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To my parents,
Jan and Susan

While I was a university student, I spent a while doing physics revision at my Nana’s house. Nana, a down-to-earth northerner, was very impressed when I told her that I was studying the structure of the atom.

‘Ooh,’ she said, ‘and what can you do when you know that?’

It is a very good question.

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 torch and set out into the darkness. The torch 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 signalling 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.

I studied physics because it explained things that I was interested in. It allowed me to look around and see the mechanisms making our everyday world tick. Best of all, it let me work some of them out for myself. Even though I’m a professional physicist now, lots of the things I’ve worked out for myself haven’t involved laboratories or complicated computer software or expensive experiments. The most satisfying discoveries have come from random things I was playing with when I wasn’t meant to be doing science at all. Knowing about some basic bits of physics turns the world into a toybox.

There is sometimes a bit of snobbery about the science found in kitchens and gardens and city streets. It’s seen as something to occupy children with, a trivial distraction which is important for the young, but of no real use to adults. An adult might buy a book about how the universe works, and that’s seen as being a proper adult topic. But that attitude misses something very important: the same physics applies everywhere. A toaster can teach you about some of the most fundamental laws of physics, and the benefit of a toaster is that you’ve probably got one, and you can see it working for yourself. Physics is awesome precisely because the same patterns are universal: they exist both in the kitchen and in the furthest reaches of the universe. The advantage of looking at the toaster first is that even if you never get to worry about the temperature of the universe, you still know why your toast is hot. But once you’re familiar with the pattern, you will recognize it in many other places, and some of those other places will be the most impressive achievements of human society. Learning the science of the everyday is a direct route to the background knowledge about the world that every citizen needs in order to participate fully in society.

Have you ever had to tell apart a raw egg and a boiled egg, without taking their shells off? There’s an easy way to do it. Put the egg down on a smooth, hard surface and set it spinning. After a few seconds, briefly touch the outside of the shell with one finger, just enough to stop the egg’s rotation. The egg might just sit there, stationary. But after a second or two, it might slowly start to spin again. Raw and boiled eggs look the same on the outside, but their insides are different and that gives the secret away. When you touched the cooked egg, you stopped a whole solid object. But when you stopped the raw egg, you only stopped the shell. The liquid inside never stopped swirling around, and so after a second or so, the shell started to rotate again, because it was being dragged around by its insides. If you don’t believe me, go and find an egg and try it. It is a principle of physics that objects tend to continue the same sort of movement unless you push or pull on them. In this case, the total amount of spin of the egg white stays the same because it had no reason to change. This is known as conservation of angular momentum. And it doesn’t just work in eggs.

The Hubble Space Telescope, an orbiting eye that has been whooshing round our planet since 1990, has produced many thousands of spectacular images of the cosmos. It has sent back pictures of Mars, the rings of Uranus, the oldest stars in the Milky Way, the wonderfully named Sombrero Galaxy and the giant Crab Nebula. But when you’re floating freely in space, how do you hold your position as you gaze on such tiny pinpoints of light? How do you know precisely which way you’re facing? Hubble has six gyroscopes, each of which is a wheel spinning at 19,200 revolutions per minute. Conservation of angular momentum means that those wheels will keep spinning at that rate because there is nothing to slow them down. And the spin axis will stay pointed in precisely the same direction, because it has no reason to move. The gyroscopes give Hubble a reference direction, so that its optics can stay locked on a distant object for as long as necessary. The physical principle used to orient one of the most advanced technologies our civilization can produce can be demonstrated with an egg in your kitchen.

This is why I love physics. Everything you learn will come in useful somewhere else, and it’s all one big adventure because you don’t know where it will take you next. As far as we know, the physical laws we observe here on Earth apply everywhere in the universe. Many of the nuts and bolts of our universe are accessible to everyone. You can test them for yourself. What you can learn with an egg hatches into a principle that applies everywhere. You step outside armed with your hatchling, and the world looks different.

