Black Hole: How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled On by Hawking Became Loved
“Like unicorns and gargoyles, black holes seem more at home in the realms of science fiction and ancient myth than in the real Universe.” Thus wrote Kip Thorne, former Feynman Professor of Theoretical Physics at the California Institute of Technology, in his acclaimed 1994 book Black Holes and Time Warps. And right he was, for black holes are weird things indeed, even weirder than unicorns and gargoyles. Moreover, while these mythical creatures are nothing but figments of our imagination, black holes are real and to be found all over in the universe. At least, that is what physicists and astronomers tell us, and we better believe them. Thorne’s book, an authoritative overview for non-scientists of general relativity theory and its many strange consequences, included a historical account of the idea of black holes, the same topic that Marcia Bartusiak presents in a very different but no less fascinating way in her Black Hole. While Thorne is a distinguished theoretical physicist, Bartusiak is an award-winning author of popular books on cosmology and astronomy, including Thursday’s Universe (1986), Through a Universe Darkly (1993), and The Day We Found the Universe (2010).
Although Bartusiak’s beautifully written book is much briefer and less demanding than Thorne’s, it is also more historically sensitive and provides a surprising wealth of well-documented information. Its popular and captivating form of presentation is based on serious research in the scientific and historical literature. Contrary to many science writers (and nearly all scientists), she is aware of and uses effectively the scholarly works written by historians of science. Moreover, she has consulted archives and made interviews with several of the scientists involved in research on general relativity. I found only very few factual errors in the book, none of them of a serious nature. On page 86 Bartusiak writes that the positron, the electron’s antiparticle predicted by Paul Dirac in 1931, was discovered the following year by means of a cosmic-ray “bubble chamber.” It should be “cloud chamber.” (The much more sophisticated bubble chamber was only introduced in the late 1950s.)
Black holes are notoriously difficult to understand and, of course, higher mathematics and technical explanations have no place in a popular book on the subject. As a substitute, Bartusiak makes frequent use of analogies and word images to comprehend what is almost incomprehensible. Here is an example, describing the collision of two black holes: “As the twirling holes are about to meet, spiraling inward faster and faster at speeds close to that of light, it’s predicted the whine will turn into a quick chirp, a birdlike trill that races up the scale in a matter of seconds.” Gravity waves will follow, and near the crash site the waves “could stretch a six-foot man to twelve feet and within a millisecond squeeze him down to three, before stretching him out again.”
As Bartusiak points out, her book is about the history of an idea, of how astronomers and physicists arrived at the astonishing insight that the universe is populated by collapsed stellar objects from which light cannot escape and consequently are invisible in a direct sense. Writing such a history typically means to trace the idea back in time, to look for earlier ideas that in some way or other are considered similar to or adumbrations of the modern idea of black holes. The approach inevitably invites what historians call “presentism,” namely to conceive events in the past in the light of the present. This again invites streamlining of history and sometimes distortions in the form of anachronistic historiography.
Bartusiak describes the collision of two black holes as, “spiraling inward faster and faster at speeds close to that of light, it’s predicted the whine will turn into a quick chirp, a birdlike trill that races up the scale in a matter of seconds.” Gravity waves will follow, and near the crash site the waves “could stretch a six-foot man to twelve feet and within a millisecond squeeze him down to three, before stretching him out again.”
Following the tradition of Thorne and other writers Bartusiak starts with the English clergyman and natural philosopher John Michell, who in 1784 suggested that particles of light would be unable to escape from very large and heavy stars, leaving them invisible. Another key person in the standard history is the brilliant German astronomer Karl Schwarzschild, who in a 1916 paper provided the exact solution to Einstein’s field equations for a uniform spherical mass, and for this reason is hailed as a founder of the black hole as a concept belonging to general relativity. But Michell worked in an entirely different, Newtonian context, and Schwarzschild had no idea that his work might have any physical or astronomical significance. To Bartusiak’s credit, she recognizes that the two scientists did not really work in what currently is called black holes physics, but she nonetheless keeps to the standard historiography, presenting them as precursors of the black hole. Michell’s dark star was a “Model-T version,” as she puts it. There is no room in this kind of history for minor or little known actors such as the Dutchman Johannes Droste, who independently derived Schwarzschild’s results (and is not mentioned by Thorne either). More surprising is it that Bartusiak, in describing how Einstein arrived at his equations of general relativity in the autumn of 1915, disregards the famous German mathematician David Hilbert and his rival work on the subject.
Rather than presenting her story in a strict chronological order, Bartusiak alternates between chapters of a predominantly theoretical nature and chapters describing relevant astrophysical and observational advances. In this way she covers both of the two major paths of the development in a reader-friendly way. The road to understanding black holes on the basis of general relativity started with Schwarzschild, but it was only with the work of John Wheeler, Roger Penrose, Stephen Hawking, and Yakov Zel’dovich in the 1960s that their true nature was gradually recognized. It was shown theoretically that real black holes must rotate, a most important insight due to the New Zealand mathematician Roy Kerr in 1963. In the words of Bartusiak, he “conquered general relativity’s Mount Everest of problems.”
