Women In STEM: Inge Lehmann (Journey to the Middle of the Earth)

STEM

STEM stands for Science, Technology, Engineering and Mathematics.  Today’s Woman in STEM mainly worked in the Science and Mathematics categories.

Introduction

Our third Woman in STEM is Inge Lehmann (May 13^th, 1888 – February 21^st, 1993).  She was a Danish mathematician and seismologist that discovered that there is a solid inner core layer of the Earth.

Early Life and College

Inge Lehmann was born on May 13, 1888 in Copenhagen, Denmark (1,2).  Her mother, Ida Sophie Tørsleff, was a housewife from a prominent family (1).  Inge’s father, Alfred Georg Ludvik Lehmann, was a pioneer experimental psychologist (1,2).  Inge had a younger sister, Harriet, who pursued a career in theatre.  Inge attended a private school with a co-gender education.  Boys and girls attended classes together and were treated equally.  This was a new idea in Denmark and not heard of at all outside of Denmark, at that time (1,2,3).  Girls were taught separately from boys, if they were taught at all.

In 1907, at 18 years old, Inge started her bachelor’s degree from the University of Copenhagen, Denmark.  She enrolled in mathematics, chemistry, and physics courses (1).  Inge then left Denmark to continue her degree at Newnham College at the University of Cambridge, UK.  During this time women were not allowed to attend the University of Cambridge or earn a degree, in fact women couldn’t earn a degree from Cambridge until 1948.  Colleges like Newnham were specifically established at the universities so women could attend lectures (4).  Although women enrolled at Newnham College could attend female designated lectures at the University, women were not allowed in the library or laboratories (3).  This was her first significant exposure to gender segregation (2).

In 1911 Inge left Cambridge to spend Christmas with family in Denmark.  It was believed that she didn’t return to Cambridge because she was too exhausted and it impacted her health so she couldn’t continue.  Although, yes, she was exhausted and it did impact her health she fully planned to return to Cambridge in January 1912.  However, those plans were stopped when her father refused to fund her studies and insisted she look for her own job in the real world (3).  It is believed that her father did this for two reasons.  First, to protect Inge’s health that was being impacted from her exhaustion.  Inge’s psychologist father believed that women did not have the mental stability to withstand the pressures of university courses.  Second, although her family had wealth, there were financial concerns with both Inge and her sister Harriet having tuition expenses.  Harriet was enrolled in acting classes at the Danish Royal theatre at this time (3).  This was also only two years before World War I started and some were already feeling the pressure leading up to the Great War.

Since she was in need of an income, she began working in an insurance office.  She continued to work there until 1918 when she decided to finish her degree in mathematics at the University of Copenhagen (1).  Inge graduated with a Bachelor’s degree in 1920, at the age of 32 (1,2).

Math to Seismology

In 1921 an act passed allowing women to seek employment in the public sector.  Before this, most women went on to be school teachers but now that act meant Inge could work at a university (3).  In 1923 Inge began working as a teaching assistant in math and statistics at the University of Copenhagen (1).  Although she was allowed to work as a teaching assistant, she received significantly less pay than the men in similar roles.  Because of this, she worked other part time jobs including occasionally assisting another math professor at the University: Niels Erik Nørlund (3).  Niels was also the director of the Danish Geodetic Service (3).  Geodetic refers to the study of numeric data collected after an earthquake.

In 1925, at 37 years old, Inge left her teaching assistant position and joined Niels as his assistant in reading and organizing geodetic data (1,2,3).  He was working to install and update the instruments that read and collect earthquake data in Denmark and Greenland.  These updates brought the Danish Geodetic Service up-to-date so that they could contribute to seismological data being collected around the world (2,3).

The Seismologist of Denmark

Denmark receives very few earthquakes so there was little interest in seismology.  There previously wasn’t even a geodetic or seismology program to study in Denmark before the Danish Geodetic Service.  Then, in 1928 Niels created a Danish Geodetic Institute connected with the University of Copenhagen. This institute was created so students and scientists could study seismology in Denmark (3).  Later that same year, at 40 years old, Inge was the second person to pass her exam from the Danish Geodetic Institute (3).  She then fully switched her field of study from Mathematics to Seismology (2).  After passing her exam Niels made her the Head of the Seismology Department at the Danish Geodetic Institute (1,2,3).  She was the only academic employee of the Seismology Department until her retirement in 1953, at age 65 (3).  She once referred to herself as the “only seismologist of Denmark”.

