame of reference to make this all work? He knew that the old picture of gravity was missing something. Intuition told him that relativity must somehow apply in this case, too— that no one observer could be “special.”

  His first breakthrough, coming to him as he sat at work in 1907, was to realize that someone falling freely in a gravitational field would sense no acceleration. Carnival rides exploit this all the time. If you “free-fall” at an amusement park ride, your stomach feels like it’s in your throat. It might be! For a brief moment you actually have no weight. Einstein’s “thought experiment” suggested that a person’s experience of free fall might be entirely equivalent to floating out in distant space, away from any gravitational pull. So, he reasoned, small frames of reference in a gravity field could indeed satisfy relativity’s requirement that no single frame of reference is unique. For my example, in that instant of nausea at the carnival, you might as well be afloat in deep space. Any unfortunate experiments going on in your stomach are indistinguishable from those performed in a place free of gravity.

  There was a problem, though. And it was a question of size. When objects fall toward the Earth, they all move in a straight line directed to the very center of the planet. This means that two opposite ends of an object are actually being pulled toward each other as the object descends. Imagine a gigantic blue whale falling toward the Earth—not because we want to damage it, but because through no fault of its own it’s rather large and convenient for an experiment. Owing to whale aerodynamics, our blue whale falls horizontally. Now, both its head and its tail are falling along lines that point directly to the center of the Earth. But that means that as it descends, its head and tail are being pushed together, since the closer we get to the planet the smaller the distance is between these two radial lines. There’s a name for this in classical Newtonian physics: the whale is feeling a gravitational tide. It also feels a stretching force between its belly and its back. The whale’s belly is closer to the Earth, and so it feels a slightly stronger gravitational pull than the whale’s back. So the poor whale is being squashed lengthwise and pulled apart bottom to top by tides. Whether it’s more squashed or more stretched depends on the precise size of the planet and the size and sh
ape of the whale.

  This represents Einstein’s dilemma. The effects felt by the whale are real, and are described easily by Newton’s theory of gravity. Yet he knew that this same theory was not compatible with relativity. Einstein could make gravity and relativity work for tiny frames of reference where tides were too small to worry about, but a universal law must apply to any situation. The only way to reconcile the problem was to throw out Newton’s theory of gravity and start over.

  Figure 6. The whale’s dilemma. Tidal forces in a gravity field squeeze the whale head to tail and stretch it top to bottom, as is well explained by Newton’s theory of gravity. But that theory is not compatible with relativity, in which measured distances depend on the frame of reference. Einstein’s solution was to rework the whole meaning of gravity. In the theory of general relativity, tidal effects are a direct consequence of the distortion of spacetime itself.

  This was and still is a conceptual leap of such insight and colossal arrogance that it boggles the mind. It profoundly altered more than three hundred years’ worth of fundamental physical research. So insistent was Einstein that relativity had to be correct that he ended up saying that Newton’s theory of gravity was fiction. In its place, he posited that spacetime was itself flexible. A mass like the Earth curves spacetime around itself and toward itself. Each part of the whale follows the shortest path through this spacetime. The tidal effect it feels pushing its head and tail together is entirely equivalent to saying that spacetime is getting more bunched up toward the Earth. Similarly, the pulling between belly and back is because spacetime is stretching in the radial direction. To solve his problem, Einstein decided that gravitational tides and the curvature of spacetime are just different descriptions of the same thing. What everyone had been calling gravity is just the way objects move in this distorted spacetime. But, although he had found the conceptual solution to incorporating gravity into relativity, he still had to formulate a mathematical framework to describe what was now a general theory of relativity. This framework had to relate the curvature or distortion of spacetime directly to the mass doing the distorting.