In the past, information was treasured more than it is now. Each nugget was hard-earned and valuable. These days, we live on the shore of an ocean of knowledge, one with regular tsunamis that threaten our sanity. If you can manage your life as you are, why seek more knowledge and therefore more complications? The Hubble Space Telescope is all very nice, but unless it’s also going to look downwards once in a while to find your keys when you’re late for a meeting, does it make any difference?

Humans are curious about the world, and we get a lot of joy from satisfying our curiosity. The process is even more rewarding if you work things out for yourself, or if you share the journey of discovery with others. And the physical principles you learn from playing also apply to new medical technologies, the weather, mobile phones, self-cleaning clothes and fusion reactors. Modern life is full of complex decisions: is it worth paying more for a compact fluorescent light bulb? Is it safe to sleep with my phone next to my bed? Should I trust the weather forecast? What difference does it make if my sunglasses have polarizing lenses? The basic principles alone often won’t provide specific answers, but they’ll provide the context needed to ask the right questions. And if we’re used to working things out for ourselves, we won’t feel helpless when the answer isn’t obvious on the first try. We’ll know that with a bit of extra thinking, we can clarify things. Critical thinking is essential to make sense of our world, especially with advertisers and politicians all telling us loudly that they know best. We need to be able to look at the evidence and work out whether we agree with them. And there’s more than our own daily lives at stake. We are responsible for our civilization. We vote, we choose what to buy and how to live, and we are collectively part of the human journey. No one can understand every single detail of our complex world, but the basic principles are fantastically valuable tools to take with you on the way.

Because of all this, I think that playing with the physical toys in the world around us is more than ‘just fun’, even though I’m a huge fan of fun for its own sake. Science isn’t just about collecting facts; it’s a logical process for working things out. The point of science is that everyone can look at the data and come to a reasoned conclusion. At first, those conclusions may differ, but then you go and collect more data that helps you decide between one description of the world and another, and eventually the conclusions converge. This is what separates science from other disciplines – a scientific hypothesis must make specific testable predictions. That means that if you have an idea about how you think something works, the next thing to do is to work out what the consequences of your idea would be. In particular, you have to look hard for consequences that you can check for, and especially for consequences that you can prove wrong. If your hypothesis passes every test we can think of, we cautiously agree that this is probably a good model for the way the world works. Science is always trying to prove itself wrong, because that’s the quickest route to finding out what’s actually going on.

You don’t have to be a qualified scientist to experiment with the world. Knowing some basic physical principles will set you on the right track to work a lot of things out for yourself. Sometimes, it doesn’t even have to be an organized process – the jigsaw pieces almost slot themselves into place.

One of my favourite voyages of discovery started with disappointment: I made blueberry jam and it turned out pink. Bright fuchsia pink. It happened a few years ago, when I was living in Rhode Island, sorting out the last bits and pieces before moving back to the UK. Most things were done, but there was one last project that I was adamant about fitting in before I left. I had always loved blueberries – they were slightly exotic, delicious, and beautifully and bizarrely blue. In most places I’ve lived they come in frustratingly small quantities, but in Rhode Island they grow in abundance. I wanted to convert some of the summer blueberry bounty into blue jam to take back to the UK. So I spent one of my last mornings there picking and sorting blueberries.

The most important and exciting thing about blueberry jam is surely that it is blue. I thought so, anyway. But nature had other ideas. The pan of bubbling jam was many things, but blue was not one of them. I filled the jam jars, and the jam really did taste lovely. But the lingering disappointment and confusion followed me and my pink jam back to the UK.

Six months later, I was asked by a friend to help with a historical conundrum. He was making a TV programme about witches, and he said that there were records of ‘wise women’ boiling verbena petals in water and putting the resulting liquid on people’s skin as a way of telling whether they were bewitched. He wondered whether they were measuring something systematically, even if it wasn’t what they intended. I did a bit of research and found that maybe they were.

Purple verbena flowers, along with red cabbage, blood oranges and lots of other red and purple plants, contain chemical compounds called anthocyanins. These anthocyanins are pigments, and they give the plants their bright colours. There are a few different versions, so the colour varies a bit, but they all have a similar molecular structure. That’s not all, though. The colour also depends on the acidity of the liquid that the molecule is in – what’s called its ‘pH value’. If you make that environment a little more acidic or a little more alkaline, the molecules change shape slightly and so their colour changes. They are indicators, nature’s version of litmus paper.

