

An excerpt from
Cracking the Einstein Code
Relativity and the Birth of Black Hole Physics
Fulvio Melia
Chapter 1
EINSTEIN’S CODE
The scene could have been straight out of Universal Studios, Hollywood. Two men are breathing rhythmically in a smokefilled modest little room facing south toward the capital of Texas. They sit quietly no more than an arm’s length apart, lost in thought. Roy Kerr, the younger of the two, is hunched over a secondhand desk with his back to the door, studying the equations he has just scribbled in a notebook. His older friend and mentor, Alfred Schild (1921–1977), puffs away at a pipe while occupying a wornout armchair to his right. It is late morning, and rays of sunlight filter through the bushes outside the window, creating a mosaic of light and shadow across the paneled walls.
At stake in this drama is the breakthrough solution to Einstein’s equations of general relativity that have defied the greatest scientific minds of the twentieth century. So impenetrable is this description of nature, that Einstein himself succeeded only partially in divining its impact on the meaning of space and time. But it is now 1963, and the freshly minted mathematician out of Cambridge University, settling in at Schild’s newly established Center for Relativity at the University of Texas at Austin, is about to crack the great physicist’s famous code.
Much has been written about Albert Einstein (1879–1955) and his profound influence on our view of the universe, but very little is known about the golden age of relativity, spanning the period 1960–75 following his death. This book is the story of the brilliant young scientists of that era who accepted the challenge of unraveling the mysteries hidden within the seemingly unfathomable language of general relativity, culminating with Kerr’s uncloaking of one of the most important and famous equations in all of science.
It is not always possible to discern the reasons why a scientific investigation meanders raggedly or slowly toward its ultimate goal, but in the development of relativity, the complexity of its mathematical formalism is certainly one of them. The difficulty of designing suitable experiments to test Einstein’s theory is another. But neither of these reasons emerged for want of interest. Einstein became an instant celebrity soon after founding general relativity in 1915–16, with the quick, auspicious confirmation of one of his predictions—that gravity should bend the path of light as well as that of any particle with mass. This result resounded across the front pages of newspapers around the world, and scientists took note of the new ideas almost right away.
Indeed, only a few months after Einstein’s publications began circulating around Europe, Karl Schwarzschild (1873–1916), a soldier on the Russian front, amazingly already succeeded in finding a description of space and time consistent with Einstein’s theory, but only for a highly idealized situation, that is, for the gravitational field surrounding a static, spherically symmetric mass. Einstein greeted Schwarzschild’s news with enthusiasm, and his solution is used to this day to describe phenomena in regions of strong gravity.
How odd, then, that arguably the most elegant scientific theory ever devised should slowly wither into the decades that followed this remarkable beginning. Those who knew him best have written that already by the 1930s Einstein’s interest in general relativity had almost completely lapsed. Having by then moved to Princeton, he could count the number of colleagues working in this field on just one hand. Relativity theory had become irrelevant to science—a situation that sadly persisted up until Einstein’s death. He would never know about the breathtaking discovery that would be announced just a few years later—a splendid confirmation of another prediction made several decades earlier.
This experimental achievement—a compelling demonstration in 1960 by the Harvard physicists Robert Pound and Glen Rebka that time slows down in the presence of gravity—sparked the revolution that followed during relativity’s golden age, leading to that special moment in Roy Kerr’s sepiatinted office shortly afterward.
In the intervening years, failure to uncover practical applications of Einstein’s theory was compounded by the lack of progress in the experimental verification of general relativity as the correct description of nature. Ironically, part of the problem was the Schwarzschild solution itself, which in time would be used to predict that truly bizarre objects, variously called dark or frozen stars, must exist somewhere in the cosmos. Today we call them black holes, but back then no one—particularly Einstein—believed they could be real. Yet the Schwarzschild solution clearly demonstrated that the end result of a gravitational collapse must be the formation of a singularity—a point of infinite density—that creates a closed pocket of space and time forever disconnected from the outside world.
Many thought that nature could not possibly create something so unreasonable, believing that no object in the universe is truly static and that, at the very least, its rotation would inhibit any collapse toward a singularity. And so began the search for the “holy grail” of relativity—a description of space and time surrounding a spinning object. Everything we see in the universe rotates, the argument went, so in order to demonstrate that Einstein’s theory is a true description of gravity, we must be able to show that his equations do in fact describe space and time surrounding a spinning mass.
But what a challenge this turned out to be! Some of the world’s most renowned physicists spent their entire careers working on this problem, making some progress, but losing interest or hope in their waning years. Of course, by the middle of the twentieth century, quantum mechanics had forged well ahead of relativity in relevance and measurability, cementing its place as the overarching theory in the physics pantheon. It didn’t help that relativity and quantum mechanics seemed to be incompatible with each other, since the former uses perfectly measurable locations and times, whereas the latter is essentially a theory of spatial imprecision.
The PoundRebka experiment changed all that, principally because even the quantum mechanicists could not easily discount its remarkable implications. In fact, among the staunchest supporters of relativity and its relevance to modern physics was Vitaly Ginzburg, corecipient of the 2003 Nobel Prize in physics for his work in the 1950s on superconductivity, a phenomenon in which some materials carry currents freely, without any resistance, by virtue of a quantum effect that becomes important at very low temperatures.
Though his interests were mainly in quantum mechanics, Ginzburg would nonetheless become an inspirational figure to many young physicists drawn to Einstein’s theory in the early 1960s. Listening to him, the twentyeightyearold Roy Kerr understood that “cracking Einstein’s code” was indeed the challenge he should ply with his mathematical talents—a task that would soon bring him to that fateful day sitting next to Alfred Schild in his smokefilled office.
But the story begins well before Kerr’s arrival in Texas, even before Einstein himself, in fact. Musings concerning the nature of space and the meaning of time began to appear thousands of years earlier, in places such as the Greek colony of Elea in southern Italy. Before we explore the evolution in Einstein’s thinking that led to his theory of general relativity, and the inspired work that followed during its golden age, we will therefore begin by tracing some of the incipient thinking that led to the problem in the first place. Our journey commences in the fifth century BC with the Greek philosopher Zeno, a man clearly far ahead of his time. Zeno realized even back then that the notion of an absolute space independent of time was paradoxical—anticipating by several thousand years the eventual unification of the two into the structure we now refer to as simply spacetime.
