by David Kaiser
Both sets of papers—by Brans and Dicke and by Higgs—received immediate attention from colleagues. One helpful measure of their impact comes from counting citations. Over the years, physicists have refined the art of citation counting, tallying the number of other scientific articles that refer to, or cite, a particular article in question. The standard citation-tracking database for high-energy physics assigns papers to various categories on the basis of their accumulated citations. Papers that have never received a single citation in the literature are assigned to the bin “unknown.” Then up march the categories: “less known” (1–9 citations), “known” (10–49), “well-known” (50–99), and so on. The highest category, “renowned,” is reserved for those rare papers that have been cited at least 500 times.10 By this measure, both the Brans-Dicke article and the Higgs articles became “renowned,” accumulating more than 500 citations by 1981; to this day, each of these papers remains within the top 0.01 percent most-cited physics articles of all time.11 (Several years ago, as my own most-cited physics article inched closer to 100 citations, administrators of the database inserted a new bin. Whereas papers with 100–499 citations had previously been labeled “famous,” now the “famous” category would be reserved for papers with at least 250 citations. I like to think of my paper, which to date has received more than 200 citations, as “almost famous.” Of course my wife, the psychologist, has no shortage of theories about all this.)
Each of these renowned papers proposed to explain the origin of mass by introducing a new field, φ, and accounting for its interactions with other types of matter. They were published around the same time, with lengthy articles appearing in the same journal, the Physical Review. And yet for twenty years, hardly any physicists considered the Brans-Dicke field and the Higgs field together. All told, 1,083 articles were published through 1981 that cited either the Brans-Dicke paper or the Higgs papers. Only 6 of these—less than 0.6 percent—cited both Brans-Dicke and Higgs, the earliest in 1972 and the rest after 1975.12 The 1,083 articles in question were written by 990 authors. Only 21 authors cited both the Brans-Dicke article and Higgs’s work—usually in separate papers—between 1961 and 1981. In other words, even as each set of ideas became “renowned” within its own subfield, nobody suggested that the Brans-Dicke field and the Higgs field might be physically similar, or even worth considering side by side, before the mid-1970s.
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The divide between particle physics and cosmology was especially sharp in the United States when Brans, Dicke, Goldstone, and Higgs introduced their respective φ fields. The Physics Survey Committee of the US National Academy of Sciences, for example, issued a policy report in 1966 entitled Physics: Survey and Outlook. The committee recommended that both funding and PhD-level personnel for American particle physics be doubled over the next few years—by far the largest increases suggested for any subfield of physics—while calling for virtually no expansion of the already-small areas of gravitation, cosmology, and astrophysics.13 At a time when some of the most influential Soviet textbooks on gravity and cosmology began by discussing the latest speculations about nuclear forces, meanwhile, such a blurring of genres remained totally absent from American textbooks.14
The US research patterns—so starkly laid out in the 1960s policy reports and mirrored in the separate treatments of the Brans-Dicke and Higgs fields—were not set in stone. Indeed, by the late 1970s the separation between cosmology and particle physics no longer seemed quite so extreme; signs of a new subfield, particle cosmology, could be discerned, as the Brans-Dicke and Higgs papers began to chalk up more shared citations. Physicists have tended to account for the rapid rise of particle cosmology by appealing to new ideas from the mid-1970s, arguing that the power of these ideas alone compelled particle physicists to begin thinking about cosmology. Those new ideas included “asymptotic freedom,” first published in 1973, and the construction of the first “grand unified theories,” or GUTs, in 1973–74.
Asymptotic freedom refers to an unexpected phenomenon within certain models of forces that remain symmetric under local transformations: the strength of the force can decrease as the particles move closer together, rather than increasing the way most other forces do. For the first time, particle theorists were able to make accurate and reliable calculations of such phenomena as the strong nuclear force—the force that keeps quarks bound within protons and neutrons—as long as they restricted their calculations to very short distances. Such short distances corresponded to interactions at very high energies, far beyond anything that had been tested experimentally.15 (H. David Politzer, David Gross, and Frank Wilczek shared the 2004 Nobel Prize for their discovery of asymptotic freedom.)