You can have lots of fun in the kitchen with this. You need to boil the plant to get the pigment out, so boil a bit of red cabbage in water, and then save the water (which is now purple). Mix some with vinegar, and it goes red. A solution of laundry powder (a strong alkali) makes it go yellow or green. You can generate a whole rainbow of outcomes just from what’s in your kitchen. I know: I did it. I love this discovery because these anthocyanins are everywhere, and accessible to anyone. No chemistry set required!

So maybe these wise women were using the verbena flowers to test for pH, not bewitchment. Your skin pH can vary naturally, and putting the verbena concoction on skin could produce different colours for different people. I could make cabbage water go from purple to blue when I was nice and sweaty after a long run, but it didn’t change colour when I hadn’t been exercising. The wise women may have noticed that different people made the verbena pigments change in different ways, and put their own interpretation on it. We’ll never know for sure, but it seems to me to be a reasonable hypothesis.

So much for history. And then I remembered the blueberries and the jam. Blueberries are blue because they contain anthocyanins. Jam has only four ingredients: fruit, sugar, water and lemon juice. The lemon juice helps the natural pectin from the fruit do its job of making the jam set. It does that because … it’s acid. My blueberry jam was pink because the boiled blueberries were acting as a saucepan-sized litmus test. It had to be pink for the jam to set properly. The excitement of working that out almost made up for the disappointment of never having made blue jam. Almost. But the discovery that there’s a whole rainbow of colour to be had from just one fruit is the sort of treasure that’s worth the sacrifice.

This book is about linking the little things we see every day with the big world we live in. It’s a romp through the physical world, showing how playing with things like popcorn, coffee stains and refrigerator magnets can shed light on Scott’s expeditions, medical tests and solving our future energy needs. Science is not about ‘them’, it’s about ‘us’, and we can all go on this adventure in our own way. Each chapter begins with something small in the everyday world, something that we will have seen many times but may never have thought about. By the end of each chapter, we’ll see the same patterns explaining some of the most important science and technology of our time. Each mini-quest is rewarding in itself, but the real payoff comes when the pieces are put together.

There’s another benefit to knowing about how the world works, and it’s one that scientists don’t talk about often enough. Seeing what makes the world tick changes your perspective. The world is a mosaic of physical patterns, and once you’re familiar with the basics, you start to see how those patterns fit together. I hope that as you read this book, the scientific hatchlings from the chapters along the way will grow into a different way of seeing the world. The final chapter of this book is an exploration of how the patterns interlock to form our three life-support systems – the human body, our planet and our civilization. But you don’t have to agree with my perspective. The essence of science is experimenting with the principles for yourself, considering all the evidence available and then reaching your own conclusions.

Popcorn and rockets

The gas laws

EXPLOSIONS IN THE kitchen are generally considered a bad idea. But just occasionally, a small one can produce something delicious. A dried corn kernel contains lots of nice food-like components – carbohydrates, proteins, iron and potassium – but they’re very densely packed and there’s a tough armoured shell in the way. The potential is tantalizing, but to make it edible you need some extreme reorganization. An explosion is just the ticket, and very conveniently, this seed carries the seeds of its own destruction within it. Last night, I did a bit of ballistic cooking and made popcorn. It’s always a relief to discover that a tough, unwelcoming exterior can conceal a softer inside – but why does this one make fluff instead of blowing itself to bits?

Once the oil in the pan was hot, I added a spoonful of kernels, put the lid on, and left it while I put the kettle on to make tea. Outside, a huge storm was raging, and chunky raindrops were hammering against the window. The corn sat in the oil and hissed gently. It looked to me as though nothing was happening, but inside the pan, the show had already started. Each corn kernel contains a germ, which is the start of a new plant, and the endosperm, which is there as food for the new plant. The endosperm is made up of starch packaged into granules, and it contains about 14 per cent water. As the kernels sat in the hot oil, that water was starting to evaporate, turning into steam. Hotter molecules move faster, so that as each kernel heated up, there were more and more water molecules whooshing around inside it as steam. The evolutionary purpose of a corn kernel’s shell is to withstand assault from outside, but it now had to contain an internal rebellion – and it was acting like a mini pressure cooker. The water molecules that had turned to steam were trapped with nowhere to go, so the pressure inside was building up. Molecules of gas were continually bumping into each other and into the walls of the container, and as the number of gas molecules increased and they moved faster, they were hammering harder and harder on the inside of the shell.

