The overlooked genius of geophysics

Many names deserve recognition on the 150th anniversary of the admission of women students to the University of Copenhagen (UCPH). But few are as remarkable as Inge Lehmann (1888–1993). She was not the first woman to study at UCPH, but she has had worldwide academic impact. Still, only a few people have heard about her in school or learned about her elsewhere.

Lehmann began studying mathematics in 1908. She chose the subject because it came easily to her. Even in her early school years, she showed a talent for maths, and continuing with it was an easy choice.

Her childhood was shaped by a home in the Copenhagen district of Østerbro, where her father, Alfred Lehmann, worked as an experimental psychologist and physiologist and attempted to make the human mind the subject of measurement and experimentation.

He took a natural science approach to the world: What could not be weighed or measured had no value. Inge Lehmann adopted this worldview. For her, it was about creating clarity, about finding answers. Either observations confirmed her ideas, or they did not. There was no middle ground.

Breakdown

In 1910, she applied for admission to the elite University of Cambridge in the UK. She was accepted at Newnham College, a women-only college, and for perhaps the first time in her life, she thrived.

She played tennis, went hiking, rowed on the River Cam, made many friends and had fun. But in the autumn of 1911, she suddenly lost her footing. A mental breakdown abruptly ended her stay, and in 1911, she returned home to Denmark shaken and depressed.

The same newspapers that had run bold headlines when she became state geodesist ignored her discovery in silence

Hanne Strager, biologist and author

Inge Lehmann was determined to return and complete her studies at Cambridge, but she never succeeded. The years until she resumed her studies at the University of Copenhagen (UCPH) in 1918 was an aimless drift between various positions as an actuarian at insurance companies and travels to Norway and England to find breathing space and respite.

In 1920, she earned her degree in mathematics from UCPH, and eight years later she received her mag.scient. degree in geodesy and was appointed state geodesist at the Danish Geodetic Institute.

It was a remarkable appointment that made headlines in the newspapers of the time. She was the first female state geodesist, and the only woman at the institute who wasn’t a secretary or a cleaner.

That geophysics became her life’s work was a coincidence. Her boss, Professor Niels Erik Nørlund, had convinced the government to establish more seismic stations in Denmark and Greenland.

When the advanced seismographs arrived at the Geodetic Institute, no one could get them to work. No one, that is, except Inge Lehmann. And from that day on, her lifelong work with earthquakes and understanding the Earth’s interior began.

The discovery

Lehmann faced resistance. Not everyone trusted women to do technical work or fieldwork, and the Geodetic Institute was mostly staffed by military officers unaccustomed to seeing women in academic or leadership roles.

She occasionally mentioned in letters that it was difficult, and sometimes she was frustrated that her gender stood in the way of promotions or prevented her from doing her job as well as she could. She dealt with this by rolling up her sleeves and digging into the geophysics research to prove her worth.

International scholarships enabled her to immerse herself in research, and in 1936 she published the work that would secure her a place in the history of science.

The article was titled P’ — a modest title for groundbreaking work. Using data from seismic stations around the world, especially from the Soviet Union, Lehmann noticed signals in the recordings of an earthquake in Murchison, New Zealand, that shouldn’t have been there.

She calculated that these could only be explained if there was a layer at the centre of the Earth’s core that was different from the solid core. P-waves would refract differently through solid versus liquid material, and this was the only way to explain the anomalous observations.

Her discovery was quickly recognised in the international scientific community, but in Denmark it was largely ignored. The same newspapers that had run bold headlines when a woman became state geodesist passed over her discovery in silence.

The oversight

The lack of attention was likely because no one else in Denmark worked with seismology, and therefore no one truly understood the significance of her discovery.

It wasn’t until after her retirement from the Geodetic Institute that her contributions to science began to receive attention.

She was the first female state geodesist, and the only woman at the institute who wasn’t a secretary or cleaner

Hanne Strager, biologist and author

Inge Lehmann started collaborating with Maurice Ewing at the Lamont Geological Observatory at Columbia University in the USA in the early 1950s. He saw her as a towering presence in geophysical research and was instrumental in much of the recognition she later received — awards, medals, and honours starting in the 1960s and continuing for years after.

She was awarded an honorary doctorate by Columbia University in 1965 and later became a member of the Royal Society in the UK. Once the door was opened, a cascade of recognition followed.

But not in Denmark. In the early 1950s, she applied for a newly established professorship in geophysics at the University of Copenhagen. On the selection committee sat, among others, her boss Niels Erik Nørlund and the famous Danish nuclear physicist Professor Niels Bohr — who happened to be Nørlund’s brother-in-law.

Their preferred candidate was another researcher, and when he withdrew, the appointment was postponed for 12 years. Inge Lehmann was never offered the position.

A not-so-subtle hint

When she received the Royal Danish Academy of Sciences and Letters’ gold medal in 1965, it was at the suggestion of her British colleague Harold Jeffreys, who wrote to Niels Bohr explaining why the medal should go to Lehmann.

In 1968, she finally received an honorary doctorate from the University of Copenhagen. But she was never admitted to the Royal Danish Academy. It almost certainly played a role that she was a woman. But the lack of recognition in Denmark perhaps had more to do with the fact that no one truly understood her work. For many decades, she was the only seismologist in Denmark.

Inge Lehmann’s discovery of the solid inner core earned her a place among the most significant and celebrated figures in the history of science. It was a pioneering insight into the structure of our planet — derived from the painstaking study of a few tiny, jittery lines on a seismogram.

She was, as one of her American colleagues put it, »a master of black art for which no amount of computerization is likely to be a complete substitute.«