University of Heidelberg, Heidelberg, Germany
After his early education in the schools of Breslau, where his father was a professor of chemistry in the university, Ladenburg went in 1900 to the University of Heidelberg.
Ludwig Maximilian University of Munich, Munich, Germany
In 1902 Ladenburg went to Munich, where he took his Ph.D. in 1906 under Roentgen with a thesis on viscosity.
University of Wroclaw, Wroclaw, Poland
Ladenburg studied at the University of Breslau (now the University of Wroclaw).
1930
Photo of Rudolf Ladenburg.
1935
Copenhagen, Denmark
Physics conference snapshot including Rudolf Ladenburg on left and Otto Stern on right. Copenhagen, c. 1935.
Rudolf Ladenburg at work.
Rudolf Ladenburg with Albert Einstein.
Rudolf Ladenburg at work.
1930
Photo of Rudolf Ladenburg.
1935
Copenhagen, Denmark
Physics conference snapshot including Rudolf Ladenburg on left and Otto Stern on right. Copenhagen, c. 1935.
University of Heidelberg, Heidelberg, Germany
After his early education in the schools of Breslau, where his father was a professor of chemistry in the university, Ladenburg went in 1900 to the University of Heidelberg.
Ludwig Maximilian University of Munich, Munich, Germany
In 1902 Ladenburg went to Munich, where he took his Ph.D. in 1906 under Roentgen with a thesis on viscosity.
University of Wroclaw, Wroclaw, Poland
Ladenburg studied at the University of Breslau (now the University of Wroclaw).
Rudolf Ladenburg at work.
Rudolf Ladenburg with Albert Einstein.
Rudolf Ladenburg at work.
Albert Einstein
Ethnicity:
Ladenburg was a descendant of the well-known Jewish Ladenburg family.
Rudolf Walter Ladenburg was born on June 6, 1882, in Kiel, Germany. He was the second of three sons of the eminent chemist Albert Ladenburg and his wife, Margarete Pringsheim.
After his early education in the schools of Breslau, where his father was a professor of chemistry in the university, Ladenburg went in 1900 to the University of Heidelberg. He returned to Breslau in 1901 and in 1902 went to Munich, where he took his Ph.D. in 1906 under Roentgen with a thesis on viscosity, a subject that held his interest until he began work in spectroscopy in 1908.
From 1906 to 1924 Ladenburg was on the staff of the University of Breslau, first as privatdocent and from 1909 as an extraordinary professor. He served in 1914 as a cavalry officer but later in the war did research in sound ranging. In 1924, at the invitation of Haber, he moved to the Kaiser Wilhelm Institute in Berlin as head of the physics division and remained there until going to Princeton in 1931.
Ladenburg is well known for his research in many fields of physics, but his most original work was done in the elucidation of anomalous dispersion in gases, in the period before 1931. To understand the originality of that work it is necessary to remember that the quantum theory had been proposed by Planck in 1901 but that its application to atomic phenomena did not become possible until after Bohr’s paper on hydrogen in 1913. It was in the interim that Ladenburg started his important work.
A paper of considerable significance to an understanding of Ladenburg’s mind must be discussed at this point. It is a sixty-page review of the photoelectric effect, published in 1909 in the Jahrbuch der Radioaktivität und Elektronik. Much of the experimental elucidation of that effect was the work of Erich Ladenburg, an older brother, whose tragic death by drowning in 1908 affected Rudolf very seriously. The paper is especially noteworthy because in it Ladenburg became one of the first physicists to accept Einstein’s view of the constitution of radiation, at least as applied to the photoelectric effect. At that time there were two competing theories of that effect, the commonly accepted resonance theory due to Lenard and the theory which depended on the acceptance of Einstein’s idea that radiation consisted of indivisible packets of energy hv. Ladenburg gives a masterly discussion of the two theories and shows conclusively that Lenard’s theory is in difficulties in explaining several of the known facts, whereas Einstein’s fits all of them in a quite natural way. At the same time, Ladenburg definitely recognized that Einstein’s theory is in contradiction to the classical theory of radiation and gives no explanation of the mechanism of the release of the electrons. In other words, Ladenburg was face to face with the fundamental problem that led to the invention of quantum mechanics some sixteen years later.
