Paul Boynton Testing the fireball hypothesis

Paul Boynton, Professor of Physics and Astronomy at the University of Washington, was in his youth a dedicated member of the Princeton Gravity Group from 1967 to 1970.

In his essay, Six Cautionary Tales for Scientists, Freeman Dyson com-pellingly warns against idolatry of "Big Science" and the unacceptable cost of failing to nurture the nimble spirit of exploration inherent in "small science" (Dyson 1992).

During the last few decades, large institutionalized scientific projects have sometimes played a productive role in extending our grasp of the natural order, but big science did not bring about the discovery of the CMBR. I believe that one could not find more compelling support for the value of quick, modest, "table-top" approaches to research than the essays collected in this volume. These accounts portray a vitally diverse community of experimentalists rapidly and resourcefully responding to a new landscape of phenomena to be observed and new hypotheses to be tested - while in constant conversation with their theoretician colleagues. This process was clearly a triumph of small physics.

Whether an experimentalist is drawn toward a career in big or small physics may be a matter of circumstance, but for some only small is beautiful. My path to the Princeton Gravity Group, where small physics was doctrine, led directly through the valley of the shadow of Big Physics.

A few miles east of the University proper, lies Princeton's James Forrestal Campus. When I was a physics graduate student, it was the site of two major research facilities: the Plasma Physics Laboratory (PPL, where the C-Stellarator was then under construction) and the Princeton-Pennsylvania Accelerator. Both would become familiar haunts.

By the time I arrived at Princeton in the fall of 1962, I was thoroughly pumped up to join the quest for controlled fusion at PPL. My under-grad senior thesis on an obscure plasma instability led to working in Jim Drummond's Plasma Physics Group at the Boeing Scientific Research Laboratories in my hometown, Seattle, during the year following graduation. In fact, this millennial dream of realizing a virtually limitless source of pollution-free energy was largely my motivation for applying to grad school, and only to Princeton.

There were several hints that PPL was truly Big Physics: (1) hundreds of scientists, engineers, and technicians were focused on the project, (2) there were many layers of administration, and (3) the lab was the terminus of two major "pipelines." Through one, invisibly flowed millions in federal funds. The other was conspicuously part of the landscape. From the eastern horizon of the Forrestal campus, marching toward the lab across the wooded New Jersey countryside was a procession of steel high-tension towers carrying 110 kV power lines that could easily meet the needs of a small city, but these lines ended abruptly at the PPL substation. Clearly, something big was underway, and I was excited to be part of it.

Within a year, however, I began to see that my research advisor and his colleagues were thinking in terms of decades to achieve controlled fusion, with commercially marketable electrical power still farther away. Not much has changed since then, for even now the best hope for meeting such a goal is project ITER, which is still years away from initial tests.

As twenty-somethings generally consider themselves both invulnerable and everlasting, it was not the abstract specter of passing decades that deflected me to another field of study. Something more immediate and visceral tarnished the luster of my vision of contributing to a new age of progress made possible by such a clever manipulation of the natural world. At PPL I had found myself in the midst of a host of earnest physicists and engineers laboring toward this common goal, yet this was just one of several such labs around the world. The prospect of being a soldier in a vast army of similarly engaged minds, all driven by committee-defined strategies and milestones, did not appeal to me. Near the end of my second year, I jumped ship and opted for a thesis topic in experimental high-energy physics.

This may seem a ludicrous choice considering the current size of typical high-energy collaborations, but this was 1964 and our PPA group was only three: Tom Devlin, my thesis advisor, Julie Solomon, and I. While writing my thesis, attending American Physical Society meetings, and visiting other laboratories, I could see that high-energy experiments were becoming more ambitious and complex. My next moment of awareness regarding the real world of Big Science occurred on a visit to Brookhaven National Laboratory, where I encountered an experiment instrumented with rows and rows of rack-mounted electronics all networked with a PDP-8 computer (or perhaps several) for data acquisition and control - and yes, tended by a vast army of grad students, postdocs, and diligent assistant professors. Once again I got the message, and this time I knew which way to turn. The choice was nearly as elementary as that between sex and death.

During my student years at Princeton, I had been fascinated by the elegant simplicity of Professor Robert Dicke's fundamental physics experiments: measurement of the gravitational redshift of solar lines, a sophisticated revisiting of the Eotvos experiment, the implication of solar oblateness for testing general relativity theory, and probing the relation between active and passive gravitational mass. All were the subject of seminars or collo-quia I attended. In each, profoundly basic hypotheses were being tested, yet none was more than a tabletop experiment - deceptively simple in construction, but combining sophisticated application of symmetry principles with electronic and mechanical tricks that yielded not only superprecise measurements, but also immunity to subtle systematic errors. Many were configured as null experiments with attendant advantages. At each turn in considering experimental design it seemed Dicke's group miraculously converted daunting challenges into distinct advantages. In conception and execution each project was impressively clever. Intimacy seems a curious label here, but a kind of intimacy between researcher and instrumentation is possible in this realm of small physics, a relationship unbuffered by committees, collaboration meetings, project management, or mega-budget politics. Small was beautiful, and it really appealed to me.

