Mark Walker
November 24th, 2015

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Bohr, photographed in 1948 at Princeton University aged 63, saw out the latter years of his career in the US, promoting the peaceful application of atomic energy (photo: Princeton University/American Institute of Physics/Science Photo Library)

For such a tiny country, Denmark certainly punches above its weight, and the sciences certainly are no exception. With Tycho Brahe’s keen eye trained on the infinite heavens above and, centuries later, Niels Henrik David Bohr’s brilliant mind engaged with the smallest known units of matter, Denmark has, at one time or another, had the entire universe covered.

Born in Copenhagen in 1885, Bohr was, like his father before him, a physicist. Dedicated to unravelling the secrets of nature, Bohr completed his studies at the University of Copenhagen and left for England. It was here that he developed ‘the Bohr model’ that sought to explain the workings of the atom and the behaviour of subatomic particles within.

Bohr followed many celebrated philosophers and physicists who had applied their learning to the question of that which everything is made of: ‘matter’. Amazingly, more than 2,000 years ago, the Greek philosopher Democritus stated that he believed there to be an indivisible unit of matter that he called ‘atomos’ (meaning ‘no-cut’). Of course, back then there were no means with which people could prove or disprove such theories, and the scientific community had believed him to be wrong for many hundreds of years. They preferred Aristotle’s theory of an infinitely divisible matter.

Footsteps of Democritus
Fast forward to the turn of the 20th century, and physicists had not only found Democritus to be correct, but were going beyond the surface of the atom to find out what happens inside it. This was the domain of quantum physics – a field of study of which Bohr would go on to be credited as the founding father, and a world which is governed by laws utterly unlike the world that we can see. You don’t have to be a physicist to get excited about quantum physics; its laws are so crazy that it frequently enters the realm of science fiction, with particles that behave unpredictably: appearing and disappearing, existing in several places at the same time, suggesting new dimensions in space and time. Bohr famously stated: “Anyone who is not shocked by quantum theory has not understood it.”

Following on from his two British mentors – JJ Thompson discovered the electron and Ernest Rutherford the proton – the stage was set for Bohr to discover how these particles worked together.

In an experiment with a hydrogen atom – a unique atom because it contains only one proton and one electron (all other types of atom contain at least two) – Bohr discovered that the negatively-charged electron travels in orbit around the positively-charged proton at the centre: a nucleus. It can also travel at different orbital heights from the nucleus depending on the energy level of the electron. To travel higher, closer to the surface of the atom, the electron requires more energy. By means of stimulating an atom with energy (i.e heat) the electron would rise to a higher orbit, but on its descent back to a lower level, it would expel the exact amount of energy it used to rise. That energy amount is expelled in the form of coloured light, which can then be measured in units known as photons. From the light, Bohr could discern the subatomic structure in correlation with the chemical identity of a given atom. Bohr also found that the light is sometimes emitted in apparently random directions. It was this research that laid the foundations for quantum physics and contributed to Bohr winning the Nobel Prize for Physics in 1922.

Heckling Heisenberg
Bohr’s life, just like his work, was littered with contradictions: positive and negative forces working together. The Bohr Model earned him the respect of Albert Einstein who, having established his widely accepted theory of relativity, represented a different approach to physics. The seemingly unpredictable nature of the quantum world was difficult to reconcile with his own classical physics, and the pair would have many friendly debates throughout their lives in an attempt to unify their theories. Einstein famously said: “I, at any rate, am convinced that he [god] does not throw dice.”

By the time Nazi Germany occupied Denmark, Bohr was running an institute for theoretical physics at his old university. Werner Heisenberg, Bohr’s one-time assistant, came visiting from Germany in 1941. He warned his dear friend of the dangers that might await Bohr, who was Jewish. Heisenberg, a brilliant physicist in his own right, was developing an atomic weapon for Hitler. Bohr was shocked. That meeting between Bohr and Heisenberg is the subject of much debate (dramatised in ‘Copenhagen’, a play by Michael Frayn, which was in 2002 adapted into a BBC film starring Daniel Craig and  Stephen Rea).

Promoting peace
It was that meeting, and the threat of being taken by the Nazis, which saw Bohr flee to Los Alamos, New Mexico in 1943 to assist with the American effort to build a weapon of their own before Hitler got there first. Although Bohr’s involvement was minimal, one can’t fail to see the contradiction in a man, who studies the world around him, contributing to a destructive device that literally tears it apart. Bohr spent many years after the war promoting the peaceful application of atomic energy.

While writing this piece, I paid a visit to the Bohr family plot in Assistens Cemetery in Nørrebro. I spotted a set of dice on Bohr’s grave below his name. I instantly thought of Einstein’s quote and smiled. The dice have spots tallying an opposite number on the opposing faces, and have been arranged with the ends mirroring each other. It may be intended to refer to Bohr’s ‘complementary theory’ in which he proposes the principle that objects may function despite containing contradictory properties. Bohr even designed himself a coat of arms featuring a yin and yang symbol. The motto reads: “Opposites are complementary.” Whatever the dice refer to, they seem a fitting gesture to a brilliant mind who dedicated his life to understanding the opposing forces in nature.