A Danish seismologist and geophysicist who was dedicated to her career, Inge Lehmann was surrounded by remarkable people from day one of her long life. Born into a family of academics, activists, and intellectuals in the city of Copenhagen, Lehmann was influenced deeply by her early education at an alternative school, which taught her to reject social prejudice. She navigated the male-dominated academic world she encountered during her studies at Copenhagen University and eventually managed several seismological stations; Her discovery of Earth's solid core occurred on her own time and was given credence due to her lasting friendships with top scientists in her field. Although Lehmann never attained an academic position, her commitment to research was unstinting, and she enriched the field of seismology with her insights. The recipient of numerous awards, she was posthumously honored by the creation of the Inge Lehmann Medal.
A Privileged Upbringing on Many Levels
Inge Lehmann was born May 13, 1888, into an intellectual family living in the Østerbro neighberhood of Copenhagen, Denmark. Her father, Alfred Georg Ludvik Lehmann, was an experimental psychologist, and her mother was Ida Sophie Tørsleff. She had a younger sister, Harriet, who had more creative interests and would eventually write filmscripts. Surrounded by activists, intellectuals, and artists as a child, Lehmann learned to think independently. She attended a progressive high school where girls and boys were treated exactly alike and studied the same subjects, be it needlepoint or soccer. The idea was to co-educate young people without prejudice toward race, gender, or social standing. Lehmann was deeply influenced by this experience, although the world awaiting her after graduation would contradict it in many ways.
Lehmann's first inkling of her future career may have occurred one Sunday morning when she was 15 or 16. Lehmann recalled sitting at home with her mother and sister when the floor moved beneath them and the ceiling lamp swayed. Her father entered the room and announced that they had just experienced an earthquake. Recalling the experience later in life, Lehmann noted that, despite scientists’ best efforts after the earthquake, they never did locate its epicenter. Her research would one day contribute to earthquake science and improve the accuracy of such efforts.
In the meantime, the idealistic Lehmann found college life a bit of a shock when she matriculated at the University of Copenhagen. Unlike her progressive high school, student behavior—particularly that of young female student—was governed by restrictive social rules. She nevertheless managed to enjoy life at college, and in 1910 she transferred to Newnham College of the University of Cambridge, in Cambridge, England, to study mathematics. Exhaustion forced Lehmann to return home to Denmark a year later.
Taking a break from rigorous academic studies, Lehmann spent several years working at an actuarial office, where risks are determined for use in determining insurance rates, and the experience built on her computational skills. Returning to Copenhagen University in 1918, she completed her degree in mathematics, taking an extra semester to study mathematics with a professor in Hamburg, Germany. With degree in hand in 1920, the 32-year-old Lehmann found work as an assistant to a professor of actuarial studies at Copenhagen University. Not the most propitious job for a would-be earth scientist, this work took advantage of her hyper-accurate mathematical abilities.
Took a Turn to Seismology
In 1925, Lehmann had her first chance to work in the field of geodesy (also called geodetics), the science of accurately measuring and representing Earth's composition, orientation in space, and gravitational field. Professor Niels Erik Nørlund, with whom she now worked at Copenhagen University, was the director of Gradmaalingen, a Danish geodetic research institution, and was planning to set up seismological stations in Denmark and Greenland. He needed someone to run them, interpreting and publishing their observations, and Lehmann jumped at the chance.
As Nørlund's assistant, Lehmann's first task was to oversee the set-up of the first station in Copenhagen. Nørlund insisted on using the best possible seismographs, which had to be placed carefully and insulated from the vibrations caused by traffic and other movements. Two ancient buildings in the massive fortress surrounding old Copenhagen served well for the purpose. The new station was inaugurated in late 1926, setting a high standard for similar installations.
For the first time, Lehmann learned that seismic observations could be used to study the earth's interior composition. Excited, she read all she could on the subject. Her travels for work, which found her visiting seismic stations in Germany, the Netherlands, and France, provided additional opportunities for learning. In Germany, she met Beno Gutenberg, a noted seismologist who, in 1914, had determined the depth of the earth's core and would and later mentor Charles Richter, inventor of the Richter Scale for measuring earthquake intensity. Gutenberg appreciated Lehmann's work and offered to mentor her in her further studies in seismology.
Lehmann left the Prague conference inspired to help solve these problems. She designed and carried out investigations into seismic station accuracy around Europe, identifying the five most accurate stations (including Copenhagen) as benchmarks for the rest. As she continued gathering and interpreting data from these stations, she pondered the question of seismic time curves. Importantly, she also continued the discussions begun in Prague by corresponding with the top scientists in attendance, among them her mentor Gutenberg and renowned British seismologist Harold Jeffreys. With access to their findings, Lehmann would build on their work and they later were able to verify the accuracy of her important discovery.
