杏吧原创

MOST people don鈥檛 get confused until after they have finished their beer. For physicists, however, the problems begin before they have even taken a sip.

Just ask Arnd Leike, a particle physicist at Ludwig Maximilians University in Munich. Last year, in what is probably the best experiment in the world, Leike took a careful look at the foaming head on top of his glass of beer. This wasn鈥檛 just a bit of fun: the idea was that by measuring how quickly the foam fades away, his students would learn how to analyse scientific data. But it appears to have reached the parts of physics that other experiments can鈥檛 reach: Leike and his students unwittingly uncovered a disturbing puzzle.

We already know that things like forest fires, Alzheimer鈥檚 disease and the stock market are underpinned by a single universal rule. Physicists call it a power law 鈥� a relationship between two quantities such that one is proportional to a fixed power of the other. Power laws are held in high regard because they can describe all sorts of complicated phenomena, from the behaviour of neutron stars to the way traffic jams form. So you might expect that fading froth would behave the same, right? Wrong. Beer foam, according to Leike鈥檚 experiment, lives by its own rules.

It鈥檚 an experiment that earned him this year鈥檚 Ig Nobel Prize for physics. The Ig Nobel prizes are awarded for scientific achievements that cannot or should not be reproduced. But at New 杏吧原创 we couldn鈥檛 give a Castlemaine XXXX about that. Armed with a measuring cylinder and stopwatch 鈥� and some beer, of course 鈥� we dared to repeat Leike鈥檚 experiment. Our mission: to find a beer whose foam really does obey nature鈥檚 universal rule.

Leike discovered that as the foamy head on his beer faded away it followed an exponential decay. This kind of law is found in simple random processes such as radioactive decay, and can easily be seen if you plot a substance鈥檚 radioactivity 鈥� which you can measure using a Geiger counter 鈥� against time.

Exponential curves have an inbuilt scale factor: the time it takes for the number of clicks to halve, for instance, is the same no matter where your starting point. If the bubbles burst randomly, then you鈥檇 expect the foam to decay exponentially, says Denis Weaire, an expert in the physics of foam at Trinity College Dublin. But they don鈥檛.

The moment you pour a glass of beer, he says, physics sets to work. Gravity pulls the liquid in the foam downwards to leave a dry froth on top. These dry bubbles are fragile and look nothing like spherical soap bubbles: they are tetrahedral and burst easily because their skin is very thin. To make things worse, gas diffuses in and out of the bubbles all the time. In contrast, the wet bubbles at the bottom of the froth are small, round and much more stable. Froth, it seems, is complex stuff with intricate interactions between the bubbles. Which is why it could well follow a power law.

All kinds of other natural systems obey power laws. Record all the earthquakes on the planet for one year, for example, plot the number of quakes of each magnitude against the magnitude itself and the kind of curve you get follows a power law: the frequency of the quakes falls off in inverse proportion to their magnitude raised to some power. The stock market and the spread of disease also behave in the same way. In fact, power laws describe any system in a 鈥渃ritical state鈥� 鈥� a state where a kind of universal order can spread through the system regardless of its details.

The only way to find out what鈥檚 really happening is to do our own experiment. But first we had to choose our beer.

A trip to the local pub reveals a huge variety of beers, with a wealth of strengths and tastes. This variation is down to the ingredients used, and the way the beer is processed. That said, lagers and ales all contain the vital ingredients for froth. The most important being the bubbles of carbon dioxide produced during fermentation.

But proteins, which act much like detergents, and polysaccharides also contribute to the natural foaminess. Brewers often add wheat and corn to beer because these cereals contain more proteins that link together to form a thin film around the bubbles, enhancing and stabilising the foam.

Armed with seven bottled and canned beers, we went to the experts for advice. Iain Low from Britain鈥檚 Campaign for Real Ale told us how to pour the perfect beer. As every beer drinker knows, the height of the head depends on the angle of the glass and the speed and height of pouring. 鈥淢ake sure the glass is perfectly clean and free from grease,鈥� recommends Low. Grease kills the head because the proteins in the foam dissolve easily in fat rather than staying stretched around the bubbles. But pouring beer into a measuring cylinder, even a squeaky clean one, isn鈥檛 the same as pouring it into a glass. For starters, the tall narrow cylinder gives a head of foam up to 30 centimetres tall, high enough to horrify most beer lovers.

Leike had another important tip for us: 鈥淒on鈥檛 test them all in one night,鈥� he advised. Throwing caution to the wind, we plotted the height of the foam against time for all seven beers (see Diagram). Stella Artois went flat rapidly, as large bubbles formed at the surface and burst quickly. Guinness did the opposite. 鈥淕uinness prides itself on the head of its beer and the company works hard to stabilise the bubbles,鈥� says Weaire. For one thing, there鈥檚 a lot more protein in Guinness than in lager. And its bubbles are small and contain a lot of nitrogen that won鈥檛 dissolve easily in water.

So do any of our beers obey a power law? Sadly all the beer foams we tested followed the exponential law, just as Leike found. Yet with over 100,000 different beers and lagers produced around the world, there鈥檚 bound to be at least one out there that fits the bill. And if we get fed up with beer, we could move on to champagne鈥� If only getting to the bottom of this problem was as easy as getting to the bottom of the glass.