It all started with a deeply original, but quite unphilosophical article from 1913, in which Niels Bohr gave a surprising, even provocative, new picture of the architecture of the atom.

2013.07.24 | AU Kommunikation

Niels Bohr's philosophical writings are now published in Danish, including his article "EPR", which is a reply to Einstein, with whom Bohr is pictured here.

The high status enjoyed by Niels Bohr in the Danish and international history of science is due, not least, to the theory of the structure of the atom suggested 100 years ago by the then 27-year-old Bohr. The theory was not only rewarded with a Nobel Prize; it also marked the start of quantum mechanics as the branch of science which, in the 1920s, came to revolutionise the physics view of the world.

In spring 1912, Bohr worked as a postdoc in Manchester under Ernest Rutherford, who the previous year had proposed the idea of the atom as a very small, positively charged nucleus orbited by negatively charged electrons. Rutherford's nuclear model of the atom attracted very little interest, but Bohr was convinced of the truth in it and decided to develop it into a theory as such.

In summer 1912, Bohr presented the outline of his new and strange theory of the structure of atoms and molecules. What he presented was merely a draft, but Bohr, a slow and methodical worker, was well aware of this.

A full year passed by before Bohr had transformed his draft into a complete and detailed theory. The result was three ground-breaking articles – known the Bohr "trilogy" – which were published in the summer and autumn of 1913 in the Philosophical Magazine, an English physics journal.

Even the physicists who were sceptical of Bohr's theory (and there were quite a few of them) had to acknowledge the considerable empirical strength of the theory: not only was the theory convincing, it also predicted a number of phenomena which were as yet unknown.

Further development

Considering the radical nature of Bohr's theory, its reception by the physics community was surprisingly positive, so that by 1920 it was regarded as the most credible model of the atom. At the time, it attained almost paradigmatic status.

This was primarily due to its instrumental virtues, i.e. its ability to explain and predict physical phenomena, often with stunning precision. Bohr himself was very keen on experiments, and always regarded his theory as being intimately associated with his work in the laboratory. He had not yet become the Socratic quantum philosopher he is now often regarded as being.

Even though most physicists acknowledged the new model of the atom, for a long time it was met with scepticism on account of Bohr's two fundamental quantum postulates which were regarded as wildly strange by many. In fact, when Bohr was nominated for the Nobel Prize in 1920, the committee in Stockholm rejected the nomination on the grounds that the two postulates were inconsistent with the concepts and laws of classical physics. But that, of course, was the whole point of Bohr's theory! In the end, he was presented with the coveted prize two years later.

A fruitful mistake

According to Bohr's model of the atom, the electrons move in neat circular or elliptical orbits around the nucleus, in much the same way as the planets orbit the sun. This model concept is fundamentally wrong, which became clear when the new theory of quantum mechanics replaced Bohr's quantum theory of the atom in 1925. If Bohr's theory was simply a mistake, why make such a fuss of it? Was he awarded the Nobel Prize for the wrong theory?

In a sense yes, but in another and more important sense, no. Even though Bohr's model of the atom is wrong, the basic idea of stationary states and quantum leaps has survived the quantum mechanical revolution. Bohr himself was well aware that his model was an analogy rather than a true reproduction of the atom. In 1918, he formulated this analogy between the quantum world and the classical world in the so-called correspondence principle, which proved extremely fruitful in the further development of nuclear physics.

So, while Bohr's famous theory of the atom from 1913 might have been wrong, it proved an extremely productive and almost necessary mistake. We should bear in mind that almost all ground-breaking theories in the history of science have been wrong. The important thing is not whether they are right or wrong, but whether they lead to an advancement of knowledge. From that perspective, there is every reason to look upon Bohr's theory of the atom as a milestone in the history of science.

This article is an extract from an anniversary article published in the Danish journal Aktuel Naturvidenskab 1/2013. Read the entire article (in Danish) by Professor Helge Kragh by clicking here – link to the A. N. aktuelnaturvidenskab.dk/fileadmin/Aktuel_Naturvidenskab/nr-1/AN_1_2013bohr100.pdf