Six-minute summary: Niels Bohr

Hello readers! It is time for another six-minute summary. This week, in response to a reader request, we will be learning about the life of Niels Bohr, a Danish physicist who made huge discoveries regarding quantum mechanics and the structure of atoms. Although his name will be very familiar to physicists (and Danes), it might only ring a distant bell for the rest of us. When you picture an atom in your head, it’s probably the atom conceptualised by Niels Bohr – but for some reason, he hasn’t become embedded in the public imagination to the same extent as Newton or Einstein.

Denmark, 1885: A Niels is Bohrn

By all accounts, Niels Bohr had a comfortable upbringing: his father was a professor of physiology at the University of Copenhagen, and they lived in the spacious house that came with his job (those were the days). According to people who knew Niels at school, he was good at maths and physics – maybe not so much when he was younger, but certainly once he was older. He never made any close friends, but this was probably because he had such a strong, close friendship with his younger brother, Harald – a friendship which would last a lifetime.

Copenhagen University

Niels Bohr studied physics at Copenhagen University. At the time, this was the only university in Denmark, and it had only one physics professor – but despite these limitations, Niels Bohr did amazingly well. His earliest notable work was in response to a prize offered by the Danish Royal Academy of Sciences, investigating surface tension using a method that had been proposed (but not sufficiently tested) by Lord Rayleigh. Copenhagen’s minimalist physics department had no laboratory space, so Bohr used his father’s lab to conduct a series of experiments. He made improvements to Rayleigh’s method, won the Royal Academy prize, and eventually had his report published by the Royal Society in London. However, the submission was actually handwritten by his brother, Harald. Niels had terrible handwriting, and struggled to string a sentence together (at least on paper), so he relied on those close to him to compile his rambling thoughts.

Niels Bohr got his master’s degree in 1909, with his thesis having been handwritten by his mother. In 1911, he got his PhD, having expanded his master’s work. His final PhD exam (in which students defend their methods and interpretations from the criticisms of expert examiners) set a record for the shortest ever conducted in Copenhagen, lasting only an hour and a half. Parts of his thesis were handwritten by his wife-to-be, Margrethe – the sister of one of his university friends, who would be his partner and intellectual collaborator for the rest of his life.

Early career

Having acquired his PhD, Niels Bohr was given a fellowship by the Carlsberg Foundation (yes, the brewery) to travel to the Cavendish Laboratory in Cambridge and work for J. J. Thomson, who had recently discovered electrons. Thomson believed that atoms had a structure like “plum pudding” – which, if I may digress, is an analogy that has aged very badly. Does anyone born after 1970 know what “plum pudding” looks like? When was the last time you saw “plum pudding” on a menu? Just try typing “plum pudding” into Google. You’ll find that “physics”, “model” and “chemistry” all come up before “recipe”. It’s hardly an analogy if nobody knows what it is. Anyway…

Niels Bohr did not make a good first impression. Having just arrived, he went into Thomson’s office, opened Thomson’s textbook on a specific page, and told Thomson that it was wrong. Thomson, who had won a Nobel prize in physics five years earlier, did not respond well to such blunt criticism. Perhaps Bohr hadn’t intended to be rude, but his English at the time was very poor. The pair never got along, and Niels Bohr was keen to leave.

Luckily for him, he wasn’t the only one who thought that Thomson was wrong. Up at Manchester University, Ernest Rutherford had just found evidence for a positively charged nucleus at the centre of the atom – a discovery that left Thomson’s plum pudding looking like a soggy, unappetising mess. Rutherford invited Bohr to work for him instead, and Bohr took this opportunity without hesitation.

Then, in 1912, Bohr went home to Denmark. He married Margrethe, with his brother Harald as his best man, then came back to the UK for his honeymoon. First, he and Margrethe went to Cambridge, where he stopped to finish a paper on the absorption of alpha particles (very romantic). Then they went to Manchester, where he handed a manuscript to Rutherford in order to get it published (even more romantic). Finally, they went to Scotland (which may well have been romantic, if Margrethe had any patience left).

The trilogy

In 1913, Niels Bohr published three papers that would become known as “the trilogy”. In these papers, he postulated that electrons within atoms can only orbit the nucleus at specific distances, equating to discrete levels of energy. If an electron jumps from a higher-level orbit into a lower one, it emits electromagnetic radiation. This was the first quantum theory for the interior of the atom – and it is still taught in schools today, because it explains most physical and chemical phenomena so simply and effectively. Older physicists didn’t like the Bohr model (especially Thomson, still clinging to his soggy plum pudding), but the younger generation loved it (especially Einstein, Hilbert and Fermi). Bohr’s model became the basis for quantum theory, which became the basis for quantum mechanics. It succeeded where other models had failed, and it predicted results that were verified by experiments.

When the Bohrs returned to Denmark, Niels was offered a permanent position at the University of Copenhagen. However, it wasn’t quite what he wanted. Rather than teaching physics, he had to lecture medical students – the disastrous results of which we can only imagine. Rutherford invited Bohr back to Manchester, and like a low-level orbiting electron, he leapt at the chance. It wasn’t long before he was famous in the scientific world, at which point, the University of Copenhagen offered him a professorship in theoretical physics.

The Bohr Institute

Now that he was a professor, Niels Bohr persuaded the University of Copenhagen to set up a new institute for theoretical physics. This institute soon became a thriving hub, with experimentalists and theoretical physicists working together to develop new models.

