Today is the birthday (1885) of Niels Henrik David Bohr, a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter of scientific research. Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another. Although the Bohr model has been supplanted by other models, its underlying principles remain valid. He also conceived the principle of complementarity: that items could be separately analyzed in terms of contradictory properties, like behaving as a wave or a stream of particles. The notion of complementarity dominated Bohr’s thinking in both science and philosophy.
I don’t want to delve too deeply into Bohr’s physics because I know I will lose a big chunk of my audience before I get started. But I will make one point before I fly off in different directions. Long-time readers know that I have a bee in my bonnet about certain superlatives – the BEST painting/painter or composer or mathematician or whatever. There are lots and lots of smart and talented people throughout history. If this were not so, this would be a very limited blog. My main limitation is that their birthdays are not spread evenly through the year, coupled with my intrinsic favoritism. In the latter case I am allowed because it is MY blog. I make the rules. What gets me wound up is the popular idea that the yardstick of hyper-genius is Einstein. He had a phenomenal mind – no question. He was, however, far from being the ONLY genius of the 20th century, yet his is the name that automatically comes to mind. I’ve always countered this prejudice when it comes up by mentioning Niels Bohr.
The first half of the 20th century was almost wallpapered with brilliant mathematicians and physicists, not to mention anthropologists, writers, philosophers, painters, and all the rest of it. Niels Bohr is one of them, but he is far from being a household name. Yet he helped usher in the age of quantum mechanics (with other brilliant minds), the dominant model of atomic physics to this day.
Bohr was born in Copenhagen, the second of three children of Christian Bohr, a professor of physiology at the University of Copenhagen, and Ellen Adler Bohr, who came from a wealthy Danish Jewish family prominent in banking and parliamentary circles. He had an elder sister, Jenny, and a younger brother Harald. Jenny became a teacher, while Harald became a mathematician and Olympic footballer who played for the Danish national team at the 1908 Summer Olympics in London. Niels was a passionate footballer as well, and the two brothers played several matches for the Copenhagen-based Akademisk Boldklub (Academic Football Club), with Niels as goalkeeper. My son and I love goalies.
In 1910, Bohr met Margrethe Nørlund, the sister of the mathematician Niels Erik Nørlund. Bohr resigned his membership in the Church of Denmark on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in Slagelse on 1 August. Their honeymoon was delayed, however, because Bohr had an insight into the nature of orbiting electrons within the atom that he felt could not wait. For reasons that are not clear to me, he was unable to sit and write the paper himself, so he dictated it to Margrethe. Maybe this was his idea of marital bliss?
Planetary models of atoms were fairly recent but not new. Bohr’s treatment was. The old planetary model could not explain why the negatively charged electron did not simply collapse into the positively charged nucleus. He advanced the theory of electrons travelling in nested orbits of different energies around the atom’s nucleus, with the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a quantum of discrete energy. This became a basis for what is now known as the old quantum theory. In 1922 Bohr received the Nobel Prize in physics for his work.
Bohr became convinced that light behaved like both waves and particles, and in 1927, experiments confirmed the de Broglie hypothesis that matter (like electrons) also behaved like waves. He conceived the philosophical principle of complementarity: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework. He felt that it was not fully understood by contemporary philosophers. Einstein never fully accepted quantum mechanics and complementarity. Einstein preferred the determinism of classical physics over the probabilistic new quantum physics to which he himself had contributed. Philosophical issues that arose from the novel aspects of quantum mechanics became widely celebrated subjects of discussion. Einstein and Bohr had good-natured arguments over such issues throughout their lives.
Bohr’s model of the atomic nucleus helped him explain the nature of nuclear fission which he published in a paper in 1939, “The Mechanism of Nuclear Fission,” along with John Wheeler. Thus the age of nuclear energy and the atom-bomb was born.
Bohr was aware of the possibility of using uranium-235 to construct an atomic bomb, referring to it in lectures in Britain and Denmark shortly before and after the war started, but he did not believe that it was technically feasible to extract a sufficient quantity of uranium-235 (fissionable material). In September 1941, Werner Heisenberg, who had become head of the German nuclear energy project, visited Bohr in Copenhagen. During this meeting the two men took a private moment outside, the content of which has caused much speculation, as both gave differing accounts. According to Heisenberg, he began to address nuclear energy, morality and the war, to which Bohr seems to have reacted by terminating the conversation abruptly while not giving Heisenberg hints about his own opinions.
In 1957, Heisenberg wrote to Robert Jungk, who was then working on the book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists. Heisenberg explained that he had visited Copenhagen to communicate to Bohr the views of several German scientists, that production of a nuclear weapon was possible with great efforts, and this raised enormous responsibilities on the world’s scientists on both sides. When Bohr saw Jungk’s depiction in the Danish translation of the book, he drafted (but never sent) a letter to Heisenberg, stating that he never understood the purpose of Heisenberg’s visit, was shocked by Heisenberg’s opinion that Germany would win the war, and that atomic weapons could be decisive.
In September 1943, word reached Bohr and his brother Harald that the Nazis considered their family to be Jewish, since their mother, Ellen Adler Bohr, had been a Jew, and that they were therefore in danger of being arrested. The Danish resistance helped Bohr and his wife escape by sea to Sweden on 29 September. The next day, Bohr persuaded King Gustaf V of Sweden to make public Sweden’s willingness to provide asylum to Jewish refugees. On 2 October 1943, Swedish radio broadcast that Sweden was ready to offer asylum, and the mass rescue of the Danish Jews by their countrymen followed swiftly thereafter. Some historians claim that Bohr’s actions led directly to the mass rescue, while others say that, though Bohr did all that he could for his countrymen, his actions were not a decisive influence on the wider events. Eventually, over 7,000 Danish Jews escaped to Sweden.
