“Extraordinary claims require extraordinary evidence” said Carl Sagan; a dictum which asks us to use the scepticism of Hume to dismiss miracles and false claims. But what counts as extraordinary, and should common sense be our guide to it? An innate scepticism at the heart of science needs to be balanced and we should instead accept that the bizarre and counter-intuitive is what modern science is all about, argues Sean McMahon.
On the Ides of March in 44 BC, Julius Caesar was murdered on the steps of the Senate Chamber in Rome. As he collapsed amid the folds of his blood-soaked toga, his lungs released their final breath: about ten thousand billion billion molecules of nitrogen, oxygen, and so on. According to a popular and venerable factoid, with every passing minute you inhale a few of those same molecules. More than two-thousand years after they left Caesar’s lungs, they entered yours, just moments ago, wherever you are on Earth! That, surely, is an extraordinary claim. It seems enormously unlikely. But you can easily prove it on your own. With a little research, you will discover that the volume of Earth’s atmosphere is about ten thousand billion billion times the volume of a human breath (in or out). Once the molecules of Caesar’s last breath had spread and mixed evenly into the atmosphere, every breathful of air contained, therefore, about one of those molecules. A chemistry textbook will tell you that nitrogen hangs around for ages, so most of those molecules are still airborne: every minute you are bound to inhale (and exhale) some. QED. No extraordinary new evidence required.
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