JORDAN, ERNST PASCUAL

(b. Hannover, Germany, 18 October 1902; d. Hamburg, Germany, 31 July 1980),

theoretical physics, biophysics, cosmology. For the original article on Jordan see DSB, vol. 17, Supplement II.

Pascual Jordan has remained an intriguing figure for historians of physics and for historians of the political relations of science in twentieth-century Germany. In studies of Jordan’s scientific work, he frequently appears as a physicist gifted in generating provocative, novel approaches, not always fully appreciated at the time but perhaps even more impressive in retrospect. Reaching a definitive interpretation of Jordan’s controversial political stances, especially vis-à-vis national socialism, remains problematic. Studies since 1990 have offered depth and complexity to scholars’ picture of Jordan’s relationship to Nazi ideology; they have also highlighted continuities between his actions during the National Socialist period and his radically conservative thought before 1933 and after 1945.

Jordan as Quantum Theorist . New assessments of Jordan’s role in the development of quantum theory have arisen from particular details about his interactions with contemporaries and from reinterpretations of the history of quantum theory as a whole. The prior consensus was that in constructing matrix mechanics and transformation theory, Jordan was largely responsible for the mathematical structures of quantum theory; several historians, however, also ascribe to him a more conceptually central role than that of a mathematical technician. Mara Beller, for example, in arguing that the “Copenhagen interpretation” of quantum theory originated from a flux of dialogues among various theorists, calls attention to Jordan’s role in formulating the indeterminacy principle. In her reading, Werner Heisenberg’s famous paper on the subject, “Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik,” published in 1927, was a direct response to Jordan’s habilitation lecture of earlier that year, “Philosophical Foundations of Quantum Theory.” Jordan’s lecture asserted that quantum theory was still incomplete, because one could imagine apparently deterministic experimental situations (e.g., tracing the trajectory of a particle in a cloud chamber) that were not described by the quantum formalism. The theory was statistical but had not yet been reduced to independent, elementary probabilities. Heisenberg was thereby inspired, according to Beller, to focus on the problem of measurement: necessarily incomplete knowledge of initial conditions produced quantum indeterminacy (Beller, 1999). These new emphases on the limitations of measurement and on essential quantum indeterminacy became fundamental to Jordan’s own work after 1927.

Although Jordan’s place in the development of matrix mechanics and transformation theory has received the most attention, Jordan himself considered his work on the “second quantization” of fields to be “his most important contribution to theoretical physics” (Schweber, 1994, p. 33). Jordan’s role was, once again, highly dialogic. A series of papers—partly by himself and partly in collaboration with Oskar Klein, Wolfgang Pauli, and Eugene Wigner, inspired other physicists, notably Heisenberg and Paul Dirac—to examine the possibilities for field quantization more closely, albeit sometimes with skepticism toward Jordan’s approach.

Jordan as Biophysicist . In the 1930s Jordan turned his attention increasingly to biophysics; indeed, based on his plans for the immediate postwar period, this was arguably his main field of interest as of 1945. Jordan’s “amplifier theory,” introduced in 1932, was an attempt to apply Niels Bohr’s complementarity idea to biology; in turn, Bohr’s subsequent statements on biology, as well as the initial work in this direction by Max Delbrück, can be understood as rejoinders to Jordan. Whereas Bohr’s application of the complementarity idea to biology was primarily analogical, Jordan attempted to locate actual quantum phenomena in organic reactions. Meanwhile, biologists such as Erwin Bünning and Max Hartmann and biochemists such as Otto Meyerhof objected that Jordan ignored the causal methodologies of the life sciences, while Jordan’s fellow positivists such as Edgar Zilsel and Otto Neurath attacked his ideas as thinly veiled neovitalism.

Undeterred, Jordan’s biophysics took on an increasingly technical character from 1936 onward. He primarily deployed the “target theory,” which had been developed since the early 1920s by radiation biologists such as Friedrich Dessauer in Germany, Fernand Holweck in France, and J. A. Crowther in Great Britain. Target theory sought to identify submicroscopic features through a statistical analysis of the effects of radiation on the organism. A target-theoretical analysis of x-ray mutagenesis formed the basis of a 1935 paper by Nikolai W. Timoféeff-Ressovsky, Karl G. Zimmer, and Delbrück, which proposed that genes were single (large) molecules. For Jordan, this signally confirmed the amplifier theory: genetic phenomena such as mutations were essentially quantum-physical in character and therefore indeterministic. Jordan sought further confirmation of the amplifier concept in immunology, protein chemistry, sense physiology, and various psychoanalytical and parapsychological phenomena. He also worked on refining the statistical methodology of target theory—for example, by mathematically accounting for the “saturation” effect of densely ionizing radiation. (From 1938 onward Zimmer and Timoféeff-Ressovsky experimented with neutron radiation to test these theoretical refinements.) Once again Jordan’s approach had its detractors; Timoféeff-Ressovsky and Zimmer, for example, while appreciating Jordan’s more technical contributions, were generally reserved about any broader philosophical implications.

