Elizabeth Palermo, Associate Editor on. Education Moseley In atom: Bohr's researches on reaction times.
While still a student, Bohr won a contest put on by the Academy of Sciences in Copenhagen for his investigation into the measurements of liquid surface tension using oscillating fluid jets. Working in the laboratory of his father a renowned physiologist , Bohr conducted several experiments and even made his own glass test tubes.
Bohr went above and beyond the current theory of liquid surface tension by taking into account the viscosity of the water as well as incorporating finite amplitudes rather than infinitesimal ones.
He submitted his essay at the last minute, winning first place and a gold medal. He improved upon these ideas and sent them to the Royal Society in London, who published them in the journal Philosophical Transactions of the Royal Society in , according to Nobelprize.
His subsequent work became increasingly theoretical. It was while conducting research for his doctoral thesis on the electron theory of metals that Bohr first came across Max Planck's early quantum theory, which described energy as tiny particles, or quanta. In , Bohr was working for the Nobel laureate J. Thompson in England when he was introduced to Ernest Rutherford, whose discovery of the nucleus and development of an atomic model had earned him a Nobel Prize in chemistry in Under Rutherford's tutelage, Bohr began studying the properties of atoms.
Bohr held a lectureship in physics at Copenhagen University from to and went on to hold a similar position at Victoria University in Manchester from to He went back to Copenhagen University in to become a professor of theoretical physics. In , he was appointed the head of the Institute for Theoretical Physics. Combining Rutherford's description of the nucleus and Planck's theory about quanta, Bohr explained what happens inside an atom and developed a picture of atomic structure.
This work earned him a Nobel Prize of his own in When challenged on this by Alfred Fowler , Bohr replied that they were caused by ionised helium , helium atoms with only one electron.
The Bohr model was found to work for such ions. Bohr did not enjoy teaching medical students. He decided to return to Manchester, where Rutherford had offered him a job as a reader in place of Darwin, whose tenure had expired.
He took a leave of absence from the University of Copenhagen, which he started by taking a holiday in Tyrol with his brother Harald and aunt Hanna Adler. There, he visited the University of Göttingen and the Ludwig Maximilian University of Munich , where he met Sommerfeld and conducted seminars on the trilogy.
The First World War broke out while they were in Tyrol, greatly complicating the trip back to Denmark and Bohr's subsequent voyage with Margrethe to England, where he arrived in October They stayed until July , by which time he had been appointed to the Chair of Theoretical Physics at the University of Copenhagen, a position created especially for him.
His docentship was abolished at the same time, so he still had to teach physics to medical students. New professors were formally introduced to King Christian X , who expressed his delight at meeting such a famous football player.
He gained the support of the Danish government and the Carlsberg Foundation, and sizeable contributions were also made by industry and private donors, many of them Jewish. Legislation establishing the Institute was passed in November His family moved into an apartment on the first floor. Bohr became widely appreciated as their congenial host and eminent colleague.
The Bohr model worked well for hydrogen, but could not explain more complex elements. By , Bohr was moving away from the idea that electrons orbited the nucleus and developed heuristics to describe them. The rare-earth elements posed a particular classification problem for chemists, because they were so chemically similar. An important development came in with Wolfgang Pauli 's discovery of the Pauli exclusion principle , which put Bohr's models on a firm theoretical footing.
Bohr was then able to declare that the as-yet-undiscovered element 72 was not a rare-earth element, but an element with chemical properties similar to those of zirconium. He was immediately challenged by the French chemist Georges Urbain , who claimed to have discovered a rare-earth element 72, which he called "celtium".
Starting with a clear idea of the chemical properties of the unknown element greatly simplified the search process. They went through samples from Copenhagen's Museum of Mineralogy looking for a zirconium-like element and soon found it. The element, which they named hafnium Hafnia being the Latin name for Copenhagen turned out to be more common than gold.
In , Bohr was awarded the Nobel Prize in Physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them".
For his Nobel lecture, Bohr gave his audience a comprehensive survey of what was then known about the structure of the atom, including the correspondence principle , which he had formulated. This states that the behaviour of systems described by quantum theory reproduces classical physics in the limit of large quantum numbers.
The discovery of Compton scattering by Arthur Holly Compton in convinced most physicists that light was composed of photons , and that energy and momentum were conserved in collisions between electrons and photons. In , Bohr, Kramers and John C. It was more a programme than a full physical theory, as the ideas it developed were not worked out quantitatively.