The research undertaken by Ladenburg in the early part of his university career was published in five papers on viscosity, including his thesis. He then turned to spectroscopy, which was just becoming of major interest to physicists. His first effort in that new field was directed to a solution of the problem of whether electrically excited hydrogen could absorb the light of the Balmer lines, a question about which there was much conflicting evidence. Ladenburg solved it very quickly and definitely by an experiment in which he ensured that the source of the Balmer lines and the absorbing column of gas were both excited at the same instant, by simply having the two tubes in series in the same induction-coil circuit. He then proceeded to resolve the question by demonstrating that anomalous dispersion could be detected in the close neighborhood of the lines by placing the absorption tube in one arm of a Jamin interferometer. A further confirming experiment was carried out to measure the magnetic rotation of the plane of polarization of the light passing in the immediate neighborhood of the absorption lines. The small but measurable effect distinguished definitely between two current theories, confirming Voigt’s theory based on the Hall effect. It also demonstrated that the oscillators were negatively charged particles. Ladenburg discussed these results in detail in a paper in Annalen der Physik.
During the years 1921-1928 Ladenburg performed a number of experiments with the aid of his students Kopfermann, Carst, and Levy that led to an important series of eight papers under the general title “Untersuchungen fiber die anomale Dispersion angeregte Gase.” They are concerned mainly with the dispersion in electrically excited neon around the familiar strong yellow to red. The new work was done with much-improved techniques and gave results of outstanding importance. A further series of five papers, written with G. Wolfsohn, under the title “Untersuchungen über die Dispersion von Gasen und Dämpfen und ihre Darstellung durch die Dispersionstheorie” was published between 1930 and 1933.
Ladenburg’s experimental work on hydrogen and, especially, neon was very important for the theory of dispersion. In addition, it showed for the first time the possibility of obtaining definite results in the extremely difficult field of electrical discharges in gases. His successful formula for anomalous dispersion, which ignored the Bohr orbital frequencies in favor of the line frequencies, was of critical importance to the later development of the new quantum theory during the 1920s.
In 1931 Ladenburg was a visiting professor at Princeton and in 1932 accepted the appointment to the Cyrus Fogg Brackett professorship of physics, a position he held until his retirement in 1950. When he accepted the call to Princeton, he was probably insufficiently aware of the radical differences in the organization of departments of physics in Germany and the United States; furthermore, it may have come as a shock to find that the research professorship did not give him the control over graduate work that it would have given in Germany. The difficulty of reorientation was increased by the change of his chief interest to nuclear physics, a field that was then rapidly displacing spectroscopy.
For Ladenburg’s use the Princeton physics department purchased, with funds from the Rockefeller Foundation, a transformer-rectifier set with a capacity of 400,000 volts and thirty milliamperes. It was considered that the high current would compensate for the low voltage, but unfortunately, this hope was not realized because of the limitations in the available accelerating tubes and ion sources.
World War II, which disrupted all physics research at Princeton, brought Ladenburg into contact with the Army Ballistic Laboratories, for which he developed a flash suppressor for rifles. This later led him to his postwar research on gas dynamics.
Refractivity changes accompanying density changes in a compressible flow field permit the use of optical methods in laboratory research. It was here that Rudolf Ladenburg made the contribution for which he is best remembered by fluid dynamicists - the introduction in the United States of optical interferometry as a quantitative tool to map the density distribution in high-speed flows. He and his associates were the first to make systematic interferometric studies of over- and underexpanded supersonic jet flows and channel flows. The method was quickly adopted by many research laboratories for use with wind tunnels, shock tubes, ballistic ranges, and similar devices and in plasma dynamics. It is evident that the method will remain a major tool for the study of the physics of gases and gas flows.
Ladenburg was an atheist.
As early as 1933, the plight of German scientists under Hitler was causing great concern, and many had to emigrate from Germany to other countries. The sympathy aroused in Ladenburg and E. P. Wigner by that situation led them to address an appeal to many physicists in American institutions for definite pledges of support for displaced colleagues, listing many by name. The work thus started was carried on by Ladenburg throughout the 1930s and 1940s by correspondence with scientists in many countries, including Germany. It was a work of the heart and showed that the human side of his nature was more important to him than the professional side.
Quotes from others about the person
Albert Einstein: “Ladenburg has been caught very suddenly by illness. He was a good human being who did not take things easily. During his last years he even fled from newspaper reading because he could not stand any more of the hypocrisy and mendacity.”
Ladenburg married Else Uhthoff in 1911. They had two children.
