Friday, October 19, 2018

What Everyone Gets Wrong About Newton's Apple

As someone whose job it is to help people understand and appreciate physics, I absolutely hate the way most people talk about Isaac Newton and how he developed his theory of gravity. It's not the apple bit that I have a problem with; that's an important part of the story, and even historically accurate! The thing that kills me is the way the idea is framed, and the gulf that it creates between his observation and his insight. What do I mean by that? Let's unpack the story, as I remember first being told it.

Young Isaac Newton is sitting under an apple tree in his yard one day, when he sees an apple fall to the ground—in some versions he's conked on the head by the falling fruit, for comedic effect. Being a brilliant thinker, or maybe mildly concussed, he starts to wonder about the why behind this everyday event. In a flash of insight, he realizes that there must be some force pulling the apple toward the earth. He calls this force gravity, and describes its principles with a simple, memorable adage: What goes up must come down! There, the story usually stops—just short of actually saying anything worthwhile.

A coloring page like this can be a great jumping-off point, but we can't stop the story there; the challenge of education is communicating at your audience's level in a way that doesn't sacrifice the important parts your message.
Image Credit: APS Physics
Hearing this version of events, with its mangled explanation of Newton's idea, a child can only walk away with a distressingly wrong idea of what science is all about. What goes up must come down isn't an insight; it's a less straightforward way of saying things fall—it's more of a lesson in archaic grammar than in physics. In that version of the story, what does Newton do besides slap a label on a phenomenon that your average squirrel, if it could talk, would be able to tell you all about?

I'm not suggesting that we start first-graders out with mathematical formulas and the inverse square law, but there's a key element missing in the version of the story above. The joy, the thrill of learning and discovery comes from the click, the act of making a mental connection. It's seeing something familiar in a new and different way, or finding a way of looking at something strange that makes it resemble the familiar. In the version of the story above, there's no click—just another vocab word, leading to a circular chain of definitions: heavy stuff gets pulled down because of gravity, gravity is a property of mass, and mass is...well...heavy stuff, unless you want to skip straight to E=MC2.

So where was the click, for young Isaac Newton? What was his big insight? The man himself is recorded as saying that his "eureka" moment really did come after seeing an apple fall from a tree.  What mental lens did he peer through to witness this ordinary thing and, in it, see the law that holds the solar system together?

The key is in one of the other laws that bears Newton's name, a simple one that's just as easy and memorable as "What goes up must come down"Every action has an equal, opposite reaction. Pushing someone away from you, you also push yourself away from them.

If we preface the story with an example like this—maybe framed in the context of introducing Newton and talking a little about his other contributions—the listener is primed not only to appreciate his stroke of genius, but maybe even to make that leap for themselves. As we tell the story of Newton seeing the apple drop from the tree, we can ask the listener the same question Newton must have asked himself: "Where's the opposite reaction to the apple being pulled to Earth? Where does the counter-force to move it come from?"

The answer—that the entire planet moves, however infinitesimally, to meet the apple—comes with a deafening click. For Isaac Newton, this simple but enormous realization blew the doors off the spatial reference frame that serves us so well in navigating the everyday world. It's a frame where up and down are absolutes, and Earth is effectively flat and immobile. But cock your head to the side, realize that down just means toward a mass, and suddenly the earth and the apple aren't fruit and firmament—they're two bodies hanging in space, each attracting the other.

(Click to enlarge)
A passage from a manuscript written by one of Newton's contemporaries, recording the version of the apple tree story as Newton tells it: "...therefore the apple draws the earth, as well as the earth draws the apple."
Image Credit: The Royal Society, Memoirs of Sir Isaac Newton's Life by Sir William Stukeley
If you can decipher that kind of handwriting, you can read the full text free online at the link above!
It couldn't have been long afterward that Newton turned his gaze to the moon in the sky, and again felt the thrill of realization as he looked at something familiar in a new and different way. Years before, the planets' orbits around the sun had been described by the astronomer and mathematician Johannes Kepler—but the paths had never been properly explained. Formalizing his new theory of universal attraction by developing the mathematics behind it, then combining it with the other laws of motion that bear his name, Newton proved that the celestial bodies circling the sun are governed by the same rules that dictate the fall of an apple, or the course of a cannonball fired here at Earth's surface.

It was a triumph of reason and imagination, and a model for the having of great ideas. Although the fabled "eureka" moment might seem to come all at once, it has to be founded on an understanding of the system, the way that Newton's knowledge of force and motion gave him the key to unlock a new way of thinking.

The way I first heard it, the story is kooky and confusing: Newton gets hit on the head, and it inspires him to give a name to something that everyone already knows. But if we really strive to put ourselves in his shoes, we can see what was so extraordinary about Newton's epiphany, and the story can become an inspirational, instructive, mind-expanding moment for listeners of all ages.

Stephen Skolnick

P.S. I can never talk about Newton without bringing up my favorite fact about his life. The story of the apple tree is set in 1666, when 23-year-old Isaac Newton unexpectedly found himself with the time to sit in his family's garden and stare at a tree because Cambridge University—where he was a student at the time—was closed for the year due to a minor outbreak of bubonic plague.

We can thank this little touch of plague for virtually all of Newton's scientific legacy: in that single impromptu gap year, he had his epiphany about gravity, discovered that white light is made up of all the colors in the spectrum, and basically invented calculus. Having a year off from studying and doing homework to actually think—to sit and sip tea and look sideways at a tree, to squint through glass panes at funny angles to study the rainbows they make, to ponder without pressure and generally explore his own ideas rather than furiously studying those of his professors and predecessors—was undoubtedly an important ingredient in producing what became known as Newton's annus mirabilis: the miracle year.

1 comment:

  1. Regarding the "annus mirabilis"... How many other greats had such a year? Einstein did in 1905 as well if I remember correctly... while sitting in a patent office doing routine mindless BS that let his mind wander rather than being constricted by the rigors of University work. Were there others?

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