Storm in a Teacup: The Physics of Everyday Life(50)

CHAPTER 6




Why Don’t Ducks

Get Cold Feet?


SALT IS OFTEN considered a mundane commodity, stored away in cupboards, never the center of attention. But if you look a little bit more closely at a handful of grains of salt, especially in bright light, you’ll notice that it’s surprisingly sparkly. And it gets better as you get closer. Peer at it through a magnifying glass, and you’ll see that the grains aren’t randomly shaped, or lumpy, or rough. Each one is a beautiful little cube with very flat sides, perhaps 0.02 inch across. This is why it sparkles; light is reflecting off those flat faces as though they were tiny mirrors, and different salt grains are glinting at you as you turn the pile in the light. The boring stuff in the salt cellar is made up of minuscule sculptures, each with the same precise shape. Salt manufacturers don’t do this deliberately—it’s just how salt forms. And it gives us a clue about what “stuff” is made of.

Salt is sodium chloride, and it’s made of equal numbers of sodium and chloride ions.* You can think of them as balls of different sizes—the chloride has almost twice the diameter of the sodium. When salt is forming, each one of its components has a fixed place in a very specific structure. Like eggs in a stack of giant egg cartons, the chloride ions assemble in rows and columns, so that they sit on a square grid. The smaller sodium ions fit into the spaces in between, so that each little box of eight chloride ions has a sodium ion in the middle. A salt crystal is just a giant grid like this, a cube that’s a million or so atoms long on each side. When the salt crystals grow, they tend to grow a new layer across an entire flat face before they start on the next layer, so the cube keeps its flat sides as it grows. It’s atomic-scale filing, each component stacked up perfectly in its place. And the flat sides of each cube can reflect light like a mirror.

We can’t see the individual atoms, but we can see the pattern of their structure because the whole salt crystal is just that same pattern repeated again and again. Salt is very simple, and a bigger salt grain is just more of the same. The flat faces that make the salt sparkle are there because individual atoms have to sit in specific places on a rigid lattice.

Sugar also sparkles, but when you look more closely at sugar crystals (especially the larger ones, like those in granulated sugar), you’ll see something even more beautiful. These crystals are six-sided pillars with pointy ends. Each sugar molecule is made up of forty-five different atoms, but those atoms are held together in a fixed way that is the same in each individual molecule. One sugar molecule is a brick in a crystalline sculpture, even though it’s a brick with quite a complicated shape. Like the simpler salt crystals, these too stack up on top of each other in a regular lattice, and there’s only one pattern for them to follow. Once again, we can’t see the atoms, but we can see the pattern because the whole crystal is just a giant stack, a skyscraper of molecules. Since the six-sided pillars have flat sides that can act as mirrors, sugar sparkles just like salt.

Flour and rice and ground spices don’t sparkle, because they have a much more complicated structure—they’re made from the tiny living factories that we call cells. The only reason sugar and salt crystals have perfectly flat sides is that they have such a simple structure: just rows and columns of atoms slotted into specific positions. And that perfect repetition is only possible because at the bottom of it all there are billions of tiny identical building blocks: atoms. The sparkling is a reminder of their existence every time you put a spoonful of sugar in your tea.

Even though we can’t see the atoms themselves, we can see the consequences of what’s happening down there in the world of the tiny. The goings-on at the bottom of the size scale directly affect what we can do at the largest scales in our society. But first, you have to believe that atoms exist.

These days, we take the existence of atoms for granted. The concept of everything being built of minuscule balls of matter is relatively simple and it makes sense to us because we’ve all grown up with it. But you only have to go back as far as 1900 to find serious debate in the scientific community about whether atoms were there at all. Photography, telephones, and radio had arrived to herald a new technological age, but there was still no agreement on what “stuff” was actually made of. To a lot of scientists, atoms seemed like a reasonable idea. For example, chemists had discovered that different elements seemed to react in fixed ratios, which made perfect sense if you needed one atom of one sort plus two atoms of another sort to make a single molecule. But the skeptics held out. How could you be sure about whether something so tiny was really there?

Many decades later, a quote was attributed to the scientist and science-fiction writer Isaac Asimov that expresses perfectly the most common path of a scientific discovery: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, ‘Hmm . . . that’s funny . . .’” The final confirmation of the existence of atoms is a perfect example of science taking that route, but it was nearly eighty years in coming. The clock started ticking in 1827, when the botanist Robert Brown was looking through a microscope at pollen grains suspended in water. Tiny particles were breaking off the pollen, and they were pretty much the smallest things that could be seen by an optical microscope, then or now. Robert Brown noticed that even when the water was perfectly still, those tiny particles were jiggling about. At first, he assumed that it was because the particles were alive, but he later observed the same thing with non-living matter. It was weird, and he didn’t have any explanation for it. But he wrote about it, and over the following decades many other people saw the same thing. The weird jiggling became known as “Brownian motion.” It never stopped, and it was only the tiniest particles that jiggled. Various people proposed explanations, but no one really got to the bottom of the mystery.