7 The Role of IUPAP in Shaping Metrological Practice: International Negotiation and Collaboration

In 1996, the then-President of the International Union of Pure and Applied Physics (IUPAP), the Swedish mathematical physicist Jan Nilsson published in Physics World—a periodical of the UK National Physics Society, the Institute of Physics (IOP)—an article addressing the question of funding for physics in the post-Cold War world.¹ Titled “What can IUPAP Do for You?,” Nilsson’s article explains how the unique governing structure of IUPAP, governed by practicing physicists rather than by “hiring a professional administrative staff,” can help the physics community obtain resources “when cash for research is in short supply.”² The lack of administrative staff increases the proportion of the IUPAP budget (drawn from national funding sources in member nations and from the United Nations Educational, Scientific and Cultural Organization (UNESCO)) which can be used for sponsoring international conferences, for providing financial support for “young scientists from developing countries to go to IUPAP’s major conferences around the world,” and for providing conference proceedings and physics journals to “physics libraries in the third world.”³ To this end, and in order to ensure that each IUPAP commission is able to communicate the needs of its participants to the council, and receive the necessary funding for its own international conferences, IUPAP had recently restructured itself so that some commission chairs could simultaneously serve as council members.

In 1969, nearly three decades before Nilsson’s article, the Canadian physicist Larkin Kerwin (the Associate Secretary General of IUPAP since 1962) published an article in Physics Today—a periodical of the US national physics society, the American Institute of Physics (AIP)—that introduces the structure and purpose of the organization to a wide audience of physicists and interested readers.⁴ The lead reads: “With national committees from 37 countries directing its policies, IUPAP fosters international meetings, spreads information and hopes to advance international understanding.” In this article packed with information about the structure and functioning of IUPAP, Kerwin gives special notice to the fact that the IUPAP committee on Symbols, Units, and Nomenclature (the SUN Commission, formed in 1931 in coordination with the International Bureau of Weights and Measures (BIPM)) is “of a more general interest and all national committees are expected to … receive and exploit” publications delineating agreed-upon standards, and should “seek to implement the adopted proposals in their countries.”⁵ In process was the updated IUPAP manual on symbols, units, and nomenclature in physics which would be published in 1978 with funding from UNESCO.⁶ Relatedly, IUPAP and the BIPM had coordinated the creation, in January 1965, of the first international journal of metrology, Metrologia, which would emphasize fundamental measurements but would also publish “reports of experiments or techniques of particular originality and importance in the area of secondary measurement,” in particular reports of “high frequency electrical measurement—where there are substantial difficulties in the way of attaining high accuracy, precision and international uniformity.”⁷

The IUPAP Commission SUN, later named Commission C2, regularly engaged with the BIPM in negotiating these definitions and in designing specialized commissions to organize and sponsor international meetings and conferences.⁸ Many of the scientists who would publish in Metrologia from its first issue onwards were actively involved in IUPAP, such as Harvard University physicist Norman Ramsey, who explained the workings of the atomic hydrogen-maser frequency standard to this broad international audience.⁹ In addition to publishing research articles, the journal would inform its readers of the “activities and decisions of the International Conference of Weights and Measures [CGPM], the International Committee of Weights and Measures [ICWM], and its Bureau at Sèvres, France, all of which operate under the oldest international scientific treaty—the Convention du Mètre of 1875—which continues to assure uniform, precise and accurate measurements throughout the world.”¹⁰ The Metric Convention of 1875, which founded the BIPM to systematize international scientific conventions for fundamental units, was part of a long 19th-century history of many negotiations over the definitions of units of measure, first focused within European nations and imperial interests and later globalized.¹¹ Already in 1921, amidst the development of novel metrological instrumentation including in timekeeping, the scope of the treaty had been extended to accommodate all physical measurements.¹² In particular, as the next section shows, the origin and continuing advancement of the International System of Unitis (SI) lay in the negotiations of physicists within IUPAP.

At issue in metrology is consistency and stability of the fundamental units and standards of measurement that enable “uniform, precise, and accurate measurements throughout the world.”¹³ Metrology spans pure and applied physics, astrophysics, space science, and engineering, and it is foundational not only for the instrumental and experimental sciences but also for the modern world’s infrastructure, industrial and commercial production, communications technology, health care, defense, and global transportation, and trade. Moreover, as Joseph Martin’s chapter in this volume notes, so-called pure physics was never completely independent from applied concerns, and applied research “often opened avenues into fundamental insight.”¹⁴ Martin argues further that IUPAP increasingly recognized the importance of industrial physics within “applied physics” in the final decades of the 20th century, in particular with regards to the needs and interests of developing nations.

