Inge Lehmann, Geophysicist & Seismologist
Inge Lehmann, a prolific Danish seismologist and geophysicist, was once described as “the master of a black art for which no amount of computerizing is likely to be a complete substitute.” This description is a testament both to her methodical and rigorous approach to manually computing and comprehending raw seismological data, but also to her strides taken in progressing women’s impact on the scientific community. The start of the 20th century was a time of rising suffrage and the empowerment of women, which Lehmann championed in her rich and fulfilling career in physical science. Not only did she revolutionize our understanding of the very planet we occupy, but she is also the longest-lived female scientist, living for nearly 105 years.
According to Lehmann herself, some of the most influential people in her intellectual development were her father, the experimental psychologist Alfred Georg Ludvik Lehmann, and the head of her school, Hanna Adler, the aunt of Niels Bohr. She attended a pedagogically progressive school Fællesskolen, which translates to “shared school,” where boys and girls are treated alike. The omnipresent force of gender discrimination in educational facilities was not a barrier in Lehmann’s formative years, giving her a firm and unbiased foundation in academics. However, she did not feel fully challenged in this environment, so in 1907, she started her freshman courses in mathematics, physics, and chemistry at the University of Copenhagen. Despite her intellect, it took her an enormous amount of time to get her degree, finally graduating in 1920 with her candidatus magisterii degree in physical science and mathematics. This can be largely attributed to the immense burnout she suffered during her year in Newman College at Cambridge University from 1910 to 1911. She worked as an actuarial assistant for an insurance company until 1918, utilizing her talent for mathematics, then returned to Copenhagen University and finished her degree in just two years. Her passion for unraveling the mysteries of the structure of the earth began when she became the assistant of Professor Niels Nørlund. He revealed the intricacies of how the relative paths and velocities of seismic waves can reveal the structure of the Earth — something we cannot directly observe or send even our most advanced probes into. So she was so captivated that from 1927–28, she achieved her magister scientiarum degree, equivalent to a masters, in geodesy. Consequently, she was offered and accepted a position as ‘State Geodesist’ and Head of the Department of Seismology at the Geodetic Institute of Denmark. This achievement was made all the more impressive considering her status as a young woman in the male-dominated field of science.
The study of seismology refers to how different seismic waves, commonly triggered by earthquakes or localized nuclear blasts, travel through the different layers of the Earth. At the time Lehmann, who once referred to herself as “the only Danish Seismologist,” was studying the accepted model of the Earth–that it contained three layers: the crust, a solid mantle and a liquid core. This was radically overturned by the readings from the 1929 earthquake near New Zealand, and it ignited Lehmann’s curiosity and provoked her into research — despite her work as department head being mainly administrative. Understanding the nature of P waves and S waves, Lehmann was perplexed by readings she received from the gradually growing global network of seismographs. P waves are known as primary waves, which are longitudinal in nature and consist of a series of compressions as rarefactions that propagate parallel to their direction of travel. In direct contrast, S waves, known as shear or secondary waves, are transverse waves with propagations perpendicular to the direction of travel. Due to these differing properties, P waves are able to travel through both solids and liquids, whereas S waves can only travel through liquids. This understanding about wave properties was key for Lehmann, as from her readings she was able to postulate the presence of a solid inner core, as opposed to the entirely molten liquid core theory, being the first scientist to do so. Her model matched the findings of P waves found in what was assumed to be a P wave shadow zone. Due to differences in density, waves can be reflected and refracted, and since she found waves where they were not supposed to be, Lehmann was able to correctly predict that the inner core must be solid to account for the degree of deflection that the seismological readings showed. Building on this, she was then able to calculate that the inner core had a radius of approximately 1,400 km. She then proceeded to publish her findings in a paper simply titled: P’. However, while it was met with wide approval, it was not universally adopted until years later with increasingly more accurate measurements from seismographs. Her discovery is astonishing and a pivotal triumph for women in science. She was able to make accurate inferences about the structure of our planet using nothing but the crude technology at the time and rigorous manual calculations.
In the years following what was arguably her most profound contribution to science, she continued to be an icon of feminist intellect as well as an invaluable member of the scientific community. She served as the chain of the Danish Geophysical Society in 1940 and 1944 and was also considered for a professorship at Copenhagen University in 1952, although she did not attain the position. Furthermore, after officially retiring from her position as head of the Geodetic institute in 1953, she was now liberated from her administrative work and could turn her focus fully to science. During this time, she discovered another seismic anomaly in the upper mantle, between 190 and 250 km deep. In her honor, it was named the “Lehmann discontinuity” as an acknowledgment of her profound contribution to her field. In addition, she won numerous awards for her contribution to science with the most prolific being the William Bowie Medal in 1971. This is the highest award in geophysics, and not only that, but she was the first woman to receive it, forever marking the history books. She also received two honorary doctorate degrees from Columbia University and the University of Copenhagen, as well as being made a fellow of the Royal Society. However, her legacy and contributions to geophysics were best eternalized through the creation of the annual ‘Inge Lehmann Medal,’ established by the American Geophysical Union, which honors: “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.” Overall, her innate curiosity, intellect and commitment to her field serve as an inspiration to all young women in science.
by Laura Jewsbury
References
American Museum of Natural History. “Inge Lehmann: Discoverer of the Earth’s Inner Core | AMNH.” American Museum of Natural History, 2020, www.amnh.org/learn-teach/curriculum-collections/earth-inside-and-out/inge-lehmann-discoverer-of-the-earth-s-inner-core. Accessed 23 May 2022.
Famous Scientists. “Inge Lehmann — Biography, Facts and Pictures.” Famousscientists.org, 2015, www.famousscientists.org/inge-lehmann/. Accessed 23 May 2022.
History of Scientific Women. “Inge LEHMANN.” Scientificwomen.net, 2015, scientificwomen.net/women/lehmann-inge-137. Accessed 23 May 2022.
Rafferty, John P. “Inge Lehmann | Danish Seismologist.” Encyclopædia Britannica, 17 Feb. 2019, www.britannica.com/biography/Inge-Lehmann. Accessed 23 May 2022.
