*** One nanometer really is tiny—there are 25.4 million of them in one inch.
??? But there’s a limit. To increase the tension in the water to pull it up farther, the stomata must get smaller. And smaller stomata let in less carbon dioxide, so there is less raw material for photosynthesis. Theory suggests that the tallest a tree could possibly grow is 400–425 feet, because beyond that it wouldn’t be able to take in enough carbon dioxide to do any actual growing.
??? There’s also some evidence to suggest that the fog might actually go the other way too—entering the stomata to keep them full of water, not just preventing evaporation.
§§§ These devices go by the catchy name of “microfluidic paper-based electrochemical devices” or uPADs for short. A non-profit organization called Diagnostics for All has been set up to move this idea into the real world.
CHAPTER 4
A Moment in Time
ON A LAZY Sunday at lunchtime, an English pub is the place to be. The innards of these establishments often give the impression of having been grown rather than designed—a cluster of oddly-shaped spaces hidden in an ancient oak skeleton. You park yourself at a table positioned between polished brass bedpans and pictures of prize pigs, and order a proper pub lunch. It always arrives with a bowl of fries and a glass bottle of ketchup, but this combination comes at a price. For decades, these oak beams have witnessed an enduring ritual. The ketchup must be extracted from the bottle, and it won’t happen without a fight.
It starts when one optimist picks up the ketchup and just holds it upside down over the bowl of fries. Nothing ever happens, but almost no one skips this step. Ketchup is thick, viscous stuff, and the feeble pull of gravity isn’t enough to shift it from the bottle. It’s made this way for two reasons. The first is that the viscosity stops the spices sinking downward if the bottle is abandoned for a while, so you don’t ever need to shake it to make sure it’s well mixed. But more importantly, people prefer a nice thick coating on each French fry and you can’t get that if the ketchup is runny. However, it’s not on the fry yet. It’s still in the bottle.
After a few seconds, having established that this bottle of ketchup is just as immune to gravity as every other one they’ve ever encountered, the hopeful fry-eater starts to shake the bottle. The shaking gets gradually more violent, until it’s time to try thumping the bottom of the bottle with the free hand. Just as the others at the table are starting to lean back to avoid the fracas, a quarter of the contents of the bottle whooshes out all at once. What’s weird is that the ketchup clearly can flow very easily and quickly—the thick blanket of ketchup now covering the bowl (and probably half the table too) provides ample evidence of that. It just doesn’t, until it does, and then it flows with considerable enthusiasm. What’s going on?
The thing about ketchup is that if you try pushing on it very slowly, it behaves almost like a solid. But once you force it to move quickly, it behaves much more like a liquid and flows very easily. When it’s sitting in the bottle or perched on a fry, it’s only being pulled weakly by gravity, so it behaves like a solid and stays put. But if you shake it hard enough and start it moving, it behaves like a liquid and moves very quickly. It’s all about time. Doing the same thing quickly and slowly gives you completely different results.
Ketchup is mostly sieved tomatoes, jazzed up by vinegar and spices. Left to itself, it’s thin and watery, and doesn’t do anything interesting. But lurking in the bottle is 0.5 per cent of something else, long molecules made up of a chain of linked sugars. This is xanthan gum: Originally grown by bacteria, it’s now a very common food additive. When the bottle is standing on the table, these long molecules have surrounded themselves with water and are slightly tangled up with other similar chains. They hold the ketchup in place. As our ketchup enthusiast shakes the bottle harder, these long molecules get slightly untangled, but they re-tangle pretty quickly. As the thumping on the bottom of the bottle shunts the ketchup around more quickly, the tangles keep breaking, and at some point they’re pushed out of place more quickly than they’re re-tangling. Once this critical point is passed, the solid-like behavior vanishes and the ketchup is on its way out of the bottle.*
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.