After World War II Jordan was in negotiation with the erstwhile Prussian Academy of Sciences, which had taken control of a number of research facilities in the Soviet Zone, including Timoféeff-Ressovsky’s at the Kaiser Wilhelm Institute for Brain Research in the Berlin suburb of Buch. Timoféeff-Ressovsky and Zimmer having both been taken to the Soviet Union, the institute needed new leadership. The academy envisioned making it into a center for biophysics and contemplated appointing Jordan as its director or rather (once this appeared barred by political complications) as its leading scientific member, with target-theoretical radiation biology as a major research emphasis. Jordan remained in West Germany, however, after several years of career uncertainty. Nevertheless, the ambitious scope of the Buch plans indicates the importance Jordan attached to his biophysical work as of the late 1940s, as well as the seriousness with which it was regarded by (at least some) of his German colleagues.

Jordan as Mathematician and Cosmologist . Jordan’s main scientific activity thereafter moved into two other fields, both of which, however, had roots in earlier work. One was algebraic theory, especially the theory of semigroups and the theory of nonassociative algebras—an interest that can be traced back to his introduction of matrices into quantum physics. Many of Jordan’s publications in this field appeared in the proceedings of the Academy of Sciences and Literature in Mainz, of which Jordan was the founding vice president (1949) and later president (1963–1967).

The second field was cosmology. Jordan made Hamburg into the leading German center for general relativity theory and its cosmological applications. Once again his approach was idiosyncratic: in the late 1930s Jordan had become interested in a suggestion by Dirac that the supposed gravitational “constant” was, rather, decreasing over time. Working out the consequences of this hypothesis, Jordan developed a cosmological model in which the universe expanded from a single atomic starting point. Matter was created continuously over time, with embryonic stars appearing in sudden, explosive bursts. Applying the idea to geophysics, Jordan concluded that Earth was expanding, as manifest in phenomena such as rift valleys. Apart from this specific hypothesis, Jordan (in collaboration with a series of students) also undertook a broader program of research into the theory of the gravitational field.

Years later Jordan’s cosmology would be commonly cited as an analogue to the 1961 theory of Carl Brans and Robert Dicke—originally articulated independently of Jordan’s work—of a varying scalar field that would account for objects’ mass. The “Jordan-Brans-Dicke field” has become one of the central theoretical concepts of particle cosmology, a burgeoning subfield of physics since the 1970s.

Jordan as Political Figure and Public Intellectual . Doubtless the most controversial aspect of Jordan’s career remains his affiliation with national socialism. Jordan’s joining the Nazi Party and the SA (Sturmabteilung, or storm troopers) in 1933 was not in itself unusual among German scientists; however, a series of publications suggested that Jordan’s affinity for the Nazi cause was more than nominal. Articles in cultural-political journals argued that academia had to reorient itself toward service to the new state following the political revolution. Physikalisches Denken in der neuen Zeit (1935) pointed out ways in which modern physics contribute to building a strong military—for example, atomic energy. In several articles and, at greater length in Physik und das Geheimnis des organischen Lebens (1941), Jordan analogized between “steering centers” in the organism, such as genes, and the dictatorial state; parliamentary democracy was, conversely, the analogue of lifeless inorganic matter.

After 1945 Jordan argued that he had joined the Nazi Party out of a sense of career pressure or obligation to work for moderation within the Nazi power system. Jordan did campaign actively against “Aryan physicists” who saw much of modern physics as Jewish influenced and hence undesirable. Anschauliche Quantentheorie (1936) was in part a sustained argument that quantum physics was not mere mathematical formalism of the sort deplored by the Aryan physicists but was, rather, based solidly on observational encounters with nature. Likewise, Jordan engaged in a running literary battle in defense of his positivistic interpretation of quantum theory against Hugo Dingler, a philosopher associated with the Aryan physicists.

However, in historical perspective, the claim that opposition to Aryan physics in itself constituted opposition to Nazism is difficult to sustain. Despite post-1945 self-representations, the mainstream physics community also made its accommodations with the Nazi regime. The circumstances behind Jordan’s receipt of the 1942 Planck Medal, for example, indicate that his colleagues were well aware of the symbolic power of having a scientist who was conspicuously friendly to the regime as the bearer of the German Physical Society’s top honor.

