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Macmillan Childrens Publishing Group

Gravity's Engines

How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

Caleb A. Scharf

Scientific American / Farrar, Straus and Giroux



A computer sits among the coffee-stained papers scattered across my desk. Its screen has been blank all morning. Suddenly it lights up and displays a pixelated image. A message is coming in from space.
A few days earlier, high above Earth's surface, a great orbiting observatory has stared for forty hours over the bows of the Milky Way galaxy. With chilled eyes it has patiently tracked a tiny patch of the cosmos, a speck of sky close to the constellation Auriga—the Charioteer. In this direction is a glorious view for a spotter peering into the abyss in the hope of finding treasure.
This remarkable instrument is called Chandra. Decades of work went into its construction, with hundreds of people toiling in multiple countries. The blood, sweat, love, and tears of a highly technological civilization produced the smooth surfaces and exquisitely precise devices inside it. Careers started and ended while it grew from a dream into a reality. Finally it was lofted into space and released with tender delicacy from the belly of NASA's space shuttle Columbia, becoming a tangible example of humanity's endless curiosity.
Now it has captured a whiff of something from the deep. Photons, particles of light, have found their way down through its mirrors and filters, forming an image on the silicon sensor of a digital camera. That image, encoded as a stream of data, has passed to Earth, first beamed as microwaves to a ground station and then relayed around the globe. Processed and sent on across a continent, another journey through hundreds of miles of wires and fiber optics, it finally re-forms as a monochrome picture on a screen in my small and untidy office ten floors above the streets of twenty-first-century Manhattan.
On any given day, we don't expect to see much that is particularly remarkable in the vast flood of incoming data that is a part of modern science. Patience is a hard-won lesson. Yet there, amid the rough noise of the image, is a structure. It's small and faint, but unmistakable. I can see a pinpoint of light surrounded by something else—a fuzzy streak jutting out to the left and right. It looks like a small dragonfly pinned to a piece of cardboard. Something is very curious about this image. It has the flavor of a new species.
Traffic out on the street echoes noisily up the canyon of buildings, but for an instant it rings hollow. My mind is not in this world anymore, but away in a very, very distant corner of the universe.
Twelve billion years ago, the photons that made this image began their journey. They are X-rays, invisible to human eyes, but able to penetrate through our soft bodies. For 12 billion years they have passed unimpeded through the cosmos. But as they have traveled, the universe has changed; space itself has expanded, stretching the photon waveforms and cooling them to a lower energy.
When they set out there is no star called the Sun, no planet called Earth. It isn't until they are two-thirds of the way through their journey that part of a collapsing nebula, a cloud of interstellar gas and dust in a still impossibly remote galaxy, produces a new star and a set of new planets that will eventually become our home.
When Earth forms, these photons are already ancient, 7-billion-year-old particles that have traversed vast stretches of the cosmos. Time passes. Somewhere on Earth a complex set of molecular structures begins to self-replicate: life begins. Two billion years later, the photons start to enter the very outer regions of what we might call the known universe. Here are the great superclusters and web-like structures of galaxies that we have mapped. Spanning tens of millions to hundreds of millions of light-years, these forms are the skeletons upon which galaxies and stars are coalescing, molded by gravity—millions of galaxies, and quintillions of stars, strung through the cosmos. On Earth, microbial evolution has just given rise to the first cells of a new type of life—the Eukarya, our direct ancestors. These busy microscopic creatures swim off in search of food.
A billion more years go by. The photons enter truly known space, a realm where our instruments have mapped great walls of galaxies and huge empty voids. Here are structures with familiar names and calling cards, like Abell 2218 and Zwicky 3146, huge gravitational swarms of galaxies known as clusters. On Earth the very first true multicellular life emerges, and the air is filling with oxygen. The chemistry of this element is ferocious. New types of metabolism are evolving in response—a revolution is under way. Just 500 million years later, the dry surfaces of Earth are covered by something exotic: plants that use the molecular tools of photosynthesis. A strange, greenish tint appears across the supercontinent Gondwana, the largest body of land on the planet.
The photons continue their patient journey, passing through regions that will be increasingly familiar to as yet unevolved astronomers. Nearby are the great galaxy clusters we will name for the constellations in which we see them: Coma, Centaurus, and Hydra. Onward the photons fly, and from the point of view of an observer standing to the side as they race past, our galaxy is now one of thousands of patches of light in the sky ahead.
It takes them another 490 million years to reach our Local Group, a ragtag band of galaxies. Some are large like Andromeda and the Milky Way, and some are small, like the dwarf galaxies Cetus, Pegasus, Fornax, and Phoenix. It is not a particularly remarkable place, perhaps a total of a few trillion or so stars altogether.
On Earth many great periods of life have come and gone. The dinosaurs have not been seen for almost 60 million years. The continents and oceans have changed dramatically, and the contours of our modern world are clearly visible. Birds and mammals are swarming across the globe. The Black, Caspian, and Aral Seas are beginning their separation from the ancient Tethys Ocean and what will become the Mediterranean Sea.
In the next few million years, the photons descend into the gravity well of our galactic neighborhood. The Milky Way is now a distinct glowing smear reaching across the sky as it gets ever closer. On the third planet from a modest G-dwarf star orbiting in one of the outer arms of this spiral galaxy, a new type of animal begins to walk upright on two legs. As it leaves its footprints in muddy volcanic ash in what is now the Olduvai Gorge in eastern Africa, the photons speed ever closer. Now in almost their 12 billionth year, they have never slowed down. As particles of light they are threaded into space and time, moving at the same constant speed as they did at their origin.
It takes them another 2 million years to reach the outermost wisps of our great Catherine wheel of a galaxy. A major glacial age is taking place on Earth. Huge ice fields grow outward from the planetary poles, engulfing the northern hemisphere. This profound change in environment impacts the behavior and fortunes of the hominids' descendants—humans. Groups of people migrate and explore. Areas that were once shallow seas are now traversable on foot. Another twelve thousand years pass and the photons fly in across the spiral arm of stars, gas, and dust in the Milky Way galaxy that is called Perseus. By now the ice has retreated, and new pockets of humanity are scattered far and wide. Great cultures have risen and vanished, and others are beginning to flourish across the planet from the Middle East to Asia, from Africa to North and South America, and in Oceania.
