Since knowledge is still growing at an impressively rapid pace, it should not be surprising that many facts people learned in school have been overturned and are now out of date. But at what rate do former facts disappear? Arbesman applies to the dissolution of facts the concept of half-life—the time required for half the atoms of a given amount of a radioactive substance to disintegrate. For example, the half-life of the radioactive isotope strontium-90 is just over 29 years. Applying the concept of half-life to facts, Arbesman cites research that looked into the decay in the truth of clinical knowledge about cirrhosis and hepatitis. “The half-life of truth was 45 years,” he found.

In other words, half of what physicians thought they knew about liver diseases was wrong or obsolete 45 years later. Similarly, ordinary people’s brains are cluttered with outdated lists of things, such as the 10 biggest cities in the United States.

Facts are being manufactured all of the time, and, as Arbesman shows, many of them turn out to be wrong. Checking each one is how the scientific process is supposed to work; experimental results need to be replicated by other researchers. So how many of the findings in 845,175 articles published in 2009 and recorded in PubMed, the free online medical database, were actually replicated? Not all that many. In 2011, a disquieting study in Nature reported that a team of researchers over 10 years was able to reproduce the results of only six out of 53 landmark papers in preclinical cancer research.