Inge Lehmann

Inge Lehmann (13 May 1888 – 21 February 1993) was a Danish seismologist and geophysicist who is known for her discovery in 1936 of the solid inner core that exists within the molten outer core of the Earth. She also discovered the seismic discontinuity in the speed of seismic waves at depths between 190 km (120 mi) and 250 km (160 mi), which is named the Lehmann discontinuity after her. Lehmann is considered to be a pioneer among women and scientists in seismology research.

Early life, family and education Inge Lehmann was born on 13 May 1888 and raised in Østerbro, a part of Copenhagen, Denmark. Her father was experimental psychologist Alfred Georg Ludvik Lehmann (1858–1921), a pioneer in the study of experimental psychology in Denmark. Her mother Ida Sophie Tørsleff was a housewife. The Lehmann family had its roots in Bohemia; the Danish branch included barristers, politicians and engineers. Inge Lehmann's paternal grandfather laid out the first Danish telegraph line (1854). Her great-grandfather was Governor of the National Bank. Her mother's father, Hans Jakob Torsleff, was from an old Danish family with a priest in every generation. Members of the Tørsleff family were well-known public leaders and activists in the women's rights movement. Inge's cousins were the Minister of Trade, the chair of the Danish Women's Society, and the leader of the Danish Girl Scouts. Her younger sister Signe, a single mother, rose to popularity as a school superintendent. Lehmann's parents enrolled both their two daughters at Fællesskolen in 1904, a liberal and progressive school that offered the same curriculum to both boys and girls — a practice uncommon at the time. Most schools of this time separated boys' and girls' education. The principles of isolation by sex went much deeper for girls who were education-oriented. It was thought to be damaging to expose girls to mental exhaustion throughout puberty. Males were believed to be more biologically adapted for such activities and therefore, allowed to take the high school entrance exam and start upper-secondary education (high school) at 15 years old, whereas females were prohibited. This policy remained in effect until 1903. This school was led by Hanna Adler, Niels Bohr's aunt, a pioneering woman scholar and firm believer in equality of the sexes. A year after earning her degree, Adler launched her school, inspired by innovative teaching practices in the US. Many of her female co-graduates, who were ineligible for many of the positions accessible to their male peers, were hired by Adler to teach. Women were prohibited from working in universities at the time, and the vast majority of female college graduates searched for employment as elementary schoolteachers despite obtaining degrees that allowed them to teach at the upper-secondary (high school) level. Lehmann credited her father and Hanna Adler as the most significant influences on her intellectual development. At age 18, Lehmann achieved a first rank mark in the entrance exam for University of Copenhagen. In 1907, she started her studies in mathematics, chemistry and physics at the University of Copenhagen. She continued her studies of mathematics at Newnham College, Cambridge, from 1910 to 1911. Lehmann faced sex-based adversities there, not being allowed to fully participate in her studies, nor to achieve higher positions of education. As a result, Lehmann had a mental breakdown during her first year in 1911. She returned to Denmark in 1912. She served as an actuarial assistant until 1918 without attending school. She developed her computational skills in the actuary office until she resumed her studies at Copenhagen University in 1918. She completed the candidata magisterii degree in physical science and mathematics in two years, graduating in 1920. This was significant, since this degree was mostly given to male students. After a short period of time, studying mathematics at the University of Hamburg, in 1923 she accepted a position at University of Copenhagen as an assistant to J.F. Steffensen, the professor of actuarial science. Lehmann's younger sister, Harriet, became an actress. Inge lived alone as an adult. She ended an engagement in March 1917 and decided to remain unmarried in order to pursue an academic career, which was not an unusual choice at the time. Lehmann once complained to her nephew Niels Groes about the incompetence of her male colleagues, writing, "You should know how many incompetent men I had to compete with — in vain."

Career

In 1925, Lehmann was assigned to be the assistant of seismologist Niels Erik Nørlund. She took an interest in his field, and she began studying it on her own. She was chosen as a delegate for Denmark to attend the International Union of Geodesy and Geophysics in 1927—a role she filled another eight times over the next forty years. By 1928, Lehmann obtained a magister scientiarum in seismology, and she was appointed head of the Geodætisk Institut's seismological department the same year. In this position, she was responsible for overseeing the operation of three seismographic observatories, two of which were in Greenland. She personally operated the one in Copenhagen, producing reports based on its readings. Though it was not part of her job, Lehmann also engaged in research at the facility. In 1929, Lehmann studied the Murchison earthquake which struck on the South Island of New Zealand. She analysed the seismic data from the earthquake and noticed that there were waves of significant amplitude recorded in the Russian cities of Sverdlovsk and Irkutsk, both unexpected locations. Lehmann noticed after reviewing the data from the earthquakes that there were waves emanating from the earthquakes, which were called seismic waves. They were unexpected locations due to the theory that S-waves and some P-waves are deflected by the core creating a shadow area in which waves are not able to pass through. The waves seemed to pass through that area to reach Russia. This led to her discovering that there is a spherical core of solid material at the Earth's centre.

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