The photons enter the Orion spur of our galaxy. To one side they pass the Orion nebula itself, a vast and beautiful cloud of gas and dust, the birthplace of new stars and the graveyard of old ones. One thousand years remain for their great migration. On Earth, Chinese and Middle Eastern astronomers observe a new bright object in the heavens. Unknown to them, they have witnessed a supernova, the explosive death of a star. A decade later, in the year we now label 1066, a duke from Normandy ingloriously named William the Bastard leads his army in the conquest of an island kingdom where he claims the throne. Preceding his arrival, and believed to presage it, a glowing comet, later to be known as Halley's, passes through the skies and is depicted in the epic Bayeux Tapestry recording these great events. This is the nineteenth time that it has been documented by human observers, each sighting some seventy-five years apart.
Kings and queens, emperors and empresses rise and fall. Wars flare up and eventually end. Humans migrate and explore the planet. Diseases, volcanoes, earthquakes, and floods ebb and flow as time goes by. Six hundred years pass in the blink of a cosmic eye. The photons enter a sphere centered on Earth that encompasses the Pleiades star cluster, the Seven Sisters. The Sun is a nondescript point of light in the distance. Galileo uses a telescope to study the moons of Jupiter, realizing that they orbit that body, and therefore Earth is not the center of all celestial paths. Half a century passes and Newton formulates physical laws that describe the properties of motion and of gravity.
The photons continue on through the great emptiness of interstellar space—far more vast compared to the sizes of the stars than intergalactic space is compared to the sizes of galaxies. Hundreds of years pass. World Wars I and II ravage the northern hemisphere of the planet. The photons begin to pass through the collection of stars that form the constellation of Auriga, as seen from the vantage point of Earth. The Vietnam War is flaring up and the Beatles are playing on every radio. Apollo 8 orbits the moon and, for the first time, human eyes see Earth rising above a new horizon.
Decades later and the photons race in through the outskirts of the solar system. Zipping through the magnetic skin of the heliopause—where the Sun's influence gives way to that of interstellar space—they have just hours to go. Finally, as if playing their part in some great cosmic tragedy, they are captured within a cylinder that is only four feet across, a mere 0.0000000000000000001 percent of the diameter of the Milky Way galaxy within which it is embedded. Instead of sailing on to infinity, the photons are caught in the high orbit of planet Earth, inside the great Chandra Observatory, where they are coaxed deep into a series of nested tubes of iridium-coated glass. In the next few nanoseconds these ancient photons of X-ray light finally encounter something in their path in their long journey through the cosmos: a piece of meticulously prepared silicon, itself composed of atoms that were forged inside another star, dead for billions of years. The silicon absorbs their energy and, where each photon lands, releases electrons into the microscopic pixels of a camera. Within a few more seconds a voltage automatically switches on, sweeping these electrons off to the side toward a line of electrodes—like a croupier gathering up the chips on a roulette table. Here, after a journey of 12 billion years, the photons are registered as electrical charges and converted into something new. They have become information.
On the screen in my office in New York this data creates an image. It is a unique and revealing fingerprint of intensity and energy. Here are the signs of a young and extraordinarily massive black hole, ferociously tearing matter apart in the skies of a distant and now ancient galaxy. Its hunger is extreme and violent. But something new and unexpected is revealed as well. A grasping presence extends further, pushing, molding, and altering the surrounding universe. Dragonfly wings of light jut out around the brightest part of the image where the black hole lurks. Their true scale is staggering: they are hundreds of thousands of light-years across. Their true brightness is immense, representing an energy output a trillion times greater than that of our Sun. They are flooding that ancient galaxy with radiation, somehow powered by the monster in the middle.
This book is, in part, about the story of this distant place. In the past few decades a remarkable and strange picture has emerged. It extends far beyond the esoteric and fantastic studies of the extremes of space and time that have been a hallmark of black hole science. Astronomy in the late twentieth and early twenty-first centuries has revealed that black holes are both varied and common. While we think that most originate as comparatively small objects, with the mass of just a few Suns, some have managed to grow far larger. The biggest are now known to be tens of billions of times the mass of our Sun. They stagger the imagination and challenge our core ideas about how all objects and structures that we see in the universe have come to be. At the same time, they do not hide away as inert bodies, invisible and aloof. We have come to realize that the science of black holes is very real and very immediate. Their presence is acutely felt across the cosmos. Black holes play a critical role in making the universe appear the way it does.
Because of this, they also profoundly influence the environments and circumstances in which planets and planetary systems are formed, and the elemental and chemical mixes that go into them. Life, the phenomenon of which we are a part, is fundamentally connected to all these chains of events. Saying that black holes have implications for life in the universe may sound outrageous and far-fetched, but it appears to be the simple truth, and we are going to follow that tale.
To begin to explain the epic cataclysm appearing on the screen in my office, I have to turn back the clock a couple of hundred years, back to a time when this small armada of photons was still streaking past the outer reaches of the Orion spur of the Milky Way galaxy. Here on Earth it was a different era, one of great change and new ideas—especially in one small corner of the planet.
* * *
With its tall and rather austere stone tower, St. Michael's parish church in the village of Thornhill in West Yorkshire, England, seems an unlikely place to nurture the secrets of the universe. Perhaps, though, there is something in the surrounding rugged green terrain or the harsh winter skies that might compel you to wrap up tightly with great cosmic thoughts. Indeed, in 1767 a remarkable thing happened in this small community. Into its midst came an extraordinary thinker, a polymath whose mind roved across the vastness of space. He also happened to be the new rector at Thornhill.
At forty-three years old, John Michell was already a highly regarded figure in British academic circles. He had spent most of his life immersed in intellectual pursuits and had risen to the title of Woodwardian Professor of Geology at the University of Cambridge. His interests were diverse, from the physics of gravitation and magnetism to the geological nature of the Earth. Despite his scientific reputation, however, little personal detail is known about Michell. Some records depict him as short and round, an eminently forgettable physical specimen. Others describe a lively and busy mind, someone who had once met with Benjamin Franklin, was fluent in ancient Greek and Hebrew, was a keen violinist, and kept a household alive with debate and inquiry.