Moreover, Jordan’s political interests were not unique to the Nazi period and so cannot be dismissed as mere opportunism. As early as 1930 Jordan (under a pseudonym) was publishing conservative-nationalist journal articles; he continued to do so under his own name after 1933. (On the connection to the “Domeier” articles see Beyler, 1994, pp. 207–224.) After World War II, Jordan’s activism for right-wing causes if anything increased. He thus may not have been committed to all aspects of Nazi ideology but apparently found in Nazism some resonances with a lifelong political philosophy: opposition to communism and suspicion of liberalism as manifestations of materialism; enthusiasm for technology; and elitism in his understanding of history and society.

In the postwar period Jordan’s activity as a public intellectual and popularizer of science flourished in the form of hundreds of books, articles, and public lectures. Above all, he sought to convey the message that modern science, and the positivist philosophy he associated with it, had brought about the end of materialism. This meant, in turn, a denial of materialism’s denial of the possibility of religious faith. Der Naturwissenschaftler vor der religiösen Frage: Abbruch einer Mauer (1963 and subsequent editions) described the breaching of the “wall” that materialism had erected between religiosity and science.

The wall metaphor had obvious political overtones. According to Jordan, the overthrow of philosophical materialism also meant its demise as a political philosophy, and he spoke and wrote vociferously against communism and socialism, presented as misguided attempts to impose philosophical dogmas on society, and in favor of conservatism, seen as an attachment to empirically grounded realities. This ideological commitment was combined with a frank endorsement of a strong military posture in the West. In Der gescheiterte Aufstand (1956), for example, Jordan portrayed a future nuclear conflict as nearly inevitable but insisted that its worst effects could be overcome by the preparation of underground cities and similar measures. The following year, an imbroglio erupted in the wake of Chancellor Konrad Adenauer’s suggestion that the German army might become equipped with nuclear weapons. The “Göttingen Eighteen”—a group of atomic physicists including Max Born and Heisenberg—issued a manifesto stating their refusal to undertake weapons research. Jordan publicly criticized his colleagues as, at best, politically naive. Results included public rejoinders and, privately, a bitter exchange of letters between Jordan and Max and Hedwig Born. Largely as a result of his advocacy for nuclear armament, Jordan was put forward as a Christian Democratic candidate for the German federal parliament in the 1957 election and won a seat that he held until 1961. This brief spell was the extent of Jordan’s activity as a politician in the strict sense, but in many publications and lectures until his death in 1980 he continued to promote various conservative positions in politics and theology.

SUPPLEMENTARY BIBLIOGRAPHY

WORKS BY JORDAN

“Philosophical Foundations of Quantum Theory.” Nature 119 (1927): 566–569, 779. Originally published as “Kausalität und Statistik in der modernen Physik.” Naturwissenschaften 15 (1927): 105–110.

Physikalisches Denken in der neuen Zeit. Hamburg, Germany: Hanseatische Verlagsanstalt, 1935.

Anschauliche Quantentheorie: Eine Einführung in die moderne Auffassung der Quantenerscheinungen. Berlin: J. Springer, 1936.

Die Physik und das Geheimnis des organischen Lebens. Braunschweig, Germany: F. Vieweg, 1941.

Der gescheiterte Aufstand: Betrachtungen zur Gegenwart. Frankfurt, Germany: Vittorio Klostermann, 1956.

Der Naturwissenschaftler vor der religiösen Frage: Abbruch einer Mauer. Oldenburg, Germany: G. Stalling, 1963.

OTHER SOURCES

Overviews

Beyler, Richard H. “From Positivism to Organicism: Pascual Jordan’s Interpretations of Modern Physics in Cultural Context.” PhD diss., Harvard University, 1994. Surveys Jordan’s career, concentrating particularly on his biophysical work and postwar popularizations of science.

Ehlers, Jürgen, and Engelbert Schücking. “‘Aber Jordan war der Erste’: Zur Erinnerung an Pascual Jordan.” Physik Journal 11 (November 2002): 71–74. Memoir by two of Jordan’s former students.

Ehlers, Jürgen, Dieter Hoffmann, and Jürgen Renn, eds. Pascual Jordan (1902–1980): Mainzer Symposium 100. Max-Planck Institut für Wissenschaftgeschichte preprint no. 329. Berlin, 2007. Includes memoirs and scientific studies by former students of Jordan, essays by historians, and a bibliography.

Hoffmann, Dieter, ed. Pascual Jordan (1902–1980): Symposium zum 100. Geburtstag des Physikers. Max-Planck-Institut für Wissenschaftsgeschichte preprint. Berlin, forthcoming 2007. Includes memoirs and scientific studies by former students of Jordan, studies by historians of physics, and an extensive bibliography.

Schücking, Engelbert L. “Jordan, Pauli, Politics, Brecht, and a Variable Gravitational Constant.” Physics Today 52 (October 1999): 26–31. Memoir by a former student; provides insight on ideology, activity as popularizer of science, and cosmological theories.

