Women in the History of Quantum Physics: The Case of Heisenberg and Mensing, a Founding Member of the Interdisciplinary Group on Heligoland

This meeting will be the high point of a 20-year research programme that set out to re-examine the development of quantum theory. Many of the women who were involved in the work have not been appreciated in the history of the field.

The standard model has been incredibly successful, culminating in the 2012 discovery of its linchpin elementary particle, the Higgs boson. But these extensions lie on less-solid theoretical ground than quantum mechanics does — and leave several phenomena unexplained, such as the nature of the ‘dark matter’ that seems to greatly outweigh conventional, visible matter in the wider cosmos. Moreover, one important phenomenon, gravity, still resists being quantized.

The quantum revolution has brought many things, but still some unfinished business. In the years in which researchers were laying the foundations of quantum mechanics, they also began to rebuild other branches of physics — such as the study of electromagnetism, and states of matter — from quantum foundations. They also looked to extend their theories to encompass objects that move at close to light speed, something that the original quantum theory did not. The scope of quantum science was expanded greatly thanks to these efforts, as well as the standard model of particles and fields that came together in the 1970s.

These “hidden figures” include Lucy Mensing, who was a member of the same group as Heisenberg and worked out some of the first applications of his quantum-mechanical theory, says Daniela Monaldi, a historian at York University in Toronto, Canada. The publication of a biographical volume of essays on Women in the History of Quantum Physics is one of the most important events of the year.

Heisenberg decided to go for a drastic approach after a couple of months after he had recovered from a bout of hayfever on the German island of Heligoland. Rather than constructing an atomic model based on the idea that electrons move along well-defined orbits in a roughly classical fashion, Heisenberg decided to develop an innovative theory of motion, a ‘quantum mechanics’ in which electrons could no longer be thought of as particles that move along continuous trajectories. On July 9, he wrote to Pauli that all of his efforts were supposed to destroy the concept of circling the Earth. This was the decisive break with classical mechanics.

A century ago, physics had its Darwinian moment — a change in perspective that was as consequential for the physical sciences as the theory of evolution by natural selection was for biology.

It was difficult to see how eliminating unobservable quantities would lead to further development of the theory. The theory would require other quantities in addition to the energies and transitions to describe phenomena such as the motion of free particles. It wasn’t clear which quantities should be considered unobservable. The electron position was observable in 1927. In 1925, the idea of eliminating unobservable quantities sounded reasonable, but in reality it was useless and even misleading.

By supposing that electrons move in elliptical orbits around an atomic nucleus, subject to certain quantization conditions, the Bohr–Sommerfeld model provided a set of rules for selecting certain ‘allowable’ orbits of a classical system (in the case of the hydrogen atom, an electron orbiting a proton), delivering calculated values in agreement with the observed energy spectrum. The model had successfully explained the spectrum of the hydrogen atom — consisting of just one proton and one electron — and the splitting of spectral lines in the presence of an applied electric field (the Stark effect) or magnetic field (the ordinary Zeeman effect). It had run into a lot of problems when dealing with atoms with more than one electron.