What is clear is that a few years earlier, in 1760, while he was a Fellow of Queens' College in Cambridge, Michell produced a study of earthquakes that established him as one of the forefathers of the modern science of seismology. A decade before that he had written a treatise on the nature and manufacture of magnets. He had also written works on navigation and astronomy, from the study of comets to stars. While he may have been short in physical stature, his sharp vision could pierce through the void.
We can only presume that in the relative calm of life at St. Michael's, Michell was able to find a secure income and home for his family. Perhaps it also gave him time to think away from his otherwise busy rounds of scientific debate in nearby Leeds, and from the great changes being wrought on the world around him. The Industrial Revolution had begun in Europe, Catherine the Great ruled Russia, and the American Revolution was gathering momentum to the west. Less than a hundred years earlier Isaac Newton had published his monumental works on the nature of forces and gravity. Science was becoming its modern self, equipped with increasingly sophisticated technological and mathematical might and emboldened by the times.
There was one problem in particular that caught John Michell's attention when he studied astronomy. It was a fundamental and practical one. While it was well understood that the stars in the night sky were cousins to our own Sun, there remained a deceptively simple question that scientists at the time were unable to answer. In our own solar system, it was clear from geometrical arguments that the Sun was vastly larger than any of the planets. This being the case, it was relatively straightforward to use the estimated distances of the planets from the Sun and the time it took them to complete an orbit—their periods—to estimate the solar mass. Newton had shown how. Newton's universal law of gravitation outlined a simple formula that related the masses of two bodies to the distance between them and the length of the orbital period of one around the other. If the mass of the planets was assumed to be negligibly small compared to the Sun, then the timing of their orbits simply revealed the Sun's true mass.
But the question that vexed Michell was not how to measure the mass of the Sun, but how to measure the mass of distant stars. No planets could yet be seen around them to serve as evidence of their gravity. The physical nature of stars themselves was still unclear. Astronomers understood that they were hot fiery objects, an inference from the way we experience the Sun here on Earth, but their true distances would not begin to be known for another seventy years. Nonetheless it was increasingly clear that the Persian and Chinese astronomers of the Middle Ages had been on the right track in believing the stars to be out in the distant universe and that they obeyed the same physical laws as those of our solar system. To know their actual sizes would help tremendously in divining their detailed nature.
Michell was an incredibly flexible thinker. In the late 1700s the term "statistics" had barely been introduced to science; the basics of probability theory had been formulated about a century earlier. The idea of applying these tools to real scientific questions was in its infancy. Yet, as he pored over astronomical charts and tables, Michell used statistical reasoning to show that the patterns of stars indicated many were not isolated in space. He proposed that some stars must occur in physically related pairs, or binaries. This observation wasn't verified until 1803, when astronomer William Herschel studied the movement of stars. If one could observe the actual orbits of binary stars, then, using Newton's formula, one could estimate their total mass. But in Michell's time such observation was not quite within the grasp of astronomers, and so he had to keep looking for another approach for measuring the mass of a single distant star.
He came up with a tremendously clever solution. A hundred years earlier Newton had proposed that light was made of "corpuscles"—tiny little particles that traveled in straight lines. Michell reasoned that if light was made of these corpuscles then they would be subject to natural forces, just like everything else. Light escaping the surface of a distant star should therefore be slowed down by gravity. In the late eighteenth century the speed of light was already known to be exceedingly fast—about 186,000 miles a second. Even the great bulk of a star like the Sun, Michell knew, would only slow the light down by a small amount. But if that change could somehow be measured, then the mass of the star could be deduced.
On November 27, 1783, Michell brought his ideas together in a presentation to the Royal Society in London. The title of his paper was a fabulous example of circumlocution and hedging: "On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose."
As he presented his work to the Society, Michell made his argument for deducing the mass of a star. His opening logic was simple: "Let us now suppose the particles of light to be attracted in the same manner as all other bodies with which we are acquainted … gravitation being, as far as we know, or have any reason to believe, a universal law of nature." The idea appealed to the audience, who were well versed in Newtonian physics, and by all accounts set them aflutter. Light being slowed by gravity was a delightful notion.
Michell's concept was audacious. The recognition that a star or other cosmic object leaves its dirty fingerprints all over the light that we eventually detect coming from it actually represented a huge leap for modern astronomy. The ability to deduce the nature of objects in the cosmos by the analysis of their light is today central to our exploration of the universe. But Michell had even more to say.
An imaginative problem solver, the rector of Thornhill was clearly feeling inspired. His next big leap was to recognize that an object might be massive enough to pull a corpuscle of light completely to a halt as it tried to fly away. With a bit of mathematical juggling, Michell computed how massive an object would have to be to halt light. He did it by turning the question around. If an object fell toward a star from an infinite distance away and reached the speed of light at the point of impact, then the star had enough gravitational might to prevent light from escaping in the reverse direction. If such a star were of the same density as the Sun, he found it would need to be five hundred times bigger in diameter. His neat summary of the situation for the Royal Society audience was clear: "… all light emitted from such a body would be made to return towards it, by its own proper gravity."
From his calculations, Michell realized that this meant there could be objects out in the universe that trapped all light coming from their surfaces and were for all intents and purposes invisible. The only way to spot them would be by detecting their gravitational influence on other objects. Such massive objects in Newtonian physics have since become known as Michell's "dark stars."
A decade after Michell formulated these ideas in the sleepy English countryside around West Yorkshire, the extraordinary French mathematician and astronomer Pierre-Simon Laplace was independently reaching a similar conclusion. Born in Normandy, Laplace was a scientific prodigy, and his mathematical prowess quickly elevated him to the higher echelons of French academia. While still in his twenties, he had single-handedly developed mathematical theories describing the stability of planetary orbits and had helped develop modern calculus. He would go on to help pioneer theories of probability and mathematical physics. What were otherwise Michell's dark stars, Laplace termed "black stars," writing in 1796 that "it is therefore possible that the greatest luminous bodies in the universe are on this account invisible."