When the news of Bohr’s escape reached Britain, Lord Cherwell sent a telegram to Bohr asking him to come to Britain. Bohr arrived in Scotland on 6 October in a de Havilland Mosquito operated by the British Overseas Airways Corporation (BOAC). The Mosquitos were unarmed high-speed bomber aircraft that had been converted to carry small, valuable cargoes or important passengers. By flying at high speed and high altitude, they could cross German-occupied Norway, and yet avoid German fighters. Bohr, equipped with parachute, flying suit and oxygen mask, spent the three-hour flight lying on a mattress in the aircraft’s bomb bay. During the flight, Bohr did not wear his flying helmet as it was too small, and consequently did not hear the pilot’s intercom instruction to turn on his oxygen supply when the aircraft climbed to high altitude to overfly Norway. He passed out from oxygen starvation and only revived when the aircraft descended to lower altitude over the North Sea.
On 8 December 1943, Bohr arrived in Washington, D.C., where he met with the director of the Manhattan Project, Brigadier General Leslie R. Groves, Jr,and went to Los Alamos in New Mexico, where the nuclear weapons were being designed. Bohr did not remain at Los Alamos, but paid a series of extended visits over the course of the next two years. Robert Oppenheimer credited Bohr with acting “as a scientific father figure to the younger men”, most notably Richard Feynman. Bohr is quoted as saying, “They didn’t need my help in making the atom bomb.”
Bohr recognized early that nuclear weapons would change international relations. In April 1944, he received a letter from Peter Kapitza, written some months before when Bohr was in Sweden, inviting him to come to the Soviet Union. The letter convinced Bohr that the Soviets were aware of the Anglo-American project, and would strive to catch up. He sent Kapitza a non-committal response, which he showed to the authorities in Britain before posting. Bohr met Churchill on 16 May 1944. Churchill disagreed with the idea of openness towards the Russians to the point that he wrote in a letter: “It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes.”
Oppenheimer suggested that Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Soviets in the hope of speeding up its results. Bohr’s friend, Supreme Court Justice Felix Frankfurter, informed President Roosevelt about Bohr’s opinions, and a meeting between them took place on 26 August 1944. Roosevelt suggested that Bohr return to the United Kingdom to try to win British approval. When Churchill and Roosevelt met at Hyde Park on 19 September 1944, they rejected the idea of informing the world about the project, and the aide-mémoire of their conversation contained a rider that “enquiries should be made regarding the activities of Professor Bohr and steps taken to ensure that he is responsible for no leakage of information, particularly to the Russians”.
Bohr died of heart failure at his home in Carlsberg on 18 November 1962. He was cremated, and his ashes were buried in the family plot in the Assistens Cemetery in the Nørrebro section of Copenhagen, along with those of his parents, his brother Harald, and his son Christian. Years later, his wife’s ashes were also interred there.
I’ve mentioned traditional Danish cuisine several times before. It shares features with the other Sandinavian countries, and, like them as well as Britain, tends to be unfairly disdained by foreigners. Danish food does not involve a lot of herbs and spices, but it is noted for combinations of flavors and colorful presentation. Historically lunch was usually an open faced sandwich known as smørrebrød. Smørrebrød (originally smør og brød, meaning “butter and bread”) usually consists of a piece of buttered rye bread (rugbrød), a dense, dark brown bread. Pålæg (“put on”), the topping, which can be cold cuts, pieces of meat or fish, cheese or spreads. More elaborate, finely decorated, varieties have contributed to the international reputation of the smørrebrød. A slice or two of pålæg is placed on the buttered bread and decorated with the right accompaniments to create a tasty and visually appealing lunch or snack. Standards include:
Dyrlægens natmad (Veterinarian’s late night snack). On a piece of dark rye bread, a layer of liver pâté (leverpostej), topped with a slice of saltkød (salted beef) and a slice of sky (meat jelly). This is all decorated with raw onion rings and garden cress.
Røget ål med røræg Smoked eel on dark rye bread, topped with scrambled eggs, herbs and a slice of lemon.
Leverpostej Warm rough-chopped liverpaste served on dark rye bread, topped with bacon, and sauteed mushrooms. Additions can include lettuce and sliced pickled cucumber.
Roast beef, thinly sliced and served on dark rye bread, topped with a portion of remoulade, and decorated with a sprinkling of shredded horseradish and crispy fried onions.
Ribbensteg (roast pork), thinly sliced and served on dark rye bread, topped with red cabbage, and decorated with a slice of orange.
Rullepølse, (rolled stuffed pork) with a slice of meat jelly, onions, tomatoes and parsley.
Tartar, with salt and pepper, served on dark rye bread, topped with raw onion rings, grated horseradish and a raw egg yolk.
Røget laks. Slices of cold-smoked salmon on white bread, topped with shrimp and decorated with a slice of lemon and fresh dill.
Stjerneskud (lit. shooting star). On a base of buttered toast, two pieces of fish: a piece of steamed white fish on one half, a piece of fried, breaded plaice or rødspætte on the other half. On top is piled a mound of shrimp, which is then decorated with a dollop of mayonnaise, sliced cucumber, caviar or blackened lumpfish roe, and a lemon slice.
Here’s a small gallery to get you thinking: