# The plague of London and the apple of Newton

In 1665, a plague swept through London, England. To avoid the plague, Newton left Cambridge University and returned to his long-standing hometown to isolate himself. Some people say that it was during his time in his hometown that Newton discovered the famous law of gravity. Is this really the case?

The most famous story is “Newton’s Apple”. French writer Voltaire once described in the article that when Newton escaped the plague in the countryside, one day he sat under an apple tree and was hit in the head by an apple that fell from the tree. Newton suddenly got inspired, realizing that Apple’s fall is controlled by gravity like the moon’s movement around the earth.

This story is so widespread that many people think that Newton discovered the law of gravity from 1665 to 1666.

However, this is not true. To be precise, Newton’s main academic achievement during the country’s escape from the plague was the invention of calculus, which was one of his great contributions to science. In time, Newton discovered that the law of gravity is much later than the invention of calculus.

After the plague ended, Newton returned to Cambridge to study for a master’s degree, and then stayed at the school with the support of teacher Barrow.

After that, Newton began to communicate with the British physicist Hook. Many of their letters have been preserved, and the contents of the letters show that Newton couldn’t explain why the planet’s trajectory was an ellipse, and even mistakenly thought that the trajectory of the projectile on the earth was a spiral. It can be seen that Newton did not know much about the phenomenon of gravitation.

Later, the British physicist and astronomer Harley visited Newton. Harley told Newton a conjecture (based on the discussion between Harley and Hook et al.): In Kepler’s first law, the trajectory of planets is an ellipse, which may indicate that gravitation is inversely proportional to the square of the distance of the object.

Newton listened to Harley’s introduction and began to use calculus to do the relevant mathematical derivation. Finally, he found that the conjectures of Harley and Hook and others were correct. So why can’t Harley and Hook discover the law of gravitation, but Newton can? Mainly because Newton’s mathematical skills are far superior to them. The mathematical tool used by Newton was the calculus he invented himself, which was a “unique skill” at the time.

In 1687, Newton’s “Mathematical Principles of Natural Philosophy” was published, and the law of gravitation was clearly derived. The publication of this book was funded by Harley, because the book was expensive at the time, and Newton’s life was relatively poor. Therefore, Newton was very grateful to Harley, and mentioned in his book the influence of Harley and Hook and others on himself. That is to say, Newton admitted that it was their enlightenment that made him discover the law of gravity.

So, why did a lot of later generations document that Newton discovered the law of gravity under apple trees while avoiding plague in the countryside?

The story behind this is related to Hook. At the time, Hook was a very famous British physicist who advocated the theory of light fluctuations, while Newton insisted on the theory of light particles. Their academic views diverged, and Hook criticized Newton for this. Newton was sensitive and suspicious by nature, and such academic criticism disgusted him.

As Newton’s status in academic circles became higher and higher, the contradiction between him and Hook became deeper and deeper. When Newton first published “The Mathematical Principles of Natural Philosophy”, Hook’s name also appeared in the book. After Hook’s death, Newton became president of the Royal Society. When he published “The Mathematical Principles of Natural Philosophy” again, Hook’s name was deleted from the book. Newton no longer mentions Hook’s enlightenment to him, and monopolizes the credit for discovering the law of gravitation. Further, Newton declared that he had discovered the law of gravitation in 1666 to completely abandon Hook’s influence on him.

This is the story behind the plague in London and the apple of Newton.