As far as astrophysics is concerned, Black Hole tells the story of the ever more dense objects contemplated by astronomers and physicists since the 1920s, from white dwarfs over neutron stars to the discovery of the enigmatic quasars—at first “quasi-stellar objects”—in the early 1960s. Among the leading scientists in this line of research were Subrahmanyan Chandrasekhar, Fritz Zwicky, and Maarten Schmidt. Bartusiak describes this development vividly and expertly, and she pays particular attention to the work in the 1930s on gravitationally collapsing stars by the odd duo of Lev Landau in Stalin’s Russia and J. Robert Oppenheimer in Roosevelt’s America. While Oppenheimer is primarily known for his seminal role in the Manhattan Project he was also an eminent theoretical physicist, and today his excursion into exotic astrophysics is regarded as a pioneering contribution. Bartusiak calls a prophetic 1939 paper by Oppenheimer and his student Hartland Snyder “the first modern description of a black hole,” an evaluation which is today generally accepted. She also points out that the paper had very little immediate impact and that not even Oppenheimer himself considered it very important.
Do black holes exist in nature or are they merely mathematical constructs residing in the minds of theoretical physicists? While Einstein resisted the idea of matter disappearing in a singularity—corresponding to a point with infinitely large density—by the 1960s it appeared that such objects were at least theoretically possible. It is widely assumed by modern physicists that if an object or phenomenon can be described by the fundamental laws of physics, then it must also exist in nature. Historians of ideas speak of this metaphysical belief as the “principle of plenitude,” the assumption that what can exist does exist. The neutrino was allowed by the laws of quantum mechanics, which was reason enough that physicists accepted the ghost-like particle as real many years before it was detected experimentally. Or think of the Higgs particle predicted in 1964 and detected nearly 50 years later.
Although the story of the black hole is somewhat different, also in this case we witness the principle of plenitude in operation. It was chiefly the majestic authority of general relativity that caused Wheeler and other physicists to think of black holes as real. In the last part of her book, Bartusiak outlines how astronomers since the early 1970s have searched for black holes, the first prime suspect being an X-ray source known as Cygnus X-1. Other and more likely suspects soon followed. By its very nature a black hole cannot be directly observed, but evidence for its existence can be so compelling that it amounts to observational proof. Today’s astronomers are confident that black holes are plentiful in the universe and that our own Milky Way harbors in its center a supermassive black hole.
Bartusiak calls a prophetic 1939 paper by Oppenheimer and his student Hartland Snyder “the first modern description of a black hole,” an evaluation which is today generally accepted. She also points out that the paper had very little immediate impact and that not even Oppenheimer himself considered it very important.
Scientists may think that words are just words, but if so they are mistaken. Because names often carry epistemic connotations with them, they are more than just neutral linguistic terms. As Bartusiak points out in her interesting chapter on the origin of the term “black hole,” it illustrates how a good name—apart from being catchy—can conjure up a mental image that emphasizes the important properties of a physical concept. Other names, such as “collapsed star” or “frozen star” (the latter preferred by Zel’dovich and other Russian theorists), tended to produce mental blocks since they failed to highlight the Schwarzschild singularity as a horizon surrounding the center of the imploding star. Wheeler introduced the name “black hole” in a talk at the end of 1967, and it was largely due to him that it caught on. However, thanks to Bartusiak’s detective work we know that the term arose earlier. She documents that it was used in the Life magazine and Science News Letter in early 1964, in both cases in reports on the important 1963 Texas Symposium on Relativistic Astrophysics. Moreover, she provides evidence that “black hole” may first have been used by the prominent Princeton physicist Robert Dicke a couple of years earlier. Of course, today the term is immensely popular and used in many other contexts than those relating to general relativity and collapsed stars.
Bartusiak’s book is more than just the story of black holes; it is also a partial story of Einstein’s general theory of relativity with which the heavenly holes are so intimately connected. General relativity, a theory which this year can celebrate its hundredth anniversary, has a curious history. Already by the mid-1920s most physicists lost interest in it, considering it to be beyond further experimental testing and irrelevant to mainstream areas of physics. Mathematicians and philosophers might find general relativity to be of value, but physicists could safely ignore it. The following three decades have been described as the theory’s “low water-mark,” but then it experienced a remarkable renaissance which soon transformed it into a hot field of both theoretical and experimental research. One of the high points of the revived interest in general relativity and gravitation studies was the 1957 Chapel Hill conference held at the University of North Carolina, and another was the 1963 Texas symposium.
Bartusiak describes in some detail the renaissance, including the fascinating story of the role played by the private (and still existing) Gravity Research Foundation—originally established with the aim of harnessing gravity and inventing devices for antigravity! She repeatedly emphasizes the low reputation of general relativity, a theory which for long “had entered a valley of despair” and become “a playtoy for mathematical physicists.” Although there is much truth in this, it is a gross exaggeration to describe Wheeler’s work in the 1950s and 1960s as “almost single-handedly taking general relativity, a field that had been stagnating for decades, and applying it to the universe at large.” In fact, during its low water-mark period general relativity was successfully used to describe the universe at large, that is, as the theoretical foundation of cosmology. At the time Wheeler entered research in general relativity, most experts thought that Einstein’s theory agreed satisfactorily with astronomical observations. Strangely, Bartusiak has nothing to say about cosmology in relation to general relativity.
Black Hole is informative and exceedingly well written, a brilliant introduction to the history of black holes and general relativity. Despite some reservations, I warmly recommend Bartusiak’s book not only to lay readers but also to scientists and historians of science and ideas.