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Then they checked to see how many were eventually recategorized as nonextinct. The answer: More than a third of all mammals that allegedly were lost to time in the past five hundred years have since been rediscovered.
    This sort of large-scale analysis is not just for understanding the nature of Lazarus taxa. It can be extended more generally to enable us to understand the entire edifice of science and how we overturn long-held scientific beliefs. By looking at how science changes overall, we can see the patterns in how scientific knowledge is revised. And it can lead us to measuring the half-life of facts.
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    AS scientific knowledge grows rapidly, it leads to a certain overturning of old truths, a churning of knowledge. While this churning is hard to deny—recall my inability to recall the health benefits of red wine despite having seen it in the newspapers many times—it is difficult to measure. But if we could quantify this churn, that could provide a handle for our uncertainty, and even a metric for how often we should revisit a subject.
    A few years ago a team of scientists at a hospital in Paris decided to actually measure this. They decided to look at fields that they specialized in: cirrhosis and hepatitis, two areas that focus on liver diseases. They took nearly five hundred articles in these fields from more than fifty years and gave them to a battery of experts to examine.
    Each expert was charged with saying whether the paper was factual, out-of-date, or disproved, according to more recent findings. Through doing this they were able to create a simple chart that showed the amount of factual content that had persisted over the previous decades. They found something striking: a clear decay in the number of papers that were still valid.
    Furthermore, they got a clear measurement for the half-lifeof facts in these fields by looking at where the curve crosses 50 percent on this chart: forty-five years. Essentially, information is like radioactive material: Medical knowledge about cirrhosis or hepatitis takes about forty-five years for half of it to be disproven or become out-of-date. This is about twice the half-life of the actual radioisotope samarium-151.

    Figure 2. Decay in the truth of knowledge in the areas of hepatitis and cirrhosis. The 50 percent mark is around forty-five years, meaning it takes about forty-five years for half of the knowledge in these fields to be overturned. From Poynard, et al. “Truth Survival in Clinical Research: An Evidence-Based Requiem?”
Annals of Internal Medicine
136, no. 12 (2002): 888–95.
    As mentioned earlier, while each individual radioactive atom’s decay is subject to a great deal of uncertainty, in the aggregate, they are far from random. They are subject to a systematic degradation and encapsulated in the shorthand of a single number—the half-life—that denotes how long it takes for half of the material to be subject to radioactive decay.
    Knowledge in a field can also decay exponentially, shrinking by a constant fraction. It is like one of Zeno’s Paradoxes, according to which we keep getting halfway closer to the finish line but neverquite reach it. In this case, the finish line is the point at which no papers from the original batch of cirrhosis and hepatitis studies are still true. While there will always be an infinitesimal number of papers cited many decades, or even centuries, from now, within a certain number of years the vast number of articles will have decayed into irrelevance. Of course, some of these are not wrong, just obsolete. These scientists noted that the effectiveness of treatments in decades past doesn’t necessarily become nullified; they simply become superseded by something newer, such as novel vaccines that make treatment of a disease no longer necessary.
    But ultimately, while we can’t predict which individual papers will be overturned, just like we can’t tell when individual radio active atoms will decay, we can