Storm in a Teacup: The Physics of Everyday Life(13)

A sparrow landed on the table to hoover up some crumbs and gave the bottle a suspicious look. My dad was giving the bottle a suspicious look from the other direction. “Does it only work with raisins?” he asked.

The answer is yes, and for a really good reason. Before you take the lid off a fizzy drink, the pressure inside is significantly higher than the pressure of the air around you. At the moment you unscrew the cap, the pressure inside the bottle drops. There’s lots of gas dissolved in the water, kept there by the higher pressure, but suddenly all that gas can escape. The problem is, it needs a route out. Starting a new gas bubble is really difficult, so the gas molecules can only join an existing bubble. What they need is a raisin. Raisins are helpfully covered in V-shaped wrinkles that won’t have been completely filled by the lemonade. Down at the bottom of each wrinkle there’s a proto-bubble, a tiny pocket of gas. This is why you need raisins, or something else that’s small, wrinkly, and just a tiny bit more dense than water. Gas floods out of the lemonade into those proto-bubbles, and each raisin grows itself a bubbly lifejacket that stays stuck to the raisin. By themselves, raisins are more dense than water, so gravity drags them down to the bottom. But after they’ve grown a few bubbles they become less dense overall, and they begin their journey up to the top. Once they get there, the bubbles that break the surface pop, and you can see the raisins tip over as the bubbles underneath lift themselves up and pop in turn. Once there is no lifejacket left, the raisin is more dense than the lemonade, so down to the bottom it sinks. This will keep going until all the excess carbon dioxide gas has come out of the lemonade.

After half an hour as the table’s centerpiece, the frantic dance of the raisins had been reduced to an occasional leisurely excursion to the surface, and the lemonade had turned an off-putting yellowish color. Beautiful buoyant exuberance had been transformed into what looked like a giant urine sample bottle with dead flies at the bottom.

Try it. It’s a good way of cheering up a slightly dull party if you can find raisins or currants in the party snacks. The key is that the bubbles and the raisin become a single object and move about as one. If you puff the raisin up with portable air pockets, you barely make it any heavier, but the whole thing takes up a lot more space. The ratio of “stuff” to space filled is density, so the raisin-plus-bubble combination is less dense than the raisin alone. Gravity can only pull on “stuff,” so things that are less dense feel less of a pull toward the Earth. This is why things can float—floating is just a gravitational hierarchy sorting itself out. Gravity pulls dense liquids downward, and any less dense object in the liquid is relegated to floating about on top. We say that anything that’s less dense than a liquid is buoyant.

Air-filled spaces are really useful for controlling relative density, and therefore buoyancy. Famously, one of the design features that was supposed to make the Titanic “unsinkable” was the large watertight compartments that took up the lower part of the ship. They were acting like the bubbles on the raisin: air-filled pockets that made the ship more buoyant and kept it afloat. When the Titanic got into trouble, those compartments turned out not to be watertight, and as they filled with water, the effect was the same as the last few bubbles popping at the surface. Just like the raisin without its lifejacket, the Titanic had to sink down to the depths.*

We accept that things sink and float, but rarely think about the real cause: gravity. The theater of our lives is played out on a stage dominated by this one ever-present force, always making it very clear which way is “down.” It’s fantastically useful—it keeps everything organized by keeping it on the floor, for a start. But it’s also the most obvious single force to play with. Forces are weird—you can’t see them and it can be hard to know what they’re up to. But gravity is always there, with the same strength (at the Earth’s surface, at least), and pointing in the same direction. If you want to play with forces, gravity is a great place to begin. And how better to begin playing than by falling?

Springboard and highboard diving sit somewhere on the scale between utter freedom and complete madness. The moment you leave the board, you are completely free of the feeling of gravity. It’s not that it’s gone away, it’s just that you’re giving in to it completely, so there’s nothing left to push against. You can rotate just like a theoretical free body, as if you were floating in space, and it’s incredibly liberating. But there is no such thing as a free lunch, and the problem comes a second or so later, when you arrive at the water’s surface. There are two ways of dealing with it. You can either make a small tunnel into the water with your hands or feet, and organize yourself so that the rest of your body slides gracefully into the tunnel, minimizing the splash. Or you can let your arms and legs, tummy or back each make their own impact, generating a very large splash. That second one hurts.

I was a springboard diver and coach for a few years in my twenties, but I hated highboard diving. The springboards are the bouncy ones, 1 and 3 meters above the pool. It’s a bit like trampolining, but with a softer landing. Highboards are the solid platforms above that, at 5, 7.5, and 10 meters. The pool I trained at only had a 5-meter platform, but I did everything I could to avoid it.

Up on the 5-meter platform, the water looks a long way away. There was always a thin stream of bubbles coming from below, so that you could see where the water surface was even if the pool was completely still. The most basic warm-up dive is a “front fall”: exactly what it says on the can. Standing on the end of the board, you lean forward into an “L” shape with your arms locked above your head, keeping your body straight apart from the bend at the hips. Things look a tiny bit less terrifying from here, because your head is closer to the water, but not much. Then you lift yourself up on tiptoe, and surrender. Suddenly, you are free. There is just you and a planet with a mass of 13 million billion billion pounds, linked only by this thing called gravity, and the laws of the universe mean that you are pulling on each other.