Although other scientists were intrigued by these ideas, there is no record that Michell and Laplace ever communicated with each other, and the concept would not be fully understood for more than another century. Newton's corpuscular theory of light fell out of favor, as it failed to explain subsequent optical experiments. Laplace even quietly removed his description of black stars from later copies of his epic work Exposition du système du monde (The System of the World). Today we know that the fundamental assumption behind Michell and Laplace's theories—that light could be slowed by gravity—is in fact wrong. The truth is far more surprising.
Nonetheless, the idea represented a turning point in thinking about massive objects in the cosmos. It was a revolutionary concept that there could be huge objects in space that are entirely hidden from sight. It was even more extraordinary to suggest that the objects that were the most massive and luminous—throwing off the greatest number of photons, or corpuscles, at any given time—might also be the darkest from our perspective. Exactly how revolutionary these ideas were would not be fully appreciated until much later.
* * *
Two pivotal events would eventually bring Michell's dark stars back into view. The first of these was to take place in a chilly basement in Cleveland, Ohio, in 1887.
By the late 1800s, remarkable advances had been made in our understanding of the properties of light and electromagnetism. Decades of experimentation had demonstrated that the flow of electrical currents produced magnetic fields, and that, conversely, moving magnetic fields, or the motion of a conducting material through a stationary magnetic field, produced electrical currents—the flow of energy. As the ability to make precise measurements of these currents, voltages, and fields improved, so did the mathematical description of the relationships between these phenomena. A turning point came in the years 1861 and 1862, when the Scottish physicist James Clerk Maxwell formulated a set of equations encapsulating all these physical relationships, and much more.
At the core of Maxwell's work are four relationships. In the language of calculus, they are partial differential equations. They describe how electrical charge and current relate to magnetic fields and flux in any situation, from a simple charge of static electricity to a complex electromagnet. Maxwell was a brilliant and persistent scientist who published his first scientific paper at the age of fourteen. As he tinkered with his equations, he found they had far broader implications. A magnetic field could typically not exist without an electric field, and vice versa. He realized that this coupling of fields implied that a wave of electrical charge could move—propagate—through a medium together with the complementary wave of a magnetic field. In its simplest form this phenomenon could be visualized as a pair of ropes being whipped into a series of hills and valleys—the shape of a sine wave. When the electric wave reaches a peak or a valley, so does the magnetic wave. The moving electric field produced a moving magnetic field and the moving magnetic field produced a moving electric field. In many senses it resembled a perpetual motion machine. Maxwell also found he could calculate the speed of the motion of this "electromagnetic radiation." To his astonishment, it was the same as the speed of light. Einstein would later write: "Imagine [Maxwell's] feelings when the differential equations he had formulated proved to him that electromagnetic fields spread in the form of polarized waves and with the speed of light!"
Maxwell had discovered, and proved, that light was a manifestation of electric and magnetic fields. It was an electromagnetic phenomenon. This was the final nail in the coffin of Newton's original corpuscular theory of light: electric and magnetic fields had no mass, and therefore light itself was "massless."
Maxwell's equations are still entirely valid today, but for all their beauty and incredible utility, they rest upon something even deeper and more surprising. Different configurations of the electric and magnetic fields do not alter the speed of propagation. Lurking in the equations is the suggestion that the speed of light is constant. There was something else, too. If light was an electromagnetic wave, then surely it needed a medium to move through. Yet light can easily travel through a vacuum. So what was the medium?
Many other physicists took up Maxwell's equations and attempted to explain the propagation of light. The most popular idea put forward by the scientific community was that of a "luminiferous aether," an unseen medium that permeated the universe and allowed electromagnetic waves to get from here to there. But there were problems with this theory. Even if light merrily wiggles its way through an invisible aether at a fixed speed, we should see changes in apparent speed. This is because we ourselves move relative to the aether. This could be on foot, on horseback, by train, or by sheer virtue of sitting on a planet that is orbiting the Sun at almost 20 miles a second. The principles of Galilean and Newtonian physics should apply, and the speed of light should appear to vary.
Testing this was an immense challenge. If light travels at 186,000 miles a second, then even the motion of the Earth around the Sun would suggest only a 0.01 percent change in the apparent speed of light in the aether. Measuring the speed of light with some precision in a laboratory is a tricky business even today. In the late 1800s the most cleverly designed experiments and state-of-the-art equipment had fallen far short of the sensitivity needed to detect such a variation between the absolute and apparent speed of light.
Then, in 1887, two American scientists, Albert Michelson and Edward Morley, constructed an ingenious apparatus designed to measure the speed of light with unprecedented precision. Michelson was a well-known optical physicist. He had already expended considerable effort trying to refine the measurement of the speed of light (it was, in fact, a lifelong obsession). He had experimented a few years earlier with a prototype apparatus for achieving a higher level of precision. Now he joined forces with Morley, a professor of chemistry and a skilled experimentalist, to construct the next version.
To avoid even the slightest distortion or vibration during the course of their investigation, they set the apparatus on a massive block of marble that floated on a shallow pool of mercury. This dense fluid supported the weight and let them easily rotate the equipment. For extra caution, the whole thing was assembled in the basement of a particularly solid dormitory building on what is now the Case Western Reserve University campus in Cleveland, Ohio. To conduct the experiment, a very fine beam of light was split by bouncing it off a partially silvered mirror (not unlike a two-way mirror) at a 45-degree angle, so that two beams were formed at right angles to each other. The beams then traveled to the first of a set of small mirrors placed toward the corners of the marble block. These mirrors reflected the beams back to others across the slab, each carefully aligned to make both beams go back and forth a total of ten times. The final reflection was arranged so that the two perpendicular beams would pass through and reflect again on the partially silvered mirror. This time the light would be brought together in one place, inside a small telescope. In this way the light would travel a much bigger total distance, thereby amplifying any variation due to different speeds in the beams.
