Storm in a Teacup: The Physics of Everyday Life(33)

In 1977, an enterprising scientist named Barrie Frost persuaded a pigeon to walk on a treadmill. This is one of those experiments that would probably win an IgNobel prize these days, as the perfect example of a piece of science that makes you laugh and then makes you think. As the treadmill belt slowly moved backward, the bird had to walk forward to stay in the same place. The pigeon apparently got the hang of it quite quickly, but something was missing as it plodded along. If you’ve ever sat in a town square and watched pigeons strut around in search of food, you’ll have noticed that their heads bob backward and forward as they walk. I’ve always thought it looks like a really uncomfortable thing to do, and it seems odd to put in all that extra effort. But the pigeon on the treadmill wasn’t bobbing its head, and that told Barrie something very important about the bobbing. The bird obviously didn’t need to do it in order to walk, so it wasn’t anything to do with the physics of locomotion. The head-bobbing was about what it could see. On the treadmill, even though the pigeon was walking, the surroundings stayed in the same place. If the pigeon held its head still, it saw exactly the same view all the time. That made the surroundings nice and easy to see. But when a pigeon is walking on land, the scenery is constantly changing as it goes past. It turns out that these birds can’t see “fast” enough to catch the changing scene. So they’re not really bobbing their heads forward and backward at all. A pigeon thrusts its head forward, and then takes a step that lets its body catch up, and then thrusts its head forward again. The head stays in the same position throughout the step, so the pigeon has more time to analyze this scene before moving on to the next one. It gets one snapshot of its surroundings, and then it jerks its head forward to get the next snapshot. If you spend a while watching a pigeon, you can convince yourself of this (although it takes a bit of patience, because they are usually quite quick).? No one seems to know exactly why some birds are so slow at gathering visual information that they need to bob their heads, and others aren’t. But the slower ones can’t keep up with their world without breaking it down into a freeze-frame movie.

Our eyes can keep up with our walking pace, but if you need to examine something close to you while you’re walking or running, you usually feel an overwhelming urge to stop for a bit to have a good look. Your eyes can’t collect information fast enough to get all the detail while you’re moving. Humans actually play exactly the same game as the pigeon (without the head-bobbing), and our brain stitches things together so that we’d never know. Our eyes dart rapidly from place to place, adding information to our mental image on each stop. If you look at yourself in a mirror and look directly at the reflection of one of your eyes and then the other, you will notice that you never see your eyes move, even though someone standing next to you will see them flick from one side to the other. Your brain has stitched your perception of the scene together in such a way that you’d never know there was a jump; but those jumps are happening all the time.

The point is that we’re only a tiny bit faster than the pigeon, and this highlights how much there must be that’s faster than us. We are used to life at a limited range of timescales—we can follow things that last from about a second to a few years—but that’s not all there is. Without science to help us, we are blind to anything happening over a few milliseconds or over a few millennia. We can only perceive our bit in the middle. That’s why computers can do so much and part of why they seem so mysterious. They can do what they need to do in tiny amounts of time, so they can get on with it and finish amazingly complex tasks before we perceive any time passing. Computers continue to get faster, but we can’t perceive why, because both a millionth and a billionth of a second are the same to us: too fast to notice. But that doesn’t mean the distinction isn’t significant.

What you see depends on the timescale on which you are looking. To grasp the contrast, let’s compare the speedy and the ponderous: a raindrop and a mountain.

A large raindrop takes one second to fall 20 feet, the height of a two-story building. What happens to it during that second? This raindrop is a jostling cluster of water molecules, each one held firmly in the grip of the group, but constantly shifting its allegiances within that group. A water molecule, as we saw in the last chapter, consists of an oxygen atom accompanied by two hydrogen atoms on either side, the trio forming a “V” shape. The whole molecule can bend and stretch as it hops through the loose network formed by billions of identical others. In that one second, this molecule may hop 200 billion times. If our molecule reaches the edge of the multitude it will find that there’s nothing outside the droplet that can compete with the huge attraction of the masses, so it’s always pulled back to the center. The cartoon raindrop shape is a fiction: Raindrops have lots of shapes but none of them have sharp points. Any pointed edges will be rapidly smoothed away, because individual molecules can’t resist the pull of the mob. But despite the strength of that pull, the perfect shape is never reached. There is constant readjustment in response to the buffeting of the air. A drop may be squashed flat, but will then pull itself back together, overshoot, become stretched into a rugby ball shape and then back again, 170 times in this one second. The globule is constantly wobbling and reinventing itself, a battleground between the external forces trying to tear it apart and the fierce pull of the mob keeping it together. Sometimes a raindrop flattens into a pancake, then stretches into a thin umbrella, and then explodes into an army of tiny droplets. All of this happens in less than a second. We can’t see any of it, but that droplet has transformed itself a billion times in the blink of an eye. Then the droplet splats down on to bare rock, and the timescales shift.