Nowhere is this more visible than in the field of metrology and in the activities of SUN Commission. When IUPAP was reformulated in 1931, electing the American experimental physicist Robert Millikan as President, IUPAP recognized the importance of universally agreed-upon units and standards of measurement by naming its second commission, “Symbols, Units, Nomenclature,” foremost in consequence only after its first commission, “Finance,” also established that year.¹⁵ As for the updates to the IUPAP report on metrology, the goal for the articles in Metrologia was to provide an international audience of physicists—not just specialists in metrology but experimental physicists across sub-disciplines—a compendium of reliable, certified, universally agreed upon definitions and standardized measurements. Already by the mid-1960s, these physicists included those from a growing number of developing countries that recognized the importance of both the standardization of instrumentation and greater accuracy in measurement for a nation’s scientific experimentation, its industrialization, and its economic growth.¹⁶ The updated IUPAP Metrology Manual was widely known as the SUN Commission Report, but, as announced at the 1966 General Assembly in Basel, Switzerland, “it will also be known as booklet IUPAP 11.”¹⁷ Because of the universal importance of the topic, Kerwin believed that SUN-sponsored conferences “should attract a wider audience from all countries than the meetings of the more specialized commissions.”¹⁸

Elisabeth Crawford, Terry Shinn, and Sverker Sörlin describe international standardization as the main (indeed the only) scientific role of such international institutions.¹⁹ As they explain, beginning in the late 19th century, international organizations for science “were set up specifically to accomplish the goals of standardization of nomenclatures, methods and units in both the laboratory and field sciences.”²⁰ The growth in homogeneity (universality) of methods and units of measurement, the increase in rapidity of communication, and the growth of standardization of scientific instruments and technological products, separately and together “furthered the standardization of scientific work.”²¹ Crawford et al. argue further that, in the early 20th century, “This wholesale intermingling of ideology and practice where lofty ideals about science promoting world peace were put on a par with the much more mundane tasks of standardization of methods and nomenclature was specific to this historic period.”²² Elements of this “internationalist ideology” continued in the post-World War II era, albeit tempered with realism about deterrence and movements for détente during the Cold War.²³ Kerwin was very aware, for example, of difficulties within the Cold War context of enabling true international exchange across the iron curtain, with visa impediments often an issue of negotiation for IUPAP leadership as reflected in archived correspondence with representatives from countries that had difficulty entering Yugoslavia for the September 1969 General Assembly in Dubrovnik.²⁴

Kerwin served as both an anchor and a driver for the work and goals of IUPAP, with a particular focus on how metrology and other sub-disciplines of physics interface and engage with wider societal concerns and applications. Continuously, for almost three decades, Kerwin had multiple official roles in IUPAP: as Associate Secretary General (1963–72 while Clifford Butler was the Secretary General); Secretary General (1972–84 while Jan Nilsson was Associate Secretary General); Vice-President (1984–87); and President (1987–90). Most significantly, at different times in his career, Kerwin’s IUPAP roles overlapped with his particular physics research interests and his activities within related professional associations, demonstrating something very important about the nature and structure of IUPAP as an association of physicists doing cutting-edge research with ever-emergent technologies, research that often interfaced multiple sub-disciplines. Kerwin served as President of the Canadian Association of Physicists (1954–55); became President of the National Research Council of Canada (1980–85), which overlapped with his assuming the role of Vice-President of IUPAP; and became the first President of the Canadian Space Agency (1989–92). Also, Kerwin’s service as President of IUPAP overlapped with his serving as President of the Canadian Academy of Engineering in 1989, where he applied his knowledge of metrology outside of physics proper. The Canadian government valued Kerwin’s skill and long-proven experience in promoting scientific research and development (R&D) such that during the recession of the early 1980s (during Kerwin’s 10th year as Secretary General of IUPAP), Kerwin represented Canada in a working group set up after the June 1982 Economic Summit to strategize how R&D could spur economic recovery and job creation globally.²⁵

