Wednesday, October 3, 2012

The apple doesn’t fall far from the Newton

Isaac Newton was sitting in his garden, contemplating nature, when an apple fell on his head. The entire notion of gravity came to him in an instant. He immediately saw how it all must fit together.

Did this really happen?

The story of Newton’s apple is a wonderful story of a sudden flash of insight, that “Eureka!” moment. But I am going to argue here that this is not exactly what happened. Here is what we know for sure.

What Newton had to say
Newton was getting on in years, in his early 80's. He was reminiscing about when he was a young man, and told friends that seeing an apple fall got him to pondering on the idea that whatever drew the apple to the Earth most likely had some effect on the moon as well. From this pondering came his theory of gravity.

There are enough independent accounts of Newton’s reminiscence that we can be sure that Newton actually said this in his old age. Voltaire wrote that he heard it from Newton’s niece, Catherine Barton Conduitt. The husband of Newton’s niece, John Conduitt, also committed this story to paper. An independent account came from William Stukeley. He wrote a biography of Newton in which he recounted that Newton himself had told him this story while they walked in the garden.

Henry Pemberton, a mathematician and a friend of Newton’s, also published a biography of Newton. In this biography he said that Newton was meditating in the garden when the idea occurred to him that the moon must be subjected to the same force that drew a falling body to the Earth.

Thus, it is quite likely that Newton told this little anecdote about how he came to discover gravity in a garden when he was 23.  
Was he hit on the head?
It is unlikely that the apple actually hit Newton on the head. None of the first-hand and second-hand accounts – people who heard the story from Newton, or from a second party – mentioned this. The embellishment was apparently the creation of Isaac D'Israeli, who was born about 40 years after Newton’s death.

A dashing gentleman

Among other things, D’Israeli wrote a book of essays called Curiosities of Literature, which contained anecdotes about a number of historical figures. D’Israeli started an essay in this book by saying that “Accident has frequently occasioned the most eminent geniuses to display their powers.” 

Isaac Newton and the alleged story of the cranial collision merited one paragraph in this D'Iraeli's book. The preceding paragraph tells of how a fellow by the name of Corneille was saved from a drab lifetime of lawyer-hood by the act of writing poetry for his mistress. The paragraph following Newton’s story tells how Ignatius Loyola founded the Jesuit Society as a result of his reading while he was convalescing from a battle wound.

D’Israeli’s book sounds pretty much like Star Magazine to me.

The modern day version of D’Israeli’s book

Was the whole apple story a fabrication?
The falling of the apple was to have occurred when Newton was 23. It is somewhat strange that Newton never mentioned the anecdote until he was 83. Then again, maybe not? It may be that he never felt it was important, or it may have been a fabrication?

Then again, it might be that Newton provided this anecdote as a way to lend credence to his claim that he discovered the law of gravity.  
Did Newton invent discover gravity?
I distinctly remember learning in third grade that Newton invented gravity. Recently I did a thorough patent search and could not find any evidence of him getting credited for this invention. My own research suggests that gravity was invented sometime around one million years ago, since at that time there were certain objects not yet brought under its control.

Two objects not subject to the force of gravity 1,002,012 years ago

But, did Newton discover gravity?  This was not exactly what Newton has been given credit for. Aristotle had written about gravity, and had his own explanation of why it exists[1]. Galileo, who lived before Newton, also had a few things to say about gravity[2].

It would be a little closer to the truth to say that Newton discovered the inverse square law of gravity – that the gravitational force between two objects is inversely proportional to the square of the distance between them.
The inverse square law as applied to light

That statement is not nearly as fun as “Newton discovered gravity”. It is shrouded in arcane mathematical verbiage. I think it puts Newton’s accomplishments just a bit out of the reach of most third grader’s, no matter how thirsty they are for knowledge. But more importantly, that silly inverse square law bit hides a lot of the historical context. There were a number of brilliant insights buried in this law:

First insight – Gravity is not limited to making objects fall to the Earth. This is a completely non-intuitive concept. Gravitational effects between two objects are just not needed when explaining our day-to-day lives, except perhaps to explain why Julia Roberts was ever attracted to Lyle Lovett. I can think of no other explanation for that attraction[3].

Do the laws of physics explain this strange attraction?

Second insight – Gravity applies to objects out in space, like the moon, the Sun, the planets, and to stars other than Julia Roberts and Lyle Lovett.

Third insight – (This is the big one.)  Kepler’s laws, which describe the elliptical paths that planets take, can be explained by an inverse square law of gravity[4].

