Theodore Arabatzis and Despina Ioannidou, University of Athens, Greece
The role of analogies in the development of the Bohr atom

Analogies are widely used in the development of scientific models and theories. Modern scholarship on analogy takes as its point of departure the work of Mary Hesse. Hesse pointed out the existence of negative analogies between two different physical systems, that is, those respects in which the two systems clearly differ. However, she underappreciated the role of negative analogies in model-building.
In our presentation we will stress the significance of negative analogy in the development of Bohr’s atom. We will argue that i t was the negative, rather than the positive, analogy between intra-atomic electrons and the constituents of a planetary system that motivated Bohr to adopt and develop Rutherford’s atomic model. The development of the negative analogy led to the conclusion that the electron could move only in certain discrete orbits and its energy and angular momentum were accordingly restricted.

Furthermore, the analogy between planets and revolving electrons played a significant role in Bohr’s subsequent articulation of the model. On the one hand, the positive analogy suggested that electrons (like planets) revolved around the center of mass of the atom (solar system). On the other hand, the extremely high speed of electrons (unlike that of planets) suggested that relativity be brought into the picture.