From Apocalypse When?:
An asteroid colliding with the earth could cause the extinction of our species. Is this a risk worth worrying about? More important, is it a risk worth doing something about? Richard A. Posner, a judge on the United States Court of Appeals for the Seventh Circuit, who produces more books in his leisure hours than most authors do working full time, thinks it is.
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When a catastrophe is really catastrophic — and Posner, it should be emphasized, isn’t writing about ”minor” disasters like the terrorist attacks of 9/11 — it can have a significant expected cost, even if the event is extremely improbable. Consider, for example, the risk that a high-energy particle accelerator will produce a ”strange matter” disaster. The official risk-assessment team for one of these accelerators, at the Brookhaven National Laboratory, offered a series of estimates, one of which puts the annual risk of a disaster at one in five million. That seems a very small risk. But since the disaster would kill six billion people, that estimate gives it an expected cost of 1,200 lives per year.