Storm in a Teacup: The Physics of Everyday Life
Helen Czerski
INTRODUCTION
WE LIVE ON the edge, perched on the boundary between planet Earth and the rest of the universe. On a clear night, anyone can admire the vast legions of bright stars, familiar and permanent, landmarks unique to our place in the cosmos. Every human civilization has seen the stars, but no one has touched them. Our home here on Earth is the opposite: messy, changeable, bursting with novelty and full of things that we touch and tweak every day. This is the place to look if you’re interested in what makes the universe tick. The physical world is full of startling variety, caused by the same principles and the same atoms combining in different ways to produce a rich bounty of outcomes. But this diversity isn’t random. Our world is full of patterns.
If you pour milk into your tea and give it a quick stir, you’ll see a swirl, a spiral of two fluids circling each other while barely touching. In your teacup, the spiral lasts just a few seconds before the two liquids mix completely. But it was there for long enough to be seen, a brief reminder that liquids mix in beautiful swirling patterns and not by merging instantaneously. The same pattern can be seen in other places too, for the same reason. If you look down on the Earth from space, you will often see very similar swirls in the clouds, made where warm air and cold air waltz around each other instead of mixing directly. In Britain, these swirls come rolling across the Atlantic from the west on a regular basis, causing our notoriously changeable weather. They form at the boundary between cold polar air to the north and warm tropical air to the south. The cool and warm air chase each other around in circles, and you can see the pattern clearly on satellite images. We know these swirls as depressions or cyclones, and we experience rapid changes between wind, rain, and sunshine as the arms of the spiral spin past.
A rotating storm might seem to have very little in common with a stirred mug of tea, but the similarity in the patterns is more than coincidence. It’s a clue that hints at something more fundamental. Hidden beneath both is a systematic basis for all such formations, one discovered and explored and tested by rigorous experiments carried out by generations of humans. This process of discovery is science: the continual refinement and testing of our understanding, alongside the digging that reveals even more to be understood.
Sometimes a pattern is easy to spot in new places. But sometimes the connection goes a little bit deeper and so it’s all the more satisfying when it finally emerges. For example, you might not think that scorpions and cyclists have much in common. But they both use the same scientific trick to survive, although in opposite ways.
A moonless night in the North American desert is cold and quiet. Finding anything out here seems close to impossible, since the ground is lit only by dim starlight. But to find one particular treasure, you equip yourself with a special flashlight and set out into the darkness. The flashlight needs to be one that produces light that is invisible to our species: ultraviolet light, or “black light.” As the beam roams across the ground, it’s impossible to tell exactly where it’s pointing because it’s invisible. Then there’s a flash, and the darkness of the desert is punctured by a surprised scuttling patch of eerie bright blue-green. It’s a scorpion.
This is how enthusiasts find scorpions. These black arachnids have pigments in their exoskeleton that take in ultraviolet light that we can’t see and give back visible light that we can see. It’s a really clever technique, although if you’re scared of scorpions to start with, your appreciation might be a little muted. The name for this trick of the light is fluorescence. The blue-green scorpion glow is thought to be an adaptation to help the scorpions find the best hiding places at dusk. Ultraviolet light is around all the time, but at dusk, when the sun has just slipped below the horizon, most of the visible light has gone and only the ultraviolet is left. So if the scorpion is out in the open, it will glow and be easy to spot because there isn’t much other blue or green light around. If the scorpion is even slightly exposed, it can detect its own glow and so it knows it needs to do a better job of hiding. It’s an elegant and effective signaling system—or was until the humans bearing ultraviolet torches turned up.
Fortunately for the arachnophobes, you don’t need to be in a scorpion-populated desert at night to see fluorescence—it’s pretty common on a dull morning in the city as well. Look again at those safety-conscious cyclists: their high-visibility jackets seem oddly bright compared with the surroundings. It looks as though they’re glowing, and that’s because they are. On cloudy days, the clouds block the visible light, but lots of ultraviolet still gets through. The pigments in the high-visibility jackets are taking in the ultraviolet and giving back visible light. It’s exactly the same trick the scorpions are playing, but for the opposite reason. The cyclists want to glow; if they’re emitting that extra light, they’re easier to see and so safer. This sort of fluorescence is pretty much a free lunch for humans; we’re not aware of the ultraviolet light in the first place, so we don’t lose anything when it gets turned into something we can use.
It’s fascinating that it happens at all, but the real joy for me is that a nugget of physics like that isn’t just an interesting fact: It’s a tool that you carry with you. It can be useful anywhere. In this case, the same bit of physics helps both scorpions and cyclists survive. It also makes tonic water glow under ultraviolet light, because the quinine in it is fluorescent. And it’s how laundry brighteners and highlighter pens work their magic. Next time you look at a highlighted paragraph, bear in mind that the highlighter ink is also acting as an ultraviolet detector; even though you can’t see the ultraviolet directly, you know it’s there because of this glow.