Storm in a Teacup: The Physics of Everyday Life(21)

* By coincidence, the distance that the Titanic sank relative to its size (14 times its length) is pretty much the same as the distance that the raisins sink in a 2-liter bottle (a large raisin is about ? inch long, and the bottle is about 12 inches deep). The Titanic was 883 feet long, and sank in water that was 12,415 feet deep.

? It’s often written: Force = mass × acceleration, or F = ma.

? If you’ve ever wondered what General Relativity is really about, the core of it is just this realization. If you’re in a closed elevator, whether you’re standing, playing catch, or doing sit-ups, you can’t tell which forces are due to “gravity” and which are because the elevator is accelerating. Einstein realized that there is a way of looking at what matter does to space which shows that these forces are indistinguishable because they’re actually exactly the same thing.

§ Yes, I know the story is apocryphal; but the fact is still true!

? Angular momentum, for the purists.

# Subsequently, she crossed with her hands and feet manacled, and also blindfolded.

** When these swim bladders evolved, they provided a huge evolutionary advantage by reducing the energy needed to stay at the same depth. But in recent years they have become a significant disadvantage, because those swim bladders are very easily detectable using acoustics. One of the major technologies that has enabled the vast overfishing of our seas is the “fish-finder,” an acoustic device that is tuned to spot air bubbles and so, by implication, fish. Whole shoals can be chased and wiped out, just because their bubble of air gives them away.

?? In 1826 Michael Faraday, the famous nineteenth-century experimentalist credited with many practical scientific discoveries, founded a series of talks at the Royal Institution in London, aimed at children, that still continues today—the RI Christmas Lectures. Among his own contributions was a series of six lectures called “The Chemical History of a Candle,” in which he discussed the science of candles, illustrating many important scientific principles that had other applications in the world. I bet he would have been astonished to hear about the nanodiamonds, and probably delighted that the simple candle was still yielding up surprises.

?? The cruising altitude of a commercial aircraft is about 33,000 feet, and the Challenger Deep, the deepest part of the Marianas Trench, is 36,070 feet deep.

§§ And also close to the coast of Antarctica.





CHAPTER 3




Small Is Beautiful


COFFEE IS A fantastically valuable global commodity, and the precise black magic needed to extract perfection from this humble bean is a constant source of debate (and some snobbishness) for connoisseurs. But my particular interest in it doesn’t depend on how it was roasted or the pressure in your espresso machine. I’m fascinated by what happens when you spill it.* It’s one of those everyday oddities that no one ever questions. A coffee puddle on a hard surface is unremarkable, just a patch of liquid in a blobby shape. But if you leave it to dry, you’ll come back to find a brown outline, reminiscent of the line drawn around the body in a detective drama from the 1970s. It was definitely filled in to start with, but during the drying process all the coffee has moved to the outside. Scrutinizing a coffee puddle to see what’s going on is the caffeine waster’s equivalent of watching paint dry, but even if you tried, you wouldn’t see very much. The physics shunting the coffee around only operates on very small scales, mostly too small for us to see directly. But we can definitely see their consequences.

If you could zoom in on the puddle, you’d see a pool of water molecules playing bumper cars, and much bigger spherical brown particles of coffee drifting around in the middle of the game. The water molecules attract each other very strongly and so if a single molecule lifts up a bit from the surface, it immediately gets pulled back down to join the horde below. This means that the water surface behaves a bit like an elastic sheet, pulling inward on the water below it so that the surface is always smooth. This apparent elasticity of the surface is known as surface tension (of which much more a little later). At the edges of the puddle, the water surface curves downward smoothly to meet the table, holding the puddle in place. But the room is probably warm, and every so often, a water molecule escapes from the surface completely and floats off into the air as water vapor. This is evaporation, and it happens gradually, and only to the water molecules. The coffee can’t evaporate, so it’s effectively trapped in the puddle.

The clever bit happens as more and more water escapes, because the water edge is pinned to the table (we’ll see why a bit later). Water is so strongly stuck to the table that the edge has to stay where it is. But evaporation is happening at the edges more quickly than from the middle, because a higher proportion of the water molecules are exposed to the air there. The bit that you can’t see (as you try to persuade your coffee companion that watching paint dry really is the latest thing) is that the contents of the puddle are on the move. Liquid coffee from the middle must flow out to the edges to replace the lost water. The water molecules carry the coffee particles along as passengers, but when it’s their turn to escape into the air, the coffee can’t join them. So the coffee particles are gradually carried out to the edges, and once the water has completely gone, all that is left is a ring of abandoned coffee.

The reason I find this so fascinating is that it happens right in front of your nose, but all the interesting bits are just too small to watch properly. This world of the small is almost a whole different place. The rules that matter are different down there. As we’ll see, the forces we’re used to, like gravity, are still present. But other forces, the ones that arise because of the way molecules dance around each other, start to matter more. When you dive down into the world of the small, things can seem very weird. It turns out that the rules that operate on this small scale explain all sorts of things in our larger-scale world: why there’s no cream on the milk anymore, why mirrors fog up, and how trees drink. But we’re also learning to use those rules to engineer our world, and we’ll see how they’re going to help us save millions of lives through improved hospital design and new medical tests.