In addition to his aptitude for managerial and diplomatic leadership, Kerwin’s lifelong career as Professor of Physics at Université Laval in Québec City researching and teaching in atomic and molecular physics draws attention to the fact that work to advance standardization and metrology was fundamental, propelling and shaping research undertaken in all areas of physics, engineering, and space science. On October 8, 1975, early during his tenure as Secretary General of IUPAP, Kerwin wrote to J. Terrien (Director of the BIPM in Paris), thanking Terrien for his “interventions” at the IUPAP General Assembly in Munich, and for Terrien’s “belle contribution” to international physics, asking whether Terrien would be able to maintain his role as representative from the BIPM to IUPAP for another three years during the final pre-publication stages of the IUPAP manual on symbols, units, and nomenclature. Terrien accepted on October 15: “j’accepte très volontiers d’être encore member associé désigné par l’IUPAC [sic IUPAP] et le BIPM à la Commission SUN,” and, as this article will show, the IUPAP committees and membership were kept informed and provided with education and training in several venues regarding these metrological advances.²⁶ The SUN report (booklet IUPAP 11) was finally published in 1978.

This chapter uses the case of light as an instrument of precision metrology for measuring distance and time as an example of the central role played by IUPAP’s Commission C2 in the development of international standards. The story of how light has come to be used as the central instrument of precision metrology, enabling international conventions to change definitions of fundamental units of time and distance to depend on the instrumentation of light, is both an international and a national story. It is international in the transnational negotiation and coordination processes necessary to formulate international conventions of fundamental units. It is national because national funding sources within countries provided the support necessary for the equipment and the research undertaken in these areas, and likewise created and funded the international conferences that enabled the exchange of ideas on fundamental physics: “Irrespective of the organizational form given government-supported science … it always remained nationally based.”²⁷ In the area of precision metrology featured in this paper, IUPAP’s funds for international conferences have come from both the individual member countries and from UNESCO. Moreover, both inside and outside IUPAP, the majority of funding for international conferences comes from state governments (civilian and military agencies), which sometimes flow through intermediary organizations (such as UNESCO) which at times remove national-based restrictions on those receiving funding.²⁸

Light has been used as an instrument of precision measurement throughout the history of the experimental physical and astronomical sciences. One of the key high-precision instruments for measuring distance is the interferometer, first developed in the 19th century to use the interference of light waves to distinguish differences in length at the smallest scales: beams of light are reflected in mirrors and split and recombined by beam splitters, and the measurement is made by observing differences in the resulting interference fringes to give information about changes in optical path lengths. Conversely, and crucially, interferometry and the velocity of light also came to play a central role in the definition and precision measurement of time, as evidenced by the long history of the development of the hydrogen maser atomic clock and later optical laser clocks, a development firmly anchored by theoretical and technical advances during the period 1945–65. A maser (microwave amplification by stimulated emission of radiation) is a quantum-electrodynamic device that establishes and sustains the frequency precision and accuracy of an atomic clock.

The origin of the SI system lay in the negotiations of physicists within IUPAP. In 1948, IUPAP “expressed to the Conference Générale their desire that a practical international system of units should be adopted, and offered some suggestions.”²⁹ Into the 1960s, negotiations within the BIPM and IUPAP gave rise to well-established definitions of length measurement, mass measurement, temperature measurement, electrical measurement, photometric measurement, time and frequency measurement, and ionizing radiation measurement.

The Michelson Interferometer, first developed by the American physicist Albert Michelson in the 1870s, enables comparison of changes in the optical path lengths of light traveling along two arms at 90 degrees to each other. The precision of the interferometer used in the Michelson Morley experiment of 1887 led to suggestions that a particular wavelength of light could be established as a standard of length. Seeking “to provide a link between the standard of length (the meter bar) and the wavelengths of spectroscopy,” the IBPM invited Michelson to “use his methods and his interferometer in collaboration with the director of the Bureau International,” and they successfully “measured with an accuracy 100 times better than the tables of Rowland, the wavelength of the red line of cadmium (1892–1893).” Similar measurements elsewhere in dry air at 15 degrees Celsius and normal atmospheric pressure gave a more accurate result that could be used as a standard for interferometric measures of length.³⁰

In 1960, the CGPM of the BIPM adopted this approach, defining the metre as 1,650,763.73 times the wavelength of light emitted during a transition of the krypton 86 atom in vacuum. After solving a number of technical problems with using laser interferometry to measure length, by 1967 the National Bureau of Standards and similar national laboratories in other countries were using laser interferometry routinely in some of their length measurements.³¹ With the development of atomic clocks in 1967 the BIPM changed the definition of the second from a fraction of the mean solar day to the duration of repeated oscillations of a cesium 133 atom. The increasing precision of atomic clocks over the next two decades would in turn enable using the second and the velocity of light together to define the metre. Indeed, in 1983, the BIPM changed the definition of metre from the wavelength of light emitted during an atomic transition instead to the distance that light travels in a vacuum during a specific time interval.

