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Evolution of Physics Sub-fields

: We study the evolution and relationships between sub-ﬁelds of Physics using the large data set of articles published in the various physical review journals from 1985-2010. Each article is assigned to some PACS codes by their authors which represent speciﬁc sub-ﬁelds of Physics. We construct a weighted network with nodes as PACS codes and there is a link between two PACS codes if there is an article assigned to both these codes. The weight of a link represents the number of articles in which both PACS codes appears. We study the time evolution of PACS network at various hierarchy levels of PACS codes. We observe that sub-ﬁelds Physics of elementary particles and ﬁelds, Nuclear Physics and Condensed matter physics have stronger connections inside the ﬁeld compared to connections to other sub-ﬁelds. We also observe that both condensed matter physics sub-ﬁelds are strongly related compared to any other pair of sub-ﬁelds.