Black Hole

How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved

Marcia Bartusiak

• Publisher: Yale University Press
• Year: 2015
• ISBN:
  9780300213638
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

The award-winning science writer "packs a lot of learning into a deceptively light and enjoyable read" exploring the contentious history of the black hole ( New Scientist ). For more than half a century, physicists and astronomers engaged in heated dispute over the possibility of black holes in the universe. The strange notion of a space-time abyss from which not even light escapes seemed to confound all logic. Now Marcia Bartusiak, author of Einstein's Unfinished Symphony and The Day We Found the Universe, recounts the frustrating, exhilarating, and at times humorous battles over one of history's most dazzling ideas. Bartusiak shows how the black hole helped revive Einstein's greatest achievement, the general theory of relativity, after decades of languishing in obscurity. Not until astronomers discovered such surprising new phenomena as neutron stars and black holes did the once-sedate universe transform into an Einsteinian cosmos, filled with sources of titanic energy that can be understood only in the light of relativity. Black Hole explains how Albert Einstein, Stephen Hawking, and other leading thinkers completely changed the way we see the universe.

1

It Is Therefore Possible That the Largest Luminous Bodies in the Universe May Be Invisible

It all began with Sir Isaac Newton.

No, let me take that back. The ancestry of the black hole can actually be traced back much earlier. You could say it began in ancient times when that distant era’s most clever thinkers—the now-forgotten Newtons and Einsteins of their day—pondered why our feet stay firmly planted on the ground. That was an obvious question for a budding scholar to ask in that bygone age.

Everything centers on gravity. Gravity controls the sweep of the planets around the Sun, as well as the fall of a fluttering leaf from an autumnal tree. It is a force we take for granted but took us centuries to understand. Why is stuff attracted downward, toward the surface of the Earth? More than two thousand years ago, Aristotle and other ancient philosophers had a ready and reasonable reply to that question: our planet resided at the very center of the universe, so naturally everything fell toward it. Humans, horses, carts, and buckets were all driven to reside at the prime position. As a consequence, we were sturdily attached to terra firma. It was the natural state of things.

This explanation made perfect sense, given our daily experience. That is, until Nicolaus Copernicus stepped in and dramatically changed the cosmic landscape once and for all. In 1543 the Polish administrative canon dared to assert that Earth was in reality orbiting the Sun with all the other planets. Others, such as Aristarchus of Samos in the third century BCE, had suggested this scheme before, but not until Copernicus introduced his heliocentric universe did it finally take root. As a result, the long-assumed setup that kept us bound to our planet had to be completely reexamined. Earth was no longer at rest in the universe’s hub, serenely waiting for objects to rain down upon it. Instead, the Earth was yanked into motion and the Sun was now at center stage. This new alignment soon motivated some of the greatest minds in Europe to reassess the rules of gravity, as well as the mechanism behind planetary motion. The challenge was on.

Inspired by Englishman William Gilbert’s claim in 1600 that the Earth was a giant magnet, the German mathematician Johannes Kepler suggested that threads of magnetic force emanating from the Sun were responsible for pushing the planets around. The French philosopher René Descartes in the 1630s, in contrast, imagined that the planets were carried around like leaves trapped within a swirling whirlpool by vortices of aether, the tenuous substance then thought to permeate the universe.

All these ideas would eventually be overturned, though, once Isaac Newton in 1687 offered a more rigorous set of rules for both gravity and planetary motion. That was the year he published his masterful Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy). We know it today as simply the Principia. Newton was forty-four years of age at the time, but his new take on gravity had been percolating in his mind long before that.

It began in 1665 during the Restoration reign of King Charles II, when the black plague was once again rampant. To wait out the epidemic, Newton temporarily left his studies at Cambridge University and retreated to his childhood home in the rural hamlet of Wools-thorpe, just east of Nottingham. It was there that the precocious student possibly watched that fabled apple fall in his country garden, which inspired him to reflect on the tendency of bodies to fall toward the Earth with a set acceleration. Did the same force acting on the apple extend to the Moon, he asked himself? A virtuoso at mathematics, largely self-taught, he computed that the Moon did seem to be continually “falling” toward the Earth—its path becoming curved—by an earthly pull that diminished outward by about the square of the distance. In other words, double the distance between two objects and the force between them is reduced to one-fourth of its original strength. Triple the distance and the force is diminished to one-ninth. Mathematically, this is a sign that a force is spreading its influence equally in all directions. But since Newton’s early figurings weren’t flawless, the young man put the problem aside for many years. “He hesitated and floundered,” wrote Newton biographer Richard Westfall, “baffled for the moment by overwhelming complexities.”