What made the use of time in the definition of distance possible? The development of highly precise atomic clocks. The origin and later development of these clocks lies in the work of several physicists who were members of IUPAP’s Commission C2. The developers of the laser, maser, and atomic clocks were mainly young physicists whose careers had been interrupted by World War II and who subsequently undertook research at specialized laboratories in industry and academia.

The physicists Norman Ramsey at Harvard University, Charles Townes at University of California Berkeley, and Robert Dicke at Princeton University—each of whom had developed microwave oscillators and waveguides for radar components during World War II—spent the post-war years applying the microwave radar technique to spectroscopy as a means to study atomic and molecular structure. By the mid-1950s, Ramsey and Townes had independently created techniques to sustain the atoms or molecules in a gas in an excited energy state, and then send a stream of photons into the gas at energies equal to that excited state. This combination of conditions results in the emission of two photons for every one sent into the chamber—serving as a novel type of atomic clock. These “masers” solved the problem of noisy vacuum tubes as amplifiers of electronic signals by finding another way to amplify a photon stream. Funded by both the Air Force Office of Scientific Research (AFOSR) and the Office of Naval Research (ONR), Townes at University of California Berkeley pioneered the ammonia maser technique in 1954, and in 1958 received AFOSR funding for laser research, arguing that “marked improvement in interferometry and measurements of length by interferometric techniques” was one of the benefits that could be derived from his research.³² His laboratory built the first working maser, an ammonia maser, and he was awarded the 1964 Nobel Prize in physics: “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle.”³³

Ramsey, who had been trained in magnetic resonance research working in Isidor I. Rabi’s laboratory at Columbia University in the latter 1930s, undertook experiments after World War II in his Harvard University laboratory on high-precision microwave and radiofrequency techniques. These experiments led to the creation in 1960 by Ramsey and his students Michael Goldenberg and Daniel Kleppner of the atomic hydrogen-maser frequency standard that would become their atomic clock; they published the “Theory of the Hydrogen Maser” in the Physical Review in 1962.³⁴ The Canadian-American physicist Robert Vessot at the Massachusetts Institute of Technology (MIT) began a collaboration with Ramsey’s laboratory in 1957, experimenting on high-precision microwave and radiofrequency techniques to perfect the hydrogen maser as a commercial laboratory time standard. The teams jointly developed design principles and techniques published in the Physical Review in 1965.³⁵ They also jointly worked on perfecting the hydrogen maser for use in space. Vessot filed an application for a National Aeronautics and Space Administration (NASA) contract in 1962 for creating a hydrogen maser clock that could be used in space missions, and his team designed and began experimental studies of frequency beat and stability of various hydrogen masers. Vessot’s team presented and published their experimental studies in the Proceedings of the 21st Annual Symposium on Frequency Control, April 24, 1967 in Ft. Monmouth, NJ. This collaboration led to their creation of the atomic hydrogen-maser frequency standard that became their atomic clock, which had the precision and stability necessary to test Einstein’s Equivalence Principle in space in NASA’s 1976 Gravity Probe A experiment, and became a model for hydrogen maser atomic clocks used in the European Space Agency’s Galileo Global Positioning System.³⁶ Vessot would also play major roles for three decades in international training of physicists and engineers in metrology related to atomic clocks.

The big umbrella of “wide-ranging subjects” that Howett specified in inaugurating the Metrologia journal in 1965 had begun coalescing around such quantum-electrodynamic devices during the 1930s in experimental studies (in laboratories across the globe) of the “atomic beams” that occur when “an atom is placed in a static magnetic field and a perpendicular rotation magnetic field,” as Ramsey explained in a detailed retrospective, “History of Atomic and Molecular Standards of Frequency and Time,” that he published (upon request) in the IEEE Transactions of Instrumentation and Measurement (1972) reporting on researches leading to atomic clocks.³⁷ In 1978, Ramsey was appointed the US representative to the Inter-Union Commission “ICSU-Spectroscopy” by IUPAP at the September 1978 council meeting in Stockholm. In this new role, Ramsey updated this history in another requested article, “History of Atomic Clocks,” which was published in the Journal of Research of the National Bureau of Standards in 1983.³⁸ In 1949, Ramsey had developed the “separated oscillatory field method,” and in 1989—six years after his updated “History of Atomic Clocks” article and forty years after the discovery of his method—Ramsey obtained a Nobel Prize in physics for this method and its use in the hydrogen maser and other atomic clocks, and for the SI definition of the second.³⁹

These innovations in the physical understanding and instrumentation of precision metrology were essential for the creation of international standards for units of time and distance at the levels of precision and accuracy for research in quantum mechanics and in gravitational physics. How did these metrological researchers communicate these developments to other practitioners?

