For decades – if not centuries – the legend has circulated that Isaac Newton would have arrived at the formulation of his own law of universal gravitation only after being hit on the head by one apple in 1666. This small incident would have made him understand that the strength that it attracted the apple towards the ground it would be the same one that allows the Moon orbiting our planet. But how much truth is there in this story? In fact, very little.
The true story of Newton’s apple
Isaac Newton he never wrote anything official regarding the role of the apple, neither to confirm nor to deny the fact – except for brief hints in some of his notes, but too vague to be considered certain proof. Consequently, the only way we have to ascertain what happened is to rely on the writings of those who knew him personally, such as William Stukeley, a friend and biographer of his.
After dinner, since the weather was mild, we went to the garden and drank some tea in the shade of some apple trees. He told me that he was in the same situation he had been in before, when the idea of gravity had first occurred to him. This had been caused by an apple falling while he was sitting in a contemplative mood. Why did that apple always have to fall perpendicular to the ground, he thought to himself…
From here it is clear how the physicist was not hit on the head, but simply observed an apple fall. The same version is also confirmed by John Conduitt, his colleague at the State Mint, who wrote in 1726 that “His first thought about his gravitational system came to him while watching an apple fall from a tree.” The same goes for Voltaire who, after a conversation with the English physicist, reported the apple falling in the same way inside his Elements of Newton’s philosophy.
Warning: it is always good to keep in mind that from when Newton had the intuition to when he managed to formulate the law of universal gravitation literally years passed. We must therefore not imagine that after seeing an apple fall he already had clear ideas after a few hours!
But if the evidence is so clear, how did this myth of the apple on the head arise and spread? The “fault”, so to speak, lies with another mathematician, Leonhard Euler.
How the apple on the head hoax was born
Euler in 1760 he wrote a letter which stated:
This great English philosopher and mathematician, finding himself lying one day in a garden under an apple tree, was struck by an apple that fell from the tree, which gave him the opportunity to formulate several reflections.
It is unclear whether this mistake was made on purpose to give a touch of color to the story or naivelydue to an error of understanding. And in reality Euler wasn’t even the only one to report the false news, so much so that the story also ended up in a work of Isaac D’Israelia writer of the time. He inserted this anecdote inside Curiosities of Literatureone of his biggest best-sellers of the time, fueling a myth that, even today, dies hard.