Pressure cookers work because hot steam cooks things very effectively, and it’s no different inside popcorn. As I searched for teabags, the starch granules were being cooked into a pressurized gelatinous goo, and the pressure kept going up. The outer shell of a popcorn kernel can withstand this stress, but only up to a point. When the temperature inside approaches 180°C and the pressure gets up to nearly ten times the normal pressure of the air around us, the goo is on the edge of victory.

I gave the pan a little shake and heard the first dull pop echoing round the inside. After a couple of seconds, it sounded as though a mini machine gun was being fired in there, and I could see the lid lifting as it got hit from underneath. Each individual pop also came with a fairly impressive puff of steam from the edge of the pan lid. I left it for a moment to pour a cup of tea, and in those few seconds, the barrage from underneath shifted the lid and fluff started taking flight.

At the moment of catastrophe, the rules change. Until that point, a fixed amount of water vapour is confined, and the pressure it exerts on the inside of the shell increases as the temperature increases. But when the hard shell finally succumbs, the insides are exposed to the atmospheric pressure in the rest of the pan and there is no volume limit any more. The starchy goo is still full of hot hammering molecules but nothing is pushing back from the other side. So it expands explosively, until the pressure inside matches the pressure outside. Compact white goo becomes expansive white fluffy foam, turning the entire kernel inside-out; and as it cools, it solidifies. The transformation is complete.

Tipping the popped corn out revealed a few casualties left behind. Dark burnt unpopped corn rattled sadly round the bottom of the pan. If the outer shell is damaged, water vapour escapes as it is heated, and the pressure never builds up. The reason that popcorn pops and other grains don’t is that all the others have porous shells. If a kernel is too dry, perhaps because it was harvested at the wrong time, there isn’t enough water inside it to build up the pressure needed to burst the shell. Without the violence of an explosion, inedible corn remains inedible.

I took the bowl of perfectly cooked fluff and the tea over to the window and stood watching the storm. Destruction doesn’t always have to be a bad thing.

There is beauty in simplicity. And it’s even more satisfying when that beauty condenses out of complexity. For me, the laws that tell us how gases behave are like one of those optical illusions where you think you’re seeing one thing, and then you blink and look again and see something completely different.

We live in a world made of atoms. Each of these tiny specks of matter is coated with a distinctive pattern of negatively charged electrons, chaperones to the heavy and positively charged nucleus within. Chemistry is the story of those chaperones sharing duties between multiple atoms, shifting formation while always obeying the strict rules of the quantum world, and holding the captive nuclei in larger patterns called molecules. In the air I’m breathing as I type this, there are pairs of oxygen atoms (each pair is one oxygen molecule) moving at 900 mph bumping into pairs of nitrogen atoms going at 200 mph, and then maybe bouncing off a water molecule going at over 1,000 mph. It’s horrifically messy and complicated – different atoms, different molecules, different speeds – and in each cubic centimetre of air there are about 30,000,000,000,000,000,000 (3 × 1019) individual molecules, each colliding about a billion times a second. You might think that the sensible approach to all that is to quit while you’re ahead and take up brain surgery or economic theory or hacking supercomputers instead. Something simpler, anyway. So it’s probably just as well that the pioneers who discovered how gases behave had no idea about any of it. Ignorance has its uses. The idea of atoms wasn’t really a part of science until the early 1800s and absolute proof of their existence didn’t turn up until around 1905. Back in 1662, all that Robert Boyle and his assistant Robert Hooke had was glassware, mercury, some trapped air and just the right amount of ignorance. They found that as the pressure on a pocket of air increased, its volume decreased. This is Boyle’s Law, and it says that gas pressure is inversely proportional to volume. A century later, Jacques Charles found that the volume of a gas is directly proportional to its temperature. If you double the temperature, you double the volume. It’s almost unbelievable. How can so much atomic complication lead to something so simple and so consistent?