Newton’s interest in gravitation didn’t fully resurface until the 1670s. That was the period when Robert Hooke, curator of experiments for Great Britain’s Royal Society, developed an appealing set of conjectures for explaining gravitation: that all celestial bodies have a gravitating power directed toward their centers; that they can attract other bodies; and that the attraction is stronger the closer you are to the body. Hooke had a general set of rules but as yet no equations. What he couldn’t figure out, as he noted in his published paper, was whether planetary motion would necessarily be “a Circle, Ellipsis, or some other more compounded Curve Line.” Triggered by an exchange of letters with Hooke on this topic in the winter of 1679–80, Newton was galvanized to return to the problem of his youth.

The famous tree (center) at Woolsthorpe Manor, England, from which Isaac Newton allegedly saw an apple fall under the influence of gravity. (Roy Bishop, Acadia University, courtesy of American Institute of Physics Emilio Segrè Visual Archives)

Yet, at first, he kept his revolutionary results to himself. That was because Newton, an intensely private man, was wary of his jealous rival Hooke. Often fearful of exposing his work to criticism, he once confessed in a letter to a colleague, “I am … shy of setting pen to paper about anything that may lead into disputes.” We largely have Edmond Halley (the famous comet’s namesake) to thank for Newton’s writing the Principia at all. While visiting in 1684, Halley asked the illustrious physicist how a planet would move under an inverse square law. Newton confidently replied, “An ellipsis,” what we now call an ellipse, noting that he had worked it out several years before.

From that very moment, Halley became Newton’s staunchest advocate. It was Halley’s persistent prodding and financial backing that finally energized Newton to write his masterpiece on gravity. And once committed, Newton didn’t hold back. Westfall notes that Newton had an enormous capacity for “ecstasy, total surrender to a commanding interest,” often forgetting to eat or sleep when in this state. Halley had now sparked that intellectual rapture once again. Newton swiftly abandoned his ongoing projects (among them, classical mathematics, theology, and alchemy) and fully applied his legendary power of concentration on completing his work on gravity. Armed with better measurements of the Earth, he was at last able to prove decisively that an inverse square law attracted the Moon to the Earth and that such a force directly leads to planets moving in elliptical orbits, just as Kepler had revealed in 1609. Kepler knew from his measurements that planets moved in elliptical orbits but did not know the reason. Decades later, Newton showed through his mathematics that such orbits were a natural consequence of the law of gravity. Observation and theory, working from opposite corners, came together and matched.

It took Newton nearly two years to complete the Principia, and for understandable reasons. Encouraged by his initial and successful calculations, Newton worked out more and more cases with his new rules, and as a result long-standing problems in astronomy seemed to dissolve away. Gravity could now explain the tides and the Earth’s precession (due to the Moon and Sun’s tugging on the Earth’s bulge), as well as the trajectory of a comet. In a grand conjectural leap, Newton was declaring that gravity was a fundamental and universal force of nature. That term universal was a key insight. What draws an apple to the ground also keeps the Moon in orbit about the Earth. “For nature is simple,” Newton wrote, “and does not indulge in the luxury of superfluous causes.” The cosmos and terra firma were no longer separate realms, as Aristotle had long reasoned; the heavens and the Earth now operated under one set of physical laws. Gravity, the attraction of one body for another, acts in a similar manner on all levels of the cosmos—on Earth, within the solar system, as well as among the stars, galaxies, and clusters of galaxies.

There was one problem with Newton’s law of gravity, though. It implied that imperceptible ribbons of attraction somehow radiated over distances, both short and long, to draw moon to planet and boulder to Earth. For many, this feat appeared more resonant with mysticism than science. Critics demanded a physical mechanism. That is what natural philosophers had been providing for centuries. In what way was gravity doing its work? What was replacing either magnetism or vortices? This led to Newton’s famous statement in the Principia: “I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses.” Newton, unlike his contemporaries, was not going to stoop to speculating or conjuring up some kind of hidden cosmic machinery. He was essentially satisfied that his laws allowed physicists to calculate, with great accuracy, the movement of a planet or the path of a cannonball. As the years passed, the rest of the physics community eventually came over to Newton’s side. What greatly helped was the visit of a celestial traveler.

After poring over historic records, Halley figured that a comet sighted in 1682 had much in common with comets previously observed in 1531 and 1607. They shared the same orbital characteristics, going around the Sun in the opposite direction to the planets, and appeared every seventy-five to seventy-six years. Upon calculating the comet’s orbit based on Newton’s laws, he predicted in 1705 that the comet would return at the end of 1758. And so it did, right on schedule sixteen years after Halley’s death, thirty-one years after Newton’s. This feat bedazzled and instantly silenced Newton’s critics. Who could argue with a theory that allowed for a spot-on prediction about the solar system’s behavior more than half a century in advance? It was at that moment that Newton’s law of gravity, despite its lack of a mechanism, was at last victorious.

