The small-world structure of sub-fields in Physics.

Soma Sanyal, Cyberinfrastructure for Network Science Center, School of Library and Information Science, Indiana University.

The Physics and Astronomy Classification Scheme (PACS) has been introduced by the American Institute of Physics (AIP) to identify sub-fields of physics. Documents published by the AIP has one or more PACS numbers. We analyse the co-occurence of PACS numbers in 263,300 documents over a period of 20 years (1985-2005). We find that the network of PACS co-occurences is a dense network with small world properties. It consists of a giant component with PACS numbers in the general category exhibiting the highest betweeness centrality. We use various clustering techniques to study the clusters of PACS numbers for each year. The clusters formed overlap with each other. We identify major fields and smaller sub-fields which change over time. We find that though different sub-fields change with time, the average path length of the network decreases due to the presence of overlapping clusters that connect most of the sub-fields to one another.