In early spring 1965, the Physics Department was abuzz with the revelation that members of Dicke's Gravity Group had been developing an apparatus to detect the remnant radiation from a putative hot big bang origin of the universe, and that researchers at Bell Labs in northern Jersey may have serendipitously planted their flag first. Nevertheless, in my mind Dicke had done it again - another simple-in-concept, yet dazzling tabletop experiment - and this time with cosmological implications. Although once more intrigued, my thesis project required me to leave for a year to set up a beam line at the Berkeley 184-inch cyclotron so that our high-energy group could attempt to measure cross sections for the excitation of isobaric analog states through pion-double-charge-exchange interactions with various complex nuclei. Perhaps another time...

The lingering appeal of the Gravity Group's activities weathered my preoccupation with thesis research and I joined the Gravity Group as a postdoc in the early summer of 1967. Professor Dicke had a spacious office on the upper floor of Princeton's historic Palmer Lab, where, in the vaguely medieval ambience of the basement, Dave Wilkinson, Jim Peebles, Mark Goldenberg, Bruce Partridge, and I, along with grad students, Bob Stokes, Paul Henry, Karl Davis, and Ed Groth, shared a large, windowless laboratory area. Rather cluttered and somewhat dimly lit, the room featured doorways along one wall that led to seldom-used nooks with black walls and ceilings for light-sensitive experiments. These darkened recesses lent a convincing touch to what might have been termed a "modern Gothic" workplace. Nonetheless, to me (and I believe to all of us) it was as comfortable and inviting as home.

Dedicated in 1908, Palmer Lab was built during Woodrow Wilson's time as president of Princeton University. Compared to the elegant, expansive grace of Jadwin Hall, built in the late 1960s to replace it, Palmer Lab's charm pales; but fares better when contrasted with the quarters it replaced, the John C. Green School of Science.

As a young faculty member in 1906, Owen Richardson (Nobel Laureate 1928) describes Princeton's 19th century physics facility in a memoir:

I remember getting quite a shock when I was first introduced to the part where I was expected to set up a research laboratory. This was a kind of dark basement, ventilated by a hole in the wall, apparently accidental in origin, and inhabited by an impressive colony of hoptoads, which enjoyed the use of a swimming pool in one corner. However, with the help of the Clerk of Works, these visitors and their amenities were got rid of and a lot of good work was done in it. Looking back on those days, I think they were in many ways the most satisfactory of my life.

Although there were no toads in our space, I do identify with Richardson's sentiment. I could not (and still can't) imagine a more exciting place, shared with these splendid colleagues amid the exhilarating conversation and consuming activity of engaging small science that loomed so very large.

In the more rarefied atmosphere of the upper floors of Palmer, I recall late-afternoon and evening informal sessions when Dicke, Peebles, Wheeler, and others would enthusiastically explore the romance of the "primeval fireball" picture - tracing out the manifold implications of a hot big bang. Everything was beginning to fit together: helium and deuterium synthesis, the CMBR, structure formation, primeval galaxies. This was a picture that had come alive - not just because of the discovery of the CMBR by Penzias and Wilson (1965a), or even the construction of a detailed expansion scenario by Peebles and Dicke, which had presaged that discovery (Dicke et al. 1965). In no small measure, the energy driving this effort emerged from the Gravity Group's firm conviction that remnant radiation from a hot big bang, if it existed, could be detected. Dave Wilkinson and Peter Roll acted on that confidence by building hardware and devising observing strategies based on microwave radiometer techniques Dicke had developed decades earlier at the MIT Radiation Lab. Jim Peebles, equally convinced, gave regular lectures on "physical cosmology" where he distributed weekly Ditto-machine notes bearing a striking resemblance to the monograph he later published under the same title (but not in that now-unfamiliar purple ink). This was the quintessential tabletop experiment, and in later years Dicke would sometimes muse: "What we need is another really good idea like the Fireball Radiation."

It was, in fact, a great idea. Developed independently at Princeton in 1964, but anticipated by Gamow and his colleagues Alpher and Herman nearly two decades earlier (Alpher and Herman 1948; Gamow 1948a,b). Unlike Dicke, Gamow did not grasp the directly observable, unique consequence of such a sweeping vision of cosmic evolution. Even in his earlier letter (Gamow 1946) motivating consideration of element production via cold neutron coagulation, he begins with a sketchy glimpse of the big bang, and clearly invokes initially hot matter subsequently cooled by expansion, suggesting an associated radiation component without explicit mention (until the 1948 publications). This letter coincidentally appears in the same volume of Physical Review as the 1946 paper describing Dicke's microwave measurement of an upper limit of 20 K to the temperature of (presciently labeled) diffuse cosmic matter (Dicke et al. 1946). Even so, Dicke remained unaware of Gamow's work for nearly 20 years. This curious disconnection is discussed elsewhere along with the observation that by 1948 technological developments would have allowed Dicke to carry out the CMBR detection program he later initiated in 1964 (Boynton 2005).