Beginning in the 1950s, metrologists seeking to increase atomic clock precision and stability would meet to explain their techniques and findings in papers given at regularly scheduled conferences, symposia, and workshops around the world. A compilation, in 1994, of the literature on time and frequency measurements indicated that by 1994, there had been forty-eight International Frequency Symposia; eight meetings of the European Frequency Control Symposium; four conferences separated by five to seven years of the Symposium on Frequency Standards and Metrology; and twenty-five meetings (through 1993) of the Annual Precise Time and Time Interval Applications and Planning Meeting, which were meetings funded by US military agencies (such as AFOSR) and civilian agencies (such as NASA) whose proceedings “have been published by NASA in recent years.”⁴⁰ The goal throughout was to share expertise and train physicists from other countries, especially in the developing world, in the on-going theorization, experimentation, and instrumentation within metrology and its applications.

In 1990, for example, both Robert Vessot, then senior physicist at the Smithsonian Astrophysical Observatory (in his Advisory Committee role as Member US Study Group 7, Comité Consultatif International de Radio Emission), and Brian Petley (in his role as Chairman of the IUPAP Commission C2) each gave invited papers based on talks presented at the 6th European Conference on Time and Frequency (1990, published the next year).

Vessot’s paper, “Applications of Highly Stable Oscillators to Scientific Measurements,” begins by explaining the technological basis for experiments with clocks, noting that “the frequency stability of highly stable oscillators has improved by a factor of about 10 every decade since the 1960 era, when atomic clocks were first introduced.” The emphasis in his talk was on “applications of highly stable oscillators, focusing on measurements using electromagnetic signals” to lead into the major focus of his own work, namely, “systems for Cancelling First-Order Doppler and Signal Propagation,” which are vital for satellite-ground communication.⁴¹

Brian Petley’s paper, “Time and Frequency in Fundamental Metrology,” which appeared directly after Vessot’s paper, discusses “the role of time and frequency in a wide range of measurements …, particularly those involving the International System of Units (SI) and fundamental physical constants.” Petley noted the importance and difficulty of formulating precise time standards: “It will probably become very important in the future to distinguish between a time standard and a possible frequency standard … We can expect … that the definitions of most of the other base units will rely on time in some way.”⁴² At that time, Petley was not only the Chairman of the IUPAP Commission C2. He also headed the Centre for Basic Metrology, Division of Quantum Metrology at the UK National Physical Laboratory and was a member of the Editorial Boards of journals including Metrologia. As Petley explains, modern “science and technology are placing increasingly stringent demands on our measurement system and associated units,” and the “ultimate arbiters” of the SI system of units “are the General Conference on Weights and Measures (CGPM), the International Committee on Weights and Measures (CIIPM), and the Consultative Committee on Units (CCU).”⁴³

These international negotiations over the precise definitions of units of measure occurred not only in the work of the BIPM, but also in the international meetings of IUPAP. At the September 1978 Council Meeting of IUPAP at the Royal Swedish Academy of Sciences in Stockholm, Sweden, the Executive Council delegated its members to the various International Commissions: “The Executive Committee decided that the following members be asked to establish liaison between the Executive Committee and the International Commissions for the period until the next General Assembly.” Kerwin, Secretary General of IUPAP, headed the newly united C2+C13 (combining “Symbols, Units, and Nomenclature” with “Atomic Masses and Fundamental Constants”).⁴⁴ These two commissions were unified due to IUPAP’s recognition of the increasing use of fundamental constants in the definitions of units and standards in this period. We have seen that, in 1967, the BIPM had changed the definition of a second from an astronomically based definition to an atomic physics definition: the duration of repeated oscillations of a cesium 133 atom within a cesium atomic clock. We have also seen that the definition of the metre in this period was based upon the interferometric measurement of the wavelength of light emitted during an atomic transition, a definition likewise depending on atomic physics. Metrologists at IUPAP and the BIPM in the 1970s debated the merits of changing the definition of the metre to depend directly upon the definition of the second, using electromagnetic radiation as the link. Ultimately this led to the 1983 shift in the definition of metre to the distance that light travels in a vacuum during a specific time interval, which was dependent on the measurements of atomic clocks and thus still relied on atomic physics.