One last intake of air, one calm flick of its fleshy tail, and the giant leaves the atmosphere behind. Everything this sperm whale needs to live for the next forty-five minutes is stored in its body, and the hunt begins. The prize is a giant squid, a rubbery monster armed with tentacles, vicious suckers and a fearsome beak. To find its prey, the whale must venture deep into the real darkness of the ocean, to the places never touched by sunlight. Routine dives will reach 500–1,000 metres, and the measured record is around 2 km. The whale probes the blackness with highly directional sonar, waiting for the faint echo that suggests dinner might be close. And the giant squid floats unaware and unsuspecting, because it is deaf.

The most precious treasure the whale carries down into the gloom is oxygen, needed to sustain the chemical reactions that power the swimming muscles, and the whale’s very life. But the gaseous oxygen supplied by the atmosphere becomes a liability in the deep – in fact, as soon as the whale leaves the surface, the air in its lungs becomes a problem. For every additional metre it swims downwards, the weight of one extra metre of water presses inwards. Nitrogen and oxygen molecules are bouncing off each other and the lung walls, and each collision provides a minuscule push. At the surface, the inward and outward pushes on the whale balance. But as the giant sinks, it is squashed by the additional weight of the water above it, and the push of the outside overwhelms the push from the inside. So the walls of the lungs move inwards until equilibrium, the point where the pushes are balanced once again. A balance is reached because as the whale’s lung compresses, each of the molecules has less space and collisions between them become more common. That means that there are more molecules hammering outwards on each bit of the lungs, so the pressure inside increases until the hammering molecules can compete equally with those outside. Ten metres of water depth is enough to exert additional pressure equivalent to a whole extra atmosphere. So even at that depth, while it could still easily see the surface (if it were looking), the whale’s lungs reduce to half the volume that they were. That means there are twice as many molecular collisions on the walls, matching the doubled pressure from outside. But the squid might be 1 km below the surface, and at that depth the vast pressure of the water means that the lungs should collapse to a mere 1 per cent of the volume they have at the surface.

Eventually, the whale hears the reflection of one of its loud clicks. With shrunken lungs, and only sonar to guide it, it must now prepare for battle in the vast darkness. The giant squid is armed, and even if it eventually succumbs, the whale may well swim away with horrific scars. Without oxygen from its lungs, how does it even have the energy to fight?

The problem of the shrunken lungs is that if their volume is only one-hundredth of what it was at the surface, the pressure of the gas in there will be one hundred times greater than atmospheric pressure. At the alveoli, the delicate part of the lungs where oxygen and carbon dioxide are exchanged into and out of the blood, this pressure would push both extra nitrogen and extra oxygen to dissolve in the whale’s bloodstream. The result would be an extreme case of what divers call ‘the bends’, and as the whale returned to the surface the extra nitrogen would bubble up in its blood, doing all sorts of damage. The evolutionary solution is to shut off the alveoli completely, from the moment the whale leaves the surface. There is no alternative. But the whale can access its energy reserves because its blood and muscles can store an extraordinary amount of oxygen. A sperm whale has twice as much haemoglobin as a human, and about ten times as much myoglobin (the protein used to store energy in the muscles). While it was at the surface, the whale was recharging these vast reservoirs. Sperm whales are never breathing from their lungs when they make these deep dives. It’s far too dangerous. And they’re not just using their one last breath while they’re underwater. They’re living – and fighting – on the surplus that’s stored in their muscles, the cache gathered during the time they spent at the surface.

No one has ever seen the battle between a sperm whale and a giant squid. But the stomachs of dead sperm whales contain collections of squid beaks, the only part of the squid that can’t be digested. So each whale carries its own internal tally of fights won. As a successful whale swims back towards the sunlight, its lungs gradually re-inflate and reconnect with its blood supply. As the pressure decreases, the volume once again increases until it has reached its original starting point.