The Michelson-Morley experiment was brilliantly conceived—in principle. In their travels through the hypothesized luminiferous aether, the beams of light moving back and forth in the same direction as the Earth's orbit would appear to travel at a different speed than the beams traveling in a direction perpendicular to the orbit. The difference in speeds would result in the waves of light from the two beams becoming misaligned. When they rejoined, a phenomenon known as interference would occur; the beams would not mesh exactly. This would be captured in a ghostly series of bright and dark rings that could be measured by the small telescope lined up with the light beams. So, in effect, Michelson and Morley used the very nature of light itself to build the exquisite ruler that they needed to make such a difficult measurement.
It was a beautiful experiment, one that would live on in the history of science forever—because it was a spectacular failure. Within the capacity of the apparatus and Michelson's and Morley's considerable skills, it was clear that the beams of light traveling in different directions had absolutely no discernible difference in speed. This was true regardless of the time of day the measurement was made, the time of year, the position of the marble block, the temperature in Cleveland, or the value of stocks and shares. Either the aether through which the light beams were traveling wasn't operating according to accepted principles of physics or it just didn't exist.
The authors described their experiment in painstaking detail in a journal article in the American Journal of Science. Desperate to understand the outcome, they made several proposals as to why they failed to achieve their anticipated result. None sounded too plausible. The only conclusion they could arrive at was that if there really was a luminiferous aether, the Earth couldn't be moving through it very fast.
Later efforts by both Michelson and Morley and others fared no better. All these brilliantly executed experiments failing to detect anything made it devastatingly hard to proceed with the aether hypothesis. Something was afoot.
The second pivotal development that would eventually return Michell's dark stars to the scientific consciousness began inside the brain of a young German patent clerk in Switzerland. Up to this point, the mysterious properties of light had continued to challenge and frustrate physicists—until Albert Einstein published his special theory of relativity in 1905. It irrevocably changed our understanding of the nature of reality. In one fell swoop, puzzles such as the fixed speed of light were turned on their heads—suddenly fitting perfectly into place. In fact, Einstein's extraordinary insight came from studying Maxwell's equations. It turned out that they already contained the correct mathematical description of nature. It just required someone to figure out what that was.
There are two fundamental postulates in special relativity. The first is that the laws of physics do not change with your frame of reference, a concept that could be traced back to the Italian astronomer Galileo Galilei. You could be sitting in a deck chair on a tropical island or strapped to a rocket traveling at tens of thousands of miles an hour, yet in either case you would deduce the same laws of physics at play anywhere in the universe around you.
With the second postulate, Einstein went out on an inspired limb. He proposed that the speed of light remains a constant, independent of the speed of its source. This is utterly counterintuitive to our everyday experience of the world and the principles of Newtonian mechanics. But it neatly deals with the agonies of Michelson and Morley, does away with the aether, and explains the validity of Maxwell's equations. It also means that the phenomenon of light is an extremely fundamental part of our universe. Today, lasers and more complex experimental arrangements can measure the speed of light with ultra-high precisions of better than approximately 2 parts in 10 trillion. Einstein was right. Light's speed in a vacuum simply doesn't change, irrespective of the motion of its source or observer.
This simple fact has many startling implications for our physical universe. Time itself becomes an important part of any system of coordinates, and the passage of time is relative—it depends on the motion between an observer and any events. The energy carried by moving objects is also modified from the simple classical physics of Newton. Einstein found that even when we see an object as stationary, it still has an energy called its rest mass energy, given by the now famous equation E = mc2. As objects with mass move faster and faster, their apparent total mass, or inertia, increases, approaching infinity if they move as fast as light itself. Einstein reasoned that this meant that no real object with mass could ever reach or exceed the speed of light, since infinite force would be needed to accelerate it to that point.
The special theory of relativity holds in situations in which any relative motion between objects, or between objects and observers, is constant (in other words, where velocities do not change). It was not until a decade later, in 1915, that Einstein published his general theory of relativity that fully incorporated modifications for acceleration and the phenomenon of gravity.
If special relativity was a revolution, general relativity was the complete and utter dismemberment of the physics that had gone before it. One of Einstein's critical insights was that if you or I were to float weightless out in the distant, empty universe, it would be entirely equivalent to falling in the gravity field of a massive object. This simple observation led him to redefine gravity itself.
The essential point for now is this: general relativity tells us that mass and energy distort the shape of both space and time, curving them as if they were part of a flexible sheet. What we call gravity is really just the way that objects move in this distorted space and time. Even light, which has no mass and a fixed velocity, is subject to its effects. If the path of light is distorted, then light too "feels" the force of this strange phenomenon as its rays are bent around massive objects. Einstein's relativity was one of the most profoundly disturbing ideas of the age. It is still considered a major conceptual challenge, but it provides the best description we have yet of the nature of the universe.
A key consequence led on from Einstein's earlier results. Special relativity had shown that the energy, or wavelength, of light changes with what we measure the velocity of its source to be. A source of light moving toward us will appear bluer, shifted to shorter wavelengths and higher energies. A source moving away will appear redder, shifted to longer wavelengths and lower energies. All the while, the speed of the light stays the same. The size of this effect in our everyday experiences is negligible. Out in the universe, however, objects can move fast enough for these effects to become starkly obvious.
General relativity demonstrated that the same effect occurs in the distorted space and time around massive objects. Light that comes from a source deep inside the distortion around a mass will be seen as shifted to lower energies, or redshifted. The effect is often termed gravitational redshift: photons have lost energy as they have "climbed" out of the gravitational "well" of an object—although their speed remains unchanged. Equally, if an observer is sitting deep inside the distorted space and time around a massive object, then the light arriving from the distant universe will appear to be shifted to higher energies—blueshifted—as it follows its path into the gravity well. Even more disturbing, the distortion of space and time results in events appearing to happen more slowly the closer they are to a large mass, as seen from a distance. Experiments have confirmed this effect. If you had the willpower to sit in a balloon for forty-eight hours roughly six miles above the Earth, you would age by almost 0.0000002 seconds more than someone who had stayed on the ground. Gravity slows time, and this is exactly the same phenomenon as the loss of energy associated with a gravitational redshift.
