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observe the aggregate and see that there are rules for how a field changes over time. The “half-life,” whether mathematically rigorous or simply conceptual, captures the regularities behind how knowledge changes. In addition, these results are nearly identical to a similar study that examined the overturning of information in surgery. Two Australian surgeons found that half of the facts in that field also become false every forty-five years. As the French scientists noted, all of these results verify the first half of a well-known medical aphorism by John Hughlings Jackson, a British neurologist in the nineteenth and early twentieth centuries: “It takes 50 years to get a wrong idea out of medicine, and 100 years a right one into medicine.”
    This means that despite the ever-expanding growth of scientific knowledge, the publication of new articles, refutations of existing theories, the bifurcations of new fields into multiple subfields, and the messy processes of grant-writing and -funding in academia, there are measurable ways in which facts are overturned and our knowledge is ever renewed. I’m not simply extrapolating from this half-life of medicine to argue that all of science is like this. Other studies have been performed about the half-lives of different types of scientific knowledge as well.
    Unfortunately, convening a panel of experts and having them comb through all of science’s past conclusions and giving a thumbs-up or thumbs-down to a paper’s validity isn’t quite feasible.So we have to sacrifice precision for our ability to look at lots and lots of science relatively quickly. One simpler way to do this is by looking at the lifetime of citations. As mentioned before, citations are the coin of the scientific realm and the metric by which we measure the impact of a paper.
    Most papers are never cited. And many more are cited only once and then forgotten. Others are only cited by their own authors, in their own other papers. But—and this is no doubt a point in favor of the scientific endeavor—there are numerous papers that are cited by others in the field. And there are the even rarer papers cited so many more times than those around them that they are truly fundamental to a field, towering well above other publications.
    To understand the decay in the “truth” of a paper, we can measure how long it takes for the citation of an average paper in a field to end. Whether it is no longer interesting, no longer relevant, or has been contradicted by new research, this paper is no longer a part of the living scientific literature. It is out-of-date. The amount of time it takes for others to stop citing half of the literature in a field is also a half-life of sorts.
    This gives us a sense of how knowledge becomes obsolete, but it also has a very practical application. Scholars in the field of information science in the 1970s were concerned with understanding the half-life of knowledge for a specific reason: protecting libraries from being overwhelmed.
    In our modern digital information age, this sounds strange. But in the 1970s librarians everywhere were coping with the very real implications of the exponential growth of knowledge: Their libraries were being inundated. They needed ways to figure out which volumes they could safely discard. If they knew the half-life of a book or article’s time to obsolescence, it would go a long way to providing a means to avoid overloading a library’s capacity. Knowing the half-lives of a library’s volumes would give a librarian a handle on how long books should be kept before they are just taking up space on the shelves, without being useful.
    So a burst of research was conducted into this area. Information scientists examined citation data, and even usage data inlibraries, in order to answer such questions as, If a book isn’t taken out for decades, is it that important anymore? And should we keep it on our shelves?
    Through this we can begin to see how the