In 1922, Niels Bohr was awarded the Nobel prize for physics, in recognition of his work on the structure of the atom. In 1923, scientists at the institute created a new element, which behaved as predicted by Bohr’s theory. They called this element hafnium, which is the Latin name for Copenhagen. Then, in 1926, Werner Heisenberg came to work of the institute. While he was there, he developed his uncertainty principle, which became one of the most fundamental concepts in quantum mechanics.

Complementarity and quantum mechanics

Heisenberg’s uncertainty principle is built on Niels Bohr’s principle of complementarity. Bohr regarded this concept as one of his greatest contributions, as it led to the development of quantum mechanics – the fundamental physical theory that describes the properties of matter and light.

One of the key ideas of complementarity is that the behaviour of an object cannot be interpreted in isolation from the measuring system that recorded it. A second key idea is that certain properties of an object cannot be measured at the same time. For example, if you set up an experiment to measure the position of an electron, you can’t use the same experiment to measure its momentum – and vice versa. If we’re dealing with light, you might set up an experiment to measure its wave-like properties, or you might set up an experiment to measure its particle-like properties – but you can’t set up an experiment that measures both.

At the time, physicists were trying to work out whether light and matter were particles or waves. There were multiple experiments in supporting both interpretations, and it was Niels Bohr who proposed that these seemingly contradictory results were actually complementary: that light and matter could display the properties of particles and waves, with the interpretation being dependent on the experimental setup. Apparently, he came up with this revolutionary concept while skiing. Must have been a very boring slope…

Bohr in the war

By the 1930s, Germany had descended into authoritarianism. Scientists were pressured, persecuted, or promoted based on their background, or by the types of research they were conducting, and in response to this increasing hostility, Denmark set up the Committee for the Support of Refugee Intellectual Workers. Niels Bohr was on the executive board, and he arranged for persecuted German physicists to be transferred to his institute in Copenhagen, before sending them to safety in the US.

In 1940, the Nazis invaded Denmark. At first, the Danish government were given political independence in domestic matters, but this lasted only until 1943, when the fragile agreement broke down. The Nazis deemed Bohr to be Jewish, as his mother had been Jewish, so he had to flee. The Danish Resistance helped him escape by boat to Sweden. From there, he was brought to the UK by military aeroplane, where he was set to work researching nuclear weapons, and sent on multiple trips to New Mexico as part of the British contribution to the Manhattan Project.

Until this point, Bohr had been convinced that nobody could make an atomic bomb soon enough to end the war. However, the atomic arms race had progressed far faster than many scientists had predicted. Bohr was only allowed to return to Denmark in 1945, after the US dropped two atomic bombs on Japan. He was welcomed home as a hero, with many newspapers taking pride in the fact that a Danish scientist had helped create the bomb that ended the war.

With the war over, Niels Bohr returned to the institute in Copenhagen. He helped to establish CERN, to further European particle physics research, and he petitioned the UN regarding the sharing of atomic research, which guided the creation of the International Atomic Energy Agency.

The Einstein disagreement

Despite Einstein being the most famous physicist to have ever lived, his long-running dispute with Niels Bohr has all but faded from public memory. The pair were of a similar age (Bohr being just six years younger), and they always ended up in arguments whenever they met. Einstein had relocated to Princeton University in the 1930s, having fled Nazi Germany, and Niels Bohr would visit often. However, their arguments never reached any sort of conclusion.

Einstein refuted the so-called “Copenhagen interpretation” developed by Bohr and his co-workers, and thought that quantum mechanics was an incomplete theory. He disliked the notion that particles only have properties once they are measured, and he really disliked the notion that the universe was probabilistic. This led to his memorably snappy assertion that “God does not play dice”. Einstein insisted that particles must have definite positions at all times, even while they aren’t observed. Bohr, by contrast, proposed that when particles are not observed, their position is not definite, but based on probability – and that their position only becomes definite in the moment that they are observed.

Ultimately, it was Bohr’s interpretation that came to be accepted. Einstein’s views on the matter have, by and large, been abandoned – although his “God playing dice” quote has lodged itself in our public memory. Elements of the Copenhagen interpretation are still debated, and quantum mechanics is far from being “solved”, but the ideas of Niels Bohr, not Einstein, formed the foundations for most modern research into sub-atomic particles.

Anything but Bohring

Niels Bohr died in 1962, aged 77, and in 1965, the Institute for Theoretical Physics at the University of Copenhagen was officially named the “Niels Bohr Institute” in his honour. His son, Aage Bohr, also went on to win a Nobel prize in physics in 1975 (which is not as unusual as it sounds: turns out that there are four father-son pairs of Nobel prize winners in physics alone). His grandson is also a professor of physics, studying fluid mechanics. Niels Bohr is arguably the grandfather of quantum mechanics, and was, without a doubt, one of the most influential scientists of the 20th century.

In summary…

Thank you to the reader who recommended Niels Bohr! I learnt a lot while researching this post, and hopefully I unearthed some new facts for all the physicist readers out there (I know there are a lot of you), who were probably well-versed in Bohr-lore already. Niels Bohr was hugely influential, but he seems to have lacked the charisma and communication skills that launched his contemporaries, such as Einstein, into the spotlight. Although the Bohr model of the atom has since been superseded by others, it remains the best known, and the only one that most of us will ever need. Happy reading, and have a lovely week!