BKS theory became the final attempt at understanding the interaction of matter and electromagnetic radiation on the basis of the old quantum theory, in which quantum phenomena were treated by imposing quantum restrictions on a classical wave description of the electromagnetic field.
Modelling atomic behaviour under incident electromagnetic radiation using "virtual oscillators" at the absorption and emission frequencies, rather than the different apparent frequencies of the Bohr orbits, led Max Born, Werner Heisenberg and Kramers to explore different mathematical models. They led to the development of matrix mechanics , the first form of modern quantum mechanics. The BKS theory also generated discussion of, and renewed attention to, difficulties in the foundations of the old quantum theory.
The next month, Bohr travelled to Leiden to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate.
When his train stopped in Hamburg , he was met by Wolfgang Pauli and Otto Stern , who asked for his opinion of the spin theory. Bohr pointed out that he had concerns about the interaction between electrons and magnetic fields. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper.
Thus, when he met Werner Heisenberg and Pascual Jordan in Göttingen on the way back, he had become, in his own words, "a prophet of the electron magnet gospel". Heisenberg first came to Copenhagen in , then returned to Göttingen in June , shortly thereafter developing the mathematical foundations of quantum mechanics.
When he showed his results to Max Born in Göttingen, Born realised that they could best be expressed using matrices. This work attracted the attention of the British physicist Paul Dirac ,  who came to Copenhagen for six months in September Austrian physicist Erwin Schrödinger also visited in His attempt at explaining quantum physics in classical terms using wave mechanics impressed Bohr, who believed it contributed "so much to mathematical clarity and simplicity that it represents a gigantic advance over all previous forms of quantum mechanics".
When Kramers left the Institute in to take up a chair as professor of theoretical physics at the Utrecht University , Bohr arranged for Heisenberg to return and take Kramers's place as a lektor at the University of Copenhagen. Bohr became convinced that light behaved like both waves and particles, and in , experiments confirmed the de Broglie hypothesis that matter like electrons also behaved like waves. In Copenhagen in Heisenberg developed his uncertainty principle ,  which Bohr embraced.
In a paper he presented at the Volta Conference at Como in September , he demonstrated that the uncertainty principle could be derived from classical arguments, without quantum terminology or matrices. 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. In , Carl Jacobsen , the heir to Carlsberg breweries , bequeathed his mansion to be used for life by the Dane who had made the most prominent contribution to science, literature or the arts, as an honorary residence Danish: He and his family moved there in By , the phenomenon of beta decay prompted Bohr to again suggest that the law of conservation of energy be abandoned, but Enrico Fermi 's hypothetical neutrino and the subsequent discovery of the neutron provided another explanation.
This prompted Bohr to create a new theory of the compound nucleus in , which explained how neutrons could be captured by the nucleus. In this model, the nucleus could be deformed like a drop of liquid. He worked on this with a new collaborator, the Danish physicist Fritz Kalckar, who died suddenly in The discovery of nuclear fission by Otto Hahn in December and its theoretical explanation by Lise Meitner generated intense interest among physicists. In April , John R. Dunning demonstrated that Bohr was correct.
In the enclosed letter, Bohr wrote, "It is the only thing I have to send home; but I do not believe that it would be very easy to find anything better I even think it is one of the most delightful things I have ever read.
There has been some dispute over the extent to which Kierkegaard influenced Bohr's philosophy and science. David Favrholdt argued that Kierkegaard had minimal influence over Bohr's work, taking Bohr's statement about disagreeing with Kierkegaard at face value,  while Jan Faye argued that one can disagree with the content of a theory while accepting its general premises and structure.
The rise of Nazism in Germany prompted many scholars to flee their countries, either because they were Jewish or because they were political opponents of the Nazi regime. In , the Rockefeller Foundation created a fund to help support refugee academics, and Bohr discussed this programme with the President of the Rockefeller Foundation, Max Mason , in May during a visit to the United States.
Bohr offered the refugees temporary jobs at the Institute, provided them with financial support, arranged for them to be awarded fellowships from the Rockefeller Foundation, and ultimately found them places at institutions around the world.
In this form, they were stored on a shelf at the Institute until after the war, when the gold was precipitated and the medals re-struck by the Nobel Foundation. Bohr kept the Institute running, but all the foreign scholars departed. Bohr was aware of the possibility of using uranium 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 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.