A year later, in 1928, Lehmann completed her magister scientiarum (master's-level degree) in geodesy. With this qualification, she was appointed by Nørlund as state geodesist and head of the department of seismology at the Geodetical Institute of Denmark, a merger of the Gradmaalingen and the government's Topographic Department. As Lehmann would later quip, she was Denmark's one seismologist.
Proved Adept at Puzzle Solving
In her new position, Lehmann focused on the issue of accuracy in the measurement of earthquakes, realizing that this would help in determining their epicenters. She explored the use of data from multiple seismic stations to obtain results impossible to get from any one station. An earthquake in Mexico in 1928 was detected by stations as far away as England, Germany, Denmark, and France. She painstakingly compared the measurements taken over time after the quake and calculated, by hand, the time curves of the quake's seismic waves. These calculations, based on readings from multiple stations at varied distances from the earthquake, proved accurate in determining the quake's epicenter.
Lehmann's most famous discovery came about after an earthquake rocked New Zealand in 1929. Leading seismologists had long since identified two types of seismic waves, or forces, occurring after an earthquake: compressional (downward) P-waves (pressure waves) and transverse (sideways) S-waves (shear waves). P-waves moved much faster than S-waves, and both types tended to disappear at a certain point in the epicentral measurement, with weak P-waves sometimes detectable beyond it. It was clear to scientists that S-waves could be explained by Earth having a liquid core, since they knew that transverse waves cannot penetrate liquid. But why were there occasional, weak P-waves?
To answer this question, Lehmann expanded her search, going over the piles of data generated from sites around Europe following the 1929 earthquake. She realized that too much attention had been paid to the horizontal S-waves, and not enough to the vertical P-waves. As she surmised, the P-waves were detected at odd times because they were running into a solid core inside the molten center of the earth. Others had noticed them, but the occurrences were so tiny that they were mostly overlooked. Lehmann not only noticed them but analyzed their occurrences meticulously, carefully building an argument for her solid-core hypothesis in a paper she published six years later, titled simply “P.”
When presented with Lehmann's theory, Gutenberg quickly recognized the validity of her reasoning, and within a few years his affirmations were supplemented by those of fellow seismologists Jeffreys and Richter, and eventually the broader scientific community. These respected scientists verified her findings, and decades later, her calculations would be verified by computer. Lehmann had made a significant breakthrough in understanding planet Earth.
Became Leading Researcher and Popular Guest
The next few decades found Lehmann attending scientific conferences on the basis of her work advancing the study of seismology. She founding the European Seismological Federation and lent her support to numerous other scientific associations, in addition to continuing her own research. Work slowed during World War II, which erupted in Europe in 1948 and affected all aspects of life there for more than half a decade. Lehmann also visited Maurice Ewing, a U.S. seismologist working at the Lamont Geological Observatory in New York, and the two compared differences between North American and European records. Through this work, they determined significant differences in the earth's upper mantle on each continent.
By the late 1940s, Lehmann was being considered for a professorship in geophysics at Copenhagen University. She would have liked nothing better, but institutional politics prevented the position being filled until well past her retirement. She retired from her post at the Geodetic Institute in 1953 and continued her research with Ewing as well as other colleagues throughout the world. Spending several years in the United States to work with Ewing, she studied the behavior of S- and P-waves at shallower levels, identifying a velocity increases at a certain level. This consistent finding of these phenomena were soon named “Lehmann discontinuities.” She also supported scientific studies of underground nuclear explosions, lending her considerable authority and credibility to arguments for a comprehensive test ban.
Starting in the 1960s, Lehmann received many awards in her field from around the world, as well as honorary doctorates from both Copenhagen University and Columbia University. These awards funded her research projects, which required that she travel. Welcomed by her fellow scientists, she maintained her professional friendships even though she was a shy person. Although Lehmann did not often speak of the struggles involved in her work as a woman in a male-dominated field, she was once quoted as saying that she had to compete with a number of incompetent men, alas, in vain.
Complete Dictionary of Scientific Biography, Charles Scribner's Sons, 2008.
Biographical Memoirs of Fellows of the Royal Society (annual), 1997, pp. 285–301.
Eos, January 20, 1987, American Geophysical Union. Inge Lehmann, “Seismology in the Days of Old,” pp 33–35.
Physics Today, January, 1994, Bruce Bolt, “Inge Lehmann,” p. 61.
American Museum of Natural History online, http://www.amnh.org/ (November 30, 2016), “Inge Lehman: Discoverer of the Earth's Inner Core.”❑