The Myth and Reality of Newton's Law of Universal Gravitation

One of the most enduring and popular anecdotes in the history of science is the story of Isaac Newton sitting under an apple tree, where an apple falls onto his head, sparking his idea of gravitational force. This narrative has long been a source of fascination, but it is quite likely that the story has been oversimplified and misunderstood. Through a detailed examination of various sources, the true story of Newton and his groundbreaking work on gravity emerges.

The Apple Tree Myth Debunked

The common belief that Newton’s idea of gravity originated from an apple falling onto his head is a misconception. In reality, the story of the apple is more of a historical footnote and a simplified metaphor for Newton's intellectual journey.

According to physicist and science popularizer Stephen Hawking, the apple story serves as a glamorized rumor that doesn't accurately reflect the true process of Newton’s discovery. The idea of gravitation, according to Hawking, was not a sudden epiphany but a culmination of years of deep thought and observation.

A More Accurate Account

William Stukeley, an English antiquarian, provides a more credible account of the incident. He recounts a conversation with Isaac Newton where the scientist mentioned that the apple incident did not happen as a sudden and dramatic event. Instead, it was a part of a broader pattern of his thoughts and observations.

In a passage from his memoirs titled 'Memoirs of Sir Isaac Newton’s Life,' Stukeley writes:

We went into the garden. We drank tea in the apple-tree shade. He told me and others present that he was just in the same situation as when formerly the notion of gravitation came into his mind. 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 up?...

The Development of Newton's Theory

The story provided by John Conduitt, Newton’s assistant at the Royal Mint, further clarifies the nature of Newton's discovery. Conduitt described how Newton, while reflecting in his garden, began to contemplate the idea of gravitational force:

In the year 1666, he retired to his mother’s residence in Lincolnshire. While walking thoughtfully in the garden, he considered the idea that the force that brings an apple from a tree to the ground might not be limited to a certain distance. He wondered whether this force might extend to influence the motion of the Moon. This led him to begin calculating the effects of such a hypothesis.

From Newton’s notebook, it is evident that his development of the theory of gravitation was a gradual process that spanned approximately two decades. The idea of universal gravitation was not instant but the result of a combination of observations, reflections, and mathematical calculations over time.

The following cartoon by Frits Ahlefeldt captures the essence of Newton's contemplative process:

Conclusion

The story of Newton under the apple tree is a valuable narrative that has inspired generations of scientists and laypeople alike. However, it is essential to separate fact from fiction. Newton's discovery of gravitation was a result of his deep intellectual journey and his persistent curiosity about the workings of the universe, not a sudden and random event. This accurate representation of Newton's process adds depth to our understanding of the scientific method and the importance of observation and reflection in the advancement of knowledge.