Oddly, the combination of complex molecular behaviour with statistics (not usually associated with simplicity) produces a relatively straightforward outcome in practice. There are indeed lots of molecules and lots of collisions and lots of different speeds, but the only two important factors are the range of speeds that the molecules are moving at, and the average number of times they collide with the walls of their container. The number of collisions, and the strength of each collision (due to the speed and mass of that molecule) determine the pressure. The push made by all that compared with the push from the outside determines the volume. And then the temperature has a slightly different effect.

‘Who would normally be worried at this point?’ Our teacher, Adam, is wearing a white tunic stretched over a happily solid belly, exactly what central casting demands of a jolly baker. The strong cockney accent is just a bonus. He pokes at the sad splat of dough on the table in front of him, and it clings on as though it’s alive – which, of course, it is. ‘What we need for good bread,’ he announces, ‘is air.’ I’m at a bakery school being taught how to make focaccia, a traditional Italian bread. I’m pretty sure I haven’t worn an apron since I was ten. And although I’ve baked lots of bread, I’ve never seen dough that looked like the splat, so I’m learning already.

Following Adam’s instructions, we obediently start our own dough from scratch. Each of us mixes fresh yeast with water and then with the flour and salt, and works the dough with therapeutic vigour to develop the gluten, the protein that gives bread its elasticity. The whole time we’re stretching and tearing the physical structure, the living yeast that’s carried along in that structure is busy fermenting sugars and making carbon dioxide. This dough, just like all the others I’ve ever made, doesn’t have any air in it at all – it just has lots of carbon dioxide bubbles. It’s a stretchy sticky golden bioreactor, and the products of the life in it are trapped, so it rises. When this first stage is done, it gets a nice bath in olive oil and keeps rising, while we clean dough off our hands, the table, and a surprising amount of the surroundings. Each individual fermentation reaction produces two molecules of carbon dioxide which are expelled by the yeast. Carbon dioxide, or CO2 – two oxygen atoms stuck to a carbon atom – is a small unreactive molecule, and at room temperature it has enough energy to float free as a gas. Once it’s found its way into a bubble with lots of other CO2 molecules, it will play bumper cars for hours. Each time it hits another molecule, there is likely to be some energy exchange, just like a cue ball hitting a snooker ball. Sometimes one will slow down almost completely and the other will take all that energy and zoom off at high speed. Sometimes the energy is shared between them. Every time a molecule bumps into the gluten-rich wall of the bubble, it pushes on the wall as it bounces off. At this stage, this is what makes the bubbles grow – as each one acquires more molecules on the inside, the push outward gets more and more insistent. So the bubble expands until the push back from the atmosphere balances the outward push of the CO2 molecules. Sometimes the CO2 molecules are travelling quickly when they hit the wall and sometimes they’re travelling slowly. Bread bakers, like physicists, don’t care which molecules hit which walls at particular speeds, because this is a game of statistics. At room temperature and atmospheric pressure, 29 per cent of them are travelling between 350 and 500 metres per second, and it doesn’t matter which ones they are.

Adam claps his hands to get our attention, and uncovers the rising dough with a magician’s flourish. And then he does something that is new to me. He stretches out the oil-covered dough and folds it over on itself, one fold from each side. The aim is to trap air between the folds. My initial unspoken response is ‘That’s cheating!’, because I had always assumed that all the ‘air’ in bread was CO2 from the yeast. I once saw an origami master in Japan enthusiastically teaching his students about the correct application of sellotape to an angular paper horse, and I felt the same unreasonable outrage then as in the bakery. But if you want air, why not use air? Once it’s cooked, no one will know. I succumb to the knowledge of the expert and meekly fold my own dough. A couple of hours later, after more rising and folding and the incorporation of more olive oil than I had believed possible, my nascent focaccia and its bubbles were ready for the oven. The ‘air’ of both types was about to have its moment.

