A car is a heavy object, so gravity makes it push down hard on the ground it’s sitting on. Cars don’t sink through the ground because the ground is solid enough to resist that push. But for a few minutes in Christchurch, that general rule was broken. Many cars that day were parked on sandy roadsides, resting on packed soil that hadn’t moved for decades. As the earthquake shook the ground, the layers of sand were forced to slide over each other from side to side extremely quickly. If this had happened slowly, the cars would have been safe. But it happened so quickly that water crept in between the sand grains and the sand grains didn’t have time to settle back into place before they were forced back in a different direction. So instead of sand resting on sand, the ground was suddenly made of a mixture of sand and water that had no fixed structure. Any car sitting on top of this mixture would sink downward into the mush as the shaking continued. But as soon as the shaking stopped, it only took a second or so for the sand grains to settle slightly, so that they were supported by other sand grains instead of water. The ground had resolidified, but by now, the car was half-buried.
This process was responsible for a lot of the damage in Christchurch. Cars sank into the silt and buildings fell because the ground couldn’t hold them up. This process is known as “liquefaction,” and it takes something as powerful as an earthquake to move the sediment fast enough to cause it. But if you move soft, sandy ground fast enough, its strength will vanish. This is also why flailing about in quicksand is such a terrible idea. If you fight and struggle, the quicksand becomes liquid-like and you’ll just sink in. Move slowly, and you stand a chance of controlling where you are. Time matters. When you change the timescale of what you’re doing, you often change the outcome.
We like to say that something was so fast “it happened in the blink of an eye.” A blink takes about a third of a second, and the average reaction time of a human is around a quarter of a second. That sounds pretty fast, but just think about what has to happen in that time if you’re taking a standard reaction test. When light rays hit your retina, specialized detection molecules twist around, and this starts a chain of chemical reactions that cause a small electric current. This signal travels through the optic nerve into the brain, stimulating brain cells to send signals to each other as they work out that this is something that requires a reaction. Then electrical signals travel out to the muscles, slowed down when they are ferried across the gaps between nerve cells by chemical diffusion. Once the order to contract has been received, molecules in the muscle fiber ratchet over each other until your hand hits the button. All that, just for you to do the fastest thing you can possibly do.
Our fabulous complexity comes at the cost of speed. I think of humans as pretty slow beasts, lumbering through the physical world, because so many different stages are involved in everything we do. While we plod through all that, many simpler physical systems are just getting on with things, lots of things. But those simple, quick processes are too fast for us to see. You can get a hint of this world if you drop a single drip of milk into your coffee from quite high up. You might just see the drop bounce right back out before falling back into the drink. It’s right on the edge of the fastest we can see. My PhD supervisor used to say that if you were quick, you could change your mind about having the milk and catch it on its way out, but I’m pretty sure you’d need the help of something smaller and faster than a human to do it.
The thought of how much we’re missing because we’re slow is what inspired my PhD. I was fascinated by the idea of a world that could be doing things right in front of my eyes, things that were too small and too fast to see. So I chose a PhD that let me play with high-speed photography, technology that let me see the parts of the world that are normally invisible because they are so fast. But cameras like that are only available to humans. What do you do if you have the same problem, but you’re a pigeon?
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.