April 15, 1726: Apple Doesn't Fall Far From Physicist

1726: Isaac Newton tells a biographer the story of how an apple falling in his garden prompted him to develop his law of universal gravitation. It will become an enduring origin story in the annals of science, and it may even be true. Newton was apparently fond of telling the tale, but written sources do […]
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1726: Isaac Newton tells a biographer the story of how an apple falling in his garden prompted him to develop his law of universal gravitation. It will become an enduring origin story in the annals of science, and it may even be true.

Newton was apparently fond of telling the tale, but written sources do not reveal a specific date for the fabled fruit-fall. We do know that on this day in 1726, William Stukeley talked with Newton in the London borough of Kensington, and Newton told him how, many years before, the idea had occurred to him.

As recounted in Stukeley's Memoirs of Sir Isaac Newton's Life:

It was occasioned by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself. Why should it not go sideways or upwards, but constantly to the earth's centre.

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Exhibit Documents Newton's PullNewton (like Ben Franklin and his kite) may have indulged in some self-mythologizing here. Surely, the puzzle was not that things fell down rather than sideways. Isn't that what the concepts "fall" and "down" are about?

Newton's breakthrough was not that things fell down, but that the force that made them fall extended upward infinitely (reduced by the square of the distance), that the force exists between any two masses, and that the same force that makes an apple fall holds the moon and planets in their courses.

John Conduitt, Newton's assistant at the Royal Mint (and also his nephew-in-law), tells the story this way:

In the year [1666] he retired again from Cambridge on account of the plague to his mother in Lincolnshire & whilst he was musing in a garden it came into his thought that the same power of gravity (which made an apple fall from the tree to the ground) was not limited to a certain distance from the earth but must extend much farther than was usually thought — Why not as high as the Moon said he to himself & if so that must influence her motion & perhaps retain her in her orbit, whereupon he fell a calculating what would be the effect of that supposition but being absent from books & taking the common estimate in use among Geographers & our sea men before Norwood had measured the earth, that 60 English miles were contained in one degree of latitude his computation did not agree with his Theory & inclined him then to entertain a notion that together with the power of gravity there might be a mixture of that force which the moon would have if it was carried along in a vortex, but when the Tract of Picard of the measure of the earth came out shewing that a degree was about 69½ English miles, He began his calculation a new & found it perfectly agreeable to his Theory.

A much finer tale: It shows one of the great minds of the millennium entertaining proper scientific doubt about his hypothesis, before better measurement and better data ultimately provide confirmation.

Voltaire also wrote of the event in 1727, the year Newton died: "Sir Isaac Newton walking in his gardens, had the first thought of his system of gravitation, upon seeing an apple falling from a tree."

Note that no one, from Newton on down (so to speak) claims the apple bopped him on the bean. Makes a good cartoon, sure, but such an event, if it happened, might have set the guy speculating instead on why — and how — pain hurts.

Source: Various

Image: Isaac Newton was 83 when he told a biographer the tale of observing an apple fall at age 23. He's 46 in this 1689 painting by Godfrey Kneller.

This article first appeared on Wired.com April 15, 2009.

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