Storm in a Teacup: The Physics of Everyday Life(15)

Fortunately for us, we’re completely free of these complications most of the time. Gravity is constant and points toward the center of the Earth. “Down” is the direction in which things fall. Even plants know that.

My mother is a keen gardener, so when I was growing up I had a lot of opportunities to plant seeds, chop up weeds, wrinkle my nose in disgust at slugs, and turn over compost heaps. I remember being fascinated by seedlings because they clearly knew up from down. Down in the darkness of the soil, a seed case would open and new roots would creep downward while a nascent shoot explored upward. You could pull up a young seedling and see that there had been no hesitation or exploration. The root just went straight down and the shoot went straight up. How did it know? When I was a bit older, I found the answer and it’s delightfully simple. It turns out that inside the seed there are specialized cells called statocytes that are mini plant snow-globes. Inside each one there are specialized starch grains that are more dense than the rest of the cell, and they settle toward the bottom of the cells. Protein networks can sense where they are, and so the seed, and later the plant, knows which way is up. Next time you plant a seed, turn it over and think about the mini snow-globe inside, and then plant it whichever way up you like, because the plant can solve the puzzle.

Gravity is a fantastically useful tool. Plumb lines and spirit levels are cheap and accurate. “Down” is universally accessible. But if everything is pulling on everything else, what about the mountain that I can see in the distance? Isn’t that pulling on me? What’s so special about the center of the planet?

I love coastlines for all sorts of reasons (waves, bubbles, sunsets, and sea breezes), but what I love most of all is the liberating, luxurious feeling of taking in the vast expanse of the sea. When I lived in California, I shared a tiny house very close to the beach, so close that we could hear the waves at night. There was an orange tree in the back garden, and a porch for watching the world go by. The ultimate luxury at the end of a busy day was to walk to the end of the road, sit on the smooth, worn rocks, and look out at the Pacific Ocean. When I did that sort of thing in England as a kid, I was just watching for fish or birds or big waves. But when I watched the ocean in San Diego, I was imagining the planet. The Pacific Ocean is vast, taking up a full third of the circumference of the Earth at the equator. Looking out toward the sunset, I could imagine the giant ball of rock I was living on, Alaska and the Arctic far away to my right, in the north, and the full length of the Andes running all the way to Antarctica on my left, south of me. I could almost give myself vertigo watching it all in my head. And once, it occurred to me that I was directly experiencing all of those places. Each one of them was tugging on me, and I was tugging on them. Every bit of mass is pulling on every other bit of mass. Gravity is a fantastically weak force—even a small child can generate the force to resist the gravitational pull of a whole planet. But nevertheless, each of those minuscule tugs is still there. Together, an uncountable number of minute tugs adds up to a single force, the gravity that we experience.

This was the step taken by the great scientist Isaac Newton when he published his Law of Universal Gravitation in Philosophiae Naturalis Principia Mathematica—the famous Principia—in 1687. Using the rule that the gravitational force between two things is inversely proportional to the square of the distance separating them, he showed that if you added up the pull of every single bit of a planet, quite a lot of those sideways pulls cancelled each other out, and the result was a single downward force, pointing toward the center of the planet and proportional to the Earth’s mass and the mass of the thing being pulled. A mountain that’s twice as far away will only pull on you with a quarter of the force. So distant objects matter less. But they still count. Sitting looking out at the sunset, I was being pulled sideways to the north and a bit downward by Alaska and sideways to the south plus a bit downward by the Andes. But the pulls to the north and the south cancelled each other out, and what was left over was downward.

So even though we’re all being pulled on (right now) by the Himalaya, the Sydney Opera House, the Earth’s inner core, and lots of marine snails, we don’t need to know the details. The complexities sort themselves out, and leave us with a simple tool. To predict the Earth’s pull on me, I just need to know how far away the center of it is, and the mass of the whole planet. The beauty of Newton’s theory was that it was simple, it was elegant, and it worked.

But it’s still true that forces are weird. In spite of its brilliance, Isaac Newton’s explanation of gravity had one major flaw: There was no mechanism. It’s straightforward to state that the Earth is pulling on an apple,§ but what is doing the pulling? Are there invisible strings? Pixies? This wasn’t sorted out satisfactorily until Einstein worked out the Theory of General Relativity, but for the 230 years in the middle, Newton’s model of gravity was accepted (and is still widely used today) because it worked so incredibly well.

You can’t see forces, but almost every kitchen has a device in it for measuring them. That’s because you need something important for cooking (and especially for baking) that no glossy recipe book ever mentions. It’s necessary because quantities matter: You have to measure “stuff,” and you have to do it accurately. The unmentioned critical ingredient that lets you do this is simple: something (anything) the size of a planet. It’s very fortunate for all fans of Eccles cakes, Victoria sponge, and chocolate gateau that we’re sitting right on top of one.