It took many years after Einstein published his theory of general relativity for some of the implications and details to be worked out. Even Einstein himself did not produce a complete model for how an object like a massive star distorts the fabric of the universe around it. However, hot on the heels of his own breakthroughs, a fellow physicist had an insight that would play a vital role in the application of relativity to this problem.
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As unlikely as it seems today, the forty-two-year-old scientist Karl Schwarzschild wrote some of the most impressive works on relativity and quantum physics while stationed at the savage Russian front of the First World War in late 1915. Born to Jewish parents in Germany, Schwarzschild, like Michell, was a polymath with a penchant for astronomy. His genius was recognized in childhood, and by his late twenties he was an established professor in the upper echelons of academia. As war broke out, Schwarzschild dutifully signed up, joining the German artillery. Somehow he continued his scientific work. In a letter to Einstein, he derived a mathematical solution that described the distortion of space and time surrounding a massive spherical object. In a second letter he derived a solution for the curvature of space and time inside such a massive object, assuming it was uniformly dense. Tragically, within six months of sending Einstein his calculations, Schwarzschild died of illness on the front, never to see the ultimate implications of his work.
Paramount in Schwarzschild's legacy is a formula that now bears his name. The Schwarzschild radius establishes a relationship between the mass of an object and its effect on light. This was the critical link that would eventually demonstrate that Michell and Laplace's dark stars might actually exist in our universe.
When Michell and Laplace thought about the properties of these massive objects, they mistakenly considered light to be made up of little bodies that would feel the pull of gravity just like a rock or a tennis ball or anything else. According to this theory, we fail to see light emerging from the surface of these stars because it has been pulled back into the star by the force of gravity. But if you could travel toward a dark star, you would begin to encounter these corpuscles of light before they fell back to its surface. If you moved a little closer still, you would see them looping past on their trajectories, like the curving flight of trillions of balls thrown upward and falling back to the ground. All you'd have to do is get close enough, and the light of the dark star would begin to reveal itself.
Remarkably, it is here that Michell's dark stars come crashing into our modern world. In Michell's language, at some distance from a sufficiently massive object the velocity required to escape the gravitational pull of that object begins to exceed the speed of light. Light is halted, and the object is dark to the outside universe. Yet we now know from experiment and fundamental theory that light has no mass, and its velocity remains unchanged. It simply follows the shortest path in time and space. Based on the principles of general relativity, what Schwarzschild's formula suggested was that there is nonetheless a distance from the center of a mass from which light cannot escape further.
Schwarzschild's radius corresponds to a singularity in his mathematical solution to the distortion of space around a spherical mass. A mathematical singularity is simply a point at which an algebraic expression provides no meaningful answer, like calculating the value of one divided by zero. In the case of Schwarzschild's wonderful formulation, such a singularity occurs at a particular distance from a massive object and indicates an extreme curvature of space and time. But, intriguing as it may be, is Schwarzschild's radius just some mathematical tomfoolery, or does it correspond to something observably real? The answer is that while the singularity can be smoothed away by the right choice of mathematical variables, there is nonetheless something remarkable about this location. All paths at this radius turn inward, even for light itself. For you as an outside observer, the light is also redshifted—its wavelength stretched—by an infinite amount. No matter how close you get, you will never see photons coming from within.
Einstein had demonstrated that light is the measuring rod of the cosmos, knit into the very web of the observable universe. It defines the way we experience the world. It defines the way that all matter and energy interact. The Schwarzschild radius is more than a point from which light cannot escape. From the frame of reference of an external observer, it represents the place where time and space seem to come to a halt. If you could place a clock at this location and observe it safely from outside, it would appear to have stopped. Strictly speaking, it would also fade entirely from view, as light coming from it is redshifted to nothingness as it climbs up to you. Anything occurring inside this point, any event, can never be seen in the external universe. For this reason, the Schwarzschild radius is also known as the event horizon.
The most obvious question, and one that came up again and again in the decades following these revelations, was whether such places could actually exist in the cosmos. The mathematical definition of the Schwarzschild radius is a very simple function, a fixed property of the mass of any spherical object. The tricky issue is that the actual value of this radius is very small. For example, while the Earth has a mass of about 13 trillion trillion pounds (6 × 1024 kilograms), its Schwarzschild radius is only about 9 millimeters—less than half an inch.
Herein lies part of the problem. You would have to pack the entire mass of the Earth within that 9-millimeter radius to create the event horizon. Given the real size of the Earth, there is clearly no point at which space and time ever become distorted enough to prevent the escape of light. Our huge Sun has a mass about 332,000 times greater than the Earth, and a radius of more than 400,000 miles. It would have to be compressed by a factor of more than 200,000 to fit inside its Schwarzschild radius of 3 kilometers, or 1.86 miles. Only then would space and time be distorted enough to prevent the escape of light.
While general relativity had provided a more complete description of the nature of gravity, and a rigorous and satisfying demonstration that dark objects could in principle exist, everyone had a very hard time believing that such nonsensical things could actually be out there.
Ironically, Einstein himself was one of the people arguing against the plausibility of such an object. What Einstein objected to, in company with the mighty English physicist Arthur Eddington and others, was the idea that real places could ever meet the necessary criteria to create an event horizon. There was also no obvious natural process by which any object could be made so compact. This was compounded by the peculiar nature of the event horizon. Time itself would slow to a halt at this point. From the viewpoint of the external universe, this might prevent anything real, with lumps and bumps, from ever vanishing entirely inside this radius. It would be stuck in stasis forever.
There were different ways of framing some of these arguments. Einstein used the example of a cloud of small masses orbiting one another, like stars orbiting in the space and time distortion, or gravity field, of their combined mass. The more compact this cloud becomes, the faster and faster the small masses need to orbit to keep the cloud from succumbing to gravity and collapsing toward its center. If the cloud becomes small enough to shrink within its Schwarzschild radius, then the little objects would have to move faster than the speed of light, which Einstein reasoned was impossible.
Over the next decades, a remarkable cadre of some of the greatest scientists of the twentieth century gradually broke through a series of highly complex and challenging physics problems that would finally resolve this issue. Other extreme environments would turn out to be far more commonplace in the universe than anyone had suspected. These would be the stepping stones to an answer.