The stimulating synergism experienced within the Princeton group of true believers spread through the larger community of cosmologists at a more measured pace, and frequently met with the skepticism of proper scientific conservatism. At an American Astronomical Society meeting in Philadelphia later that year, I witnessed a particularly memorable instance of our confident enthusiasm regarding these new ideas encountering a dubious scientific establishment. On a gray, wet day in late fall the entire Gravity Group, Dicke included, boarded a chartered bus to mount a major presence at sessions devoted to the current status of various CMBR/big bang issues.

I vividly recall Jim Peebles giving a characteristically animated presentation regarding some aspect of big bang cosmology, waving the chalk about as his tall frame enthusiastically strode back and forth in front of the board while the Princeton contingent nodded and smiled encouragingly. Just as Jim was pulling together several points to form a particularly insightful synthesis, Professor George McVittie, eminent elder astrophysicist who was seated directly in front of me, could contain himself no longer. In a clearly exasperated tone he interrupted Jim exclaiming: "One can make any point at all with a little slap-dash arithmetic!" Jim turned with a flourish and grinning broadly replied, "My arithmetic may seem slapdash, but I can assure you it is impeccable."

McVittie recoiled only by silently sinking a bit in his chair as Peebles moved confidently on without pausing. Startled by this exchange, my full attention was riveted on McVittie's next move. There was nary a twitch, but in the intensity of the moment I was certain I saw a phantom curl of smoke rising above his shining, pulsing pate. It was an extraordinary rejoinder and our hero never broke stride. Forty years later, upon my inquiry regarding this encounter, Jim surprised me by producing a copy of McVittie's hand-written apology penned that same evening.

The group's continuing sense of mission sprang largely from the fact that in late 1966 only two radiometric observations of the microwave background were in hand, both made from New Jersey: the Bell Labs detection and antenna temperature measurement at A = 7.35 cm, and the Princeton follow-up at A = 3.2 cm. These data were consistent with a A-2 power-law brightness spectrum, and therefore suggestive of the long-wavelength, Rayleigh-Jeans tail of thermal radiation - with implications of considerable interest to the Gravity Group. Of course, this could indicate either thermal-equilibrium (blackbody) emission, or a dilute, higher-temperature (gray-body) source. Wilkinson, now joined by Bruce Partridge and graduate student Robert Stokes, laid plans to examine the spectrum in more detail.

During the summer of 1967, Wilkinson, Partridge, and Stokes embarked on their seminal expedition to a high-altitude research station on White Mountain Peak above Owens Valley, California. They toted redesigned, refined, microwave radiometers operating at wavelengths A = 3.2 cm, 1.58 cm, and 8.56 mm, to better establish the spectral distribution of the CMBR by observing at three wavelengths with three independent instruments of the same design, calibration method and observing technique.

These wavelengths largely avoid thermal emission from broad atmospheric O2 and H2O molecular resonance lines that constitute a major radiation background when attempting ground-based measurements. In addition to evading these lines, by observing from an altitude of 12,000 ft, above much of the integrated atmospheric water vapor density profile, the most problematic aspect of this local microwave background should be reduced: a random, time-variable contribution to the radiometer signal from molecular H2O due to its inhomogeneous distribution coupled with atmospheric currents, which we commonly referred to as "atmospheric noise."

This expedition was the Gravity Group's second-generation refinement of their resolute goal to test the primeval fireball hypothesis. Theoreticians in the community at large had come up with various nonthermal, even non-cosmological mechanisms to explain this large, isotropic, apparently cosmic background, but none of these alternative processes filled the universe with blackbody radiation as would be expected of a hot big bang fireball. Bruce Partridge sets out an excellent, comprehensive account of both theoretical and experimental activities associated with the discovery and early history of the CMBR in his monograph 3K: The Cosmic Microwave Background Radiation (Partridge 1995). See also the superb contributions to this volume by Jim Peebles and Bruce Partridge.

Although the basic goal that summer focused on better establishing the brightness spectrum of the microwave background over a limited range of wavelengths, the real prize would be to distinguish a generic A-2 power law tentatively indicated by that pair of earlier measurements, from the unique behavior of the Planck law: that the radiation brightness begins to drop below the power-law extrapolation of the Rayleigh-Jeans tail as one proceeds to shorter wavelengths, as is illustrated in Figure 4.36. At that time, detecting this blackbody signature was the obvious, feasible test of the cosmological origin of the CMBR, other than more precisely measuring the background

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