Inside the oven, heat energy flowed into the bread. The pressure in the oven was still the same as the pressure outside, but the temperature in the bread had suddenly gone up from 20°C to 250°C. In absolute units, that’s from 293 Kelvin to 523 Kelvin, almost a doubling of temperature.^fn1 In a gas, that means that the molecules speed up. The bit that’s counter-intuitive is that no individual molecule has its own temperature. A gas – a cluster of molecules – can have a temperature, but an individual molecule within it can’t. Gas temperature is just a way of expressing how much movement energy the molecules have on average, but each individual molecule is constantly speeding up and slowing down, exchanging its energy with the others as they collide. Any individual molecule is just playing bumper cars with the energy it’s got right now. The faster they travel, the harder they bump into the sides of the bubbles, so the greater the pressure they generate. As the bread went into the oven, gas molecules suddenly gained lots more heat energy and so they sped up. The average speed shifted from 480 metres per second to 660 metres per second. So the outward push on the bubble walls got much harder and the outsides weren’t pushing back. Each bubble expanded in proportion to the temperature, pushing outward on the dough and forcing it to expand. And here’s the thing … the air bubbles (mostly nitrogen and oxygen) expanded in exactly the same way as the CO2 bubbles. This is the last piece of the puzzle. It turns out that it doesn’t matter what the molecules are. If you double the temperature you still double the volume (if you keep the pressure constant). Or, if you keep the volume constant and double the temperature, the pressure will double. The complication of having a mix of different atoms present is irrelevant, because the statistics are the same for any mixture. No one looking at the final bread could ever tell which bubbles had been CO2 and which ones had been air. And then the protein and carbohydrate matrix surrounding the bubbles cooked and solidified. The bubble size was fixed. Fluffy white focaccia was assured.

The way that gases behave is described by something called ‘the ideal gas law’, and the idealism is justified by the fact that it works. It works spectacularly well. It says that for a fixed mass of gas the pressure is inversely proportional to the volume (if you double the pressure, you halve the volume), the temperature is proportional to the pressure (if you double the temperature, you double the pressure) and the volume is proportional to the temperature, at fixed pressure. It doesn’t matter what the gas is, only how many molecules of it there are. The ideal gas law is what drives the internal combustion engine, hot air balloons – and popcorn. And it applies not only when things heat up, but also when they cool down.

Reaching the South Pole was a major landmark in human history. The great polar explorers – Amundsen, Scott, Shackleton and others – are legendary figures, and the books about their achievements and failures are some of the greatest adventure stories of all time. And as if it wasn’t enough to deal with unimaginable cold, lack of food, fierce oceans and clothing that wasn’t up to the task, the mighty ideal gas law was against them, quite literally.

The centre of Antarctica is a high, dry plateau. It is covered in deep ice, but it hardly ever snows. The bright white surface reflects almost all of the feeble sunlight back into space, and temperatures can drop below −80°C. It is quiet. At an atomic level, the atmosphere here is sluggish, because the air molecules have little energy (due to the cold) and are moving relatively slowly. Air from above descends on to the plateau, and the ice steals its heat. Cold air becomes colder. The pressure is fixed, so this air shrinks in volume and becomes more dense. The molecules are closer together, moving more slowly, unable to push outwards hard enough to compete with the air around them pushing inwards. As the land slopes away from the centre of the continent towards the ocean, so this cold, dense air also slithers away from the centre along the surface, unstoppable, like a slow waterfall of air. It is funnelled through vast valleys, picking up speed as the funnels descend outwards, always outwards towards the ocean. These are the katabatic winds of Antarctica, and if you want to walk to the South Pole, they will be in your face all the way. It’s hard to think of a worse trick nature could have played on those polar explorers.

‘Katabatic’ is just a name for this sort of wind, and it’s found in many places, not all of them cold. As they descend, those sluggish molecules do warm up, just a little bit. And the consequences of that warming can be dramatic.

In 2007, I was living in San Diego and working at the Scripps Institution of Oceanography. As a northerner I was slightly suspicious of the eternal sunshine, but I got to swim in a 50-metre outdoor pool every morning so I couldn’t really complain. And the sunsets were amazing. San Diego is on the coast with a clear view west across the Pacific Ocean, and the evening skyline was reliably stunning.