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Beginning in the early 1930s, another revolution in science was well under way. This was the formulation of quantum mechanics, the physics of atomic and subatomic scales and the dual nature of matter as both particle-like and wave-like. If general relativity had toppled our previously tidy picture of the nature of reality off its perch, quantum mechanics took it on an extended bender that few people, if anyone, could or still can completely comprehend.
Many scientists played key roles in the development of this new physics, from Einstein himself to Max Planck, Niels Bohr, Werner Heisenberg, and others. In 1927, Heisenberg was the first to formulate one of the most philosophically challenging and strange parts of quantum mechanics—the uncertainty principle. At the core of this extraordinary description of the physical world was the fact that at microscopic scales nature has an inherent "fuzziness." For example, it is impossible to precisely measure both the location of something and its momentum—its mass multiplied by its velocity. If the location of an object like an electron, which occupies scales on the order of femto-meters (10-15 meters), is known to high precision, then its momentum will be very poorly constrained. Because "measurement" always involves actual interaction—for example, trapping the electron in a tiny space—there is no getting around this. This intrinsic uncertainty to the world opens up all manner of deeply disturbing phenomena, from parallel realities to virtual particles, appearing out of nothingness and vanishing again. Nonetheless, seen through the protective shielding of mathematics, quantum theory is clearly a good description of the universe around us. The behavior of atoms and electrons, of atomic nuclei, and of light and electromagnetism is accurately described by quantum mechanics.
As this deeply weird subatomic reality was revealing itself, other critical developments were taking place in stellar astrophysics. It became increasingly clear throughout the early 1900s that stars and star-like objects were not permanent fixtures but constantly evolving. Not only did they come in a wide variety of sizes and colors, but somehow they also represented different stages in the life cycle of a single phenomenon. And the only known energy source capable of powering stars had to be that of nuclear fusion, in which matter is transformed to energy—now described by the special theory of relativity and quantum mechanics.
By the late 1950s the major pieces had fallen into place. We knew that stars were objects in which gravity competes against pressure—the pressure of a mix of electrons and atomic nuclei known as plasma, and even the pressure of light itself. Gravity "tries" to compress, or collapse, matter inward. The outward pressure tries to keep matter from collapsing. This competition results in the cores of stars reaching temperatures of tens of millions of degrees. Such extreme conditions are sufficient for the nuclei of elements to bind or fuse together, forming or synthesizing heavier elements and releasing energy. This is critical, or else life-forms such as ourselves could never exist.
Most of the visible matter in the universe still consists of hydrogen and helium. These are the primordial elemental remains of the hot young universe following what has come to be known as the Big Bang. All the carbon, nitrogen, oxygen, and every other heavy element in the universe came along later. The stars are responsible. By fusing hydrogen and helium into larger and larger atomic nuclei, they act as cosmic pressure cookers, serving up new elements.
The recipes get complicated, but the more massive a star is, the heavier the elements it can eventually synthesize. Also, the greater the mass of a star, the faster it can "burn" up the lighter elements that serve as fuel. While a star like our Sun may cook atomic nuclei for a total of about 10 billion years, a star that is twenty times more massive may eat through its fuel in just a few million years. The least massive stars, a mere tenth of the mass of our Sun, can quietly burn for a trillion years or more.
The ultimate fate of stars was a critical part of these discoveries. A star bereft of its central source of energy is an object in which gravity might win its war with pressure once and for all. This too is a complex problem, but there are signposts in nature. The decades following the early 1900s saw a steady stream of increasingly sophisticated and challenging observations about the universe around us—in particular, the discovery and characterization of distant astrophysical objects that were clearly nothing like our Sun, or its familiar stellar neighbors. Among these were bodies called white dwarf stars. Despite being extremely dim, they exhibited the colors of light that one would expect from a big, very luminous, and very hot star. In the 1920s astronomers realized that these were actually tiny objects, far smaller and far, far denser than typical stars. We now know that their density is such that a mere cubic centimeter, about the size of the tip of your pinky, can have a mass of millions of grams. To put that in some kind of perspective, a cube of white-dwarf material only about thirteen feet on each side would have the same mass as all of humanity.
Stellar astrophysics provided an explanation for the origin of such objects as the remnants, the burnt-out husks, of stars like the Sun. However, explaining how such a dense object—although still much larger than its Schwarzschild radius—could exist in a stable state was a much trickier question. For an object as compact as a white dwarf, the normal pressure forces, the same push and shove of atoms that keeps our Sun from imploding because of gravity, simply do not suffice.
The first critical insight into this problem came from the English physicist Ralph Fowler. An athletic and vigorous Cambridge scientist, Fowler had moved hungrily through mathematics to physics and chemistry. In the 1920s he deftly applied the newly minted quantum mechanics to the question. The equations revealed that as matter is forced into denser states, a new type of pressure, with a barely noticeable role in "normal" environments, such as here on the surface of Earth, must come into play. As the atoms in a white dwarf are squeezed together, the electrons are increasingly confined, boosting their momentum and unveiling more and more of their wave-like nature. Quantum mechanics dictates that the little electron waves are not allowed to impinge on each other; the particles must remain distinct. This creates a force known as degeneracy pressure that pushes back against gravity in the white dwarf, far exceeding the pressure of a normal gas. Fowler understood that this pressure didn't even depend on temperature. In fact, a white dwarf could, given enough time, cool off to absolute zero and its electron degeneracy pressure could still support it! But was there some limit? How massive could a white dwarf be and still not collapse under its own gravity?
It took the genius of a young physicist training in Madras in southeast India, named Subramanyan Chandrasekhar, to crack the problem, with a piece of insight that effectively married the varied findings of relativity, quantum mechanics, and gravity.
In any stable object with the density of a white dwarf, the electrons end up fizzing about in their tiny compressed volumes exceedingly fast. Speeds well over 50 percent of the speed of light are common. The more massive a white dwarf, the higher this speed gets as the electrons get squashed into less and less space, and their wave-like nature takes over more and more. There are two remarkable consequences. The first is that in contrast to mundane objects like normal stars, the more massive a white dwarf is, the smaller it gets. The second is that since nothing can travel faster than the speed of light, there is a very real limit to how massive the dwarf can be. Eventually the electrons cannot fizz any faster, their degeneracy pressure cannot increase any further, and gravity must overwhelm the object.