A position as a department head was very uncommon for a woman at this time. However, since this position was mostly paperwork and since earthquakes were uncommon in Denmark, it was unlikely that there were men interested in the position.  As part of her new position, Inge reviewed and filed all of the station earthquake readings.  This gave her significant experience interpreting, comparing, and visually calibrating seismological data readings from different stations (1,2).  Since computers did not yet exist this was all done on paper.

Seismology Data

When an Earthquake hits the movement that we feel as shaking also travels into the Earth as something called P-Waves.  These P-waves will travel in a straight line unless they hit something solid.  When a P-wave hits a solid object, it will bounce off of it and travel in a new direction.  The seismology data that Inge and other scientists read include the directions of these P-waves.

The World at the Center of the Earth

Until 1936 the scientific community believed that the Earth had two layers: the outer solid mantle and the inner molten (hot liquid) core.  It was called the 2-layer theory (1,2).  But let’s back up a few years.  In 1929, when an earthquake struck New Zealand, the data readings that Inge received did not make sense based on this 2-layer theory (5).  In the data from the New Zealand earthquake Inge noticed that some of the P-waves that should have moved through the liquid core actually appeared to have moved into the core, hit something solid, and bounced back to the surface (5).  Under the 2-layer theory this would be impossible as the Earth’s core wouldn’t be solid.  Over the next few years Inge started to notice similar P-wave data from other earthquakes.  This led to her publishing in a well-accepted scientific paper in 1936, at age 48.  She theorized a 3-layer hypothesis with a solid inner core, a liquid outer core and then the mantle (1,3,5).  Her theory was strongly supported by other scientists.  Later, in 1970 her theory was further supported with the use of more sensitive seismography equipment that didn’t yet exist in 1936 (5).  In Inge Lehmann’s honor, the area that separates the solid inner core and the molten outer core has become known as the Lehmann Discontinuity (5).

Retirement and Beyond

In 1953 Inge retired from her 25-year role as the Department Head of the Seismology Department of the Danish Geodetic Institute.  She didn’t stop working as a seismologist though.  Instead, Inge moved to the United States of America. Here she left her administrative duties behind and spent the next 20 years fully immersed in seismology research.  In 1971, at the age of 83, Inge received the William Bowie medal. This is considered the highest honor by the American Geophysical Union (1,5).  In 1987, at age 99, Inge published her last research paper.  The next year she returned to the Danish Geodetic Institute to celebrate her 100^th birthday (1).  Overall, Inge lived to be 104 years old!  Just under 3 months from her 105^th birthday (1,2).  Inge never got married or had any children (1).

Timeline

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See the list of all the scientists in this Women in STEM series here

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Citations

1. “Inge Lehmann” (On-line) Famous Scientists: The Art of Genius. Accessed in 2023 at https://www.famousscientists.org/inge-lehmann/.
2. Zeldovich, L. “The Woman Who Found the Earth’s Inner Core” (On-line) JSTOR Daily. Accessed in 2023 at https://daily.jstor.org/the-woman-who-found-the-earths-inner-core/.
3. Jacobsen, L.L. (2022). Intellectually gifted but inherently fragile – society’s view of female scientists as experienced by seismologist Inge Lehmann up to 1930. Copernicus 13: 13-83. https://hgss.copernicus.org/articles/13/83/2022/
4. “Newnham College University of Cambridge” (On-line) Newnham College. Accessed in 2023 at https://newn.cam.ac.uk/about/history/.
5. “Inge Lehmann: Discoverer of the Earth’s Inner Core” (On-line) American museum of Natural History. Accessed in 2023 at https://www.amnh.org/learn-teach/curriculum-collections/earth-inside-and-out/inge-lehmann-discoverer-of-the-earth-s-inner-core