The Well Balanced Universe


Newton, the apple, and the moon

By Edmund Wood

Everybody knows that gravity was sorted out about 300 years ago. We have all heard the legend of Isaac Newton watching an apple fall from a tree and then forming his theory about a force of attraction. Sometimes the story relates that the apple actually fell on his head. The fact is that even the former version is unlikely to be true.

The earliest account of the famous piece of fruit comes from Newton's friend and biographer, William Stukeley. In his Memoirs of Sir Isaac Newton's Life, Stukeley claims he got the story from Newton himself, and he writes:"The notion of gravitation... was occasioned by the fall of an apple, as he sat in a contemplative mood."

Now, it is not easy to watch an apple falling of its own accord. For one thing, it is an infrequent occurrence: you would probably have to stare at a tree intently for several hours, and it is a safe bet that after only a short period of time your attention would start to wander, however purposeful you were. For another, it is fairly easy to work out that the event would last less than a second, so unless you were staring at pretty much the right spot, you would miss it.

If we go back to our illustrious scientist in his garden; we know that Newton was not noted for his alertness, but rather for his ability to meditate deeply about abstract ideas for long periods of time. If he says that the notion of gravitation came to him while "in a contemplative mood", then we can be pretty sure from the preceding arguments that he did not see any apple fall. What is far more likely (assuming that he was telling the truth) is that he heard the sound of an apple hitting the ground.

Anyone who has sat meditatively in a tranquil garden and has heard that particular sudden THUMP! will know that it is quite startling. Your immediate reaction is to search around for some lurking fiend in the act of aiming another projectile your way.

Only when you are sure that there is no one about do you begin to contemplate an alternative explanation. It can still take a few moments of mental sleuthing to home in on the group of innocent-looking apples scattered on the ground, all of which have the air of having been sitting there for hours, if not days. When your brain finally settles on this harmless explanation as the most likely one, you smile about your initial alarm.

Perhaps at this point you remember the story about Newton, and start to wonder how on earth he went from thoughts about a falling apple to a theory involving a force of attraction. After all, you say to yourself, people had known about things falling since... well, since the beginning of people. What was it about Newton's train of thoughts that was so different from anyone else's?

The answer is that he first posed a most particular question. He said: “If an apple falls of its own accord, why then does the Moon not fall?” And then came the flash of inspiration: "The Moon is falling. If it wasn't, it would move off in a straight line. The continuous falling, combined with a steady horizontal speed of just the right amount, results in a perpetual orbit around the Earth. In the same way, the planets must be falling towards the Sun."

Newton went on to reason that, if everything is falling towards everything else, there must be some kind of force of attraction between all matter: "Every particle of matter in the universe," he wrote, "attracts every other particle with a force whose direction is that of the line joining the two and whose magnitude is directly as a product of their masses and inversely as the square of the distance from each other."

However, it was a great jump to go from his limited observations in the local neighbourhood to a law for every particle of matter in the universe. Was he right?

Copyright © March 2007 Edmund F Wood



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