Inge Lehmann and the Center of the Earth

By Jaime Seltzer

Inge Lehmann was born in 1888 in Copenhagen. Her parents were both of an intellectual bent; her father was Professor of Psychology at the University of Copenhagen, where he taught classes on the battle between superstition and reason. The principal of Inge’s secondary school was Hanna Alder, Niels Bohr’s aunt, who had a very evenhanded approach to education. Everyone who attended her school learned the same skills, whether that was needlepoint, football, or physics. As a result, Inge never learned that less was expected of girls in her society until she had graduated and begun to work in the wider world: things at home and at school simply never prepared her for the attitudes of her time.

Inge attended the University of Copenhagen beginning in 1907, and obtained her degree three years later. Then, she applied for and was accepted to Newnman College, where she was in an accelerated degree program for only a year. Acquaintances agreed that Inge had pushed herself too hard and had developed ‘nervous exhaustion’; but considering she was out of academia for the next eight years, it may be that she had developed a chronic illness that was not well-understood at the time. Regardless, she rallied or recovered, and returned to school in 1918, finishing her degree in 1920.

She had laid out her reasoning so clearly and her data so cleanly that it seemed impossible to disagree with her findings.

In 1925, she first worked with Professor N.E. Nörland as his assistant. This was Inge’s introduction to seismology, the study of earthquakes. In 1928, she earned her master’s degree by submitting a paper on seismology, and that year she was made chief of the Seismological Department, a position she would hold for nearly twenty years. She was responsible for monitoring and gathering data from three seismological stations, and helping to interpret that data.

This was where Lehmann shone. She kept all of her seismological records on cards that she made herself, and stored them in old oatmeal boxes. She would sit in her garden and sort them, looking for connections with scientific rigor, determination, and a touch of intuition. This quality caused a friend to say that Inge was the “master of a black art for which no amount of computerizing is likely to be a complete substitution.”

Her incredible ability to sort, connect and understand vast heaps of information was rewarded in 1936, when she began to realize that the data did not support the idea of an earth with a liquid core and a solid crust and mantle, as had been previously believed. Instead, the P waves she was measuring appeared to shift direction in an unexpected manner as they traveled through the earth. This caused her to develop the theory that the earth had a solid, inner core, surrounded by a liquid outer core. Her groundbreaking paper was entitled simply, ‘P’ – and, unlike many other groundbreaking papers of the time, was heralded as a success almost right away. She had laid out her reasoning so clearly and her data so cleanly that it seemed impossible to disagree with her findings.

Inge Lehmann

Inge Lehmann and her picture in the Women in Science card game.

World War II had the opposite effect on Lehmann and her colleagues that it did on the science of many others. Rather than be well-funded by a government suddenly intrigued with science and what it could bring to the war effort, Lehmann and other seismologists were shoved out of the loop. What did waves traveling through the earth matter, when everyone was racing towards the development of the atomic bomb? Of course, it doesn’t take much imagination to see that seismology does have applications in giant explosions, and everyone realized that soon enough. However, during the War, Lehmann was not only underfunded but cut off from the rest of the planet as Danish scientists slowly lost contact with the rest of the scientific world.

Partly to encourage the connection she and other Danish scientists were starving for, she and some of her colleagues formed the Danish Geophysical Society. The DGS was expanded to become a society that included all of Europe, but when the International Association of Seismology and Physics of the Earth’s Interior (IASPEI) became involved, Inge bowed out rather abruptly. She made no secret of how distasteful she found bureaucracy.

Inge went on to prove that her solid core existed at approximately a depth of 220-km. This location is now known as the 200-km Discontinuity; when she turned 100, the inner-core boundary was named the Lehmann Discontinuity to honor her. Lehmann worked in seismology well into her 70s, and lived to be 105.

References
Jui, C. (2007, November 14). Inge Lehmann “The Only Danish Seismologist”. In Department of Physics and Astronomy at the University of Utah. Retrieved October 6, 2015, from http://www.physics.utah.edu/~jui/3375/Class%20Materials%20Files/y2007m11d14/Inge%20Lehmann.pdf
Mathez, E. A. (Ed.). (2000). Profile – Inge Lehmann: Discoverer of the Earth’s Inner Core. In Earth: Inside and Out. New York, NY: the New Press. Retrieved October 6, 2015, from http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
Williams, C. A., & Hudson, J. A. (1994). Inge Lehmann (1888–1993). Quarterly Journal of the Royal Astronomical Society, 35, 231-234. Retrieved October 6, 2015, from https://www.aip.org/sites/default/files/history/files/LessonPlan_Lehmann.pdf

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