Storm in a Teacup: The Physics of Everyday Life(16)

I’ve got a book full of handwritten recipes that I’ve been adding to since I was eight or nine years old, and I love being able to go straight back to the ones of my childhood. Carrot cake is one of those, scribbled on a page smudged with the years, and the recipe starts with an instruction to procure 8 ounces of plain flour. So the baker does something very clever that we all take completely for granted. He puts some flour in a bowl and measures directly how much the Earth is pulling on it. That’s what scales do. You put them in the gap between the vast planet and the tiny bowl, and measure the squeeze. The pull between an object and our planet is directly proportional to the mass of both the object and the Earth. Since the mass of the Earth isn’t changing, that pull depends solely on the mass of the flour that went into the bowl. Scales measure weight, which is the force between flour and planet. But the weight is just the mass of the flour multiplied by the strength of gravity, which is a constant in our kitchens. So if you measure the weight and you know the strength of gravity, you can work out the mass of flour in the bowl. Next you need 4 ounces of butter, so you put butter into the bowl until the squeezing force is half what it was before. This is a fantastically useful and very simple technique for getting at how much stuff you have, and it works for everyone on the planet. Heavy objects are heavy only because they consist of more “stuff,” so Earth is pulling harder on them. Nothing is heavy in space because the local gravity is too weak to pull noticeably on things, unless you’re very close to a planet or a star.

But what those kitchen scales are really telling you is that gravity, the grand force that holds together our planet and our solar system and dominates our civilization, is unbelievably weak and weedy. The Earth has a mass of 1.3 × 1025 pounds (six thousand billion billion tons, if you prefer those units), and it can only pull on your bowl of flour with the force of a small elastic band. It’s just as well, otherwise life wouldn’t be able to exist, but it does put things into perspective a bit. Every time you pick up an object, you are resisting the gravitational pull of a whole planet. The solar system is large because gravity is weak. Gravity does have one major advantage over all the other fundamental forces, though, and that’s its reach. It may be weak, and getting even weaker as you travel farther from Earth, but it stretches out across the vast distances of space, tugging on other planets and suns and galaxies. Each tug is tiny, but it’s this frail force field that gives our universe its structure.

Still, even picking up the finished carrot cake does take some effort. When it’s sitting on the table, the table surface is pushing upward on the cake just enough to exactly balance out the pull between cake and planet. To pick it up, you have to provide that much force plus a tiny bit more, enough so that the overall force on the cake is upward. Our lives are controlled not by what individual forces are acting, but on what’s left over on balance. And that simplifies things a lot. Massive forces can be made irrelevant by setting them in opposition to other massive forces. The easiest place to start thinking about this is with solid objects, because they keep their shape while they’re being pulled. And London’s Tower Bridge is very solid indeed.

Gravity can be a nuisance, because sometimes you want to hold things in the air. To do that, you need to resist the downward pull. If you couldn’t, everything would slither around on the floor. Fluids flow downward, and that’s just the way it is. For solids, things are different. One single concept, the pivot, lets us effectively neutralize gravity by turning stupidly heavy things into one half of a seesaw. The mysterious other half is often cunningly hidden away, and there’s no better example of this than the two graceful towers of Tower Bridge in London. Built on two man-made islands, each a third of the way across the Thames, these towers stand guard at the entrance into London from the sea, and carry the road linking the north of the city to the south.

The pavement is a noisy circus of tourists engaged in camera choreography, while London’s taxis, souvenir merchants, coffee stalls, dog-walkers, and buses just get on with things in the background. Our tour guide strides through all the chaos and we toddle along behind him like a line of obedient ducklings. He opens an iron gate at the base of one of the towers, ushers us around the corner into a sort of posh garden shed made of stone, and suddenly it’s calm. You can almost hear the sigh of relief as his flock realizes they’ve survived the tourist gauntlet and have arrived at their reward: the brass dials, giant levers, and reassuringly robust-looking valves of solid Victorian engineering. The pretty and delicate fairy-castle exterior of Tower Bridge is famous around the world, but we are here to see what’s lurking inside: the massive steel guts of this elegant and powerful beast.

London has been a port for two thousand years, and the nice thing about having a city on a river is that you have two banks to play with, not just one bit of coastline. But the Thames is both a vital highway for anything that floats and a massive obstacle for anything that walks or rolls. Many bridges have come and gone over the millennia, and by the 1870s the city was crying out for another one. The problem was: How do you satisfy the owner of the horse and cart without cutting off the river to the tall ships? Tower Bridge is the ingenious solution.

The small stone shed squats on top of a spiral staircase that leads downward into a series of improbably large brick grottos, hidden inside the foundations of one of the towers. It’s like the wardrobe that leads into Narnia, except that this is Narnia for engineers. The first grotto contains the original hydraulic pumps and the next (much bigger) one is mostly filled by a wooden monster: a two-story-high barrel that used to act as a temporary energy store—a non-electric battery. But it’s the third one, the largest of all, that I’ve really come to see. This is the chamber that houses the counterweight.