The introduction of GUTs also helped to point particle theorists’ attention toward very high energies. As physicists were piecing together the Standard Model, several noticed that the strengths of each of the three forces described by the model—electromagnetism, the weak nuclear force, and the strong nuclear force—might become equal at very high energies. Theorists hypothesized that above that energy, the three forces would act as a single, undifferentiated force—hence the “grand unification.” Below those energies, the GUT symmetry would be spontaneously broken, leaving three distinct symmetries describing three separate forces, each with its characteristic strength.16
The energy scale at which grand unification might set in was literally astronomical: more than one trillion times higher than anything particle physicists had been able to probe using Earth-bound particle accelerators. Physicists had no possible way of accessing such energies via their traditional route; even with four decades of improvements in the underlying technology, today’s most powerful particle accelerators have increased the energies under study by a factor of a thousand, a far cry from a trillion. So GUT-scale energies could never be created in physicists’ laboratories. But some began to realize that if the entire universe had begun in a hot big bang, then the average energy of particles in the universe would have been extraordinarily high at early times in cosmic history, cooling over time as the universe expanded. With the advent of asymptotic freedom and GUTs, particle physicists therefore had a “natural” reason to begin asking about the high-energy early universe: cosmology would provide what many came to call “the poor man’s accelerator.”17
Is this the whole story? Although certainly important, these new ideas, taken on their own, cannot explain why the new subfield, particle cosmology, emerged and grew as it did. For one thing, the timing is a bit off. Publications on cosmology (worldwide as well as in the United States) began a steep rise before 1973–74, and the rate of increase was completely unaffected by the appearance of the papers on asymptotic freedom and GUTs. Moreover, although GUTs were introduced in 1973–74, they did not receive much attention—even from particle theorists—until the late 1970s and early 1980s. Three of the earliest review articles on the emerging field of particle cosmology, published between 1978 and 1980, ignored asymptotic freedom and GUTs altogether, highlighting other work instead, some of it dating back to 1972, before either asymptotic freedom or GUTs had even been introduced.18
More than just ideas were at stake in the creation of particle cosmology. Politics, institutions, and infrastructure played major roles as well. When the Cold War bubble burst, right around 1970, academic physics fell into a tailspin. Nearly all fields of science and engineering entered a period of decline around that time; yet physics fell faster and deeper than any other field. Funding for physics fell about as quickly as enrollments did, plummeting by more than one-third between 1967 and 1976 (in constant dollars). By the early 1970s, physicists in the United States faced the worst crisis their discipline had ever seen.19
The cuts did not fall evenly across the discipline. Particle physics was hit hardest by far. Federal spending on particle physics fell by half between 1970 and 1974 (a combination of direct cutbacks and inflation), combined with a sudden drop in government demand for high-energy physicists.20 The sudden cuts drove a rapid
outflow of particle physicists: between 1968 and 1970, twice as many physicists left particle physics as entered it in the United States. The downward slide continued into the 1970s: the number of new particle physics PhDs trained per year in the United States fell by 44 percent between 1969 and 1975—the fastest decline of any subfield. As particle physicists’ fortunes tumbled, meanwhile, astrophysics and gravitation became some of the fastest-growing subfields in American physics. Spurred in part by a series of new discoveries during the mid-1960s (such as quasars, pulsars, and the cosmic microwave background radiation), as well as by innovations in experimental design, the number of new PhDs in this area per year grew by 60 percent between 1968 and 1970 and by another 33 percent between 1971 and 1976—even as the total number of physics PhDs fell sharply.21
Surveying the wreckage a few years into the slump, the Physics Survey Committee released a new report, Physics in Perspective (1972). The committee noted that theoretical particle physicists had fared worst of all when the cutbacks hit. When demand for particle physicists fell off, too many of the young particle theorists had difficulty switching their research efforts elsewhere. The nation’s physics departments needed to revamp how particle theorists were trained, urged the elite committee:
The employment problem for theoretical particle physicists appears to be even more serious than it is for other physicists. The large number of such theorists produced in recent years and their high degree of specialization are often given as the causes of this difficulty. This narrow specialization is already an indication that the student of particle theory has been allowed to choose unwisely, because real success in any part of physics requires more breadth. . . . University groups have a responsibility to expose their most brilliant and able students to the opportunities in all subfields of physics.22
Particle theorists were the only subfield singled out for such criticism in the entire 2,500-page report. Curricular changes quickly followed, aimed to broaden graduate students’ exposure to other areas of physics, including more emphasis on gravitation and cosmology. Across the country, physics departments began to offer new courses on the subject. American publishers pumped out scores of new textbooks on gravitation and cosmology—having all but ignored the topic for decades—to meet the sudden demand. Whereas a major textbook publisher had advised series editors to proceed with caution when considering textbooks in the tiny field back in 1959—“There is probably not a vast market for a [general] relativity book, however good,” one noted—publishers in the United States brought out twenty-six new graduate-level textbooks on the subject during the 1970s. Amid the fast-changing curricula, physicists sometimes decided not to wait for formal textbooks to be published. In 1971, for example, Caltech began to circulate mimeographed copies of the lecture notes from Richard Feynman’s 1962–63 course on gravitation, while a Boston-based publisher rushed out another physicist’s informal lecture notes on general relativity in 1974.23