Although it would suffer tremendous criticism and take many years to become fully accepted and recognized, in 1935 Chandrasekhar presented his complete theory explaining the behavior of all white dwarfs. He also predicted the maximum mass that they could ever attain. He had realized that this new degeneracy pressure was only enough to prevent a white dwarf from collapsing under its own weight if the white dwarf never exceeded a mass about 1.4 times that of our Sun.
There are many other fascinating threads to this tale, but Chandrasekhar's beautiful insight was pivotal. Here were hints at an answer to the puzzlement and distaste felt by Einstein and other physicists over how any real object could come close to inhabiting its Schwarzschild radius. Here too was a linchpin in understanding the life cycles of stars themselves—many of which end up as white dwarfs. It is not surprising that the great modern observatory of X-ray photons, capturing light from 12 billion years across the universe, was affectionately christened "Chandra."
Dissecting white dwarfs was just the beginning. As the nature of stars yielded more and more secrets to human understanding, so did the nature of the subatomic realm. The twentieth century saw an unprecedented entwining of science with the development of weapons and the politics of war and economics. As physicists on both western and eastern sides of the planet raced to build increasingly devastating nuclear bombs, they also pushed forward the science of extreme states of matter. The next piece of the puzzle for dark stars was the realization that an even denser state of matter could exist. Beyond white dwarfs was another possibility, where the electrons were subsumed into the nuclear particles themselves, turning protons into neutrons, to form an object that was in essence a giant and peculiar atomic nucleus—a neutron star. It would be far, far denser and more compact than anything seen before. The American physicist J. Robert Oppenheimer, who had played a central role in the development of the atomic bomb, was one of those who developed the physics necessary to describe such an extraordinary object. Just like the white dwarfs, neutron stars had a limit to their mass. Beyond two or three times the mass of the Sun, gravity would overwhelm them.
Unlike white dwarfs, however, neutron stars had never been observed in nature. This changed in the late 1960s with several intriguing astronomical measurements. The culmination was the spectacular discovery by the scientists Jocelyn Bell and Antony Hewish of a distant neutron star spinning around its axis roughly once a second, assigned to a class of objects subsequently named pulsars. The detection of this object came from a giant array of radio antennae, covering about four acres of land amid the fields a couple of miles west of Cambridge in England. Aided by the lawn-mowing skills of a flock of dedicated local sheep, the Belfast-born Bell and her English thesis advisor Hewish were originally planning to study radio emissions from objects in the distant universe. They were shocked when they found this new pulsing signal. As scientists puzzled over the nature of this object, they realized that the only conceivable explanation was a very, very small and very rapidly spinning body sending out a lighthouse-like beam of radiation. The only astrophysical object that could be this small yet tough enough to withstand spinning this fast was the conjectured neutron star.
Neutron stars make white dwarfs look positively tenuous. A mere cubic centimeter—about the size of a sugar cube—of neutron star material has the same mass as all of humanity. While a white dwarf may contain the mass of the Sun within a sphere roughly the size of the Earth or a little larger, a neutron star can contain twice the mass of the Sun within its radius of about 7.5 miles.
In a neutron star, gravity is resisted by the same kind of degeneracy pressure as in white dwarfs, except that it is now the neutrons themselves, rather than electrons, providing the force. The incredible compactness of neutron stars brings them much, much closer to being contained inside their Schwarzschild radius. To escape from the surface of one of these objects you would need to move at a substantial fraction of the speed of light—as much as 30 percent, or 62,000 miles a second. Space and time are so distorted or curved that if you fell from an altitude of one meter, you would crash into the surface traveling at roughly 1,200 miles a second.
Finally, here were objects in the universe that hovered on the edge of darkness. And together with more detailed and better-understood models of how stellar remains could implode, they provided the final impetus to let go of the cherished belief that nothing could ever really collapse to within its event horizon. If more matter were to be piled onto these bizarre spheres, there is no known pressure force that could prevent utter collapse to within the Schwarzschild radius, and inwards to a single point that is, to all intents and purposes, of infinite density—an inner singularity. By the late 1960s, the reality that such places existed within the cosmos was generally accepted, and observations of the universe were beginning to turn up some intriguing candidates.
In 1967, the American physicist John Wheeler gave a talk at what is now the NASA Goddard Institute for Space Studies at Columbia University in New York. In this nondescript building, which also houses on the ground floor a restaurant immortalized by the singer Suzanne Vega as "Tom's Diner," the charismatic Wheeler used the term "black hole" to characterize an object collapsed within its Schwarzschild radius. It stuck. After a journey of two hundred years, Michell's dark stars had finally become black holes.
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Since then we have learned more and more about these extraordinary objects. Earlier on I stated that black holes play a critically important role in making the universe the way it is, and in setting the stage for life itself. That may sound pretty outrageous, but this universe of ours is far more interconnected and far more nuanced than we might have suspected even ten years ago. The concepts that help us get a grip on it are also some of the most important and critical ideas in the physical sciences over the past century or so. We've encountered a few of them above. They include the finite and unchanging speed of light; the nature of space and time, mass and gravity; and, of course, the finite age and scale of the observable universe. I've touched upon others: the nature of stars, the quantum universe, and the synthesis of elements from primordial hydrogen and helium. Beyond those are further components, ideas that are still at the very cutting edge of human understanding: the ways in which this universe makes stars in the first place; the formation of worlds; the molecular structures that pervade interstellar space yet are the same flavor as those that make life on a planet. It's a remarkably diverse set of ideas, and so some clear perspective would be a good thing.
We have already traversed the cosmos from a colossal black hole in a now-ancient galaxy to our own microscopic speck of rock and metal. But what do we know about the size and shape of the observable universe? What does it look, feel, and smell like? If we are to understand what forms it, what makes it appear the way it does, and how to navigate its highways and byways, peaks and plains, nooks and crannies, we will need to begin with a very, very good map.

Copyright © 2012 by Caleb Scharf