With Newton’s laws in place, scientists in the eighteenth century came to view the universe as intrinsically knowable, ticking away like a well-oiled timepiece. Many astronomers began spending long hours huddled at their desks using Newton’s mathematical rules to work out planetary motions and to forecast the tides. Stars, as well, became convenient objects for testing the laws of gravity. And it was during such a stellar endeavor that a precursor to the black hole—the Model-T version, in a way—emerged. The possible existence of such a bizarre object arose when an Englishman named John Michell applied Newton’s laws to the most extreme case imaginable.

Michell stood in the very thick of a wondrous age of scientific discovery, and he dabbled in it all. He was a geologist, astronomer, mathematician, and theorist who regularly hobnobbed with the greats of the Royal Society of London, such men as Henry Cavendish, Joseph Priestley, and even the society’s American fellow Benjamin Franklin (during the diplomat’s two long stays in London). The claim could be made, science historian Russell McCormmach has written, that Michell was “the most inventive of the eighteenth-century natural philosophers.” He recognized early on, for example, that Earth’s strata could bend, fold, rise, and dip. If Michell is remembered at all today, it is for his suggestion in 1760 that earthquakes propagate as elastic waves through the Earth’s crust. That earned Michell the title “father of modern seismology.” By deeply scrutinizing and comparing various accounts of the great temblor that leveled Lisbon, Portugal, in 1755, Michell was able to compute the time, location, and depth of the quake’s epicenter, situated to the west in the Atlantic Ocean.

Michell also designed a delicate instrument that could measure the gravitational constant in Newton’s equations, allowing him in essence to “weigh” the Earth. He died before he could carry out the experiment, but his friend Cavendish ultimately obtained the torsion balance, made modifications, and successfully measured our planet’s mass with it.

The torsion balance, based on a design by John Michell, that was used by Henry Cavendish in 1797–98 to weigh the Earth. (Philosophical Transactions of the Royal Society of London)

Despite such accomplishments, however, Michell had the unfortunate habit of burying original insights (such as the inverse-square law of magnetic force decades before it was rediscovered) in journal papers that focused on more mundane research, and so received little notice. Some of his greatest ideas were casually mentioned in asides or footnotes. As a consequence, long-lasting fame eluded him.

Michell had begun his scientific investigations at Queens’ College in Cambridge. Son of an Anglican rector, he entered Queens’ in 1742 at the age of seventeen and after graduation remained there to teach for many years. A contemporary described him as a “short Man, of a black Complexion, and fat. … He was esteemed a very ingenious Man, and an excellent Philosopher.” While in Cambridge, Michell even tutored a young Erasmus Darwin, Charles Darwin’s grandfather, who called his mentor “a comet of the first magnitude.”

But by 1763, ready to marry, Michell decided to leave teaching and devote himself to the church. He ultimately settled in the village of Thornhill in West Yorkshire, where he served as a clergyman until his death in 1793 at the age of sixty-eight. Yet, over those decades with the Church of England, the reverend continued to indulge his wide-ranging scientific curiosity. He had a nose for interesting questions and was willing to stick his neck out in speculation, though always grounded in his first-rate mathematical skills. One of Michell’s more intriguing conjectures at this time, right when Great Britain was recovering from its war with colonial America, was imagining what today we call a black hole.

This idea grew out of an earlier prediction that Michell had made. Astronomers in the eighteenth century were starting to see more and more double stars as they scanned the celestial sky with their ever-improving telescopes. The common wisdom of the time declared that such stars were actually at varying distances from Earth and closely aligned in the sky by chance alone—that it was just an illusion that they were connected in any way. But, with remarkable insight, Michell argued that nearly all those doubles had to be gravitationally bound together.

He was suggesting that some stars exist in pairs, a completely novel notion for astronomers in those days. In a groundbreaking paper published in 1767, Michell worked out the high probability that, given how most other stars were arranged on the sky, the twin stars were physically near each other—“the odds against the contrary opinion,” he stressed, “being many million millions to one.” (As usual, he displayed his computations in a footnote.) In carrying out this calculation, Michell was the first person to add statistics to astronomy’s repertoire of mathematical tools. This paper, according to astronomy historian Michael Hoskin, was “arguably the most innovative and perceptive contribution to stellar astronomy … in the eighteenth century.”

At the same time, Michell recognized that double stars would be quite handy for learning lots of good things about the properties of stars—how bright they are, how much they weigh, how vast is their girth. Michell suspected that there were stars both brighter and dimmer than our Sun. He cunningly ventured that a white star was brighter than a red one. (“Those fires, which produce the whitest light,” he pointed out in his paper, “are much the brightest.”) So, two stars orbiting each other were the perfect laboratory for testing his ideas from afar and arri...