It’s the single most famous story of scientific discovery: in 1666, Isaac Newton was walking in his garden outside Cambridge, England – he was avoiding the city because of the plague – when he saw an apple fall from a tree. The fruit fell straight to the earth, as if tugged by an invisible force. (Subsequent versions of the story had the apple hitting Newton on the head.) This mundane observation led Newton to devise the concept of universal gravitation, which explained everything from the falling apple to the orbit of the moon.
There is something appealing about such narratives. They reduce the scientific process to a sudden epiphany: There is no sweat or toil, just a new idea, produced by a genius. Everybody knows that things fall – it took Newton to explain why.
Unfortunately, the story of the apple is almost certainly false; Voltaire probably made it up. Even if Newton started thinking about gravity in 1666, it took him years of painstaking work before he understood it. He filled entire vellum notebooks with his scribbles and spent weeks recording the exact movements of a pendulum. (It made, on average, 1,512 ticks per hour.) The discovery of gravity, in other words, wasn’t a flash of insight – it required decades of effort, which is one of the reasons Newton didn’t publish his theory until 1687, in the “Principia.