Between these two events, amazing developments had taken place: our understanding of the smallest building blocks of the world was completely transformed, and the world had to grapple with the enormous forces that this new knowledge had placed in human hands.

The story began quite peacefully, in 1912, when Niels Bohr, after earning his doctorate, went to Manchester to work with the physicist Ernest Rutherford (1871–1937). Here Bohr developed his new atomic theory, inspired by Rutherford’s discovery of the atomic nucleus.

Bohr’s theory was the first to explain why atoms emit light in particular colours. In Bohr’s model, electrons orbited around the nucleus and jumped between orbits by absorbing or emitting energy in the form of light.

Bohr’s theory was a radical departure from classical physics and marked the beginning of modern quantum physics. When it was published, in 1913, it was met with scepticism, because it relied on postulates of quantum leaps between fixed electron orbits. However, it proved very fruitful in explaining experimental results.

Later, Bohr formulated his principle of complementarity, stating that quantum phenomena may involve seemingly contradictory properties, as electrons behave simultaneously as particles and waves. No single model can capture the full picture, but together they provide a more complete representation of nature.

After his return to Copenhagen, Bohr was appointed to the post of professor, and in 1921 he founded the Institute for Theoretical Physics, which soon became an international powerhouse of the new science of physics. Leading physicists stopped by to discuss quantum mechanics and epistemology – Heisenberg, Pauli and Schrödinger among many others.

The open dialogue that Bohr cultivated became known as the Copenhagen Spirit. The founding of the institute and Bohr’s continued research were faithfully supported by the Carlsberg Foundation, and from 1932 Bohr lived with his family in Carlsberg’s honorary residence. Here too, scientific discussions merged seamlessly with philosophical and political reflections.

Before and during World War II, Bohr aided exiled scientists, many of Jewish background, before he himself fled to the United States. Here he was involved in the development of the atom bomb and was an experienced and respected interlocutor for the many younger physicists. He made theoretical contributions to the implosion mechanism and the neutron initiator that triggered the bomb’s chain reaction.

After the war, he was a prominent advocate for international control with nuclear weapons. Today, Bohr is remembered both as one of the greatest physicists of the 20th century and as a strong ethical voice during a highly dramatic time.

Even before his father’s death, Aage Bohr had established himself as a researcher at the institute where, since childhood, he had been around some of the most prominent physicists of the time – many of whom he thought of as his uncles. His path to the Nobel Prize, in 1975, began with a stay at Columbia University in 1949. Here he met the physicist James Rainwater (1917–1986), who later shared the Nobel Prize with Bohr and Mottelson.

At the time, Rainwater had just proposed that the nucleus of the atom is not necessarily spherical but may be deformed and move as a liquid, both vibrating and rotating. This challenged the common perception of the nucleus as a fixed system.

Back in Copenhagen, Bohr and Mottelson explored Rainwater’s ideas. Together, they created the so-called Bohr-Mottelson model – also known as the collective nucleus model – which combined the movements of individual particles with the collective dynamics of the nucleus.

It offered a brand-new understanding of the nucleus as a dynamic system with both particulate and fluid properties. It showed how nucleons both affect and are affected by the collective movements of the nucleus. Bohr and Mottelson presented their ideas in the monumental two-volume publication Nuclear Structure from 1969, which for decades was the most important reference book on nuclear physics.

The collective-core model soon became a key tool in nuclear physics. It made it possible to predict how atomic nuclei vibrate, rotate and change shape and thus to explain the characteristic patterns in the radiation emitted by the nucleus. It also enabled scientists to understand the properties of both light and heavy nuclei – from stable elements to the many new, short-lived nuclei that were generated in the large particle accelerators constructed after the war.

Today, Bohr’s reputation continues to attract scientists from all over the world, and the Niels Bohr Institute remains one of the leading centres for nuclear physics – a living testimony to the enduring legacy which began with a young physicist’s bold idea that light held the key to the structure of the atom. 

The chapter is written by Asser Pelle.