The Doomsday Calculation

April 15, 2019
An amusing, entertaining effort to answer the unanswerable. In his latest thought-provoking journey, journalist Poundstone (Head in the Cloud: Why Knowing Things Still Matters When Facts Are so Easy to Look up, 2016, etc.) writes about how modern scientists have applied the mathematical theorem of obscure 18th-century British clergyman Thomas Bayes to a host of important questions and come up with unsettling answers. "By applying Thomas Bayes' rules to the technique of self-sampling," writes the author, "we can address cosmic mysteries. Was life on Earth probable or a rare accident? Why don't we see any evidence of extraterrestrials? Is the world we see real or a simulation? Is the universe we observe all there is?" There is perhaps less than meets the eye because the results are always in probabilities, but many defy common sense, and the enthusiastic Poundstone delivers a steady stream of delicious jolts. Bayes' theorem finds the probability of something if one knows other probabilities. Sound boring? Here's an example: A woman learns that her mammogram is positive. What are the odds that she has breast cancer? The known probabilities are that 1 percent of women have breast cancer and that mammograms are 90 percent accurate. The startling answer: The odds are only about 1 in 9. Since the test is only 90 percent accurate, 10 percent of the 99 healthy women will also be positive. Poundstone turns up brilliant scientists who have applied Bayes' theorem to unanswerable questions and then delivered answers. One determined that, with a 95 percent confidence level, the human race will survive at least 5,100 but not more than 7.8 million years. Another charming example of the Malcolm Gladwell school of writing: making an implausible statement and then producing evidence that it's true--maybe. Poundstone's examples mix statistics and serious philosophical arguments, and readers who pay close attention will be rewarded.

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