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.
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A MAP OF FOREVER
Making a map of nature is such an instinctive and appealing notion. As a species, we have found it incredibly difficult to resist mapping and charting everything we see around us. Not only do we like to know where we are at any given moment, we also like to know the arrangement of the world far beyond our visible horizon. Maps help us organize our model of the world, and they can help us test whether that model fits reality.
For more than ten thousand years we have scraped, chiseled, drawn, and painted charts of our immediate surroundings and of the larger regions beyond. Driven to explore, we have expanded the boundaries of the known world from our every point of origin. What was once a blank expanse labeled with helpful information like “Here Be Dragons,” we have transformed into familiar and well-known terrain. Over time, though, terrestrial maps have reached their limits—the inevitable consequence of living on a sphere. Bereft of new continents, eager cartographers have looked at finer and finer levels of detail, filling in ever more minutiae. Today, many of us can sit in front of a computer and zoom instantly to whatever distant corner of the globe our whimsy takes us to. We can even descend to a street’s-eye view of places we may never travel to in person. Just like all the humans before us, we may long in vain for a still closer look: a peek inside that shop in an unknown town, a glimpse of the headlines on the newsstand in a country we’ll never visit.
Charting the objects we see in the sky has both paralleled and diverged from terrestrial mapmaking. Astronomy is the most ancient of sciences, and it has always been about mapping, and relating to, the shapes and rhythms of nature. The wonderful propensity of the human brain for pattern recognition has enabled us to imagine great richness in the night sky as well. A grouping of stars like the constellation we call Orion is linked to several different myths in cultures throughout the world. The Australian aborigines looked at Orion and saw a canoe carrying two banished brothers. The Finns saw a scythe. In India, that pattern of stars was obviously a deer. For the Babylonians it was the heavenly shepherd, and for the Greeks it was the hunter, a primordial giant.
Astronomy has long motivated both precision and imaginative abstraction in a multitude of societies and civilizations. We find in the recorded histories of practically every known culture that people have developed methods to plot out the locations of stars and planets. Indeed, from Oceania to Asia, from the Middle East and Africa to Europe, and in the Americas, we have keenly observed and depicted the night skies. Also, archaeologists think that Orion is one of the oldest recognizable celestial representations, carved into the ivory of a thirty-three-thousand-year-old mammoth tusk in Germany; another is the Pleiades star cluster in the paintings of the Lascaux caves in France, made some twenty thousand years ago. In the southern planetary hemisphere the indigenous peoples of Australia have complex pictorial and oral depictions of the stars that are a part of their forty-thousand-year-old cultural history.
The curved surface of the Earth has prevented us from easily mapping the globe. But the skies have always been fully accessible to anyone with an interest in making sense of our place in the cosmos, or simply dreaming about what magical realms might exist elsewhere. However, our eyes are limited in their sensitivity. Fainter celestial objects become invisible, and cl
ose-knit points of light become impossibly tangled, often merging into confusing blobs of luminosity. A vast and three-dimensional universe is projected onto our retinas as a flat image. What is far may seem near, and what is near may seem far. Mapping the cosmos has therefore been a gradual progression, both outward and across, locating the brighter objects and then filling in the gaps. We have helped our eyes along by constructing telescopes that can capture far more light than our little biological lenses can, and that bring that light to a much sharper and finer focus. These instruments have also allowed us to overcome one of our greatest scientific handicaps. Evolution has equipped us with remarkable senses, yet has blinded us to most of the universe by giving us eyes that detect only a narrow range of photon wavelengths. Eventually, though, we figured out how to build telescopes that reveal much more than just visible light, opening up the whole electromagnetic spectrum and illuminating phenomena beyond our wildest imaginations.
Seeing the cosmos for what it is has required us to overcome many other blind spots, including a perpetually sticky one for mapmaking: we place ourselves at the center. It is a natural assumption and often a practical necessity, but it has also been an enormous hindrance to developing an accurate model of the universe. It took the insight and intellectual conviction of Galileo and Copernicus to challenge the orthodoxy that our home the Earth, like us, was at the center of the cosmos. Relinquishing that child’s-eye view was no small accomplishment, but the notion that our whole solar system was nonetheless located somewhere at the center of the visible universe was still very much in vogue even into the first decades of the twentieth century. As so often in science, only a major advance in the observation of nature would convince the world otherwise.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 4