These massive changes in American physics left their mark on the way theorists handled such esoteric ideas as the Brans-Dicke and Higgs fields. In 1979, two physicists in the United States independently suggested that the Brans-Dicke and Higgs fields might be one and the same—this after two decades in which virtually no one had even mentioned the two fields in the same paper, let alone considered them to be physically similar. Anthony Zee and Lee Smolin separately introduced a “broken-symmetric theory of gravity” by combining the Brans-Dicke gravitational equations with a Goldstone-Higgs symmetry-breaking potential, in effect gluing the two aspects of the φ fields together.24 (Similar ideas had been broached tentatively by theorists in Tokyo, Kiev, Brussels, and Bern between 1974 and 1978, though they received very little attention at the time.)25 In this model not only could the local strength of gravity, governed by Newton’s “constant,” G ~ 1/φ2, vary over space and time (as in the Brans-Dicke work), but its present-day value emerged only after φ settled into a minimum of its symmetry-breaking potential (as in the Goldstone-Higgs work). In this way, Zee and Smolin aimed to explain why the gravitational force is so weak compared with other forces: when the field settles into its final state, φ = ±v, it anchors φ to some large, nonzero value, pushing G ~ 1/v2 to a small value.26
Anthony Zee’s path to uniting the two φ fields illustrates one way in which physicists in the United States wandered into cosmology from particle theory after the collapse of the Cold War bubble. He had worked with gravitation expert John Wheeler as an undergraduate at Princeton in the mid-1960s before pursuing his PhD in particle theory at Harvard, earning his degree in 1970 just as the biggest declines in that area began. As he later recalled, cosmology had never even been mentioned while he was in graduate school. After postdoctoral work, Zee began teaching at Princeton. He swapped apartments with a French physicist while on sabbatical in Paris in 1974, and in his borrowed quarters he stumbled upon a stack of papers by European physicists who tried to use ideas from particle physics to explain various cosmological features, such as why our universe contains more matter than antimatter. Although he found the particular ideas in the papers unconvincing, the chance encounter reignited Zee’s earlier interest in gravitation. Returning from his sabbatical, and back in touch with Wheeler, Zee began to redirect his research interests more and more toward particle cosmology.27
Lee Smolin, on the other hand, entered graduate school at Harvard in 1975, just as the curricular changes began to take effect. Unlike Zee, Smolin took courses in gravitation and cosmology alongside his coursework in particle theory—he didn’t need to stumble into one area from the other. Smolin worked closely with Stanley Deser (based at nearby Brandeis University), who was visiting Harvard’s department at the time. Deser was one of the few American physicists who had taken an interest in quantum gravity by the 1960s—attempting to formulate a description of gravitation that would be compatible with quantum mechanics. He was also the very first physicist in the entire world to publish an article that cited both the Brans-Dicke work and the Higgs work (although he treated the two fields rather differently and in separate parts of his 1972 paper). Smolin’s other main adviser was Sidney Coleman, a particle theorist who just a few years earlier had begun teaching the first course on general relativity to be offered in Harvard’s physics department for nearly twenty years. Smolin completed coursework with Steven Weinberg, whose influential textbook, Gravitation and Cosmology (1972), had recently appeared. Meanwhile, Smolin also took intense courses on Standard Model physics and GUTs with several architects of the new material, including Howard Georgi and visiting professor Gerard ’t Hooft. Building on this broader range of courses, Smolin focused on quantum gravity for his research and suggested that the Brans-Dicke and Higgs fields might be the same just as he was finishing his dissertation in 1979.28
Smolin’s experiences marked the new routine for his generation of theorists, trained during the mid- and late 1970s to work at the interface of gravitation and particle theory. Theorists like Paul Steinhardt, Michael Turner, Edward “Rocky” Kolb, and others—each of whom, like Smolin, received his PhD between 1978 and 1979—devoted formal study to gravitation as well as to particle theory in graduate school. Soon Smolin, Steinhardt, Turner, Kolb, and others were training their own graduate students to work in the new hybrid area. For these young theorists and their growing numbers of students, it became “natural” to associate the Brans-Dicke and Higgs fields with each other. Turner, Kolb, and Steinhardt each led groups that pursued further links between the two φ fields during the 1980s, constructing cosmological models in which the Brans-Dicke and Higgs fields either appeared side by side or were identified as one and the same. Some became avid program builders as well. Kolb and Turner, for example, established the first Center for Particle Astrophysics in 1983, carving out space for the new types of studies within Fermilab. They went on to write the first textbook for the new subfield, The Early Universe, which appeared in 1990.29
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I was a sophomore in college when Kolb and Turner’s The
Early Universe came out. Thanks to books like theirs, students like me could begin taking courses in particle cosmology as undergraduates. I was hooked immediately, thanks in no small measure to inspiring teachers—younger physicists who had studied in places like Fermilab’s Center for Particle Astrophysics during their own training—and to Kolb and Turner’s book. (I eventually purchased three copies, so that at least one would be near my fingertips at any moment.) For students of my own generation, it became routine, even second nature, to draw upon theoretical objects like the Brans-Dicke and Higgs fields in our research, hardly thinking twice about a move that had been so novel a few decades earlier. Indeed, from today’s vantage point, it seems downright bizarre that physicists never considered the Higgs and Brans-Dicke fields in the light of one another for so long. The fields’ union moved from unthinkable to unnoticeable within a few academic generations.
That seeming naturalness—the banality of combining those fields today and the strangeness of holding them at arm’s length—illustrates how the contours of intellectual life can be reshaped by rapid changes in institutions and infrastructure, ultimately shifting the boundaries of what young physicists come to find compelling or worth pursuing. Hence the immediate appreciation I felt—tinged with a touch of sadness—when I stumbled upon the “Higgs inflation” paper while browsing the arXiv preprint server that morning, back in the autumn of 2007. Why didn’t I think of that?30
COSMOS
14
Guess Who’s Coming to Dinner