The council at the 1978 Stockholm meeting also assigned IUPAP representatives to inter-union commissions. The only inter-union commission with more than two representatives appointed was “ICSU-Spectroscopy,” and of its four appointed representatives, Norman Ramsey represented the United States.⁴⁵ Ramsey, Vessot, Kerwin, and Petley shared a commitment to international cooperation in the determination of measurement units and the progress of high-precision measurement techniques. For instance, at the 1990 IUPAP council meeting in Dresden, Petley “spoke about the favorable experience of C2 with small conferences” that brought international groups of scientists together for focused workshops on subtopics within the field of metrology.⁴⁶ The results of these meetings often took the form of concrete guidebooks, training materials for universities and industrial laboratories, and direct education of younger colleagues. At the February 1997 IUPAP Executive Council Meeting at CERN in Geneva, Switzerland, Petley “described work through the SUNAMCO Commission with other international organizations in producing two guides in metrology” (The Guide to the Expression of the Uncertainties of Measurement and The International Vocabulary on Metrology) which “will likely have a wide impact on industrial physics.”⁴⁷

In recent years, atomic clocks have achieved ever-greater precision and stability over long time periods due to the collaborative work of scientists and engineers in national laboratories, industrial laboratories, and university laboratories using cool atoms instead of hot atoms. This innovation was made possible from the research beginning in the 1980s carried out by Steven Chu at Bell Labs funded by AFOSR; William Phillips at the National Bureau of Standards (now the National Institute of Standards and Technology), funded in part by the Office of Naval Research; and Claude Cohen-Tannoudji at the Collège de France on the cooling and trapping of atoms with laser light, for which they shared the 1997 Nobel Prize in physics.⁴⁸ Given the importance of their researches for instrumentation worldwide, these three scientists communicated with each other during their extended years of research and with teams at other laboratories such as the Max Planck Institute for Quantum Optics in Germany.

Both Cohen-Tannoudji and Phillips emphasized in their Nobel lectures the inter-institutional and international collaborations that made possible their metrological research. Cohen-Tannoudji recalled attending lectures by Ramsey and many other leading physicists (including Schwinger and Pauli) at the 1955 Les Houches summer school, founded in 1951 and led for twenty-two years by Cécile DeWitt-Morette, based on funding she obtained from a French government ministry.⁴⁹ Phillips had carried out his PhD research in Daniel (Dan) Kleppner’s lab at MIT, who was working “on a hydrogen maser experiment” (building on his work with Robert Vessot) when Phillips arrived in 1970.⁵⁰

As part of Kleppner’s lab, Phillips learned “a way of thinking about physics intuitively, and a way of inquiring about a problem that has shaped the way I approach physics to this day:”

The style of open and lively discussion of physics problems that I found in Dan’s group is one that I have tried to emulate in my own group at NIST [National Institute of Standards and Technology]. I also try to follow the principle Dan taught by example: that one can do physics at the frontiers, competing with the best in the world, and do it with openness, humanity and cooperation.⁵¹

In his Nobel biographical statement, Phillips recalled Bengt Nagel’s formal remarks to the three Nobel laureates in Stockholm (December 10, 1997) “that we were being recognized as leaders and representatives of our groups,” groups that consisted of many collaborators who together developed the methods of laser cooling.⁵² This collaborative approach served Phillips well when he carried out his photonics research at NIST and “the tremendously fruitful collaboration” he had “with Claude Cohen-Tannoudji’s research group” in France in the 1980s, while Steven Chu’s group at Bell Labs carried out a parallel research program. Like Kerwin, Ramsey, Petley, and other physicists serving roles in IUPAP, for many years, Phillips actively participated in the C2 commission of IUPAP, as a member since 2011 and as Vice-Chair of Commission C2 between 2014 and 2017.

In answer to Nilsson’s question with which this chapter began—“What can IUPAP [Commission C2] do for you?”—the IUPAP Commission C2 played a central role in shaping modern metrological practice in three primary ways. First, through the international negotiations by scientists within the IUPAP that coordinated with the IBPM in defining and redefining fundamental units. Second, through the international collaborations made possible with the international conferences put together under the organizational guidance of IUPAP (and other international societies). Third, through the emphasis IUPAP placed on bringing leading metrological researchers into direct contact with researchers at differing stages of their careers and from countries at varying stages of development. While much of the funding for the metrological research itself, and for the international conferences and workshops at which this research was shared, was from national funders, the IUPAP Commission C2 served the international metrological community as an organizational framework which sought to expand and democratize the access to metrological techniques and technologies across the globe.