Newton gives this account of his discovery:
In the same year I began to think of gravity extending to the orb of the Moon and (having found out how to estimate the force with which globe revolving within a sphere presses the surface of a sphere) from Kepler's rule of the periodical times of the Planets being in sesquialternate proportion to their distances from the centres of their Orbs, I deduced that the forces which keep the Planets in their Orbs must reciprocally as the squares of their distances from the centres about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth, and found them answer pretty nearly.
This is quite a mouthful for one sentence. Newton could have been a great patent writer, if he decided to turn his hand to that occupation[5]. Maybe if he had, he would have filed a patent for the invention of gravity.

Priority of Hooke
But this was Newton’s account, which doesn’t quite square with other facts about the discovery of the inverse square law.

Newton said his ruminations and discovery of the inverse square law date back to a period from 1665 to 1666, when Newton was 23. But, historical evidence contradicts this. In particular, there were a series of letters between Robert Hooke and Newton starting in 1679 that show that Newton had not yet put all the pieces together. In fact, Hooke may have had a better grasp at that time.

Robert Hooke, another dashing man

In the first letter between them, Hooke asserted without proof, that the elliptical motion of the planets around the Sun was the result of a force pulling them toward the Sun. In the ensuing letters, Hooke stated that an inverse square law of gravity would lead to an elliptical orbit.

Newton replied in one letter that an object falling to the center of the Earth, if unobstructed, would follow a spiral. Later he asserted that with a constant force of gravity the object would follow a clover leaf path. Clearly, he was not thinking about an inverse square law and ellipses at this time.

In 1684, Hooke told Edmund Halley that he (I mean Hooke) had developed a proof that an inverse square law of gravitational attraction between celestial objects would lead to elliptical orbits. He never produced the proof.

Edmund Halley (left), not to be confused with Alex Haley, author of Roots (right)

This idea intrigued Halley. Learned men of his time (sadly) did not have lofty discussion topics like Romney’s tax returns and Obama’s birth certificate. The men[6] in his circles were unfortunately limited to mundane topics like Copernicus’ revolutionary idea that the Earth was not the center of the universe about which the Sun and all that stuff revolved[7]. And the exciting idea of Kepler that Ptolemy’s ridiculously complicated mathematical model of the orbits of the planets could be explained by three simple laws[8].

Halley sought out Newton, and posed the question to him. In (Doctor) Halley’s words:
Sr Isaac replied immediately that it would be an Ellipsis, the Doctor struck with joy and amasement asked him how he knew it, why, said he I have calculated it, whereupon Dr Halley asked him for his calculation without any farther delay, Sr Isaac looked among his papers but could not find it, but he promised him to renew it, and then to send it him.
Newton was not immediately able to provide the proof, but shortly after produced a nine page demonstration. At Halley’s urging[9], this was fleshed out and was eventually published as Philosophiae Naturalis Principia Mathematica[10]. This was the first proof that an inverse square law of gravity implied elliptical orbits.

The acrimonious battle with Hooke over priority ensued, but that will be the topic of another blog.

My conclusions from this historical lesson
The bit about Newton being hit by an apple in the garden is clearly a fabrication. Newton did indeed discover the “law of gravity”, which is to say, he is the first to provide a proof that the inverse square law of gravity explained the paths of the planets. It is not likely, however, that he developed the full idea back when he was 23 sitting in a garden, pondering a falling apple.     

[1] This is fascinating history that will be left to a future blog.
[2] Remember the guy who dropped the balls from the Leaning Tower of Pisa? Well, it probably wasn’t Galileo, but it will be another fascinating future blog.
[3] In a previous blog, “Flies walk on the ceiling”, I suggested that a hypothetical Flysaac Newton might have discovered the laws of surface tension instead of gravity.
[4] This sounds like yet another topic for a future blog.
[5] I introduce this as silliness, but patents did exist in England back to the year 1449, when King Henry IV granted a patent for the making of stained glass.
[6] Sadly, these were mostly men. That sounds like another blog topic.
[7] Grist for a future blog?
[8] Another blog? I sure hope I live long enough to write all the blogs that I have half-written in my head.
[9] Although Newton was a prolific writer, he was extremely reluctant o publish, partly due to the mockery he received from his first publication on the nature of light.
[10] It is a shame that Newton did not have the benefit of Google blogs to publish.


  1. Proud to say that parts of the movie with the (ex) young girl that isn't affected by gravity, a part of her is evident out of this law, were recorded at Canary Islands. I got a collection of international film productions where the scenario is at those islands, and girls share similar characteristics on gravity behaviour, so I would say that is a geologic characteristic of the volcanic origin as actual landscape.
    Girls in Canary Islands...