Storm in a Teacup: The Physics of Everyday Life(31)

There is a way around this problem, but considering how much time the British spend eating fries with ketchup on top, it’s surprisingly rare to see it. The tactic of turning the bottle upside down and thumping it on the bottom doesn’t help much, because the ketchup that is forced to become liquid is all up near where you’re thumping. The neck of the bottle is still blocked by thick gloop that isn’t going anywhere. The solution is to make the ketchup in the neck liquid, so the thing to do is to hold the bottle at an angle and tap the neck. The amount that will come out is limited, because only the ketchup there is liquid. Surrounding diners will be saved from your elbows (and a potential ketchup spray), and the fries will be saved from drowning.

Time is important in the physical world, because the speed at which things happen matters. If you do something at twice the speed, sometimes you get the same result in half the time. But quite often you get a completely different result. This is pretty useful, and we use it to control our world in all sorts of ways. There’s also a lot of time to play with, in the sense that there are a lot of different timescales on which things can happen. Time matters for coffee and pigeons and tall buildings, and the timescale that matters is different for each of them. This isn’t just about tweaking the mundane things in our lives for the sake of convenience. It turns out that life is only possible at all because the physical world never really catches up with itself. But let’s begin at the beginning. We’ll start with a creature that famously never catches up with anything, the mascot for those who are always last.


ONE SUNNY DAY in Cambridge, I finally had to admit that I had been defeated by a snail.

It’s not traditional to take up gardening during your final year as an undergraduate, but the house I was sharing with three friends had a garden, and the temptation was too much. In the odd spare hours between work and sport that year, I enthusiastically chopped down the huge forest of nettles that had taken over, and discovered buried treasure in the form of rhubarb plants and rose bushes. My dad laughed at me for planting potatoes (“typical Pole,” he said), but they were only part of my new vegetable patch. Most exciting of all, there was a grimy greenhouse, filled with rubble and a grapevine. Seedlings (leeks and beetroot, I think) could grow in shelter before joining the vegetable patch in the spring. In late February I sowed seeds in trays, and waited for new plants to grow.

After a while, it was noticeable that there weren’t any seedlings, but there were a lot of snails. I’d arrive with my watering can to find a smug mollusk parked in the middle of each tray, surrounded by bare soil and the occasional green hint of a chewed-off shoot. Not to be defeated, I chucked the snails outside, resowed the seeds, and placed the trays on top of bricks to make it harder for the snails to crawl in. Two weeks later, the nascent seedlings were gone, and there were more snails than ever. I tried a few different approaches, none successful, until I had only one idea left. This time, I took pairs of empty flower pots and balanced upside-down tea trays on top, so they were like giant mushrooms with two stalks. I greased the edges of each one, and put the seedling trays on top of the tea-tray mushrooms. After replacing the compost, I sowed the last of the seeds, crossed my fingers, and went back to studying condensed matter physics.

The seedlings grew undisturbed for about three weeks. And then the inevitable day came when I found one fat, happy snail where the seedlings should have been. I remember standing in the greenhouse, analyzing in forensic detail the possible routes that this creature could have taken. There were only two. Option one: It could have crawled up the inside walls of the greenhouse and out across the underside of the roof, and then somehow have dropped off at exactly the right location to land on a seed tray. This seemed unlikely. Option two: It had crawled along the bench and up the flower-pot sides, slimed its way upside down to the outer edge of the tea tray, crawled around the edge without falling off, and then trekked along the top of the tea tray to the seedlings. In either case, I had to admit that it had probably earned the bounty.? How could a snail do that? Both cases involved it crawling upside down, while glued to the surface only by its own mucus. If you watch a snail move, it’s different from a caterpillar—it doesn’t lift itself off the surface as it goes. It’s just stuck to its slime, and yet somehow it manages to shift itself along. But that slime is the snail’s secret weapon, because it behaves just like ketchup.

If you watch a snail moving, you won’t see very much because the outer rim of its foot is just moving at a constant slow speed. Everything on the edges is happening slowly, so the mucus is like the stationary ketchup: thick and gloopy and hard to move. But underneath, in the middle, muscular waves are traveling from the back of the snail to the front. Each wave is pushing forward on the mucus really hard, and it’s forcing the mucus to shift very quickly. And just like the ketchup, the mucus is shear-thinning, so if you’re shunting it very fast it suddenly flows very easily. The snail is sailing over the top of this liquid mucus on those muscular waves, taking advantage of the lower resistance. It needs the thick slime as well, so that it has something to push against. The only reason why snails (and slugs) can move is that the same mucus can behave either like a solid or a liquid, depending on how fast they force it to move. The huge advantage of this method is that they don’t fall off the underside of things because they never lift themselves away from the surface.

How does the slime manage this trick? It’s a gel of very long molecules called glycoproteins, all mixed up together. When it’s just sitting still, chemical links form between the chains, so it behaves like a solid. But when you push hard enough, the links suddenly break, and all the long molecules can slither over each other like strands of spaghetti. Let it sit still again, and the links will re-form; and after only a second, you’ll have a gel again.