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

by Scharf, Caleb

  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.

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  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 com
puted 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.

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  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 i
n 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.

  Figure 2. Illustrating the idea behind the Michelson-Morley experiment. Imagine two fish in a flowing river (the aether). They both always swim through their medium by pushing with the same constant force. One fish (1) swims toward and back from a buoy anchored a distance across the river; the other fish (2) swims to a buoy the same distance away, but upstream. Michelson and Morley realized that it takes the fish different amounts of time to complete their round-trip swims, and that photons should behave similarly if they are interacting with a medium. On the right-hand side is a diagram plotting the relative travel position of each fish versus time. The cross-stream fish (1) fights against the sideways current equally on its trips to and from the buoy. The upstream fish (2) must first fight hard as it swims headfirst into the current, but then it speeds back to its starting point. Nonetheless, the cross-stream fish (1) will always manage to arrive back at the starting point first—it appears to have swum faster. This is exactly the principle Michelson and Morley sought to exploit by bouncing rays of light back and forth—upstream and across the stream of the hypothetical aether.