I have had peanut butter, honey, pesto sauce, toothpaste and, most painfully, a bottle of single-malt whisky all confiscated at airport security. I inevitably lose the plot in situations like this. I say things like ‘I want to see your supervisor’, or ‘peanut butter is not a liquid’, even though I know it is. Peanut butter flows and assumes the shape of its container – that is what liquids do – and so peanut butter is one. Even so, it just infuriates me that in a world full of ‘smart’ technology airport security still can’t tell the difference between a liquid spread and a liquid explosive.
Although bringing more than 100 ml of liquid through security has been banned since 2006, our detection technology has not improved much in that time. X- ray machines can see through your luggage to detect objects. In particular they alert security to suspicious shapes: distinguishing guns from hairdryers, and knives from pens. But liquids don’t have a shape. They just take the form of whatever is containing them.
Read more from the Royal Society Science Book Prize:
- The Immunotherapy Revolution
- A fond farewell to the humble kilo
- Can an algorithm deliver justice?
- What we got wrong about pandas and teenagers [Science Focus Podcast]
Airport scanning technology can also detect density and a range of chemical elements. But here again they run into problems. The molecular make-up of the explosive nitroglycerine, for instance, is similar to peanut butter’s. They’re both made from carbon, hydrogen, nitrogen and oxygen – and yet one is a liquid explosive while the other is just, well, delicious. There are an enormous number of dangerous toxins, poisons, bleaches and pathogens that are incredibly difficult to distinguish from more innocent liquids in a quick and reliable manner. And it is this line of argument, which I’ve heard from many security guards (and their supervisors), that usually persuades me to agree – begrudgingly – that my peanut butter, or one of the other liquids I seem to regularly forget to take out of my carry-on luggage, is a significant risk.
Liquids are the alter ego of dependable solid stuff. While solid materials are humanity’s faithful friend, taking on the permanent shapes of clothes, shoes, phones, cars and indeed airports, liquids are fluid; they will take on any shape, but only while contained. When they’re not contained, they are always on the move, seeping, corroding, dripping and escaping our control.
If you put a solid material somewhere it stays there – barring forcible removal – often doing something very useful, like holding up a building or supplying electricity for a whole community. Liquids, on the other hand, are anarchic: they have a knack of destroying things. In a bathroom, for instance, it is a constant battle to keep water from seeping into cracks and pooling under the floor, where it gets up to no good, rotting and undermining the wooden joists; on a smooth tile floor, water is the perfect slip hazard, causing an enormous number of injuries; and when it gathers in the corners of the bathroom, it can harbour black slimy fungus and bacteria, which threaten to infiltrate our bodies and make us sick.
And yet, despite all this treachery, we love the stuff; we love bathing in water, and showering in it, drenching our whole body. And what bathroom is complete without a cornucopia of bottled liquid soaps, shampoos and conditioners, jars of cream and tubes of toothpaste? We delight in these miraculous liquids and yet we worry about them too: are they bad for us? Do they cause cancer? Do they ruin the environment? With liquids, delight and suspicion go hand in hand. They are duplicitous by nature, neither a gas nor a solid but something in- between, something inscrutable and mysterious.
Liquid: The Delightful and Dangerous Substances That Flow Through Our Lives by Mark Miodownik is out now (£16.99, Viking)
The Royal Society Insight Investment Science Book Prize celebrates the best of science writing for a non-specialist audience. The winner of the 2018 Prize will be announced on 1 October 2018.