I really missed the seasons, though. It seemed as though time never moved on, almost like living in a dream. But then the Santa Ana winds came, and it went from sunny and warm and cheerful to sullenly hot and dry. The Santa Ana winds come every autumn, as air pours off the high deserts and flows over the coast of California out towards the ocean. These are also katabatic winds, just like the ones in Antarctica. But by the time they reach the ocean, the air is much hotter at the coast than it was on the high plateau. One memorable day, I was driving north up the I-5 freeway, towards one of the big valleys that funnelled the hot air out to sea. There was a river of low cloud sitting in the valley. My boyfriend at the time was driving. ‘Can you smell smoke?’ I asked. ‘Don’t be silly,’ he said. But the next morning, I woke up in a weird world. There were huge wildfires to the north of San Diego, marching across the valleys, and there was ash in the air. A campfire had got out of control in the hot, dry conditions, and the winds were blowing the fire towards the coast. That river of cloud had been smoke. People went to work, and either were sent home or sat in huddles listening to the radio and wondering whether their houses were safe. We waited. The horizon was hazy because of ash clouds you could see from space, but the sunsets were spectacular. After three days, the smoke started to lift. People I knew had lost their houses to the flames. Everything had a layer of ash on it and health officials were advising against any outdoor exercise for a week.

Up on the high plateau, hot desert air had cooled, become more dense, and slithered downslope, just like the winds that faced Scott in Antarctica. But the wildfires started because that air wasn’t only dry, it was hot. Why would it get hotter as it came downhill? Where does the energy come from? The ideal gas law still applies – this was a fixed mass of air, and it was moving so quickly that there was no time for it to exchange energy with its surroundings. As that stream of dense air made its way downhill, the atmosphere that was already at the bottom of the hill pushed on it, because the pressure down there was higher. Pushing on something is a way of giving it energy. You can imagine individual air molecules hitting the wall of a balloon that is moving towards them. They’ll bounce off with more energy than they had to start with, because they’re bouncing off a moving surface. So the volume of the air in the Santa Ana winds decreased because it was squeezed inwards by the surrounding atmosphere. That squeezing gave the travelling air molecules extra energy, and so the temperature of the wind increased. It’s called adiabatic heating. Every year, when the Santa Ana winds come, everyone in California is extra vigilant about open fires. After a few days of such hot, dry air stealing the moisture from the landscape, sparks can easily turn into wildfires. And the heat doesn’t just come from the California sun – it also comes from the extra energy given to the gas molecules as they are compressed by denser air closer to the ocean. Anything that changes the average speed of air molecules will change the temperature.

The same thing happens in reverse when you squirt whipped cream out of a can. The air that comes out in the cream has expanded suddenly and pushed on its surroundings, so it has given away energy and cooled down. The nozzle of the squirty cream canister feels cold to the touch for this reason – the gas that’s coming through it is giving away its energy as it reaches the free atmosphere. Less energy is left behind, so the can feels cool.

Air pressure is just a measure of how hard all those tiny molecules are hammering on a surface. Normally we don’t notice it much because the hammering is the same from every direction – if I hold up a piece of paper, it doesn’t move because it’s getting pushed equally from both sides. Each one of us is getting pushed by air all the time, and we hardly feel it at all. So it took people a long time to work out how hard that push actually is, and when it came along, the answer was a bit of a shock. The magnitude of the discovery was easy to appreciate because the demonstration was unusually memorable. It’s not often that an important scientific experiment is also set up to be a theatrical spectacle, but this one had all the proper ingredients: horses, suspense, an astonishing outcome and the Holy Roman Emperor looking on.

The difficulty was that to work out how hard air is pushing on something, you really need to take away all the air on the other side of it, leaving behind a vacuum. In the fourth century BC, Aristotle had declared that ‘nature abhors a vacuum’, and that was still the prevailing view nearly a thousand years later. Creating a vacuum seemed out of the question. But some time around 1650, Otto von Guericke invented the first vacuum pump. Instead of writing a technical paper about it and disappearing into obscurity, he chose spectacle to make his point.^fn2 It probably helped that he was a well-known politician and diplomat, and was on good terms with the rulers of his day.

STORM IN A TEACUP

The Physics of Everyday Life

HELEN CZERSKI

Introduction

1

Popcorn and rockets

The gas laws