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The aging of physics journals (and indeed of all scientific journals) has been a much discussed, and often debated, subject (Burton & Kebler, 1960; Brookes, 1970; MacRae, 1969; Line, 1970; 1974; Sandison, 1971a; 1971b; 1974; 1975; Chen, 1972; Documentation, 1960; Seymour, 1972; Rao, 1973; Ash, 1974; Griffith, 1979; Miyamoto, 1981; McCain & Bobbick, 1981; Midorikawa, 1983; Hodowanec, 1983; Lambert, 1985; Wallace, 1986). It has been known for a long time that the use of scientific literature decreases with the age of the literature. Burton and Kebler (1960) introduced the term “half-life” to quantitatively describe the rate of obsolescence of the scientific literature (in analogy with the term “half-life” used in nuclear physics to describe radioactive decay). While there is ambiguity in the meaning of the term “obsolescence,” Burton and Kebler took the working definition of half-life as “the time during which one-half of the currently active literature was published.” Currently active literature was assumed to follow an exponential decay curve. Brookes (1970) gave an alternative model of literature obsolescence in which the residual utility of the literature after some time was given in terms of an aging factor. As might be suspected, Burton and Kebler’s half-life and Brookes’ aging factor are related to each other (Gupta & Gupta, 1991). Around 1969-1970 MacRae (1969), Line (1970), and Sandison (1971a; 1971b) correctly pointed out that when considering the age distribution of citations or that of jour-

Obsolescence of physics literature: Exponential decrease of the density of citations to Physical Review articles with age