Jordan and Quantum Physics

Beller, Mara. Quantum Dialogue: The Making of a Revolution. Chicago: University of Chicago Press, 1999. Attempts a historically contingent interpretation of the rise of the “Copenhagen interpretation” of quantum theory; considers Jordan in dialogue with other figures.

Darrigol, Olivier. From c- Numbers to q-Numbers: The Classical Analogy in the History of Quantum Theory. Berkeley: University of California Press, 1992. Examines the use of correspondence principles in the development of quantum theory; considers role of Jordan among other figures.

Greenspan, Nancy Thorndike. The End of the Certain World: The Life and Science of Max Born. New York: Basic Books, 2004. Includes discussion of Jordan’s interactions with Born as a student and during the nuclear weapons controversy of the 1950s.

Schweber, Silvan S. QED and the Men Who Made It: Dyson, Feynman, Schwinger, and Tomonaga. Princeton, NJ: Princeton University Press, 1994. Includes a section on Jordan as progenitor of quantum field theory.

Jordan’s Work in Other Scientific Fields

Aaserud, Finn. Redirecting Science: Niels Bohr, Philanthropy, and the Rise of Nuclear Physics. Cambridge, U.K., and New York: Cambridge University Press, 1990. Describes the origin of Jordan’s biological theories and disagreements with Bohr thereon.

Beyler, Richard H. “Targeting the Organism: The Scientific and Cultural Context of Pascual Jordan’s Quantum Biology, 1932–1947.” Isis 87 (1996): 248–273. Analyzes Jordan’s contributions to target theory in the context of his ideological stances before, during, and after the National Socialist era.

———. “Evolution als Problem für Quantenphysiker,” translated by Rainer Brömer. In Evolutionsbiologie von Darwin bis heute, edited by Rainer Brömer, Uwe Hossfeld, and Nicolaas Rupke, 137–160. Berlin: VNB, 1999. Examines Jordan’s biological theories in contrast to competing views presented by Erwin Schrödinger.

Kragh, Helge. Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton, NJ: Princeton University Press, 1994. Includes discussion of Jordan’s cosmological models.

Jordan’s Political and Philosophical Commitments

Beyler, Richard H. “The Demon of Technology, Mass Society, and Atomic Physics in West Germany, 1945–1957.” History and Technology 19 (2003): 227–239. Discusses pessimism about and enthusiasm for technology, in particular nuclear technology, in postwar Germany, with Jordan as major example.

———. “Pascual Jordan: Freedom vs. Materialism.” In Eminent

Lives in Twentieth-Century Science and Religion, edited by Nicolaas A. Rupke, 157–176. Frankfurt, Germany: Peter Lang, 2007. Considers Jordan’s religious views in the context of Cold War Germany.

———, Michael Eckert, and Dieter Hoffmann. “Die Planck-Medaille.” In Physiker zwischen Autonomie und Anpassung: Die Deutsche Physikalische Gesellschaft im Dritten Reich, 217–235. Weinheim, Germany: Wiley-VCH, 2006. Analyzes the politics behind the granting of the Planck Medal of the German Physical Society, including Jordan’s award in 1942.

Danneberg, Lutz. “Logischer Empirismus in Deutschland.” In Wien-Berlin-Prag: Der Aufstieg der wissenschaftlichen Philosophie, edited by Rudolf Haller and Friedrich Stadler, 320–361. Vienna: Hölder-Pichler-Tempsky, 1993. Surveys controversies over positivism in Germany in the 1920s and 1930s, including Jordan’s involvement.

Hentschel, Klaus, ed. Physics and National Socialism. Basel, Switzerland: Birkhäuser, 1996. Extensive documentation, background, and interpretation of primary texts, with Jordan among many other authors.

Hoffmann, Dieter. Pascual Jordan im Dritten Reich: Schlaglichter. Max-Planck-Institut für Wissenschaftsgeschichte preprint no. 248. Berlin, 2003. Includes reproductions of several articles and letters by Jordan from the National Socialist era.

Schirrmacher, Arne. Dreier Männer Arbeit in der frühen Bundesrepublik: Max Born, Werner Heisenberg and Pascual Jordan als politische Grenzgänger. Max-Planck-Institut für Wissenschaftsgeschichte preprint no. 296. Berlin, 2005. Examines Jordan as public and political figure in the 1950s, in comparison with Born and Heisenberg.

Wise, M. Norton. “Pascual Jordan: Quantum Mechanics, Psychology, and National Socialism.” In Science, Technology, and National Socialism, edited by Monika Renneberg and Mark Walker, 224–254. Cambridge, U.K., and New York: Cambridge University Press, 1994. Seminal article analyzing Jordan’s political thought in juxtaposition to his psychological and biological theories.

Richard H. Beyler