Albert Einstein announced his greatest achievement, the general theory of relativity, in Berlin a century ago, on Nov. 25, 1915. For many years, hardly any physicist could understand it. But, since the 1960s, following decades of controversy, most cosmologists have regarded general relativity as the best available explanation, if not the complete description, of the observed structure of the universe, including black holes.

And yet, even today, hardly anyone apart from specialists understands general relativity — unlike, say, the theory of natural selection, the periodic table of the elements and the wave/particle duality in quantum theory. So why is Einstein the world's most famous and most quoted (and misquoted) scientist — far ahead of Isaac Newton or Stephen Hawking — as well as a universal byword for genius?

Einstein's fame is indeed puzzling. When he gave lectures about general relativity at Oxford University in 1931, the academic audience packed the hall, only to ebb away, baffled by his mathematics and his German, leaving only a small core of experts. Afterward, a cleaner rubbed the equations off the blackboard (though thankfully one blackboard was saved and is on display in Oxford's Museum of the History of Science).