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. Michael Frayn 's play Copenhagen explores what might have happened at the meeting between Heisenberg and Bohr.
In September , word reached Bohr and his brother Harald that the Nazis considered their family to be Jewish, since their mother was Jewish, 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. On 2 October , 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.
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. He passed out from oxygen starvation and only revived when the aircraft descended to lower altitude over the North Sea. Bohr was warmly received by James Chadwick and Sir John Anderson , but for security reasons Bohr was kept out of sight. He was given an apartment at St James's Palace and an office with the British Tube Alloys nuclear weapons development team.
Bohr, Heisenberg, and a few others then went on to develop what came to be known as the Copenhagen interpretation of quantum mechanics, which still provides a conceptual basis for the theory. According to complementarity, on the atomic level a physical phenomenon expresses itself differently depending on the experimental setup used to observe it. Thus, light appears sometimes as waves and sometimes as particles. For a complete explanation, both aspects, which according to classical physics are contradictory, need to be taken into account.
For the rest of his life, Bohr worked to generalize complementarity as a guiding idea applying far beyond physics. In the early s Bohr found use once more for his fund-raising abilities and his vision of a fruitful combination of theory and experiment. He realized early that the research front in theoretical physics was moving from the study of the atom as a whole to the study of its nucleus.
Together with Hevesy and the Danish physiologist August Krogh , Bohr applied for support to build a cyclotron —a kind of particle accelerator recently invented by Ernest O.
Lawrence in the United States—as a means to pursue biological studies. Although Bohr intended to use the cyclotron primarily for investigations in nuclear physics, it could also produce isotopes of elements involved in organic processes, making it possible in particular to extend the radioactive indicator method, invented and promoted by Hevesy, to biological purposes. In addition to the support from the Rockefeller Foundation, funds for the cyclotron and other equipment for studying the nucleus were also granted to Bohr from Danish sources.
Just as the close connection between theory and experiment had proved fruitful for atomic physics , so now the same connection came to work well in the study of the nucleus. By that time, at the beginning of , Bohr was in the United States, where a fierce race to confirm experimentally the so-called fission of the nucleus began after the news of the German experiments and their explanation had become known. Bohr had felt the consequences of the Nazi regime almost as soon as Adolf Hitler came to power in Germany in , as several of his colleagues there were of Jewish descent and lost their jobs without any prospect of a future in their home country.
After the discovery of fission, Bohr was acutely aware of the theoretical possibility of making an atomic bomb. However, as he announced in lectures in Denmark and in Norway just before the German occupation of both countries in April , he considered the practical difficulties so prohibitive as to prevent the realization of a bomb until well after the war could be expected to end. Even when Heisenberg at his visit to Copenhagen in told Bohr about his role in a German atomic bomb project, Bohr did not waver from that conviction.
In early Bohr received a secret message from his British colleague James Chadwick , inviting Bohr to join him in England to do important scientific work. Still convinced of the infeasibility of such a project, Bohr answered that there was greater need for him in occupied Denmark. After being warned about his imminent arrest, Bohr escaped by boat with his family across the narrow sound to Sweden.
In Stockholm the invitation to England was repeated, and Bohr was brought by a military airplane to Scotland and then on to London. Upon being briefed about the state of the Allied atomic bomb project on his arrival in London, Bohr changed his mind immediately about its feasibility.
Concerned about a corresponding project being pursued in Germany, Bohr willingly joined the Allied project. Taking part for several weeks at a time in the work in Los Alamos, New Mexico , to develop the atomic bomb, he made significant technical contributions, notably to the design of the so-called initiator for the plutonium bomb. His most-important role, however, was to serve, in J. Bohr set out on a solitary campaign, during which he even succeeded in obtaining personal interviews with British Prime Minister Winston Churchill and U.
He was unable to convince either of them of his viewpoint, however, instead being suspected by Churchill of spying for the Russians. Bohr was allowed to return home only after the atomic bomb had been dropped on Japan in August In Denmark he was greeted as a hero, some newspapers even welcoming him with pride as the Dane who had invented the atomic bomb.
He continued to run and expand his institute, and he was central in postwar institution building for physics. Internationally, he took part in the establishment of CERN , the European experimental particle physics facility near Geneva , Switzerland, as well as of the Nordic Institute for Atomic Physics Nordita adjacent to his institute.