Niels Bohr (1885-1962)

The Danish physicist Niels Bohr is most famous for his contributions to our understanding of atomic structure. He was born in Copenhagen, where his father, Christian Bohr, was an eminent physiologist and inspired his son’s early interest in science.

Bohr completed his master’s and doctor’s degrees at the University of Copenhagen, then travelled to Cambridge to work with J J Thompson at the Cavendish Laboratory in 1911. The following year he moved to Ernest Rutherford’s laboratory in Manchester, where he first conducted a theoretical study into the absorption of alpha radiation, before moving on to concentrate on the structure of atoms.

In 1913, Bohr published his highly influential theory of atomic structure, building on Rutherford’s work which described the atom as a positively charged nucleus with negatively charged electrons orbiting around it. Bohr incorporated elements of Max Planck’s quantum theory to propose that electrons are confined to clearly defined orbits, and need to emit or absorb specific amounts of energy to jump between them. This model, with some later amendments, remains valid to this day.

Bohr was appointed Professor of Theoretical Physics at Copenhagen University in 1916, and in 1920 became the director of the university’s newly created Institute of Theoretical Physics.

Scholars from around the world travelled to Copenhagen to discuss their theories with Bohr, who won a Nobel Prize in 1922 for his work on atomic structure. Bohr and his collaborators, including his assistant Werner Heisenberg, proposed the ‘Copenhagen interpretation’ of quantum physics, an attempt to explain experimental results and their mathematical formulations. This concept - that the very act of observing something changes what is being observed, and we can only measure probabilities, not absolutes - was difficult for many scientists to accept. Even Albert Einstein was deeply troubled by the implications, but it is now the standard interpretation of quantum physics.

Bohr’s Jewish ancestry meant he was forced to flee Denmark in 1943 after the Nazi occupation. He escaped to Sweden by fishing boat and was flown to England, where he began working on the Tube Alloys project, the British nuclear weapon programme. As part of the Tube Alloys team he travelled to the United States to work on the Manhattan Project. However, in later years he campaigned for the peaceful use of atomic technologies.